[1/8] kcsan: Add Kernel Concurrency Sanitizer infrastructure
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Message ID 20191016083959.186860-2-elver@google.com
State New
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Series
  • Add Kernel Concurrency Sanitizer (KCSAN)
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Commit Message

Marco Elver Oct. 16, 2019, 8:39 a.m. UTC
Kernel Concurrency Sanitizer (KCSAN) is a dynamic data-race detector for
kernel space. KCSAN is a sampling watchpoint-based data-race detector.
See the included Documentation/dev-tools/kcsan.rst for more details.

This patch adds basic infrastructure, but does not yet enable KCSAN for
any architecture.

Signed-off-by: Marco Elver <elver@google.com>
---
 Documentation/dev-tools/kcsan.rst | 202 +++++++++++++
 MAINTAINERS                       |  11 +
 Makefile                          |   3 +-
 include/linux/compiler-clang.h    |   9 +
 include/linux/compiler-gcc.h      |   7 +
 include/linux/compiler.h          |  35 ++-
 include/linux/kcsan-checks.h      | 116 ++++++++
 include/linux/kcsan.h             |  85 ++++++
 include/linux/sched.h             |   7 +
 init/init_task.c                  |   6 +
 init/main.c                       |   2 +
 kernel/Makefile                   |   1 +
 kernel/kcsan/Makefile             |  14 +
 kernel/kcsan/atomic.c             |  21 ++
 kernel/kcsan/core.c               | 458 ++++++++++++++++++++++++++++++
 kernel/kcsan/debugfs.c            | 225 +++++++++++++++
 kernel/kcsan/encoding.h           |  94 ++++++
 kernel/kcsan/kcsan.c              |  81 ++++++
 kernel/kcsan/kcsan.h              | 140 +++++++++
 kernel/kcsan/report.c             | 307 ++++++++++++++++++++
 kernel/kcsan/test.c               | 117 ++++++++
 lib/Kconfig.debug                 |   2 +
 lib/Kconfig.kcsan                 |  88 ++++++
 lib/Makefile                      |   3 +
 scripts/Makefile.kcsan            |   6 +
 scripts/Makefile.lib              |  10 +
 26 files changed, 2041 insertions(+), 9 deletions(-)
 create mode 100644 Documentation/dev-tools/kcsan.rst
 create mode 100644 include/linux/kcsan-checks.h
 create mode 100644 include/linux/kcsan.h
 create mode 100644 kernel/kcsan/Makefile
 create mode 100644 kernel/kcsan/atomic.c
 create mode 100644 kernel/kcsan/core.c
 create mode 100644 kernel/kcsan/debugfs.c
 create mode 100644 kernel/kcsan/encoding.h
 create mode 100644 kernel/kcsan/kcsan.c
 create mode 100644 kernel/kcsan/kcsan.h
 create mode 100644 kernel/kcsan/report.c
 create mode 100644 kernel/kcsan/test.c
 create mode 100644 lib/Kconfig.kcsan
 create mode 100644 scripts/Makefile.kcsan

Comments

Boqun Feng Oct. 16, 2019, 9:42 a.m. UTC | #1
Hi Marco,

On Wed, Oct 16, 2019 at 10:39:52AM +0200, Marco Elver wrote:
[...]
> --- /dev/null
> +++ b/kernel/kcsan/kcsan.c
> @@ -0,0 +1,81 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +/*
> + * The Kernel Concurrency Sanitizer (KCSAN) infrastructure. For more info please
> + * see Documentation/dev-tools/kcsan.rst.
> + */
> +
> +#include <linux/export.h>
> +
> +#include "kcsan.h"
> +
> +/*
> + * Concurrency Sanitizer uses the same instrumentation as Thread Sanitizer.

Is there any documentation on the instrumentation? Like a complete list
for all instrumentation functions plus a description of where the
compiler will use those functions. Yes, the names of the below functions
are straightforward, but an accurate doc on the instrumentation will
cerntainly help people review KCSAN.

Regards,
Boqun

> + */
> +
> +#define DEFINE_TSAN_READ_WRITE(size)                                           \
> +	void __tsan_read##size(void *ptr)                                      \
> +	{                                                                      \
> +		__kcsan_check_access(ptr, size, false);                        \
> +	}                                                                      \
> +	EXPORT_SYMBOL(__tsan_read##size);                                      \
> +	void __tsan_write##size(void *ptr)                                     \
> +	{                                                                      \
> +		__kcsan_check_access(ptr, size, true);                         \
> +	}                                                                      \
> +	EXPORT_SYMBOL(__tsan_write##size)
> +
> +DEFINE_TSAN_READ_WRITE(1);
> +DEFINE_TSAN_READ_WRITE(2);
> +DEFINE_TSAN_READ_WRITE(4);
> +DEFINE_TSAN_READ_WRITE(8);
> +DEFINE_TSAN_READ_WRITE(16);
> +
> +/*
> + * Not all supported compiler versions distinguish aligned/unaligned accesses,
> + * but e.g. recent versions of Clang do.
> + */
> +#define DEFINE_TSAN_UNALIGNED_READ_WRITE(size)                                 \
> +	void __tsan_unaligned_read##size(void *ptr)                            \
> +	{                                                                      \
> +		__kcsan_check_access(ptr, size, false);                        \
> +	}                                                                      \
> +	EXPORT_SYMBOL(__tsan_unaligned_read##size);                            \
> +	void __tsan_unaligned_write##size(void *ptr)                           \
> +	{                                                                      \
> +		__kcsan_check_access(ptr, size, true);                         \
> +	}                                                                      \
> +	EXPORT_SYMBOL(__tsan_unaligned_write##size)
> +
> +DEFINE_TSAN_UNALIGNED_READ_WRITE(2);
> +DEFINE_TSAN_UNALIGNED_READ_WRITE(4);
> +DEFINE_TSAN_UNALIGNED_READ_WRITE(8);
> +DEFINE_TSAN_UNALIGNED_READ_WRITE(16);
> +
> +void __tsan_read_range(void *ptr, size_t size)
> +{
> +	__kcsan_check_access(ptr, size, false);
> +}
> +EXPORT_SYMBOL(__tsan_read_range);
> +
> +void __tsan_write_range(void *ptr, size_t size)
> +{
> +	__kcsan_check_access(ptr, size, true);
> +}
> +EXPORT_SYMBOL(__tsan_write_range);
> +
> +/*
> + * The below are not required KCSAN, but can still be emitted by the compiler.
> + */
> +void __tsan_func_entry(void *call_pc)
> +{
> +}
> +EXPORT_SYMBOL(__tsan_func_entry);
> +void __tsan_func_exit(void)
> +{
> +}
> +EXPORT_SYMBOL(__tsan_func_exit);
> +void __tsan_init(void)
> +{
> +}
> +EXPORT_SYMBOL(__tsan_init);
[...]
Marco Elver Oct. 16, 2019, 10:06 a.m. UTC | #2
On Wed, 16 Oct 2019 at 11:42, Boqun Feng <boqun.feng@gmail.com> wrote:
>
> Hi Marco,
>
> On Wed, Oct 16, 2019 at 10:39:52AM +0200, Marco Elver wrote:
> [...]
> > --- /dev/null
> > +++ b/kernel/kcsan/kcsan.c
> > @@ -0,0 +1,81 @@
> > +// SPDX-License-Identifier: GPL-2.0
> > +
> > +/*
> > + * The Kernel Concurrency Sanitizer (KCSAN) infrastructure. For more info please
> > + * see Documentation/dev-tools/kcsan.rst.
> > + */
> > +
> > +#include <linux/export.h>
> > +
> > +#include "kcsan.h"
> > +
> > +/*
> > + * Concurrency Sanitizer uses the same instrumentation as Thread Sanitizer.
>
> Is there any documentation on the instrumentation? Like a complete list
> for all instrumentation functions plus a description of where the
> compiler will use those functions. Yes, the names of the below functions
> are straightforward, but an accurate doc on the instrumentation will
> cerntainly help people review KCSAN.

As far as I'm aware neither GCC nor Clang have documentation on the
emitted instrumentation that we could reference (other than look into
the compiler passes).

However it is as straightforward as it seems: the compiler emits
instrumentation calls for all loads and stores that the compiler
generates; inline asm is not instrumented. I will add a comment to
that effect for v2.

Thanks,
-- Marco

> Regards,
> Boqun
>
> > + */
> > +
> > +#define DEFINE_TSAN_READ_WRITE(size)                                           \
> > +     void __tsan_read##size(void *ptr)                                      \
> > +     {                                                                      \
> > +             __kcsan_check_access(ptr, size, false);                        \
> > +     }                                                                      \
> > +     EXPORT_SYMBOL(__tsan_read##size);                                      \
> > +     void __tsan_write##size(void *ptr)                                     \
> > +     {                                                                      \
> > +             __kcsan_check_access(ptr, size, true);                         \
> > +     }                                                                      \
> > +     EXPORT_SYMBOL(__tsan_write##size)
> > +
> > +DEFINE_TSAN_READ_WRITE(1);
> > +DEFINE_TSAN_READ_WRITE(2);
> > +DEFINE_TSAN_READ_WRITE(4);
> > +DEFINE_TSAN_READ_WRITE(8);
> > +DEFINE_TSAN_READ_WRITE(16);
> > +
> > +/*
> > + * Not all supported compiler versions distinguish aligned/unaligned accesses,
> > + * but e.g. recent versions of Clang do.
> > + */
> > +#define DEFINE_TSAN_UNALIGNED_READ_WRITE(size)                                 \
> > +     void __tsan_unaligned_read##size(void *ptr)                            \
> > +     {                                                                      \
> > +             __kcsan_check_access(ptr, size, false);                        \
> > +     }                                                                      \
> > +     EXPORT_SYMBOL(__tsan_unaligned_read##size);                            \
> > +     void __tsan_unaligned_write##size(void *ptr)                           \
> > +     {                                                                      \
> > +             __kcsan_check_access(ptr, size, true);                         \
> > +     }                                                                      \
> > +     EXPORT_SYMBOL(__tsan_unaligned_write##size)
> > +
> > +DEFINE_TSAN_UNALIGNED_READ_WRITE(2);
> > +DEFINE_TSAN_UNALIGNED_READ_WRITE(4);
> > +DEFINE_TSAN_UNALIGNED_READ_WRITE(8);
> > +DEFINE_TSAN_UNALIGNED_READ_WRITE(16);
> > +
> > +void __tsan_read_range(void *ptr, size_t size)
> > +{
> > +     __kcsan_check_access(ptr, size, false);
> > +}
> > +EXPORT_SYMBOL(__tsan_read_range);
> > +
> > +void __tsan_write_range(void *ptr, size_t size)
> > +{
> > +     __kcsan_check_access(ptr, size, true);
> > +}
> > +EXPORT_SYMBOL(__tsan_write_range);
> > +
> > +/*
> > + * The below are not required KCSAN, but can still be emitted by the compiler.
> > + */
> > +void __tsan_func_entry(void *call_pc)
> > +{
> > +}
> > +EXPORT_SYMBOL(__tsan_func_entry);
> > +void __tsan_func_exit(void)
> > +{
> > +}
> > +EXPORT_SYMBOL(__tsan_func_exit);
> > +void __tsan_init(void)
> > +{
> > +}
> > +EXPORT_SYMBOL(__tsan_init);
> [...]
Andrey Konovalov Oct. 16, 2019, 11:49 a.m. UTC | #3
On Wed, Oct 16, 2019 at 10:41 AM Marco Elver <elver@google.com> wrote:
>
> Kernel Concurrency Sanitizer (KCSAN) is a dynamic data-race detector for
> kernel space. KCSAN is a sampling watchpoint-based data-race detector.
> See the included Documentation/dev-tools/kcsan.rst for more details.
>
> This patch adds basic infrastructure, but does not yet enable KCSAN for
> any architecture.
>
> Signed-off-by: Marco Elver <elver@google.com>
> ---
>  Documentation/dev-tools/kcsan.rst | 202 +++++++++++++
>  MAINTAINERS                       |  11 +
>  Makefile                          |   3 +-
>  include/linux/compiler-clang.h    |   9 +
>  include/linux/compiler-gcc.h      |   7 +
>  include/linux/compiler.h          |  35 ++-
>  include/linux/kcsan-checks.h      | 116 ++++++++
>  include/linux/kcsan.h             |  85 ++++++
>  include/linux/sched.h             |   7 +
>  init/init_task.c                  |   6 +
>  init/main.c                       |   2 +
>  kernel/Makefile                   |   1 +
>  kernel/kcsan/Makefile             |  14 +
>  kernel/kcsan/atomic.c             |  21 ++
>  kernel/kcsan/core.c               | 458 ++++++++++++++++++++++++++++++
>  kernel/kcsan/debugfs.c            | 225 +++++++++++++++
>  kernel/kcsan/encoding.h           |  94 ++++++
>  kernel/kcsan/kcsan.c              |  81 ++++++
>  kernel/kcsan/kcsan.h              | 140 +++++++++
>  kernel/kcsan/report.c             | 307 ++++++++++++++++++++
>  kernel/kcsan/test.c               | 117 ++++++++
>  lib/Kconfig.debug                 |   2 +
>  lib/Kconfig.kcsan                 |  88 ++++++
>  lib/Makefile                      |   3 +
>  scripts/Makefile.kcsan            |   6 +
>  scripts/Makefile.lib              |  10 +
>  26 files changed, 2041 insertions(+), 9 deletions(-)
>  create mode 100644 Documentation/dev-tools/kcsan.rst
>  create mode 100644 include/linux/kcsan-checks.h
>  create mode 100644 include/linux/kcsan.h
>  create mode 100644 kernel/kcsan/Makefile
>  create mode 100644 kernel/kcsan/atomic.c
>  create mode 100644 kernel/kcsan/core.c
>  create mode 100644 kernel/kcsan/debugfs.c
>  create mode 100644 kernel/kcsan/encoding.h
>  create mode 100644 kernel/kcsan/kcsan.c
>  create mode 100644 kernel/kcsan/kcsan.h
>  create mode 100644 kernel/kcsan/report.c
>  create mode 100644 kernel/kcsan/test.c
>  create mode 100644 lib/Kconfig.kcsan
>  create mode 100644 scripts/Makefile.kcsan
>
> diff --git a/Documentation/dev-tools/kcsan.rst b/Documentation/dev-tools/kcsan.rst
> new file mode 100644
> index 000000000000..5b46cc5593c3
> --- /dev/null
> +++ b/Documentation/dev-tools/kcsan.rst
> @@ -0,0 +1,202 @@
> +The Kernel Concurrency Sanitizer (KCSAN)
> +========================================
> +
> +Overview
> +--------
> +
> +*Kernel Concurrency Sanitizer (KCSAN)* is a dynamic data-race detector for
> +kernel space. KCSAN is a sampling watchpoint-based data-race detector -- this
> +is unlike Kernel Thread Sanitizer (KTSAN), which is a happens-before data-race
> +detector. Key priorities in KCSAN's design are lack of false positives,
> +scalability, and simplicity. More details can be found in `Implementation
> +Details`_.
> +
> +KCSAN uses compile-time instrumentation to instrument memory accesses. KCSAN is
> +supported in both GCC and Clang. With GCC it requires version 7.3.0 or later.
> +With Clang it requires version 7.0.0 or later.
> +
> +Usage
> +-----
> +
> +To enable KCSAN configure kernel with::
> +
> +    CONFIG_KCSAN = y
> +
> +KCSAN provides several other configuration options to customize behaviour (see
> +their respective help text for more info).
> +
> +debugfs
> +~~~~~~~
> +
> +* The file ``/sys/kernel/debug/kcsan`` can be read to get stats.
> +
> +* KCSAN can be turned on or off by writing ``on`` or ``off`` to
> +  ``/sys/kernel/debug/kcsan``.
> +
> +* Writing ``!some_func_name`` to ``/sys/kernel/debug/kcsan`` adds
> +  ``some_func_name`` to the report filter list, which (by default) blacklists
> +  reporting data-races where either one of the top stackframes are a function
> +  in the list.
> +
> +* Writing either ``blacklist`` or ``whitelist`` to ``/sys/kernel/debug/kcsan``
> +  changes the report filtering behaviour. For example, the blacklist feature
> +  can be used to silence frequently occurring data-races; the whitelist feature
> +  can help with reproduction and testing of fixes.
> +
> +Error reports
> +~~~~~~~~~~~~~
> +
> +A typical data-race report looks like this::
> +
> +    ==================================================================
> +    BUG: KCSAN: data-race in generic_permission / kernfs_refresh_inode
> +
> +    write to 0xffff8fee4c40700c of 4 bytes by task 175 on cpu 4:
> +     kernfs_refresh_inode+0x70/0x170
> +     kernfs_iop_permission+0x4f/0x90
> +     inode_permission+0x190/0x200
> +     link_path_walk.part.0+0x503/0x8e0
> +     path_lookupat.isra.0+0x69/0x4d0
> +     filename_lookup+0x136/0x280
> +     user_path_at_empty+0x47/0x60
> +     vfs_statx+0x9b/0x130
> +     __do_sys_newlstat+0x50/0xb0
> +     __x64_sys_newlstat+0x37/0x50
> +     do_syscall_64+0x85/0x260
> +     entry_SYSCALL_64_after_hwframe+0x44/0xa9
> +
> +    read to 0xffff8fee4c40700c of 4 bytes by task 166 on cpu 6:
> +     generic_permission+0x5b/0x2a0
> +     kernfs_iop_permission+0x66/0x90
> +     inode_permission+0x190/0x200
> +     link_path_walk.part.0+0x503/0x8e0
> +     path_lookupat.isra.0+0x69/0x4d0
> +     filename_lookup+0x136/0x280
> +     user_path_at_empty+0x47/0x60
> +     do_faccessat+0x11a/0x390
> +     __x64_sys_access+0x3c/0x50
> +     do_syscall_64+0x85/0x260
> +     entry_SYSCALL_64_after_hwframe+0x44/0xa9
> +
> +    Reported by Kernel Concurrency Sanitizer on:
> +    CPU: 6 PID: 166 Comm: systemd-journal Not tainted 5.3.0-rc7+ #1
> +    Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.12.0-1 04/01/2014
> +    ==================================================================
> +
> +The header of the report provides a short summary of the functions involved in
> +the race. It is followed by the access types and stack traces of the 2 threads
> +involved in the data-race.
> +
> +The other less common type of data-race report looks like this::
> +
> +    ==================================================================
> +    BUG: KCSAN: racing read in e1000_clean_rx_irq+0x551/0xb10

Do we want to have a different bug title here? Can we also report this
as a data-race to simplify report parsing rules?

> +
> +    race at unknown origin, with read to 0xffff933db8a2ae6c of 1 bytes by interrupt on cpu 0:
> +     e1000_clean_rx_irq+0x551/0xb10
> +     e1000_clean+0x533/0xda0
> +     net_rx_action+0x329/0x900
> +     __do_softirq+0xdb/0x2db
> +     irq_exit+0x9b/0xa0
> +     do_IRQ+0x9c/0xf0
> +     ret_from_intr+0x0/0x18
> +     default_idle+0x3f/0x220
> +     arch_cpu_idle+0x21/0x30
> +     do_idle+0x1df/0x230
> +     cpu_startup_entry+0x14/0x20
> +     rest_init+0xc5/0xcb
> +     arch_call_rest_init+0x13/0x2b
> +     start_kernel+0x6db/0x700
> +
> +    Reported by Kernel Concurrency Sanitizer on:
> +    CPU: 0 PID: 0 Comm: swapper/0 Not tainted 5.3.0-rc7+ #2
> +    Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.12.0-1 04/01/2014
> +    ==================================================================
> +
> +This report is generated where it was not possible to determine the other
> +racing thread, but a race was inferred due to the data-value of the watched
> +memory location having changed. These can occur either due to missing
> +instrumentation or e.g. DMA accesses.
> +
> +Data-Races
> +----------
> +
> +Informally, two operations *conflict* if they access the same memory location,
> +and at least one of them is a write operation. In an execution, two memory
> +operations from different threads form a **data-race** if they *conflict*, at
> +least one of them is a *plain access* (non-atomic), and they are *unordered* in
> +the "happens-before" order according to the `LKMM
> +<../../tools/memory-model/Documentation/explanation.txt>`_.
> +
> +Relationship with the Linux Kernel Memory Model (LKMM)
> +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> +
> +The LKMM defines the propagation and ordering rules of various memory
> +operations, which gives developers the ability to reason about concurrent code.
> +Ultimately this allows to determine the possible executions of concurrent code,
> +and if that code is free from data-races.
> +
> +KCSAN is aware of *atomic* accesses (``READ_ONCE``, ``WRITE_ONCE``,
> +``atomic_*``, etc.), but is oblivious of any ordering guarantees. In other
> +words, KCSAN assumes that as long as a plain access is not observed to race
> +with another conflicting access, memory operations are correctly ordered.
> +
> +This means that KCSAN will not report *potential* data-races due to missing
> +memory ordering. If, however, missing memory ordering (that is observable with
> +a particular compiler and architecture) leads to an observable data-race (e.g.
> +entering a critical section erroneously), KCSAN would report the resulting
> +data-race.
> +
> +Implementation Details
> +----------------------
> +
> +The general approach is inspired by `DataCollider
> +<http://usenix.org/legacy/events/osdi10/tech/full_papers/Erickson.pdf>`_.
> +Unlike DataCollider, KCSAN does not use hardware watchpoints, but instead
> +relies on compiler instrumentation. Watchpoints are implemented using an
> +efficient encoding that stores access type, size, and address in a long; the
> +benefits of using "soft watchpoints" are portability and greater flexibility in
> +limiting which accesses trigger a watchpoint.
> +
> +More specifically, KCSAN requires instrumenting plain (unmarked, non-atomic)
> +memory operations; for each instrumented plain access:
> +
> +1. Check if a matching watchpoint exists; if yes, and at least one access is a
> +   write, then we encountered a racing access.
> +
> +2. Periodically, if no matching watchpoint exists, set up a watchpoint and
> +   stall some delay.
> +
> +3. Also check the data value before the delay, and re-check the data value
> +   after delay; if the values mismatch, we infer a race of unknown origin.
> +
> +To detect data-races between plain and atomic memory operations, KCSAN also
> +annotates atomic accesses, but only to check if a watchpoint exists
> +(``kcsan_check_atomic(..)``); i.e.  KCSAN never sets up a watchpoint on atomic
> +accesses.
> +
> +Key Properties
> +~~~~~~~~~~~~~~
> +
> +1. **Performance Overhead:** KCSAN's runtime is minimal, and does not require
> +   locking shared state for each access. This results in significantly better
> +   performance in comparison with KTSAN.
> +
> +2. **Memory Overhead:** No shadow memory is required. The current
> +   implementation uses a small array of longs to encode watchpoint information,
> +   which is negligible.
> +
> +3. **Memory Ordering:** KCSAN is *not* aware of the LKMM's ordering rules. This
> +   may result in missed data-races (false negatives), compared to a
> +   happens-before data-race detector such as KTSAN.
> +
> +4. **Accuracy:** Imprecise, since it uses a sampling strategy.
> +
> +5. **Annotation Overheads:** Minimal annotation is required outside the KCSAN
> +   runtime. With a happens-before data-race detector, any omission leads to
> +   false positives, which is especially important in the context of the kernel
> +   which includes numerous custom synchronization mechanisms. With KCSAN, as a
> +   result, maintenance overheads are minimal as the kernel evolves.
> +
> +6. **Detects Racy Writes from Devices:** Due to checking data values upon
> +   setting up watchpoints, racy writes from devices can also be detected.

This part compares KCSAN with KTSAN, do we need it here? I think it
might be better to move this to the cover letter as a rationale as to
why we went with the watchpoint based approach, instead of the
happens-before one.

Some performance numbers comparing KCSAN with a non instrumented
kernel would be more useful here.

> diff --git a/MAINTAINERS b/MAINTAINERS
> index 0154674cbad3..71f7fb625490 100644
> --- a/MAINTAINERS
> +++ b/MAINTAINERS
> @@ -8847,6 +8847,17 @@ F:       Documentation/kbuild/kconfig*
>  F:     scripts/kconfig/
>  F:     scripts/Kconfig.include
>
> +KCSAN
> +M:     Marco Elver <elver@google.com>
> +R:     Dmitry Vyukov <dvyukov@google.com>
> +L:     kasan-dev@googlegroups.com
> +S:     Maintained
> +F:     Documentation/dev-tools/kcsan.rst
> +F:     include/linux/kcsan*.h
> +F:     kernel/kcsan/
> +F:     lib/Kconfig.kcsan
> +F:     scripts/Makefile.kcsan
> +
>  KDUMP
>  M:     Dave Young <dyoung@redhat.com>
>  M:     Baoquan He <bhe@redhat.com>
> diff --git a/Makefile b/Makefile
> index ffd7a912fc46..ad4729176252 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -478,7 +478,7 @@ export KBUILD_HOSTCXXFLAGS KBUILD_HOSTLDFLAGS KBUILD_HOSTLDLIBS LDFLAGS_MODULE
>
>  export KBUILD_CPPFLAGS NOSTDINC_FLAGS LINUXINCLUDE OBJCOPYFLAGS KBUILD_LDFLAGS
>  export KBUILD_CFLAGS CFLAGS_KERNEL CFLAGS_MODULE
> -export CFLAGS_KASAN CFLAGS_KASAN_NOSANITIZE CFLAGS_UBSAN
> +export CFLAGS_KASAN CFLAGS_KASAN_NOSANITIZE CFLAGS_UBSAN CFLAGS_KCSAN
>  export KBUILD_AFLAGS AFLAGS_KERNEL AFLAGS_MODULE
>  export KBUILD_AFLAGS_MODULE KBUILD_CFLAGS_MODULE KBUILD_LDFLAGS_MODULE
>  export KBUILD_AFLAGS_KERNEL KBUILD_CFLAGS_KERNEL
> @@ -900,6 +900,7 @@ endif
>  include scripts/Makefile.kasan
>  include scripts/Makefile.extrawarn
>  include scripts/Makefile.ubsan
> +include scripts/Makefile.kcsan
>
>  # Add user supplied CPPFLAGS, AFLAGS and CFLAGS as the last assignments
>  KBUILD_CPPFLAGS += $(KCPPFLAGS)
> diff --git a/include/linux/compiler-clang.h b/include/linux/compiler-clang.h
> index 333a6695a918..a213eb55e725 100644
> --- a/include/linux/compiler-clang.h
> +++ b/include/linux/compiler-clang.h
> @@ -24,6 +24,15 @@
>  #define __no_sanitize_address
>  #endif
>
> +#if __has_feature(thread_sanitizer)
> +/* emulate gcc's __SANITIZE_THREAD__ flag */
> +#define __SANITIZE_THREAD__
> +#define __no_sanitize_thread \
> +               __attribute__((no_sanitize("thread")))
> +#else
> +#define __no_sanitize_thread
> +#endif
> +
>  /*
>   * Not all versions of clang implement the the type-generic versions
>   * of the builtin overflow checkers. Fortunately, clang implements
> diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler-gcc.h
> index d7ee4c6bad48..de105ca29282 100644
> --- a/include/linux/compiler-gcc.h
> +++ b/include/linux/compiler-gcc.h
> @@ -145,6 +145,13 @@
>  #define __no_sanitize_address
>  #endif
>
> +#if __has_attribute(__no_sanitize_thread__) && defined(__SANITIZE_THREAD__)
> +#define __no_sanitize_thread                                                   \
> +       __attribute__((__noinline__)) __attribute__((no_sanitize_thread))
> +#else
> +#define __no_sanitize_thread
> +#endif
> +
>  #if GCC_VERSION >= 50100
>  #define COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW 1
>  #endif
> diff --git a/include/linux/compiler.h b/include/linux/compiler.h
> index 5e88e7e33abe..0a7467477f84 100644
> --- a/include/linux/compiler.h
> +++ b/include/linux/compiler.h
> @@ -178,6 +178,7 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val,
>  #endif
>
>  #include <uapi/linux/types.h>
> +#include <linux/kcsan-checks.h>
>
>  #define __READ_ONCE_SIZE                                               \
>  ({                                                                     \
> @@ -193,12 +194,6 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val,
>         }                                                               \
>  })
>
> -static __always_inline
> -void __read_once_size(const volatile void *p, void *res, int size)
> -{
> -       __READ_ONCE_SIZE;
> -}
> -
>  #ifdef CONFIG_KASAN
>  /*
>   * We can't declare function 'inline' because __no_sanitize_address confilcts
> @@ -211,14 +206,38 @@ void __read_once_size(const volatile void *p, void *res, int size)
>  # define __no_kasan_or_inline __always_inline
>  #endif
>
> -static __no_kasan_or_inline
> +#ifdef CONFIG_KCSAN
> +# define __no_kcsan_or_inline __no_sanitize_thread notrace __maybe_unused
> +#else
> +# define __no_kcsan_or_inline __always_inline
> +#endif
> +
> +#if defined(CONFIG_KASAN) || defined(CONFIG_KCSAN)
> +/* Avoid any instrumentation or inline. */
> +#define __no_sanitize_or_inline                                                \
> +       __no_sanitize_address __no_sanitize_thread notrace __maybe_unused
> +#else
> +#define __no_sanitize_or_inline __always_inline
> +#endif
> +
> +static __no_kcsan_or_inline
> +void __read_once_size(const volatile void *p, void *res, int size)
> +{
> +       kcsan_check_atomic((const void *)p, size, false);
> +       __READ_ONCE_SIZE;
> +}
> +
> +static __no_sanitize_or_inline
>  void __read_once_size_nocheck(const volatile void *p, void *res, int size)
>  {
>         __READ_ONCE_SIZE;
>  }
>
> -static __always_inline void __write_once_size(volatile void *p, void *res, int size)
> +static __no_kcsan_or_inline
> +void __write_once_size(volatile void *p, void *res, int size)
>  {
> +       kcsan_check_atomic((const void *)p, size, true);
> +
>         switch (size) {
>         case 1: *(volatile __u8 *)p = *(__u8 *)res; break;
>         case 2: *(volatile __u16 *)p = *(__u16 *)res; break;
> diff --git a/include/linux/kcsan-checks.h b/include/linux/kcsan-checks.h
> new file mode 100644
> index 000000000000..bee619b66e1c
> --- /dev/null
> +++ b/include/linux/kcsan-checks.h
> @@ -0,0 +1,116 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +
> +#ifndef _LINUX_KCSAN_CHECKS_H
> +#define _LINUX_KCSAN_CHECKS_H
> +
> +#include <linux/types.h>
> +
> +/*
> + * __kcsan_*: Always available when KCSAN is enabled. This may be used
> + * even in compilation units that selectively disable KCSAN, but must use KCSAN
> + * to validate access to an address.   Never use these in header files!
> + */
> +#ifdef CONFIG_KCSAN
> +/**
> + * __kcsan_check_watchpoint - check if a watchpoint exists
> + *
> + * Returns true if no race was detected, and we may then proceed to set up a
> + * watchpoint after. Returns false if either KCSAN is disabled or a race was
> + * encountered, and we may not set up a watchpoint after.
> + *
> + * @ptr address of access
> + * @size size of access
> + * @is_write is access a write
> + * @return true if no race was detected, false otherwise.
> + */
> +bool __kcsan_check_watchpoint(const volatile void *ptr, size_t size,
> +                             bool is_write);
> +
> +/**
> + * __kcsan_setup_watchpoint - set up watchpoint and report data-races
> + *
> + * Sets up a watchpoint (if sampled), and if a racing access was observed,
> + * reports the data-race.
> + *
> + * @ptr address of access
> + * @size size of access
> + * @is_write is access a write
> + */
> +void __kcsan_setup_watchpoint(const volatile void *ptr, size_t size,
> +                             bool is_write);
> +#else
> +static inline bool __kcsan_check_watchpoint(const volatile void *ptr,
> +                                           size_t size, bool is_write)
> +{
> +       return true;
> +}
> +static inline void __kcsan_setup_watchpoint(const volatile void *ptr,
> +                                           size_t size, bool is_write)
> +{
> +}
> +#endif
> +
> +/*
> + * kcsan_*: Only available when the particular compilation unit has KCSAN
> + * instrumentation enabled. May be used in header files.
> + */
> +#ifdef __SANITIZE_THREAD__
> +#define kcsan_check_watchpoint __kcsan_check_watchpoint
> +#define kcsan_setup_watchpoint __kcsan_setup_watchpoint
> +#else
> +static inline bool kcsan_check_watchpoint(const volatile void *ptr, size_t size,
> +                                         bool is_write)
> +{
> +       return true;
> +}
> +static inline void kcsan_setup_watchpoint(const volatile void *ptr, size_t size,
> +                                         bool is_write)
> +{
> +}
> +#endif
> +
> +/**
> + * __kcsan_check_access - check regular access for data-races
> + *
> + * Full access that checks watchpoint and sets up a watchpoint if this access is
> + * sampled.
> + *
> + * @ptr address of access
> + * @size size of access
> + * @is_write is access a write
> + */
> +#define __kcsan_check_access(ptr, size, is_write)                              \
> +       do {                                                                   \
> +               if (__kcsan_check_watchpoint(ptr, size, is_write) &&           \
> +                   !(IS_ENABLED(CONFIG_KCSAN_PLAIN_WRITE_PRETEND_ONCE) &&     \
> +                     is_write))                                               \
> +                       __kcsan_setup_watchpoint(ptr, size, is_write);         \
> +       } while (0)
> +/**
> + * kcsan_check_access - check regular access for data-races
> + *
> + * @ptr address of access
> + * @size size of access
> + * @is_write is access a write
> + */
> +#define kcsan_check_access(ptr, size, is_write)                                \
> +       do {                                                                   \
> +               if (kcsan_check_watchpoint(ptr, size, is_write) &&             \
> +                   !(IS_ENABLED(CONFIG_KCSAN_PLAIN_WRITE_PRETEND_ONCE) &&     \
> +                     is_write))                                               \
> +                       kcsan_setup_watchpoint(ptr, size, is_write);           \
> +       } while (0)
> +
> +/*
> + * Check for atomic accesses: if atomics are not ignored, this simply aliases to
> + * kcsan_check_watchpoint, otherwise becomes a no-op.
> + */
> +#ifdef CONFIG_KCSAN_IGNORE_ATOMICS
> +#define kcsan_check_atomic(...)                                                \
> +       do {                                                                   \
> +       } while (0)
> +#else
> +#define kcsan_check_atomic kcsan_check_watchpoint
> +#endif
> +
> +#endif /* _LINUX_KCSAN_CHECKS_H */
> diff --git a/include/linux/kcsan.h b/include/linux/kcsan.h
> new file mode 100644
> index 000000000000..18c660628376
> --- /dev/null
> +++ b/include/linux/kcsan.h
> @@ -0,0 +1,85 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +
> +#ifndef _LINUX_KCSAN_H
> +#define _LINUX_KCSAN_H
> +
> +#include <linux/types.h>
> +#include <linux/kcsan-checks.h>
> +
> +#ifdef CONFIG_KCSAN
> +
> +/**
> + * kcsan_init - initialize KCSAN runtime
> + */
> +void kcsan_init(void);
> +
> +/**
> + * kcsan_disable_current - disable KCSAN for the current context
> + *
> + * Supports nesting.
> + */
> +void kcsan_disable_current(void);
> +
> +/**
> + * kcsan_enable_current - re-enable KCSAN for the current context
> + *
> + * Supports nesting.
> + */
> +void kcsan_enable_current(void);
> +
> +/**
> + * kcsan_begin_atomic - use to denote an atomic region
> + *
> + * Accesses within the atomic region may appear to race with other accesses but
> + * should be considered atomic.
> + *
> + * @nest true if regions may be nested, or false for flat region
> + */
> +void kcsan_begin_atomic(bool nest);
> +
> +/**
> + * kcsan_end_atomic - end atomic region
> + *
> + * @nest must match argument to kcsan_begin_atomic().
> + */
> +void kcsan_end_atomic(bool nest);
> +
> +/**
> + * kcsan_atomic_next - consider following accesses as atomic
> + *
> + * Force treating the next n memory accesses for the current context as atomic
> + * operations.
> + *
> + * @n number of following memory accesses to treat as atomic.
> + */
> +void kcsan_atomic_next(int n);
> +
> +#else /* CONFIG_KCSAN */
> +
> +static inline void kcsan_init(void)
> +{
> +}
> +
> +static inline void kcsan_disable_current(void)
> +{
> +}
> +
> +static inline void kcsan_enable_current(void)
> +{
> +}
> +
> +static inline void kcsan_begin_atomic(bool nest)
> +{
> +}
> +
> +static inline void kcsan_end_atomic(bool nest)
> +{
> +}
> +
> +static inline void kcsan_atomic_next(int n)
> +{
> +}
> +
> +#endif /* CONFIG_KCSAN */
> +
> +#endif /* _LINUX_KCSAN_H */
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 2c2e56bd8913..34a1d9310304 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -1171,6 +1171,13 @@ struct task_struct {
>  #ifdef CONFIG_KASAN
>         unsigned int                    kasan_depth;
>  #endif
> +#ifdef CONFIG_KCSAN
> +       /* See comments at kernel/kcsan/core.c: struct cpu_state. */
> +       int                             kcsan_disable;
> +       int                             kcsan_atomic_next;
> +       int                             kcsan_atomic_region;
> +       bool                            kcsan_atomic_region_flat;
> +#endif
>
>  #ifdef CONFIG_FUNCTION_GRAPH_TRACER
>         /* Index of current stored address in ret_stack: */
> diff --git a/init/init_task.c b/init/init_task.c
> index 9e5cbe5eab7b..f98fc4c9f635 100644
> --- a/init/init_task.c
> +++ b/init/init_task.c
> @@ -161,6 +161,12 @@ struct task_struct init_task
>  #ifdef CONFIG_KASAN
>         .kasan_depth    = 1,
>  #endif
> +#ifdef CONFIG_KCSAN
> +       .kcsan_disable                  = 1,
> +       .kcsan_atomic_next              = 0,
> +       .kcsan_atomic_region            = 0,
> +       .kcsan_atomic_region_flat       = 0,
> +#endif
>  #ifdef CONFIG_TRACE_IRQFLAGS
>         .softirqs_enabled = 1,
>  #endif
> diff --git a/init/main.c b/init/main.c
> index 91f6ebb30ef0..4d814de017ee 100644
> --- a/init/main.c
> +++ b/init/main.c
> @@ -93,6 +93,7 @@
>  #include <linux/rodata_test.h>
>  #include <linux/jump_label.h>
>  #include <linux/mem_encrypt.h>
> +#include <linux/kcsan.h>
>
>  #include <asm/io.h>
>  #include <asm/bugs.h>
> @@ -779,6 +780,7 @@ asmlinkage __visible void __init start_kernel(void)
>         acpi_subsystem_init();
>         arch_post_acpi_subsys_init();
>         sfi_init_late();
> +       kcsan_init();
>
>         /* Do the rest non-__init'ed, we're now alive */
>         arch_call_rest_init();
> diff --git a/kernel/Makefile b/kernel/Makefile
> index daad787fb795..74ab46e2ebd1 100644
> --- a/kernel/Makefile
> +++ b/kernel/Makefile
> @@ -102,6 +102,7 @@ obj-$(CONFIG_TRACEPOINTS) += trace/
>  obj-$(CONFIG_IRQ_WORK) += irq_work.o
>  obj-$(CONFIG_CPU_PM) += cpu_pm.o
>  obj-$(CONFIG_BPF) += bpf/
> +obj-$(CONFIG_KCSAN) += kcsan/
>
>  obj-$(CONFIG_PERF_EVENTS) += events/
>
> diff --git a/kernel/kcsan/Makefile b/kernel/kcsan/Makefile
> new file mode 100644
> index 000000000000..c25f07062d26
> --- /dev/null
> +++ b/kernel/kcsan/Makefile
> @@ -0,0 +1,14 @@
> +# SPDX-License-Identifier: GPL-2.0
> +KCSAN_SANITIZE := n
> +KCOV_INSTRUMENT := n
> +
> +CFLAGS_REMOVE_kcsan.o = $(CC_FLAGS_FTRACE)
> +CFLAGS_REMOVE_core.o = $(CC_FLAGS_FTRACE)
> +CFLAGS_REMOVE_atomic.o = $(CC_FLAGS_FTRACE)
> +
> +CFLAGS_kcsan.o = $(call cc-option, -fno-conserve-stack -fno-stack-protector)
> +CFLAGS_core.o = $(call cc-option, -fno-conserve-stack -fno-stack-protector)
> +CFLAGS_atomic.o = $(call cc-option, -fno-conserve-stack -fno-stack-protector)
> +
> +obj-y := kcsan.o core.o atomic.o debugfs.o report.o
> +obj-$(CONFIG_KCSAN_SELFTEST) += test.o
> diff --git a/kernel/kcsan/atomic.c b/kernel/kcsan/atomic.c
> new file mode 100644
> index 000000000000..dd44f7d9e491
> --- /dev/null
> +++ b/kernel/kcsan/atomic.c
> @@ -0,0 +1,21 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +#include <linux/jiffies.h>
> +
> +#include "kcsan.h"
> +
> +/*
> + * List all volatile globals that have been observed in races, to suppress
> + * data-race reports between accesses to these variables.
> + *
> + * For now, we assume that volatile accesses of globals are as strong as atomic
> + * accesses (READ_ONCE, WRITE_ONCE cast to volatile). The situation is still not
> + * entirely clear, as on some architectures (Alpha) READ_ONCE/WRITE_ONCE do more
> + * than cast to volatile. Eventually, we hope to be able to remove this
> + * function.
> + */
> +bool kcsan_is_atomic(const volatile void *ptr)
> +{
> +       /* only jiffies for now */
> +       return ptr == &jiffies;
> +}
> diff --git a/kernel/kcsan/core.c b/kernel/kcsan/core.c
> new file mode 100644
> index 000000000000..e8c3823bf7c4
> --- /dev/null
> +++ b/kernel/kcsan/core.c
> @@ -0,0 +1,458 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +#include <linux/atomic.h>
> +#include <linux/bug.h>
> +#include <linux/delay.h>
> +#include <linux/export.h>
> +#include <linux/init.h>
> +#include <linux/percpu.h>
> +#include <linux/preempt.h>
> +#include <linux/random.h>
> +#include <linux/sched.h>
> +#include <linux/uaccess.h>
> +
> +#include "kcsan.h"
> +#include "encoding.h"
> +
> +/*
> + * Helper macros to iterate slots, starting from address slot itself, followed
> + * by the right and left slots.
> + */
> +#define CHECK_NUM_SLOTS (1 + 2 * KCSAN_CHECK_ADJACENT)
> +#define SLOT_IDX(slot, i)                                                      \
> +       ((slot + (((i + KCSAN_CHECK_ADJACENT) % CHECK_NUM_SLOTS) -             \
> +                 KCSAN_CHECK_ADJACENT)) %                                     \
> +        KCSAN_NUM_WATCHPOINTS)
> +
> +bool kcsan_enabled;
> +
> +/*
> + * Per-CPU state that should be used instead of 'current' if we are not in a
> + * task.
> + */
> +struct cpu_state {
> +       int disable; /* disable counter */
> +       int atomic_next; /* number of following atomic ops */
> +
> +       /*
> +        * We use separate variables to store if we are in a nestable or flat
> +        * atomic region. This helps make sure that an atomic region with
> +        * nesting support is not suddenly aborted when a flat region is
> +        * contained within. Effectively this allows supporting nesting flat
> +        * atomic regions within an outer nestable atomic region. Support for
> +        * this is required as there are cases where a seqlock reader critical
> +        * section (flat atomic region) is contained within a seqlock writer
> +        * critical section (nestable atomic region), and the "mismatching
> +        * kcsan_end_atomic()" warning would trigger otherwise.
> +        */
> +       int atomic_region;
> +       bool atomic_region_flat;
> +};
> +static DEFINE_PER_CPU(struct cpu_state, this_state) = {
> +       .disable = 0,
> +       .atomic_next = 0,
> +       .atomic_region = 0,
> +       .atomic_region_flat = 0,
> +};
> +
> +/*
> + * Watchpoints, with each entry encoded as defined in encoding.h: in order to be
> + * able to safely update and access a watchpoint without introducing locking
> + * overhead, we encode each watchpoint as a single atomic long. The initial
> + * zero-initialized state matches INVALID_WATCHPOINT.
> + */
> +static atomic_long_t watchpoints[KCSAN_NUM_WATCHPOINTS];
> +
> +/*
> + * Instructions skipped counter; see should_watch().
> + */
> +static DEFINE_PER_CPU(unsigned long, kcsan_skip);
> +
> +static inline atomic_long_t *find_watchpoint(unsigned long addr, size_t size,
> +                                            bool expect_write,
> +                                            long *encoded_watchpoint)
> +{
> +       const int slot = watchpoint_slot(addr);
> +       const unsigned long addr_masked = addr & WATCHPOINT_ADDR_MASK;
> +       atomic_long_t *watchpoint;
> +       unsigned long wp_addr_masked;
> +       size_t wp_size;
> +       bool is_write;
> +       int i;
> +
> +       for (i = 0; i < CHECK_NUM_SLOTS; ++i) {
> +               watchpoint = &watchpoints[SLOT_IDX(slot, i)];
> +               *encoded_watchpoint = atomic_long_read(watchpoint);
> +               if (!decode_watchpoint(*encoded_watchpoint, &wp_addr_masked,
> +                                      &wp_size, &is_write))
> +                       continue;
> +
> +               if (expect_write && !is_write)
> +                       continue;
> +
> +               /* Check if the watchpoint matches the access. */
> +               if (matching_access(wp_addr_masked, wp_size, addr_masked, size))
> +                       return watchpoint;
> +       }
> +
> +       return NULL;
> +}
> +
> +static inline atomic_long_t *insert_watchpoint(unsigned long addr, size_t size,
> +                                              bool is_write)
> +{
> +       const int slot = watchpoint_slot(addr);
> +       const long encoded_watchpoint = encode_watchpoint(addr, size, is_write);
> +       atomic_long_t *watchpoint;
> +       int i;
> +
> +       for (i = 0; i < CHECK_NUM_SLOTS; ++i) {
> +               long expect_val = INVALID_WATCHPOINT;
> +
> +               /* Try to acquire this slot. */
> +               watchpoint = &watchpoints[SLOT_IDX(slot, i)];
> +               if (atomic_long_try_cmpxchg_relaxed(watchpoint, &expect_val,
> +                                                   encoded_watchpoint))
> +                       return watchpoint;
> +       }
> +
> +       return NULL;
> +}
> +
> +/*
> + * Return true if watchpoint was successfully consumed, false otherwise.
> + *
> + * This may return false if:
> + *
> + *     1. another thread already consumed the watchpoint;
> + *     2. the thread that set up the watchpoint already removed it;
> + *     3. the watchpoint was removed and then re-used.
> + */
> +static inline bool try_consume_watchpoint(atomic_long_t *watchpoint,
> +                                         long encoded_watchpoint)
> +{
> +       return atomic_long_try_cmpxchg_relaxed(watchpoint, &encoded_watchpoint,
> +                                              CONSUMED_WATCHPOINT);
> +}
> +
> +/*
> + * Return true if watchpoint was not touched, false if consumed.
> + */
> +static inline bool remove_watchpoint(atomic_long_t *watchpoint)
> +{
> +       return atomic_long_xchg_relaxed(watchpoint, INVALID_WATCHPOINT) !=
> +              CONSUMED_WATCHPOINT;
> +}
> +
> +static inline bool is_atomic(const volatile void *ptr)
> +{
> +       if (in_task()) {
> +               if (unlikely(current->kcsan_atomic_next > 0)) {
> +                       --current->kcsan_atomic_next;
> +                       return true;
> +               }
> +               if (unlikely(current->kcsan_atomic_region > 0 ||
> +                            current->kcsan_atomic_region_flat))
> +                       return true;
> +       } else { /* interrupt */
> +               if (unlikely(this_cpu_read(this_state.atomic_next) > 0)) {
> +                       this_cpu_dec(this_state.atomic_next);
> +                       return true;
> +               }
> +               if (unlikely(this_cpu_read(this_state.atomic_region) > 0 ||
> +                            this_cpu_read(this_state.atomic_region_flat)))
> +                       return true;
> +       }
> +
> +       return kcsan_is_atomic(ptr);
> +}
> +
> +static inline bool should_watch(const volatile void *ptr)
> +{
> +       /*
> +        * Never set up watchpoints when memory operations are atomic.
> +        *
> +        * We need to check this first, because: 1) atomics should not count
> +        * towards skipped instructions below, and 2) to actually decrement
> +        * kcsan_atomic_next for each atomic.
> +        */
> +       if (is_atomic(ptr))
> +               return false;
> +
> +       /*
> +        * We use a per-CPU counter, to avoid excessive contention; there is
> +        * still enough non-determinism for the precise instructions that end up
> +        * being watched to be mostly unpredictable. Using a PRNG like
> +        * prandom_u32() turned out to be too slow.
> +        */
> +       return (this_cpu_inc_return(kcsan_skip) %
> +               CONFIG_KCSAN_WATCH_SKIP_INST) == 0;
> +}
> +
> +static inline bool is_enabled(void)
> +{
> +       return READ_ONCE(kcsan_enabled) &&
> +              (in_task() ? current->kcsan_disable :
> +                           this_cpu_read(this_state.disable)) == 0;
> +}
> +
> +static inline unsigned int get_delay(void)
> +{
> +       unsigned int max_delay = in_task() ? CONFIG_KCSAN_UDELAY_MAX_TASK :
> +                                            CONFIG_KCSAN_UDELAY_MAX_INTERRUPT;
> +       return IS_ENABLED(CONFIG_KCSAN_DELAY_RANDOMIZE) ?
> +                      ((prandom_u32() % max_delay) + 1) :
> +                      max_delay;
> +}
> +
> +/* === Public interface ===================================================== */
> +
> +void __init kcsan_init(void)
> +{
> +       BUG_ON(!in_task());
> +
> +       kcsan_debugfs_init();
> +       kcsan_enable_current();
> +#ifdef CONFIG_KCSAN_EARLY_ENABLE
> +       /*
> +        * We are in the init task, and no other tasks should be running.
> +        */
> +       WRITE_ONCE(kcsan_enabled, true);
> +#endif
> +}
> +
> +/* === Exported interface =================================================== */
> +
> +void kcsan_disable_current(void)
> +{
> +       if (in_task())
> +               ++current->kcsan_disable;
> +       else
> +               this_cpu_inc(this_state.disable);
> +}
> +EXPORT_SYMBOL(kcsan_disable_current);
> +
> +void kcsan_enable_current(void)
> +{
> +       int prev = in_task() ? current->kcsan_disable-- :
> +                              (this_cpu_dec_return(this_state.disable) + 1);
> +       if (prev == 0) {
> +               kcsan_disable_current(); /* restore to 0 */
> +               kcsan_disable_current();
> +               WARN(1, "mismatching %s", __func__);
> +               kcsan_enable_current();
> +       }
> +}
> +EXPORT_SYMBOL(kcsan_enable_current);
> +
> +void kcsan_begin_atomic(bool nest)
> +{
> +       if (nest) {
> +               if (in_task())
> +                       ++current->kcsan_atomic_region;
> +               else
> +                       this_cpu_inc(this_state.atomic_region);
> +       } else {
> +               if (in_task())
> +                       current->kcsan_atomic_region_flat = true;
> +               else
> +                       this_cpu_write(this_state.atomic_region_flat, true);
> +       }
> +}
> +EXPORT_SYMBOL(kcsan_begin_atomic);
> +
> +void kcsan_end_atomic(bool nest)
> +{
> +       if (nest) {
> +               int prev =
> +                       in_task() ?
> +                               current->kcsan_atomic_region-- :
> +                               (this_cpu_dec_return(this_state.atomic_region) +
> +                                1);
> +               if (prev == 0) {
> +                       kcsan_begin_atomic(true); /* restore to 0 */
> +                       kcsan_disable_current();
> +                       WARN(1, "mismatching %s", __func__);
> +                       kcsan_enable_current();
> +               }
> +       } else {
> +               if (in_task())
> +                       current->kcsan_atomic_region_flat = false;
> +               else
> +                       this_cpu_write(this_state.atomic_region_flat, false);
> +       }
> +}
> +EXPORT_SYMBOL(kcsan_end_atomic);
> +
> +void kcsan_atomic_next(int n)
> +{
> +       if (in_task())
> +               current->kcsan_atomic_next = n;
> +       else
> +               this_cpu_write(this_state.atomic_next, n);
> +}
> +EXPORT_SYMBOL(kcsan_atomic_next);
> +
> +bool __kcsan_check_watchpoint(const volatile void *ptr, size_t size,
> +                             bool is_write)
> +{
> +       atomic_long_t *watchpoint;
> +       long encoded_watchpoint;
> +       unsigned long flags;
> +       enum kcsan_report_type report_type;
> +
> +       if (unlikely(!is_enabled()))
> +               return false;
> +
> +       watchpoint = find_watchpoint((unsigned long)ptr, size, !is_write,
> +                                    &encoded_watchpoint);
> +       if (watchpoint == NULL)
> +               return true;
> +
> +       flags = user_access_save();
> +       if (!try_consume_watchpoint(watchpoint, encoded_watchpoint)) {
> +               /*
> +                * The other thread may not print any diagnostics, as it has
> +                * already removed the watchpoint, or another thread consumed
> +                * the watchpoint before this thread.
> +                */
> +               kcsan_counter_inc(kcsan_counter_report_races);
> +               report_type = kcsan_report_race_check_race;
> +       } else {
> +               report_type = kcsan_report_race_check;
> +       }
> +
> +       /* Encountered a data-race. */
> +       kcsan_counter_inc(kcsan_counter_data_races);
> +       kcsan_report(ptr, size, is_write, raw_smp_processor_id(), report_type);
> +
> +       user_access_restore(flags);
> +       return false;
> +}
> +EXPORT_SYMBOL(__kcsan_check_watchpoint);
> +
> +void __kcsan_setup_watchpoint(const volatile void *ptr, size_t size,
> +                             bool is_write)
> +{
> +       atomic_long_t *watchpoint;
> +       union {
> +               u8 _1;
> +               u16 _2;
> +               u32 _4;
> +               u64 _8;
> +       } expect_value;
> +       bool is_expected = true;
> +       unsigned long ua_flags = user_access_save();
> +       unsigned long irq_flags;
> +
> +       if (!should_watch(ptr))
> +               goto out;
> +
> +       if (!check_encodable((unsigned long)ptr, size)) {
> +               kcsan_counter_inc(kcsan_counter_unencodable_accesses);
> +               goto out;
> +       }
> +
> +       /*
> +        * Disable interrupts & preemptions, to ignore races due to accesses in
> +        * threads running on the same CPU.
> +        */
> +       local_irq_save(irq_flags);
> +       preempt_disable();
> +
> +       watchpoint = insert_watchpoint((unsigned long)ptr, size, is_write);
> +       if (watchpoint == NULL) {
> +               /*
> +                * Out of capacity: the size of `watchpoints`, and the frequency
> +                * with which `should_watch()` returns true should be tweaked so
> +                * that this case happens very rarely.
> +                */
> +               kcsan_counter_inc(kcsan_counter_no_capacity);
> +               goto out_unlock;
> +       }
> +
> +       kcsan_counter_inc(kcsan_counter_setup_watchpoints);
> +       kcsan_counter_inc(kcsan_counter_used_watchpoints);
> +
> +       /*
> +        * Read the current value, to later check and infer a race if the data
> +        * was modified via a non-instrumented access, e.g. from a device.
> +        */
> +       switch (size) {
> +       case 1:
> +               expect_value._1 = READ_ONCE(*(const u8 *)ptr);
> +               break;
> +       case 2:
> +               expect_value._2 = READ_ONCE(*(const u16 *)ptr);
> +               break;
> +       case 4:
> +               expect_value._4 = READ_ONCE(*(const u32 *)ptr);
> +               break;
> +       case 8:
> +               expect_value._8 = READ_ONCE(*(const u64 *)ptr);
> +               break;
> +       default:
> +               break; /* ignore; we do not diff the values */
> +       }
> +
> +#ifdef CONFIG_KCSAN_DEBUG
> +       kcsan_disable_current();
> +       pr_err("KCSAN: watching %s, size: %zu, addr: %px [slot: %d, encoded: %lx]\n",
> +              is_write ? "write" : "read", size, ptr,
> +              watchpoint_slot((unsigned long)ptr),
> +              encode_watchpoint((unsigned long)ptr, size, is_write));
> +       kcsan_enable_current();
> +#endif
> +
> +       /*
> +        * Delay this thread, to increase probability of observing a racy
> +        * conflicting access.
> +        */
> +       udelay(get_delay());
> +
> +       /*
> +        * Re-read value, and check if it is as expected; if not, we infer a
> +        * racy access.
> +        */
> +       switch (size) {
> +       case 1:
> +               is_expected = expect_value._1 == READ_ONCE(*(const u8 *)ptr);
> +               break;
> +       case 2:
> +               is_expected = expect_value._2 == READ_ONCE(*(const u16 *)ptr);
> +               break;
> +       case 4:
> +               is_expected = expect_value._4 == READ_ONCE(*(const u32 *)ptr);
> +               break;
> +       case 8:
> +               is_expected = expect_value._8 == READ_ONCE(*(const u64 *)ptr);
> +               break;
> +       default:
> +               break; /* ignore; we do not diff the values */
> +       }
> +
> +       /* Check if this access raced with another. */
> +       if (!remove_watchpoint(watchpoint)) {
> +               /*
> +                * No need to increment 'race' counter, as the racing thread
> +                * already did.
> +                */
> +               kcsan_report(ptr, size, is_write, smp_processor_id(),
> +                            kcsan_report_race_setup);
> +       } else if (!is_expected) {
> +               /* Inferring a race, since the value should not have changed. */
> +               kcsan_counter_inc(kcsan_counter_races_unknown_origin);
> +#ifdef CONFIG_KCSAN_REPORT_RACE_UNKNOWN_ORIGIN
> +               kcsan_report(ptr, size, is_write, smp_processor_id(),
> +                            kcsan_report_race_unknown_origin);
> +#endif
> +       }
> +
> +       kcsan_counter_dec(kcsan_counter_used_watchpoints);
> +out_unlock:
> +       preempt_enable();
> +       local_irq_restore(irq_flags);
> +out:
> +       user_access_restore(ua_flags);
> +}
> +EXPORT_SYMBOL(__kcsan_setup_watchpoint);
> diff --git a/kernel/kcsan/debugfs.c b/kernel/kcsan/debugfs.c
> new file mode 100644
> index 000000000000..6ddcbd185f3a
> --- /dev/null
> +++ b/kernel/kcsan/debugfs.c
> @@ -0,0 +1,225 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +#include <linux/atomic.h>
> +#include <linux/bsearch.h>
> +#include <linux/bug.h>
> +#include <linux/debugfs.h>
> +#include <linux/init.h>
> +#include <linux/kallsyms.h>
> +#include <linux/mm.h>
> +#include <linux/seq_file.h>
> +#include <linux/sort.h>
> +#include <linux/string.h>
> +#include <linux/uaccess.h>
> +
> +#include "kcsan.h"
> +
> +/*
> + * Statistics counters.
> + */
> +static atomic_long_t counters[kcsan_counter_count];
> +
> +/*
> + * Addresses for filtering functions from reporting. This list can be used as a
> + * whitelist or blacklist.
> + */
> +static struct {
> +       unsigned long *addrs; /* array of addresses */
> +       size_t size; /* current size */
> +       int used; /* number of elements used */
> +       bool sorted; /* if elements are sorted */
> +       bool whitelist; /* if list is a blacklist or whitelist */
> +} report_filterlist = {
> +       .addrs = NULL,
> +       .size = 8, /* small initial size */
> +       .used = 0,
> +       .sorted = false,
> +       .whitelist = false, /* default is blacklist */
> +};
> +static DEFINE_SPINLOCK(report_filterlist_lock);
> +
> +static const char *counter_to_name(enum kcsan_counter_id id)
> +{
> +       switch (id) {
> +       case kcsan_counter_used_watchpoints:
> +               return "used_watchpoints";
> +       case kcsan_counter_setup_watchpoints:
> +               return "setup_watchpoints";
> +       case kcsan_counter_data_races:
> +               return "data_races";
> +       case kcsan_counter_no_capacity:
> +               return "no_capacity";
> +       case kcsan_counter_report_races:
> +               return "report_races";
> +       case kcsan_counter_races_unknown_origin:
> +               return "races_unknown_origin";
> +       case kcsan_counter_unencodable_accesses:
> +               return "unencodable_accesses";
> +       case kcsan_counter_encoding_false_positives:
> +               return "encoding_false_positives";
> +       case kcsan_counter_count:
> +               BUG();
> +       }
> +       return NULL;
> +}
> +
> +void kcsan_counter_inc(enum kcsan_counter_id id)
> +{
> +       atomic_long_inc(&counters[id]);
> +}
> +
> +void kcsan_counter_dec(enum kcsan_counter_id id)
> +{
> +       atomic_long_dec(&counters[id]);
> +}
> +
> +static int cmp_filterlist_addrs(const void *rhs, const void *lhs)
> +{
> +       const unsigned long a = *(const unsigned long *)rhs;
> +       const unsigned long b = *(const unsigned long *)lhs;
> +
> +       return a < b ? -1 : a == b ? 0 : 1;
> +}
> +
> +bool kcsan_skip_report(unsigned long func_addr)
> +{
> +       unsigned long symbolsize, offset;
> +       unsigned long flags;
> +       bool ret = false;
> +
> +       if (!kallsyms_lookup_size_offset(func_addr, &symbolsize, &offset))
> +               return false;
> +       func_addr -= offset; /* get function start */
> +
> +       spin_lock_irqsave(&report_filterlist_lock, flags);
> +       if (report_filterlist.used == 0)
> +               goto out;
> +
> +       /* Sort array if it is unsorted, and then do a binary search. */
> +       if (!report_filterlist.sorted) {
> +               sort(report_filterlist.addrs, report_filterlist.used,
> +                    sizeof(unsigned long), cmp_filterlist_addrs, NULL);
> +               report_filterlist.sorted = true;
> +       }
> +       ret = !!bsearch(&func_addr, report_filterlist.addrs,
> +                       report_filterlist.used, sizeof(unsigned long),
> +                       cmp_filterlist_addrs);
> +       if (report_filterlist.whitelist)
> +               ret = !ret;
> +
> +out:
> +       spin_unlock_irqrestore(&report_filterlist_lock, flags);
> +       return ret;
> +}
> +
> +static void set_report_filterlist_whitelist(bool whitelist)
> +{
> +       unsigned long flags;
> +
> +       spin_lock_irqsave(&report_filterlist_lock, flags);
> +       report_filterlist.whitelist = whitelist;
> +       spin_unlock_irqrestore(&report_filterlist_lock, flags);
> +}
> +
> +static void insert_report_filterlist(const char *func)
> +{
> +       unsigned long flags;
> +       unsigned long addr = kallsyms_lookup_name(func);
> +
> +       if (!addr) {
> +               pr_err("KCSAN: could not find function: '%s'\n", func);
> +               return;
> +       }
> +
> +       spin_lock_irqsave(&report_filterlist_lock, flags);
> +
> +       if (report_filterlist.addrs == NULL)
> +               report_filterlist.addrs = /* initial allocation */
> +                       kvmalloc_array(report_filterlist.size,
> +                                      sizeof(unsigned long), GFP_KERNEL);
> +       else if (report_filterlist.used == report_filterlist.size) {
> +               /* resize filterlist */
> +               unsigned long *new_addrs;
> +
> +               report_filterlist.size *= 2;
> +               new_addrs = kvmalloc_array(report_filterlist.size,
> +                                          sizeof(unsigned long), GFP_KERNEL);
> +               memcpy(new_addrs, report_filterlist.addrs,
> +                      report_filterlist.used * sizeof(unsigned long));
> +               kvfree(report_filterlist.addrs);
> +               report_filterlist.addrs = new_addrs;
> +       }
> +
> +       /* Note: deduplicating should be done in userspace. */
> +       report_filterlist.addrs[report_filterlist.used++] =
> +               kallsyms_lookup_name(func);
> +       report_filterlist.sorted = false;
> +
> +       spin_unlock_irqrestore(&report_filterlist_lock, flags);
> +}
> +
> +static int show_info(struct seq_file *file, void *v)
> +{
> +       int i;
> +       unsigned long flags;
> +
> +       /* show stats */
> +       seq_printf(file, "enabled: %i\n", READ_ONCE(kcsan_enabled));
> +       for (i = 0; i < kcsan_counter_count; ++i)
> +               seq_printf(file, "%s: %ld\n", counter_to_name(i),
> +                          atomic_long_read(&counters[i]));
> +
> +       /* show filter functions, and filter type */
> +       spin_lock_irqsave(&report_filterlist_lock, flags);
> +       seq_printf(file, "\n%s functions: %s\n",
> +                  report_filterlist.whitelist ? "whitelisted" : "blacklisted",
> +                  report_filterlist.used == 0 ? "none" : "");
> +       for (i = 0; i < report_filterlist.used; ++i)
> +               seq_printf(file, " %ps\n", (void *)report_filterlist.addrs[i]);
> +       spin_unlock_irqrestore(&report_filterlist_lock, flags);
> +
> +       return 0;
> +}
> +
> +static int debugfs_open(struct inode *inode, struct file *file)
> +{
> +       return single_open(file, show_info, NULL);
> +}
> +
> +static ssize_t debugfs_write(struct file *file, const char __user *buf,
> +                            size_t count, loff_t *off)
> +{
> +       char kbuf[KSYM_NAME_LEN];
> +       char *arg;
> +       int read_len = count < (sizeof(kbuf) - 1) ? count : (sizeof(kbuf) - 1);
> +
> +       if (copy_from_user(kbuf, buf, read_len))
> +               return -EINVAL;
> +       kbuf[read_len] = '\0';
> +       arg = strstrip(kbuf);
> +
> +       if (!strncmp(arg, "on", sizeof("on") - 1))
> +               WRITE_ONCE(kcsan_enabled, true);
> +       else if (!strncmp(arg, "off", sizeof("off") - 1))
> +               WRITE_ONCE(kcsan_enabled, false);
> +       else if (!strncmp(arg, "whitelist", sizeof("whitelist") - 1))
> +               set_report_filterlist_whitelist(true);
> +       else if (!strncmp(arg, "blacklist", sizeof("blacklist") - 1))
> +               set_report_filterlist_whitelist(false);
> +       else if (arg[0] == '!')
> +               insert_report_filterlist(&arg[1]);
> +       else
> +               return -EINVAL;
> +
> +       return count;
> +}
> +
> +static const struct file_operations debugfs_ops = { .read = seq_read,
> +                                                   .open = debugfs_open,
> +                                                   .write = debugfs_write,
> +                                                   .release = single_release };
> +
> +void __init kcsan_debugfs_init(void)
> +{
> +       debugfs_create_file("kcsan", 0644, NULL, NULL, &debugfs_ops);
> +}
> diff --git a/kernel/kcsan/encoding.h b/kernel/kcsan/encoding.h
> new file mode 100644
> index 000000000000..8f9b1ce0e59f
> --- /dev/null
> +++ b/kernel/kcsan/encoding.h
> @@ -0,0 +1,94 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +
> +#ifndef _MM_KCSAN_ENCODING_H
> +#define _MM_KCSAN_ENCODING_H
> +
> +#include <linux/bits.h>
> +#include <linux/log2.h>
> +#include <linux/mm.h>
> +
> +#include "kcsan.h"
> +
> +#define SLOT_RANGE PAGE_SIZE
> +#define INVALID_WATCHPOINT 0
> +#define CONSUMED_WATCHPOINT 1
> +
> +/*
> + * The maximum useful size of accesses for which we set up watchpoints is the
> + * max range of slots we check on an access.
> + */
> +#define MAX_ENCODABLE_SIZE (SLOT_RANGE * (1 + KCSAN_CHECK_ADJACENT))
> +
> +/*
> + * Number of bits we use to store size info.
> + */
> +#define WATCHPOINT_SIZE_BITS bits_per(MAX_ENCODABLE_SIZE)
> +/*
> + * This encoding for addresses discards the upper (1 for is-write + SIZE_BITS);
> + * however, most 64-bit architectures do not use the full 64-bit address space.
> + * Also, in order for a false positive to be observable 2 things need to happen:
> + *
> + *     1. different addresses but with the same encoded address race;
> + *     2. and both map onto the same watchpoint slots;
> + *
> + * Both these are assumed to be very unlikely. However, in case it still happens
> + * happens, the report logic will filter out the false positive (see report.c).
> + */
> +#define WATCHPOINT_ADDR_BITS (BITS_PER_LONG - 1 - WATCHPOINT_SIZE_BITS)
> +
> +/*
> + * Masks to set/retrieve the encoded data.
> + */
> +#define WATCHPOINT_WRITE_MASK BIT(BITS_PER_LONG - 1)
> +#define WATCHPOINT_SIZE_MASK                                                   \
> +       GENMASK(BITS_PER_LONG - 2, BITS_PER_LONG - 2 - WATCHPOINT_SIZE_BITS)
> +#define WATCHPOINT_ADDR_MASK                                                   \
> +       GENMASK(BITS_PER_LONG - 3 - WATCHPOINT_SIZE_BITS, 0)
> +
> +static inline bool check_encodable(unsigned long addr, size_t size)
> +{
> +       return size <= MAX_ENCODABLE_SIZE;
> +}
> +
> +static inline long encode_watchpoint(unsigned long addr, size_t size,
> +                                    bool is_write)
> +{
> +       return (long)((is_write ? WATCHPOINT_WRITE_MASK : 0) |
> +                     (size << WATCHPOINT_ADDR_BITS) |
> +                     (addr & WATCHPOINT_ADDR_MASK));
> +}
> +
> +static inline bool decode_watchpoint(long watchpoint,
> +                                    unsigned long *addr_masked, size_t *size,
> +                                    bool *is_write)
> +{
> +       if (watchpoint == INVALID_WATCHPOINT ||
> +           watchpoint == CONSUMED_WATCHPOINT)
> +               return false;
> +
> +       *addr_masked = (unsigned long)watchpoint & WATCHPOINT_ADDR_MASK;
> +       *size = ((unsigned long)watchpoint & WATCHPOINT_SIZE_MASK) >>
> +               WATCHPOINT_ADDR_BITS;
> +       *is_write = !!((unsigned long)watchpoint & WATCHPOINT_WRITE_MASK);
> +
> +       return true;
> +}
> +
> +/*
> + * Return watchpoint slot for an address.
> + */
> +static inline int watchpoint_slot(unsigned long addr)
> +{
> +       return (addr / PAGE_SIZE) % KCSAN_NUM_WATCHPOINTS;
> +}
> +
> +static inline bool matching_access(unsigned long addr1, size_t size1,
> +                                  unsigned long addr2, size_t size2)
> +{
> +       unsigned long end_range1 = addr1 + size1 - 1;
> +       unsigned long end_range2 = addr2 + size2 - 1;
> +
> +       return addr1 <= end_range2 && addr2 <= end_range1;
> +}
> +
> +#endif /* _MM_KCSAN_ENCODING_H */
> diff --git a/kernel/kcsan/kcsan.c b/kernel/kcsan/kcsan.c
> new file mode 100644
> index 000000000000..ce13e0b38ba2
> --- /dev/null
> +++ b/kernel/kcsan/kcsan.c
> @@ -0,0 +1,81 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +/*
> + * The Kernel Concurrency Sanitizer (KCSAN) infrastructure. For more info please
> + * see Documentation/dev-tools/kcsan.rst.
> + */
> +
> +#include <linux/export.h>
> +
> +#include "kcsan.h"
> +
> +/*
> + * Concurrency Sanitizer uses the same instrumentation as Thread Sanitizer.
> + */
> +
> +#define DEFINE_TSAN_READ_WRITE(size)                                           \
> +       void __tsan_read##size(void *ptr)                                      \
> +       {                                                                      \
> +               __kcsan_check_access(ptr, size, false);                        \
> +       }                                                                      \
> +       EXPORT_SYMBOL(__tsan_read##size);                                      \
> +       void __tsan_write##size(void *ptr)                                     \
> +       {                                                                      \
> +               __kcsan_check_access(ptr, size, true);                         \
> +       }                                                                      \
> +       EXPORT_SYMBOL(__tsan_write##size)
> +
> +DEFINE_TSAN_READ_WRITE(1);
> +DEFINE_TSAN_READ_WRITE(2);
> +DEFINE_TSAN_READ_WRITE(4);
> +DEFINE_TSAN_READ_WRITE(8);
> +DEFINE_TSAN_READ_WRITE(16);
> +
> +/*
> + * Not all supported compiler versions distinguish aligned/unaligned accesses,
> + * but e.g. recent versions of Clang do.
> + */
> +#define DEFINE_TSAN_UNALIGNED_READ_WRITE(size)                                 \
> +       void __tsan_unaligned_read##size(void *ptr)                            \
> +       {                                                                      \
> +               __kcsan_check_access(ptr, size, false);                        \
> +       }                                                                      \
> +       EXPORT_SYMBOL(__tsan_unaligned_read##size);                            \
> +       void __tsan_unaligned_write##size(void *ptr)                           \
> +       {                                                                      \
> +               __kcsan_check_access(ptr, size, true);                         \
> +       }                                                                      \
> +       EXPORT_SYMBOL(__tsan_unaligned_write##size)
> +
> +DEFINE_TSAN_UNALIGNED_READ_WRITE(2);
> +DEFINE_TSAN_UNALIGNED_READ_WRITE(4);
> +DEFINE_TSAN_UNALIGNED_READ_WRITE(8);
> +DEFINE_TSAN_UNALIGNED_READ_WRITE(16);
> +
> +void __tsan_read_range(void *ptr, size_t size)
> +{
> +       __kcsan_check_access(ptr, size, false);
> +}
> +EXPORT_SYMBOL(__tsan_read_range);
> +
> +void __tsan_write_range(void *ptr, size_t size)
> +{
> +       __kcsan_check_access(ptr, size, true);
> +}
> +EXPORT_SYMBOL(__tsan_write_range);
> +
> +/*
> + * The below are not required KCSAN, but can still be emitted by the compiler.
> + */
> +void __tsan_func_entry(void *call_pc)
> +{
> +}
> +EXPORT_SYMBOL(__tsan_func_entry);
> +void __tsan_func_exit(void)
> +{
> +}
> +EXPORT_SYMBOL(__tsan_func_exit);
> +void __tsan_init(void)
> +{
> +}
> +EXPORT_SYMBOL(__tsan_init);
> diff --git a/kernel/kcsan/kcsan.h b/kernel/kcsan/kcsan.h
> new file mode 100644
> index 000000000000..429479b3041d
> --- /dev/null
> +++ b/kernel/kcsan/kcsan.h
> @@ -0,0 +1,140 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +
> +#ifndef _MM_KCSAN_KCSAN_H
> +#define _MM_KCSAN_KCSAN_H
> +
> +#include <linux/kcsan.h>
> +
> +/*
> + * Total number of watchpoints. An address range maps into a specific slot as
> + * specified in `encoding.h`. Although larger number of watchpoints may not even
> + * be usable due to limited thread count, a larger value will improve
> + * performance due to reducing cache-line contention.
> + */
> +#define KCSAN_NUM_WATCHPOINTS 64
> +
> +/*
> + * The number of adjacent watchpoints to check; the purpose is 2-fold:
> + *
> + *     1. the address slot is already occupied, check if any adjacent slots are
> + *        free;
> + *     2. accesses that straddle a slot boundary due to size that exceeds a
> + *        slot's range may check adjacent slots if any watchpoint matches.
> + *
> + * Note that accesses with very large size may still miss a watchpoint; however,
> + * given this should be rare, this is a reasonable trade-off to make, since this
> + * will avoid:
> + *
> + *     1. excessive contention between watchpoint checks and setup;
> + *     2. larger number of simultaneous watchpoints without sacrificing
> + *        performance.
> + */
> +#define KCSAN_CHECK_ADJACENT 1
> +
> +/*
> + * Globally enable and disable KCSAN.
> + */
> +extern bool kcsan_enabled;
> +
> +/*
> + * Helper that returns true if access to ptr should be considered as an atomic
> + * access, even though it is not explicitly atomic.
> + */
> +bool kcsan_is_atomic(const volatile void *ptr);
> +
> +/*
> + * Initialize debugfs file.
> + */
> +void kcsan_debugfs_init(void);
> +
> +enum kcsan_counter_id {
> +       /*
> +        * Number of watchpoints currently in use.
> +        */
> +       kcsan_counter_used_watchpoints,
> +
> +       /*
> +        * Total number of watchpoints set up.
> +        */
> +       kcsan_counter_setup_watchpoints,
> +
> +       /*
> +        * Total number of data-races.
> +        */
> +       kcsan_counter_data_races,
> +
> +       /*
> +        * Number of times no watchpoints were available.
> +        */
> +       kcsan_counter_no_capacity,
> +
> +       /*
> +        * A thread checking a watchpoint raced with another checking thread;
> +        * only one will be reported.
> +        */
> +       kcsan_counter_report_races,
> +
> +       /*
> +        * Observed data value change, but writer thread unknown.
> +        */
> +       kcsan_counter_races_unknown_origin,
> +
> +       /*
> +        * The access cannot be encoded to a valid watchpoint.
> +        */
> +       kcsan_counter_unencodable_accesses,
> +
> +       /*
> +        * Watchpoint encoding caused a watchpoint to fire on mismatching
> +        * accesses.
> +        */
> +       kcsan_counter_encoding_false_positives,
> +
> +       kcsan_counter_count, /* number of counters */
> +};
> +
> +/*
> + * Increment/decrement counter with given id; avoid calling these in fast-path.
> + */
> +void kcsan_counter_inc(enum kcsan_counter_id id);
> +void kcsan_counter_dec(enum kcsan_counter_id id);
> +
> +/*
> + * Returns true if data-races in the function symbol that maps to addr (offsets
> + * are ignored) should *not* be reported.
> + */
> +bool kcsan_skip_report(unsigned long func_addr);
> +
> +enum kcsan_report_type {
> +       /*
> +        * The thread that set up the watchpoint and briefly stalled was
> +        * signalled that another thread triggered the watchpoint, and thus a
> +        * race was encountered.
> +        */
> +       kcsan_report_race_setup,
> +
> +       /*
> +        * A thread encountered a watchpoint for the access, therefore a race
> +        * was encountered.
> +        */
> +       kcsan_report_race_check,
> +
> +       /*
> +        * A thread encountered a watchpoint for the access, but the other
> +        * racing thread can no longer be signaled that a race occurred.
> +        */
> +       kcsan_report_race_check_race,
> +
> +       /*
> +        * No other thread was observed to race with the access, but the data
> +        * value before and after the stall differs.
> +        */
> +       kcsan_report_race_unknown_origin,
> +};
> +/*
> + * Print a race report from thread that encountered the race.
> + */
> +void kcsan_report(const volatile void *ptr, size_t size, bool is_write,
> +                 int cpu_id, enum kcsan_report_type type);
> +
> +#endif /* _MM_KCSAN_KCSAN_H */
> diff --git a/kernel/kcsan/report.c b/kernel/kcsan/report.c
> new file mode 100644
> index 000000000000..1a0f34b623bf
> --- /dev/null
> +++ b/kernel/kcsan/report.c
> @@ -0,0 +1,307 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +#include <linux/kernel.h>
> +#include <linux/preempt.h>
> +#include <linux/printk.h>
> +#include <linux/sched.h>
> +#include <linux/spinlock.h>
> +#include <linux/stacktrace.h>
> +
> +#include "kcsan.h"
> +#include "encoding.h"
> +
> +/*
> + * Max. number of stack entries to show in the report.
> + */
> +#define NUM_STACK_ENTRIES 16
> +
> +/*
> + * Other thread info: communicated from other racing thread to thread that set
> + * up the watchpoint, which then prints the complete report atomically. Only
> + * need one struct, as all threads should to be serialized regardless to print
> + * the reports, with reporting being in the slow-path.
> + */
> +static struct {
> +       const volatile void *ptr;
> +       size_t size;
> +       bool is_write;
> +       int task_pid;
> +       int cpu_id;
> +       unsigned long stack_entries[NUM_STACK_ENTRIES];
> +       int num_stack_entries;
> +} other_info = { .ptr = NULL };
> +
> +static DEFINE_SPINLOCK(other_info_lock);
> +static DEFINE_SPINLOCK(report_lock);
> +
> +static bool set_or_lock_other_info(unsigned long *flags,
> +                                  const volatile void *ptr, size_t size,
> +                                  bool is_write, int cpu_id,
> +                                  enum kcsan_report_type type)
> +{
> +       if (type != kcsan_report_race_check && type != kcsan_report_race_setup)
> +               return true;
> +
> +       for (;;) {
> +               spin_lock_irqsave(&other_info_lock, *flags);
> +
> +               switch (type) {
> +               case kcsan_report_race_check:
> +                       if (other_info.ptr != NULL) {
> +                               /* still in use, retry */
> +                               break;
> +                       }
> +                       other_info.ptr = ptr;
> +                       other_info.size = size;
> +                       other_info.is_write = is_write;
> +                       other_info.task_pid =
> +                               in_task() ? task_pid_nr(current) : -1;
> +                       other_info.cpu_id = cpu_id;
> +                       other_info.num_stack_entries = stack_trace_save(
> +                               other_info.stack_entries, NUM_STACK_ENTRIES, 1);
> +                       /*
> +                        * other_info may now be consumed by thread we raced
> +                        * with.
> +                        */
> +                       spin_unlock_irqrestore(&other_info_lock, *flags);
> +                       return false;
> +
> +               case kcsan_report_race_setup:
> +                       if (other_info.ptr == NULL)
> +                               break; /* no data available yet, retry */
> +
> +                       /*
> +                        * First check if matching based on how watchpoint was
> +                        * encoded.
> +                        */
> +                       if (!matching_access((unsigned long)other_info.ptr &
> +                                                    WATCHPOINT_ADDR_MASK,
> +                                            other_info.size,
> +                                            (unsigned long)ptr &
> +                                                    WATCHPOINT_ADDR_MASK,
> +                                            size))
> +                               break; /* mismatching access, retry */
> +
> +                       if (!matching_access((unsigned long)other_info.ptr,
> +                                            other_info.size,
> +                                            (unsigned long)ptr, size)) {
> +                               /*
> +                                * If the actual accesses to not match, this was
> +                                * a false positive due to watchpoint encoding.
> +                                */
> +                               other_info.ptr = NULL; /* mark for reuse */
> +                               kcsan_counter_inc(
> +                                       kcsan_counter_encoding_false_positives);
> +                               spin_unlock_irqrestore(&other_info_lock,
> +                                                      *flags);
> +                               return false;
> +                       }
> +
> +                       /*
> +                        * Matching access: keep other_info locked, as this
> +                        * thread uses it to print the full report; unlocked in
> +                        * end_report.
> +                        */
> +                       return true;
> +
> +               default:
> +                       BUG();
> +               }
> +
> +               spin_unlock_irqrestore(&other_info_lock, *flags);
> +       }
> +}
> +
> +static void start_report(unsigned long *flags, enum kcsan_report_type type)
> +{
> +       switch (type) {
> +       case kcsan_report_race_setup:
> +               /* irqsaved already via other_info_lock */
> +               spin_lock(&report_lock);
> +               break;
> +
> +       case kcsan_report_race_unknown_origin:
> +               spin_lock_irqsave(&report_lock, *flags);
> +               break;
> +
> +       default:
> +               BUG();
> +       }
> +}
> +
> +static void end_report(unsigned long *flags, enum kcsan_report_type type)
> +{
> +       switch (type) {
> +       case kcsan_report_race_setup:
> +               other_info.ptr = NULL; /* mark for reuse */
> +               spin_unlock(&report_lock);
> +               spin_unlock_irqrestore(&other_info_lock, *flags);
> +               break;
> +
> +       case kcsan_report_race_unknown_origin:
> +               spin_unlock_irqrestore(&report_lock, *flags);
> +               break;
> +
> +       default:
> +               BUG();
> +       }
> +}
> +
> +static const char *get_access_type(bool is_write)
> +{
> +       return is_write ? "write" : "read";
> +}
> +
> +/* Return thread description: in task or interrupt. */
> +static const char *get_thread_desc(int task_id)
> +{
> +       if (task_id != -1) {
> +               static char buf[32]; /* safe: protected by report_lock */
> +
> +               snprintf(buf, sizeof(buf), "task %i", task_id);
> +               return buf;
> +       }
> +       return in_nmi() ? "NMI" : "interrupt";
> +}
> +
> +/* Helper to skip KCSAN-related functions in stack-trace. */
> +static int get_stack_skipnr(unsigned long stack_entries[], int num_entries)
> +{
> +       char buf[64];
> +       int skip = 0;
> +
> +       for (; skip < num_entries; ++skip) {
> +               snprintf(buf, sizeof(buf), "%ps", (void *)stack_entries[skip]);
> +               if (!strnstr(buf, "csan_", sizeof(buf)) &&
> +                   !strnstr(buf, "tsan_", sizeof(buf)) &&
> +                   !strnstr(buf, "_once_size", sizeof(buf))) {
> +                       break;
> +               }
> +       }
> +       return skip;
> +}
> +
> +/* Compares symbolized strings of addr1 and addr2. */
> +static int sym_strcmp(void *addr1, void *addr2)
> +{
> +       char buf1[64];
> +       char buf2[64];
> +
> +       snprintf(buf1, sizeof(buf1), "%pS", addr1);
> +       snprintf(buf2, sizeof(buf2), "%pS", addr2);
> +       return strncmp(buf1, buf2, sizeof(buf1));
> +}
> +
> +/*
> + * Returns true if a report was generated, false otherwise.
> + */
> +static bool print_summary(const volatile void *ptr, size_t size, bool is_write,
> +                         int cpu_id, enum kcsan_report_type type)
> +{
> +       unsigned long stack_entries[NUM_STACK_ENTRIES] = { 0 };
> +       int num_stack_entries =
> +               stack_trace_save(stack_entries, NUM_STACK_ENTRIES, 1);
> +       int skipnr = get_stack_skipnr(stack_entries, num_stack_entries);
> +       int other_skipnr;
> +
> +       /* Check if the top stackframe is in a blacklisted function. */
> +       if (kcsan_skip_report(stack_entries[skipnr]))
> +               return false;
> +       if (type == kcsan_report_race_setup) {
> +               other_skipnr = get_stack_skipnr(other_info.stack_entries,
> +                                               other_info.num_stack_entries);
> +               if (kcsan_skip_report(other_info.stack_entries[other_skipnr]))
> +                       return false;
> +       }
> +
> +       /* Print report header. */
> +       pr_err("==================================================================\n");
> +       switch (type) {
> +       case kcsan_report_race_setup: {
> +               void *this_fn = (void *)stack_entries[skipnr];
> +               void *other_fn = (void *)other_info.stack_entries[other_skipnr];
> +               int cmp;
> +
> +               /*
> +                * Order functions lexographically for consistent bug titles.
> +                * Do not print offset of functions to keep title short.
> +                */
> +               cmp = sym_strcmp(other_fn, this_fn);
> +               pr_err("BUG: KCSAN: data-race in %ps / %ps\n",
> +                      cmp < 0 ? other_fn : this_fn,
> +                      cmp < 0 ? this_fn : other_fn);
> +       } break;
> +
> +       case kcsan_report_race_unknown_origin:
> +               pr_err("BUG: KCSAN: racing %s in %pS\n",
> +                      get_access_type(is_write),
> +                      (void *)stack_entries[skipnr]);
> +               break;
> +
> +       default:
> +               BUG();
> +       }
> +
> +       pr_err("\n");
> +
> +       /* Print information about the racing accesses. */
> +       switch (type) {
> +       case kcsan_report_race_setup:
> +               pr_err("%s to 0x%px of %zu bytes by %s on cpu %i:\n",
> +                      get_access_type(other_info.is_write), other_info.ptr,
> +                      other_info.size, get_thread_desc(other_info.task_pid),
> +                      other_info.cpu_id);
> +
> +               /* Print the other thread's stack trace. */
> +               stack_trace_print(other_info.stack_entries + other_skipnr,
> +                                 other_info.num_stack_entries - other_skipnr,
> +                                 0);
> +
> +               pr_err("\n");
> +               pr_err("%s to 0x%px of %zu bytes by %s on cpu %i:\n",
> +                      get_access_type(is_write), ptr, size,
> +                      get_thread_desc(in_task() ? task_pid_nr(current) : -1),
> +                      cpu_id);
> +               break;
> +
> +       case kcsan_report_race_unknown_origin:
> +               pr_err("race at unknown origin, with %s to 0x%px of %zu bytes by %s on cpu %i:\n",
> +                      get_access_type(is_write), ptr, size,
> +                      get_thread_desc(in_task() ? task_pid_nr(current) : -1),
> +                      cpu_id);
> +               break;
> +
> +       default:
> +               BUG();
> +       }
> +       /* Print stack trace of this thread. */
> +       stack_trace_print(stack_entries + skipnr, num_stack_entries - skipnr,
> +                         0);
> +
> +       /* Print report footer. */
> +       pr_err("\n");
> +       pr_err("Reported by Kernel Concurrency Sanitizer on:\n");
> +       dump_stack_print_info(KERN_DEFAULT);
> +       pr_err("==================================================================\n");
> +
> +       return true;
> +}
> +
> +void kcsan_report(const volatile void *ptr, size_t size, bool is_write,
> +                 int cpu_id, enum kcsan_report_type type)
> +{
> +       unsigned long flags = 0;
> +
> +       if (type == kcsan_report_race_check_race)
> +               return;
> +
> +       kcsan_disable_current();
> +       if (set_or_lock_other_info(&flags, ptr, size, is_write, cpu_id, type)) {
> +               start_report(&flags, type);
> +               if (print_summary(ptr, size, is_write, cpu_id, type) &&
> +                   panic_on_warn)
> +                       panic("panic_on_warn set ...\n");
> +               end_report(&flags, type);
> +       }
> +       kcsan_enable_current();
> +}
> diff --git a/kernel/kcsan/test.c b/kernel/kcsan/test.c
> new file mode 100644
> index 000000000000..68c896a24529
> --- /dev/null
> +++ b/kernel/kcsan/test.c
> @@ -0,0 +1,117 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +#include <linux/init.h>
> +#include <linux/kernel.h>
> +#include <linux/printk.h>
> +#include <linux/random.h>
> +#include <linux/types.h>
> +
> +#include "encoding.h"
> +
> +#define ITERS_PER_TEST 2000
> +
> +/* Test requirements. */
> +static bool test_requires(void)
> +{
> +       /* random should be initialized */
> +       return prandom_u32() + prandom_u32() != 0;
> +}
> +
> +/* Test watchpoint encode and decode. */
> +static bool test_encode_decode(void)
> +{
> +       int i;
> +
> +       for (i = 0; i < ITERS_PER_TEST; ++i) {
> +               size_t size = prandom_u32() % MAX_ENCODABLE_SIZE + 1;
> +               bool is_write = prandom_u32() % 2;
> +               unsigned long addr;
> +
> +               prandom_bytes(&addr, sizeof(addr));
> +               if (WARN_ON(!check_encodable(addr, size)))
> +                       return false;
> +
> +               /* encode and decode */
> +               {
> +                       const long encoded_watchpoint =
> +                               encode_watchpoint(addr, size, is_write);
> +                       unsigned long verif_masked_addr;
> +                       size_t verif_size;
> +                       bool verif_is_write;
> +
> +                       /* check special watchpoints */
> +                       if (WARN_ON(decode_watchpoint(
> +                                   INVALID_WATCHPOINT, &verif_masked_addr,
> +                                   &verif_size, &verif_is_write)))
> +                               return false;
> +                       if (WARN_ON(decode_watchpoint(
> +                                   CONSUMED_WATCHPOINT, &verif_masked_addr,
> +                                   &verif_size, &verif_is_write)))
> +                               return false;
> +
> +                       /* check decoding watchpoint returns same data */
> +                       if (WARN_ON(!decode_watchpoint(
> +                                   encoded_watchpoint, &verif_masked_addr,
> +                                   &verif_size, &verif_is_write)))
> +                               return false;
> +                       if (WARN_ON(verif_masked_addr !=
> +                                   (addr & WATCHPOINT_ADDR_MASK)))
> +                               goto fail;
> +                       if (WARN_ON(verif_size != size))
> +                               goto fail;
> +                       if (WARN_ON(is_write != verif_is_write))
> +                               goto fail;
> +
> +                       continue;
> +fail:
> +                       pr_err("%s fail: %s %zu bytes @ %lx -> encoded: %lx -> %s %zu bytes @ %lx\n",
> +                              __func__, is_write ? "write" : "read", size,
> +                              addr, encoded_watchpoint,
> +                              verif_is_write ? "write" : "read", verif_size,
> +                              verif_masked_addr);
> +                       return false;
> +               }
> +       }
> +
> +       return true;
> +}
> +
> +static bool test_matching_access(void)
> +{
> +       if (WARN_ON(!matching_access(10, 1, 10, 1)))
> +               return false;
> +       if (WARN_ON(!matching_access(10, 2, 11, 1)))
> +               return false;
> +       if (WARN_ON(!matching_access(10, 1, 9, 2)))
> +               return false;
> +       if (WARN_ON(matching_access(10, 1, 11, 1)))
> +               return false;
> +       if (WARN_ON(matching_access(9, 1, 10, 1)))
> +               return false;
> +       return true;
> +}
> +
> +static int __init kcsan_selftest(void)
> +{
> +       int passed = 0;
> +       int total = 0;
> +
> +#define RUN_TEST(do_test)                                                      \
> +       do {                                                                   \
> +               ++total;                                                       \
> +               if (do_test())                                                 \
> +                       ++passed;                                              \
> +               else                                                           \
> +                       pr_err("KCSAN selftest: " #do_test " failed");         \
> +       } while (0)
> +
> +       RUN_TEST(test_requires);
> +       RUN_TEST(test_encode_decode);
> +       RUN_TEST(test_matching_access);
> +
> +       pr_info("KCSAN selftest: %d/%d tests passed\n", passed, total);
> +       if (passed != total)
> +               panic("KCSAN selftests failed");
> +       return 0;
> +}
> +postcore_initcall(kcsan_selftest);
> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> index 93d97f9b0157..35accd1d93de 100644
> --- a/lib/Kconfig.debug
> +++ b/lib/Kconfig.debug
> @@ -2086,6 +2086,8 @@ source "lib/Kconfig.kgdb"
>
>  source "lib/Kconfig.ubsan"
>
> +source "lib/Kconfig.kcsan"
> +
>  config ARCH_HAS_DEVMEM_IS_ALLOWED
>         bool
>
> diff --git a/lib/Kconfig.kcsan b/lib/Kconfig.kcsan
> new file mode 100644
> index 000000000000..b532d0d98f7a
> --- /dev/null
> +++ b/lib/Kconfig.kcsan
> @@ -0,0 +1,88 @@
> +# SPDX-License-Identifier: GPL-2.0-only
> +
> +config HAVE_ARCH_KCSAN
> +       bool
> +
> +menuconfig KCSAN
> +       bool "KCSAN: watchpoint-based dynamic data-race detector"
> +       depends on HAVE_ARCH_KCSAN && !KASAN && STACKTRACE
> +       default n
> +       help
> +         Kernel Concurrency Sanitizer is a dynamic data-race detector, which
> +         uses a watchpoint-based sampling approach to detect races.
> +
> +if KCSAN
> +
> +config KCSAN_SELFTEST
> +       bool "KCSAN: perform short selftests on boot"
> +       default y
> +       help
> +         Run KCSAN selftests on boot. On test failure, causes kernel to panic.
> +
> +config KCSAN_EARLY_ENABLE
> +       bool "KCSAN: early enable"
> +       default y
> +       help
> +         If KCSAN should be enabled globally as soon as possible. KCSAN can
> +         later be enabled/disabled via debugfs.
> +
> +config KCSAN_UDELAY_MAX_TASK
> +       int "KCSAN: maximum delay in microseconds (for tasks)"
> +       default 80
> +       help
> +         For tasks, the max. microsecond delay after setting up a watchpoint.
> +
> +config KCSAN_UDELAY_MAX_INTERRUPT
> +       int "KCSAN: maximum delay in microseconds (for interrupts)"
> +       default 20
> +       help
> +         For interrupts, the max. microsecond delay after setting up a watchpoint.
> +
> +config KCSAN_DELAY_RANDOMIZE
> +       bool "KCSAN: randomize delays"
> +       default y
> +       help
> +         If delays should be randomized; if false, the chosen delay is simply
> +         the maximum values defined above.
> +
> +config KCSAN_WATCH_SKIP_INST
> +       int "KCSAN: watchpoint instruction skip"
> +       default 2000
> +       help
> +         The number of per-CPU memory operations to skip watching, before
> +         another watchpoint is set up; in other words, 1 in
> +         KCSAN_WATCH_SKIP_INST per-CPU memory operations are used to set up a
> +         watchpoint. A smaller value results in more aggressive race
> +         detection, whereas a larger value improves system performance at the
> +         cost of missing some races.
> +
> +config KCSAN_REPORT_RACE_UNKNOWN_ORIGIN
> +       bool "KCSAN: report races of unknown origin"
> +       default y
> +       help
> +         If KCSAN should report races where only one access is known, and the
> +         conflicting access is of unknown origin. This type of race is
> +         reported if it was only possible to infer a race due to a data-value
> +         change while an access is being delayed on a watchpoint.
> +
> +config KCSAN_IGNORE_ATOMICS
> +       bool "KCSAN: do not instrument atomic accesses"
> +       default n
> +       help
> +         If enabled, never instruments atomic accesses. This results in not
> +         reporting data-races where one access is atomic and the other is a
> +         plain access.
> +
> +config KCSAN_PLAIN_WRITE_PRETEND_ONCE
> +       bool "KCSAN: pretend plain writes are WRITE_ONCE"
> +       default n
> +       help
> +         This option makes KCSAN pretend that all plain writes are WRITE_ONCE.
> +         This option should only be used to prune initial data-races found in
> +         existing code.
> +
> +config KCSAN_DEBUG
> +       bool "Debugging of KCSAN internals"
> +       default n
> +
> +endif # KCSAN
> diff --git a/lib/Makefile b/lib/Makefile
> index c5892807e06f..778ab704e3ad 100644
> --- a/lib/Makefile
> +++ b/lib/Makefile
> @@ -24,6 +24,9 @@ KASAN_SANITIZE_string.o := n
>  CFLAGS_string.o := $(call cc-option, -fno-stack-protector)
>  endif
>
> +# Used by KCSAN while enabled, avoid recursion.
> +KCSAN_SANITIZE_random32.o := n
> +
>  lib-y := ctype.o string.o vsprintf.o cmdline.o \
>          rbtree.o radix-tree.o timerqueue.o xarray.o \
>          idr.o extable.o \
> diff --git a/scripts/Makefile.kcsan b/scripts/Makefile.kcsan
> new file mode 100644
> index 000000000000..caf1111a28ae
> --- /dev/null
> +++ b/scripts/Makefile.kcsan
> @@ -0,0 +1,6 @@
> +# SPDX-License-Identifier: GPL-2.0
> +ifdef CONFIG_KCSAN
> +
> +CFLAGS_KCSAN := -fsanitize=thread
> +
> +endif # CONFIG_KCSAN
> diff --git a/scripts/Makefile.lib b/scripts/Makefile.lib
> index 179d55af5852..0e78abab7d83 100644
> --- a/scripts/Makefile.lib
> +++ b/scripts/Makefile.lib
> @@ -152,6 +152,16 @@ _c_flags += $(if $(patsubst n%,, \
>         $(CFLAGS_KCOV))
>  endif
>
> +#
> +# Enable ConcurrencySanitizer flags for kernel except some files or directories
> +# we don't want to check (depends on variables KCSAN_SANITIZE_obj.o, KCSAN_SANITIZE)
> +#
> +ifeq ($(CONFIG_KCSAN),y)
> +_c_flags += $(if $(patsubst n%,, \
> +       $(KCSAN_SANITIZE_$(basetarget).o)$(KCSAN_SANITIZE)y), \
> +       $(CFLAGS_KCSAN))
> +endif
> +
>  # $(srctree)/$(src) for including checkin headers from generated source files
>  # $(objtree)/$(obj) for including generated headers from checkin source files
>  ifeq ($(KBUILD_EXTMOD),)
> --
> 2.23.0.700.g56cf767bdb-goog
>
Marco Elver Oct. 16, 2019, 1:52 p.m. UTC | #4
> > diff --git a/Documentation/dev-tools/kcsan.rst b/Documentation/dev-tools/kcsan.rst
> > new file mode 100644
> > index 000000000000..5b46cc5593c3
> > --- /dev/null
> > +++ b/Documentation/dev-tools/kcsan.rst
> > @@ -0,0 +1,202 @@
> > +The Kernel Concurrency Sanitizer (KCSAN)
> > +========================================
> > +
> > +Overview
> > +--------
> > +
> > +*Kernel Concurrency Sanitizer (KCSAN)* is a dynamic data-race detector for
> > +kernel space. KCSAN is a sampling watchpoint-based data-race detector -- this
> > +is unlike Kernel Thread Sanitizer (KTSAN), which is a happens-before data-race
> > +detector. Key priorities in KCSAN's design are lack of false positives,
> > +scalability, and simplicity. More details can be found in `Implementation
> > +Details`_.
> > +
> > +KCSAN uses compile-time instrumentation to instrument memory accesses. KCSAN is
> > +supported in both GCC and Clang. With GCC it requires version 7.3.0 or later.
> > +With Clang it requires version 7.0.0 or later.
> > +
> > +Usage
> > +-----
> > +
> > +To enable KCSAN configure kernel with::
> > +
> > +    CONFIG_KCSAN = y
> > +
> > +KCSAN provides several other configuration options to customize behaviour (see
> > +their respective help text for more info).
> > +
> > +debugfs
> > +~~~~~~~
> > +
> > +* The file ``/sys/kernel/debug/kcsan`` can be read to get stats.
> > +
> > +* KCSAN can be turned on or off by writing ``on`` or ``off`` to
> > +  ``/sys/kernel/debug/kcsan``.
> > +
> > +* Writing ``!some_func_name`` to ``/sys/kernel/debug/kcsan`` adds
> > +  ``some_func_name`` to the report filter list, which (by default) blacklists
> > +  reporting data-races where either one of the top stackframes are a function
> > +  in the list.
> > +
> > +* Writing either ``blacklist`` or ``whitelist`` to ``/sys/kernel/debug/kcsan``
> > +  changes the report filtering behaviour. For example, the blacklist feature
> > +  can be used to silence frequently occurring data-races; the whitelist feature
> > +  can help with reproduction and testing of fixes.
> > +
> > +Error reports
> > +~~~~~~~~~~~~~
> > +
> > +A typical data-race report looks like this::
> > +
> > +    ==================================================================
> > +    BUG: KCSAN: data-race in generic_permission / kernfs_refresh_inode
> > +
> > +    write to 0xffff8fee4c40700c of 4 bytes by task 175 on cpu 4:
> > +     kernfs_refresh_inode+0x70/0x170
> > +     kernfs_iop_permission+0x4f/0x90
> > +     inode_permission+0x190/0x200
> > +     link_path_walk.part.0+0x503/0x8e0
> > +     path_lookupat.isra.0+0x69/0x4d0
> > +     filename_lookup+0x136/0x280
> > +     user_path_at_empty+0x47/0x60
> > +     vfs_statx+0x9b/0x130
> > +     __do_sys_newlstat+0x50/0xb0
> > +     __x64_sys_newlstat+0x37/0x50
> > +     do_syscall_64+0x85/0x260
> > +     entry_SYSCALL_64_after_hwframe+0x44/0xa9
> > +
> > +    read to 0xffff8fee4c40700c of 4 bytes by task 166 on cpu 6:
> > +     generic_permission+0x5b/0x2a0
> > +     kernfs_iop_permission+0x66/0x90
> > +     inode_permission+0x190/0x200
> > +     link_path_walk.part.0+0x503/0x8e0
> > +     path_lookupat.isra.0+0x69/0x4d0
> > +     filename_lookup+0x136/0x280
> > +     user_path_at_empty+0x47/0x60
> > +     do_faccessat+0x11a/0x390
> > +     __x64_sys_access+0x3c/0x50
> > +     do_syscall_64+0x85/0x260
> > +     entry_SYSCALL_64_after_hwframe+0x44/0xa9
> > +
> > +    Reported by Kernel Concurrency Sanitizer on:
> > +    CPU: 6 PID: 166 Comm: systemd-journal Not tainted 5.3.0-rc7+ #1
> > +    Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.12.0-1 04/01/2014
> > +    ==================================================================
> > +
> > +The header of the report provides a short summary of the functions involved in
> > +the race. It is followed by the access types and stack traces of the 2 threads
> > +involved in the data-race.
> > +
> > +The other less common type of data-race report looks like this::
> > +
> > +    ==================================================================
> > +    BUG: KCSAN: racing read in e1000_clean_rx_irq+0x551/0xb10
>
> Do we want to have a different bug title here? Can we also report this
> as a data-race to simplify report parsing rules?

Changed to just "data-race in" as well.

> > +
> > +    race at unknown origin, with read to 0xffff933db8a2ae6c of 1 bytes by interrupt on cpu 0:
> > +     e1000_clean_rx_irq+0x551/0xb10
> > +     e1000_clean+0x533/0xda0
> > +     net_rx_action+0x329/0x900
> > +     __do_softirq+0xdb/0x2db
> > +     irq_exit+0x9b/0xa0
> > +     do_IRQ+0x9c/0xf0
> > +     ret_from_intr+0x0/0x18
> > +     default_idle+0x3f/0x220
> > +     arch_cpu_idle+0x21/0x30
> > +     do_idle+0x1df/0x230
> > +     cpu_startup_entry+0x14/0x20
> > +     rest_init+0xc5/0xcb
> > +     arch_call_rest_init+0x13/0x2b
> > +     start_kernel+0x6db/0x700
> > +
> > +    Reported by Kernel Concurrency Sanitizer on:
> > +    CPU: 0 PID: 0 Comm: swapper/0 Not tainted 5.3.0-rc7+ #2
> > +    Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.12.0-1 04/01/2014
> > +    ==================================================================
> > +
> > +This report is generated where it was not possible to determine the other
> > +racing thread, but a race was inferred due to the data-value of the watched
> > +memory location having changed. These can occur either due to missing
> > +instrumentation or e.g. DMA accesses.
> > +
> > +Data-Races
> > +----------
> > +
> > +Informally, two operations *conflict* if they access the same memory location,
> > +and at least one of them is a write operation. In an execution, two memory
> > +operations from different threads form a **data-race** if they *conflict*, at
> > +least one of them is a *plain access* (non-atomic), and they are *unordered* in
> > +the "happens-before" order according to the `LKMM
> > +<../../tools/memory-model/Documentation/explanation.txt>`_.
> > +
> > +Relationship with the Linux Kernel Memory Model (LKMM)
> > +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > +
> > +The LKMM defines the propagation and ordering rules of various memory
> > +operations, which gives developers the ability to reason about concurrent code.
> > +Ultimately this allows to determine the possible executions of concurrent code,
> > +and if that code is free from data-races.
> > +
> > +KCSAN is aware of *atomic* accesses (``READ_ONCE``, ``WRITE_ONCE``,
> > +``atomic_*``, etc.), but is oblivious of any ordering guarantees. In other
> > +words, KCSAN assumes that as long as a plain access is not observed to race
> > +with another conflicting access, memory operations are correctly ordered.
> > +
> > +This means that KCSAN will not report *potential* data-races due to missing
> > +memory ordering. If, however, missing memory ordering (that is observable with
> > +a particular compiler and architecture) leads to an observable data-race (e.g.
> > +entering a critical section erroneously), KCSAN would report the resulting
> > +data-race.
> > +
> > +Implementation Details
> > +----------------------
> > +
> > +The general approach is inspired by `DataCollider
> > +<http://usenix.org/legacy/events/osdi10/tech/full_papers/Erickson.pdf>`_.
> > +Unlike DataCollider, KCSAN does not use hardware watchpoints, but instead
> > +relies on compiler instrumentation. Watchpoints are implemented using an
> > +efficient encoding that stores access type, size, and address in a long; the
> > +benefits of using "soft watchpoints" are portability and greater flexibility in
> > +limiting which accesses trigger a watchpoint.
> > +
> > +More specifically, KCSAN requires instrumenting plain (unmarked, non-atomic)
> > +memory operations; for each instrumented plain access:
> > +
> > +1. Check if a matching watchpoint exists; if yes, and at least one access is a
> > +   write, then we encountered a racing access.
> > +
> > +2. Periodically, if no matching watchpoint exists, set up a watchpoint and
> > +   stall some delay.
> > +
> > +3. Also check the data value before the delay, and re-check the data value
> > +   after delay; if the values mismatch, we infer a race of unknown origin.
> > +
> > +To detect data-races between plain and atomic memory operations, KCSAN also
> > +annotates atomic accesses, but only to check if a watchpoint exists
> > +(``kcsan_check_atomic(..)``); i.e.  KCSAN never sets up a watchpoint on atomic
> > +accesses.
> > +
> > +Key Properties
> > +~~~~~~~~~~~~~~
> > +
> > +1. **Performance Overhead:** KCSAN's runtime is minimal, and does not require
> > +   locking shared state for each access. This results in significantly better
> > +   performance in comparison with KTSAN.
> > +
> > +2. **Memory Overhead:** No shadow memory is required. The current
> > +   implementation uses a small array of longs to encode watchpoint information,
> > +   which is negligible.
> > +
> > +3. **Memory Ordering:** KCSAN is *not* aware of the LKMM's ordering rules. This
> > +   may result in missed data-races (false negatives), compared to a
> > +   happens-before data-race detector such as KTSAN.
> > +
> > +4. **Accuracy:** Imprecise, since it uses a sampling strategy.
> > +
> > +5. **Annotation Overheads:** Minimal annotation is required outside the KCSAN
> > +   runtime. With a happens-before data-race detector, any omission leads to
> > +   false positives, which is especially important in the context of the kernel
> > +   which includes numerous custom synchronization mechanisms. With KCSAN, as a
> > +   result, maintenance overheads are minimal as the kernel evolves.
> > +
> > +6. **Detects Racy Writes from Devices:** Due to checking data values upon
> > +   setting up watchpoints, racy writes from devices can also be detected.
>
> This part compares KCSAN with KTSAN, do we need it here? I think it
> might be better to move this to the cover letter as a rationale as to
> why we went with the watchpoint based approach, instead of the
> happens-before one.

Removed mentions of KTSAN where it doesn't add very much.

These are properties of the design that, if not summarized here, would
be lost and we'd have to look at the code. This is also for the
benefit of developers using KCSAN to detect races, highlighting the
pros and cons in the inherent design they should be aware of. I prefer
keeping this information here, as otherwise it will get lost and we
will have no central place to refer to.

> Some performance numbers comparing KCSAN with a non instrumented
> kernel would be more useful here.

I've added a sentence with some empirical data.

Changes queued for v2.

Many thanks,
-- Marco
Mark Rutland Oct. 16, 2019, 3:16 p.m. UTC | #5
On Wed, Oct 16, 2019 at 10:39:52AM +0200, Marco Elver wrote:
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 2c2e56bd8913..34a1d9310304 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -1171,6 +1171,13 @@ struct task_struct {
>  #ifdef CONFIG_KASAN
>  	unsigned int			kasan_depth;
>  #endif
> +#ifdef CONFIG_KCSAN
> +	/* See comments at kernel/kcsan/core.c: struct cpu_state. */
> +	int				kcsan_disable;
> +	int				kcsan_atomic_next;
> +	int				kcsan_atomic_region;
> +	bool				kcsan_atomic_region_flat;
> +#endif

Should these be unsigned?

> +/*
> + * Per-CPU state that should be used instead of 'current' if we are not in a
> + * task.
> + */
> +struct cpu_state {
> +	int disable; /* disable counter */
> +	int atomic_next; /* number of following atomic ops */
> +
> +	/*
> +	 * We use separate variables to store if we are in a nestable or flat
> +	 * atomic region. This helps make sure that an atomic region with
> +	 * nesting support is not suddenly aborted when a flat region is
> +	 * contained within. Effectively this allows supporting nesting flat
> +	 * atomic regions within an outer nestable atomic region. Support for
> +	 * this is required as there are cases where a seqlock reader critical
> +	 * section (flat atomic region) is contained within a seqlock writer
> +	 * critical section (nestable atomic region), and the "mismatching
> +	 * kcsan_end_atomic()" warning would trigger otherwise.
> +	 */
> +	int atomic_region;
> +	bool atomic_region_flat;
> +};
> +static DEFINE_PER_CPU(struct cpu_state, this_state) = {
> +	.disable = 0,
> +	.atomic_next = 0,
> +	.atomic_region = 0,
> +	.atomic_region_flat = 0,
> +};

These are the same as in task_struct, so I think it probably makes sense
to have a common structure for these, e.g.

| struct kcsan_ctx {
| 	int	disable;
| 	int	atomic_next;
| 	int	atomic_region;
| 	bool	atomic_region_flat;
| };

... which you then place within task_struct, e.g.

| #ifdef CONFIG_KCSAN
| 	struct kcsan_ctx	kcsan_ctx;
| #endif

... and here, e.g.

| static DEFINE_PER_CPU(struct kcsan_ctx, kcsan_cpu_ctx);

That would simplify a number of cases below where you have to choose one
or the other, as you can choose the pointer, then handle the rest in a
common way.

e.g. for:

> +static inline bool is_atomic(const volatile void *ptr)
> +{
> +	if (in_task()) {
> +		if (unlikely(current->kcsan_atomic_next > 0)) {
> +			--current->kcsan_atomic_next;
> +			return true;
> +		}
> +		if (unlikely(current->kcsan_atomic_region > 0 ||
> +			     current->kcsan_atomic_region_flat))
> +			return true;
> +	} else { /* interrupt */
> +		if (unlikely(this_cpu_read(this_state.atomic_next) > 0)) {
> +			this_cpu_dec(this_state.atomic_next);
> +			return true;
> +		}
> +		if (unlikely(this_cpu_read(this_state.atomic_region) > 0 ||
> +			     this_cpu_read(this_state.atomic_region_flat)))
> +			return true;
> +	}
> +
> +	return kcsan_is_atomic(ptr);
> +}

... you could have something like:

| struct kcsan_ctx *kcsan_get_ctx(void)
| {
| 	return in_task() ? &current->kcsan_ctx : this_cpu_ptr(kcsan_cpu_ctx);
| }
|
| static inline bool is_atomic(const volatile void *ptr)
| {
| 	struct kcsan_ctx *ctx = kcsan_get_ctx();
|	if (unlikely(ctx->atomic_next > 0) {
|		--ctx->atomic_next;
| 		return true;
| 	}
| 	if (unlikely(ctx->atomic_region > 0 || ctx->atomic_region_flat))
| 		return true;
|
| 	return kcsan_is_atomic(ptr);
| }

... avoiding duplicating the checks for task/irq contexts.

It's not clear to me how either that or the original code works if a
softirq is interrupted by a hardirq. IIUC most of the fields should
remain stable over that window, since the hardirq should balance most
changes it makes before returning, but I don't think that's true for
atomic_next. Can't that be corrupted from the PoV of the softirq
handler?

[...]

> +void kcsan_begin_atomic(bool nest)
> +{
> +	if (nest) {
> +		if (in_task())
> +			++current->kcsan_atomic_region;
> +		else
> +			this_cpu_inc(this_state.atomic_region);
> +	} else {
> +		if (in_task())
> +			current->kcsan_atomic_region_flat = true;
> +		else
> +			this_cpu_write(this_state.atomic_region_flat, true);
> +	}
> +}

Assuming my suggestion above wasn't bogus, this can be:

| void kcsan_begin_atomic(boot nest)
| {
| 	struct kcsan_ctx *ctx = kcsan_get_ctx();
| 	if (nest)
| 		ctx->atomic_region++;
| 	else
| 		ctx->atomic_region_flat = true;
| }

> +void kcsan_end_atomic(bool nest)
> +{
> +	if (nest) {
> +		int prev =
> +			in_task() ?
> +				current->kcsan_atomic_region-- :
> +				(this_cpu_dec_return(this_state.atomic_region) +
> +				 1);
> +		if (prev == 0) {
> +			kcsan_begin_atomic(true); /* restore to 0 */
> +			kcsan_disable_current();
> +			WARN(1, "mismatching %s", __func__);
> +			kcsan_enable_current();
> +		}
> +	} else {
> +		if (in_task())
> +			current->kcsan_atomic_region_flat = false;
> +		else
> +			this_cpu_write(this_state.atomic_region_flat, false);
> +	}
> +}

... similarly:

| void kcsan_end_atomic(bool nest)
| {
| 	struct kcsan_ctx *ctx = kcsan_get_ctx();
| 
| 	if (nest)
| 		if (ctx->kcsan_atomic_region--) {
| 			kcsan_begin_atomic(true); /* restore to 0 */
| 			kcsan_disable_current();
| 			WARN(1, "mismatching %s"\ __func__);
| 			kcsan_enable_current();
| 		}
| 	} else {
| 		ctx->atomic_region_flat = true;
| 	}
| }

> +void kcsan_atomic_next(int n)
> +{
> +	if (in_task())
> +		current->kcsan_atomic_next = n;
> +	else
> +		this_cpu_write(this_state.atomic_next, n);
> +}

... and:

| void kcsan_atomic_nextint n)
| {
| 	kcsan_get_ctx()->atomic_next = n;
| }

Thanks,
Mark.
Marco Elver Oct. 16, 2019, 3:53 p.m. UTC | #6
On Wed, 16 Oct 2019 at 17:16, Mark Rutland <mark.rutland@arm.com> wrote:
>
> On Wed, Oct 16, 2019 at 10:39:52AM +0200, Marco Elver wrote:
> > diff --git a/include/linux/sched.h b/include/linux/sched.h
> > index 2c2e56bd8913..34a1d9310304 100644
> > --- a/include/linux/sched.h
> > +++ b/include/linux/sched.h
> > @@ -1171,6 +1171,13 @@ struct task_struct {
> >  #ifdef CONFIG_KASAN
> >       unsigned int                    kasan_depth;
> >  #endif
> > +#ifdef CONFIG_KCSAN
> > +     /* See comments at kernel/kcsan/core.c: struct cpu_state. */
> > +     int                             kcsan_disable;
> > +     int                             kcsan_atomic_next;
> > +     int                             kcsan_atomic_region;
> > +     bool                            kcsan_atomic_region_flat;
> > +#endif
>
> Should these be unsigned?

I prefer to keep them int, as they can become negative (rather than
underflow with unsigned), if we e.g. have unbalanced
kcsan_enable_current etc. Since we do not need the full unsigned range
(these values should stay relatively small), int is more than enough.

> > +/*
> > + * Per-CPU state that should be used instead of 'current' if we are not in a
> > + * task.
> > + */
> > +struct cpu_state {
> > +     int disable; /* disable counter */
> > +     int atomic_next; /* number of following atomic ops */
> > +
> > +     /*
> > +      * We use separate variables to store if we are in a nestable or flat
> > +      * atomic region. This helps make sure that an atomic region with
> > +      * nesting support is not suddenly aborted when a flat region is
> > +      * contained within. Effectively this allows supporting nesting flat
> > +      * atomic regions within an outer nestable atomic region. Support for
> > +      * this is required as there are cases where a seqlock reader critical
> > +      * section (flat atomic region) is contained within a seqlock writer
> > +      * critical section (nestable atomic region), and the "mismatching
> > +      * kcsan_end_atomic()" warning would trigger otherwise.
> > +      */
> > +     int atomic_region;
> > +     bool atomic_region_flat;
> > +};
> > +static DEFINE_PER_CPU(struct cpu_state, this_state) = {
> > +     .disable = 0,
> > +     .atomic_next = 0,
> > +     .atomic_region = 0,
> > +     .atomic_region_flat = 0,
> > +};
>
> These are the same as in task_struct, so I think it probably makes sense
> to have a common structure for these, e.g.
>
> | struct kcsan_ctx {
> |       int     disable;
> |       int     atomic_next;
> |       int     atomic_region;
> |       bool    atomic_region_flat;
> | };
>
> ... which you then place within task_struct, e.g.
>
> | #ifdef CONFIG_KCSAN
> |       struct kcsan_ctx        kcsan_ctx;
> | #endif
>
> ... and here, e.g.
>
> | static DEFINE_PER_CPU(struct kcsan_ctx, kcsan_cpu_ctx);
>
> That would simplify a number of cases below where you have to choose one
> or the other, as you can choose the pointer, then handle the rest in a
> common way.
>
> e.g. for:
>
> > +static inline bool is_atomic(const volatile void *ptr)
> > +{
> > +     if (in_task()) {
> > +             if (unlikely(current->kcsan_atomic_next > 0)) {
> > +                     --current->kcsan_atomic_next;
> > +                     return true;
> > +             }
> > +             if (unlikely(current->kcsan_atomic_region > 0 ||
> > +                          current->kcsan_atomic_region_flat))
> > +                     return true;
> > +     } else { /* interrupt */
> > +             if (unlikely(this_cpu_read(this_state.atomic_next) > 0)) {
> > +                     this_cpu_dec(this_state.atomic_next);
> > +                     return true;
> > +             }
> > +             if (unlikely(this_cpu_read(this_state.atomic_region) > 0 ||
> > +                          this_cpu_read(this_state.atomic_region_flat)))
> > +                     return true;
> > +     }
> > +
> > +     return kcsan_is_atomic(ptr);
> > +}
>
> ... you could have something like:
>
> | struct kcsan_ctx *kcsan_get_ctx(void)
> | {
> |       return in_task() ? &current->kcsan_ctx : this_cpu_ptr(kcsan_cpu_ctx);
> | }
> |
> | static inline bool is_atomic(const volatile void *ptr)
> | {
> |       struct kcsan_ctx *ctx = kcsan_get_ctx();
> |       if (unlikely(ctx->atomic_next > 0) {
> |               --ctx->atomic_next;
> |               return true;
> |       }
> |       if (unlikely(ctx->atomic_region > 0 || ctx->atomic_region_flat))
> |               return true;
> |
> |       return kcsan_is_atomic(ptr);
> | }
>
> ... avoiding duplicating the checks for task/irq contexts.
>
> It's not clear to me how either that or the original code works if a
> softirq is interrupted by a hardirq. IIUC most of the fields should
> remain stable over that window, since the hardirq should balance most
> changes it makes before returning, but I don't think that's true for
> atomic_next. Can't that be corrupted from the PoV of the softirq
> handler?

As you say, these fields should balance. So far I have not observed
any issues. For atomic_next I'm not concerned as it is an
approximation either way (see seqlock patch), and it's fine if there
is a small error.

> [...]
>
> > +void kcsan_begin_atomic(bool nest)
> > +{
> > +     if (nest) {
> > +             if (in_task())
> > +                     ++current->kcsan_atomic_region;
> > +             else
> > +                     this_cpu_inc(this_state.atomic_region);
> > +     } else {
> > +             if (in_task())
> > +                     current->kcsan_atomic_region_flat = true;
> > +             else
> > +                     this_cpu_write(this_state.atomic_region_flat, true);
> > +     }
> > +}
>
> Assuming my suggestion above wasn't bogus, this can be:
>
> | void kcsan_begin_atomic(boot nest)
> | {
> |       struct kcsan_ctx *ctx = kcsan_get_ctx();
> |       if (nest)
> |               ctx->atomic_region++;
> |       else
> |               ctx->atomic_region_flat = true;
> | }
>
> > +void kcsan_end_atomic(bool nest)
> > +{
> > +     if (nest) {
> > +             int prev =
> > +                     in_task() ?
> > +                             current->kcsan_atomic_region-- :
> > +                             (this_cpu_dec_return(this_state.atomic_region) +
> > +                              1);
> > +             if (prev == 0) {
> > +                     kcsan_begin_atomic(true); /* restore to 0 */
> > +                     kcsan_disable_current();
> > +                     WARN(1, "mismatching %s", __func__);
> > +                     kcsan_enable_current();
> > +             }
> > +     } else {
> > +             if (in_task())
> > +                     current->kcsan_atomic_region_flat = false;
> > +             else
> > +                     this_cpu_write(this_state.atomic_region_flat, false);
> > +     }
> > +}
>
> ... similarly:
>
> | void kcsan_end_atomic(bool nest)
> | {
> |       struct kcsan_ctx *ctx = kcsan_get_ctx();
> |
> |       if (nest)
> |               if (ctx->kcsan_atomic_region--) {
> |                       kcsan_begin_atomic(true); /* restore to 0 */
> |                       kcsan_disable_current();
> |                       WARN(1, "mismatching %s"\ __func__);
> |                       kcsan_enable_current();
> |               }
> |       } else {
> |               ctx->atomic_region_flat = true;
> |       }
> | }
>
> > +void kcsan_atomic_next(int n)
> > +{
> > +     if (in_task())
> > +             current->kcsan_atomic_next = n;
> > +     else
> > +             this_cpu_write(this_state.atomic_next, n);
> > +}
>
> ... and:
>
> | void kcsan_atomic_nextint n)
> | {
> |       kcsan_get_ctx()->atomic_next = n;
> | }

Otherwise, yes, this makes much more sense and I will just introduce
the struct and integrate the above suggestions for v2.

Many thanks,
-- Marco
Peter Zijlstra Oct. 16, 2019, 6:43 p.m. UTC | #7
On Wed, Oct 16, 2019 at 10:39:52AM +0200, Marco Elver wrote:

> +bool __kcsan_check_watchpoint(const volatile void *ptr, size_t size,
> +			      bool is_write)
> +{
> +	atomic_long_t *watchpoint;
> +	long encoded_watchpoint;
> +	unsigned long flags;
> +	enum kcsan_report_type report_type;
> +
> +	if (unlikely(!is_enabled()))
> +		return false;
> +
> +	watchpoint = find_watchpoint((unsigned long)ptr, size, !is_write,
> +				     &encoded_watchpoint);
> +	if (watchpoint == NULL)
> +		return true;
> +
> +	flags = user_access_save();

Could use a comment on why find_watchpoint() is save to call without
user_access_save() on.

> +	if (!try_consume_watchpoint(watchpoint, encoded_watchpoint)) {
> +		/*
> +		 * The other thread may not print any diagnostics, as it has
> +		 * already removed the watchpoint, or another thread consumed
> +		 * the watchpoint before this thread.
> +		 */
> +		kcsan_counter_inc(kcsan_counter_report_races);
> +		report_type = kcsan_report_race_check_race;
> +	} else {
> +		report_type = kcsan_report_race_check;
> +	}
> +
> +	/* Encountered a data-race. */
> +	kcsan_counter_inc(kcsan_counter_data_races);
> +	kcsan_report(ptr, size, is_write, raw_smp_processor_id(), report_type);
> +
> +	user_access_restore(flags);
> +	return false;
> +}
> +EXPORT_SYMBOL(__kcsan_check_watchpoint);
> +
> +void __kcsan_setup_watchpoint(const volatile void *ptr, size_t size,
> +			      bool is_write)
> +{
> +	atomic_long_t *watchpoint;
> +	union {
> +		u8 _1;
> +		u16 _2;
> +		u32 _4;
> +		u64 _8;
> +	} expect_value;
> +	bool is_expected = true;
> +	unsigned long ua_flags = user_access_save();
> +	unsigned long irq_flags;
> +
> +	if (!should_watch(ptr))
> +		goto out;
> +
> +	if (!check_encodable((unsigned long)ptr, size)) {
> +		kcsan_counter_inc(kcsan_counter_unencodable_accesses);
> +		goto out;
> +	}
> +
> +	/*
> +	 * Disable interrupts & preemptions, to ignore races due to accesses in
> +	 * threads running on the same CPU.
> +	 */
> +	local_irq_save(irq_flags);
> +	preempt_disable();

Is there a point to that preempt_disable() here?
Marco Elver Oct. 16, 2019, 7:34 p.m. UTC | #8
On Wed, 16 Oct 2019 at 20:44, Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Wed, Oct 16, 2019 at 10:39:52AM +0200, Marco Elver wrote:
>
> > +bool __kcsan_check_watchpoint(const volatile void *ptr, size_t size,
> > +                           bool is_write)
> > +{
> > +     atomic_long_t *watchpoint;
> > +     long encoded_watchpoint;
> > +     unsigned long flags;
> > +     enum kcsan_report_type report_type;
> > +
> > +     if (unlikely(!is_enabled()))
> > +             return false;
> > +
> > +     watchpoint = find_watchpoint((unsigned long)ptr, size, !is_write,
> > +                                  &encoded_watchpoint);
> > +     if (watchpoint == NULL)
> > +             return true;
> > +
> > +     flags = user_access_save();
>
> Could use a comment on why find_watchpoint() is save to call without
> user_access_save() on.

Thanks, will add a comment for v2.

> > +     if (!try_consume_watchpoint(watchpoint, encoded_watchpoint)) {
> > +             /*
> > +              * The other thread may not print any diagnostics, as it has
> > +              * already removed the watchpoint, or another thread consumed
> > +              * the watchpoint before this thread.
> > +              */
> > +             kcsan_counter_inc(kcsan_counter_report_races);
> > +             report_type = kcsan_report_race_check_race;
> > +     } else {
> > +             report_type = kcsan_report_race_check;
> > +     }
> > +
> > +     /* Encountered a data-race. */
> > +     kcsan_counter_inc(kcsan_counter_data_races);
> > +     kcsan_report(ptr, size, is_write, raw_smp_processor_id(), report_type);
> > +
> > +     user_access_restore(flags);
> > +     return false;
> > +}
> > +EXPORT_SYMBOL(__kcsan_check_watchpoint);
> > +
> > +void __kcsan_setup_watchpoint(const volatile void *ptr, size_t size,
> > +                           bool is_write)
> > +{
> > +     atomic_long_t *watchpoint;
> > +     union {
> > +             u8 _1;
> > +             u16 _2;
> > +             u32 _4;
> > +             u64 _8;
> > +     } expect_value;
> > +     bool is_expected = true;
> > +     unsigned long ua_flags = user_access_save();
> > +     unsigned long irq_flags;
> > +
> > +     if (!should_watch(ptr))
> > +             goto out;
> > +
> > +     if (!check_encodable((unsigned long)ptr, size)) {
> > +             kcsan_counter_inc(kcsan_counter_unencodable_accesses);
> > +             goto out;
> > +     }
> > +
> > +     /*
> > +      * Disable interrupts & preemptions, to ignore races due to accesses in
> > +      * threads running on the same CPU.
> > +      */
> > +     local_irq_save(irq_flags);
> > +     preempt_disable();
>
> Is there a point to that preempt_disable() here?

We want to avoid being preempted while the watchpoint is set up;
otherwise, we would report data-races for CPU-local data, which is
incorrect. An alternative would be adding the source CPU to the
watchpoint, and checking that the CPU != this_cpu. There are several
problems with that alternative:
1. We do not want to steal more bits from the watchpoint encoding for
things other than read/write, size, and address, as not only does it
affect accuracy, it would also increase performance overhead in the
fast-path.
2. As a consequence, if we get a preemption and run a task on the same
CPU, and there *is* a genuine data-race, we would *not* report it; and
since this is the common case (and not accesses to CPU-local data), it
makes more sense (from a data-race detection PoV) to simply disable
preemptions and ensure that all tasks are run on other CPUs as well as
avoid the problem of point (1).

I can add a comment to that effect here for v2.

Thanks,
-- Marco
Boqun Feng Oct. 17, 2019, 12:25 a.m. UTC | #9
On Wed, Oct 16, 2019 at 12:06:51PM +0200, Marco Elver wrote:
> On Wed, 16 Oct 2019 at 11:42, Boqun Feng <boqun.feng@gmail.com> wrote:
> >
> > Hi Marco,
> >
> > On Wed, Oct 16, 2019 at 10:39:52AM +0200, Marco Elver wrote:
> > [...]
> > > --- /dev/null
> > > +++ b/kernel/kcsan/kcsan.c
> > > @@ -0,0 +1,81 @@
> > > +// SPDX-License-Identifier: GPL-2.0
> > > +
> > > +/*
> > > + * The Kernel Concurrency Sanitizer (KCSAN) infrastructure. For more info please
> > > + * see Documentation/dev-tools/kcsan.rst.
> > > + */
> > > +
> > > +#include <linux/export.h>
> > > +
> > > +#include "kcsan.h"
> > > +
> > > +/*
> > > + * Concurrency Sanitizer uses the same instrumentation as Thread Sanitizer.
> >
> > Is there any documentation on the instrumentation? Like a complete list
> > for all instrumentation functions plus a description of where the
> > compiler will use those functions. Yes, the names of the below functions
> > are straightforward, but an accurate doc on the instrumentation will
> > cerntainly help people review KCSAN.
> 
> As far as I'm aware neither GCC nor Clang have documentation on the
> emitted instrumentation that we could reference (other than look into
> the compiler passes).
> 

Yeah, I don't find them either, which makes me surprised, because I
think the thread sanitizer has been there for a while...

> However it is as straightforward as it seems: the compiler emits
> instrumentation calls for all loads and stores that the compiler
> generates; inline asm is not instrumented. I will add a comment to
> that effect for v2.
> 

Or you can push the compiler people to document it, and we can simply
reference it in kernel ;-)

Regards,
Boqun

> Thanks,
> -- Marco
> 
> > Regards,
> > Boqun
> >
> > > + */
> > > +
> > > +#define DEFINE_TSAN_READ_WRITE(size)                                           \
> > > +     void __tsan_read##size(void *ptr)                                      \
> > > +     {                                                                      \
> > > +             __kcsan_check_access(ptr, size, false);                        \
> > > +     }                                                                      \
> > > +     EXPORT_SYMBOL(__tsan_read##size);                                      \
> > > +     void __tsan_write##size(void *ptr)                                     \
> > > +     {                                                                      \
> > > +             __kcsan_check_access(ptr, size, true);                         \
> > > +     }                                                                      \
> > > +     EXPORT_SYMBOL(__tsan_write##size)
> > > +
> > > +DEFINE_TSAN_READ_WRITE(1);
> > > +DEFINE_TSAN_READ_WRITE(2);
> > > +DEFINE_TSAN_READ_WRITE(4);
> > > +DEFINE_TSAN_READ_WRITE(8);
> > > +DEFINE_TSAN_READ_WRITE(16);
> > > +
> > > +/*
> > > + * Not all supported compiler versions distinguish aligned/unaligned accesses,
> > > + * but e.g. recent versions of Clang do.
> > > + */
> > > +#define DEFINE_TSAN_UNALIGNED_READ_WRITE(size)                                 \
> > > +     void __tsan_unaligned_read##size(void *ptr)                            \
> > > +     {                                                                      \
> > > +             __kcsan_check_access(ptr, size, false);                        \
> > > +     }                                                                      \
> > > +     EXPORT_SYMBOL(__tsan_unaligned_read##size);                            \
> > > +     void __tsan_unaligned_write##size(void *ptr)                           \
> > > +     {                                                                      \
> > > +             __kcsan_check_access(ptr, size, true);                         \
> > > +     }                                                                      \
> > > +     EXPORT_SYMBOL(__tsan_unaligned_write##size)
> > > +
> > > +DEFINE_TSAN_UNALIGNED_READ_WRITE(2);
> > > +DEFINE_TSAN_UNALIGNED_READ_WRITE(4);
> > > +DEFINE_TSAN_UNALIGNED_READ_WRITE(8);
> > > +DEFINE_TSAN_UNALIGNED_READ_WRITE(16);
> > > +
> > > +void __tsan_read_range(void *ptr, size_t size)
> > > +{
> > > +     __kcsan_check_access(ptr, size, false);
> > > +}
> > > +EXPORT_SYMBOL(__tsan_read_range);
> > > +
> > > +void __tsan_write_range(void *ptr, size_t size)
> > > +{
> > > +     __kcsan_check_access(ptr, size, true);
> > > +}
> > > +EXPORT_SYMBOL(__tsan_write_range);
> > > +
> > > +/*
> > > + * The below are not required KCSAN, but can still be emitted by the compiler.
> > > + */
> > > +void __tsan_func_entry(void *call_pc)
> > > +{
> > > +}
> > > +EXPORT_SYMBOL(__tsan_func_entry);
> > > +void __tsan_func_exit(void)
> > > +{
> > > +}
> > > +EXPORT_SYMBOL(__tsan_func_exit);
> > > +void __tsan_init(void)
> > > +{
> > > +}
> > > +EXPORT_SYMBOL(__tsan_init);
> > [...]
Peter Zijlstra Oct. 17, 2019, 7:47 a.m. UTC | #10
On Wed, Oct 16, 2019 at 09:34:05PM +0200, Marco Elver wrote:
> On Wed, 16 Oct 2019 at 20:44, Peter Zijlstra <peterz@infradead.org> wrote:
> > > +     /*
> > > +      * Disable interrupts & preemptions, to ignore races due to accesses in
> > > +      * threads running on the same CPU.
> > > +      */
> > > +     local_irq_save(irq_flags);
> > > +     preempt_disable();
> >
> > Is there a point to that preempt_disable() here?
> 
> We want to avoid being preempted while the watchpoint is set up;
> otherwise, we would report data-races for CPU-local data, which is
> incorrect.

Disabling IRQs already very much disables preemption. There is
absolutely no point in doing preempt_disable() when the whole section
already runs with IRQs disabled.
Marco Elver Oct. 17, 2019, 7:49 a.m. UTC | #11
On Thu, 17 Oct 2019 at 09:47, Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Wed, Oct 16, 2019 at 09:34:05PM +0200, Marco Elver wrote:
> > On Wed, 16 Oct 2019 at 20:44, Peter Zijlstra <peterz@infradead.org> wrote:
> > > > +     /*
> > > > +      * Disable interrupts & preemptions, to ignore races due to accesses in
> > > > +      * threads running on the same CPU.
> > > > +      */
> > > > +     local_irq_save(irq_flags);
> > > > +     preempt_disable();
> > >
> > > Is there a point to that preempt_disable() here?
> >
> > We want to avoid being preempted while the watchpoint is set up;
> > otherwise, we would report data-races for CPU-local data, which is
> > incorrect.
>
> Disabling IRQs already very much disables preemption. There is
> absolutely no point in doing preempt_disable() when the whole section
> already runs with IRQs disabled.

Ah thanks for the clarification, in that case I assume it's safe to
remove preempt_disable() for v2.

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Patch
diff mbox series

diff --git a/Documentation/dev-tools/kcsan.rst b/Documentation/dev-tools/kcsan.rst
new file mode 100644
index 000000000000..5b46cc5593c3
--- /dev/null
+++ b/Documentation/dev-tools/kcsan.rst
@@ -0,0 +1,202 @@ 
+The Kernel Concurrency Sanitizer (KCSAN)
+========================================
+
+Overview
+--------
+
+*Kernel Concurrency Sanitizer (KCSAN)* is a dynamic data-race detector for
+kernel space. KCSAN is a sampling watchpoint-based data-race detector -- this
+is unlike Kernel Thread Sanitizer (KTSAN), which is a happens-before data-race
+detector. Key priorities in KCSAN's design are lack of false positives,
+scalability, and simplicity. More details can be found in `Implementation
+Details`_.
+
+KCSAN uses compile-time instrumentation to instrument memory accesses. KCSAN is
+supported in both GCC and Clang. With GCC it requires version 7.3.0 or later.
+With Clang it requires version 7.0.0 or later.
+
+Usage
+-----
+
+To enable KCSAN configure kernel with::
+
+    CONFIG_KCSAN = y
+
+KCSAN provides several other configuration options to customize behaviour (see
+their respective help text for more info).
+
+debugfs
+~~~~~~~
+
+* The file ``/sys/kernel/debug/kcsan`` can be read to get stats.
+
+* KCSAN can be turned on or off by writing ``on`` or ``off`` to
+  ``/sys/kernel/debug/kcsan``.
+
+* Writing ``!some_func_name`` to ``/sys/kernel/debug/kcsan`` adds
+  ``some_func_name`` to the report filter list, which (by default) blacklists
+  reporting data-races where either one of the top stackframes are a function
+  in the list.
+
+* Writing either ``blacklist`` or ``whitelist`` to ``/sys/kernel/debug/kcsan``
+  changes the report filtering behaviour. For example, the blacklist feature
+  can be used to silence frequently occurring data-races; the whitelist feature
+  can help with reproduction and testing of fixes.
+
+Error reports
+~~~~~~~~~~~~~
+
+A typical data-race report looks like this::
+
+    ==================================================================
+    BUG: KCSAN: data-race in generic_permission / kernfs_refresh_inode
+
+    write to 0xffff8fee4c40700c of 4 bytes by task 175 on cpu 4:
+     kernfs_refresh_inode+0x70/0x170
+     kernfs_iop_permission+0x4f/0x90
+     inode_permission+0x190/0x200
+     link_path_walk.part.0+0x503/0x8e0
+     path_lookupat.isra.0+0x69/0x4d0
+     filename_lookup+0x136/0x280
+     user_path_at_empty+0x47/0x60
+     vfs_statx+0x9b/0x130
+     __do_sys_newlstat+0x50/0xb0
+     __x64_sys_newlstat+0x37/0x50
+     do_syscall_64+0x85/0x260
+     entry_SYSCALL_64_after_hwframe+0x44/0xa9
+
+    read to 0xffff8fee4c40700c of 4 bytes by task 166 on cpu 6:
+     generic_permission+0x5b/0x2a0
+     kernfs_iop_permission+0x66/0x90
+     inode_permission+0x190/0x200
+     link_path_walk.part.0+0x503/0x8e0
+     path_lookupat.isra.0+0x69/0x4d0
+     filename_lookup+0x136/0x280
+     user_path_at_empty+0x47/0x60
+     do_faccessat+0x11a/0x390
+     __x64_sys_access+0x3c/0x50
+     do_syscall_64+0x85/0x260
+     entry_SYSCALL_64_after_hwframe+0x44/0xa9
+
+    Reported by Kernel Concurrency Sanitizer on:
+    CPU: 6 PID: 166 Comm: systemd-journal Not tainted 5.3.0-rc7+ #1
+    Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.12.0-1 04/01/2014
+    ==================================================================
+
+The header of the report provides a short summary of the functions involved in
+the race. It is followed by the access types and stack traces of the 2 threads
+involved in the data-race.
+
+The other less common type of data-race report looks like this::
+
+    ==================================================================
+    BUG: KCSAN: racing read in e1000_clean_rx_irq+0x551/0xb10
+
+    race at unknown origin, with read to 0xffff933db8a2ae6c of 1 bytes by interrupt on cpu 0:
+     e1000_clean_rx_irq+0x551/0xb10
+     e1000_clean+0x533/0xda0
+     net_rx_action+0x329/0x900
+     __do_softirq+0xdb/0x2db
+     irq_exit+0x9b/0xa0
+     do_IRQ+0x9c/0xf0
+     ret_from_intr+0x0/0x18
+     default_idle+0x3f/0x220
+     arch_cpu_idle+0x21/0x30
+     do_idle+0x1df/0x230
+     cpu_startup_entry+0x14/0x20
+     rest_init+0xc5/0xcb
+     arch_call_rest_init+0x13/0x2b
+     start_kernel+0x6db/0x700
+
+    Reported by Kernel Concurrency Sanitizer on:
+    CPU: 0 PID: 0 Comm: swapper/0 Not tainted 5.3.0-rc7+ #2
+    Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.12.0-1 04/01/2014
+    ==================================================================
+
+This report is generated where it was not possible to determine the other
+racing thread, but a race was inferred due to the data-value of the watched
+memory location having changed. These can occur either due to missing
+instrumentation or e.g. DMA accesses.
+
+Data-Races
+----------
+
+Informally, two operations *conflict* if they access the same memory location,
+and at least one of them is a write operation. In an execution, two memory
+operations from different threads form a **data-race** if they *conflict*, at
+least one of them is a *plain access* (non-atomic), and they are *unordered* in
+the "happens-before" order according to the `LKMM
+<../../tools/memory-model/Documentation/explanation.txt>`_.
+
+Relationship with the Linux Kernel Memory Model (LKMM)
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+The LKMM defines the propagation and ordering rules of various memory
+operations, which gives developers the ability to reason about concurrent code.
+Ultimately this allows to determine the possible executions of concurrent code,
+and if that code is free from data-races.
+
+KCSAN is aware of *atomic* accesses (``READ_ONCE``, ``WRITE_ONCE``,
+``atomic_*``, etc.), but is oblivious of any ordering guarantees. In other
+words, KCSAN assumes that as long as a plain access is not observed to race
+with another conflicting access, memory operations are correctly ordered.
+
+This means that KCSAN will not report *potential* data-races due to missing
+memory ordering. If, however, missing memory ordering (that is observable with
+a particular compiler and architecture) leads to an observable data-race (e.g.
+entering a critical section erroneously), KCSAN would report the resulting
+data-race.
+
+Implementation Details
+----------------------
+
+The general approach is inspired by `DataCollider
+<http://usenix.org/legacy/events/osdi10/tech/full_papers/Erickson.pdf>`_.
+Unlike DataCollider, KCSAN does not use hardware watchpoints, but instead
+relies on compiler instrumentation. Watchpoints are implemented using an
+efficient encoding that stores access type, size, and address in a long; the
+benefits of using "soft watchpoints" are portability and greater flexibility in
+limiting which accesses trigger a watchpoint.
+
+More specifically, KCSAN requires instrumenting plain (unmarked, non-atomic)
+memory operations; for each instrumented plain access:
+
+1. Check if a matching watchpoint exists; if yes, and at least one access is a
+   write, then we encountered a racing access.
+
+2. Periodically, if no matching watchpoint exists, set up a watchpoint and
+   stall some delay.
+
+3. Also check the data value before the delay, and re-check the data value
+   after delay; if the values mismatch, we infer a race of unknown origin.
+
+To detect data-races between plain and atomic memory operations, KCSAN also
+annotates atomic accesses, but only to check if a watchpoint exists
+(``kcsan_check_atomic(..)``); i.e.  KCSAN never sets up a watchpoint on atomic
+accesses.
+
+Key Properties
+~~~~~~~~~~~~~~
+
+1. **Performance Overhead:** KCSAN's runtime is minimal, and does not require
+   locking shared state for each access. This results in significantly better
+   performance in comparison with KTSAN.
+
+2. **Memory Overhead:** No shadow memory is required. The current
+   implementation uses a small array of longs to encode watchpoint information,
+   which is negligible.
+
+3. **Memory Ordering:** KCSAN is *not* aware of the LKMM's ordering rules. This
+   may result in missed data-races (false negatives), compared to a
+   happens-before data-race detector such as KTSAN.
+
+4. **Accuracy:** Imprecise, since it uses a sampling strategy.
+
+5. **Annotation Overheads:** Minimal annotation is required outside the KCSAN
+   runtime. With a happens-before data-race detector, any omission leads to
+   false positives, which is especially important in the context of the kernel
+   which includes numerous custom synchronization mechanisms. With KCSAN, as a
+   result, maintenance overheads are minimal as the kernel evolves.
+
+6. **Detects Racy Writes from Devices:** Due to checking data values upon
+   setting up watchpoints, racy writes from devices can also be detected.
diff --git a/MAINTAINERS b/MAINTAINERS
index 0154674cbad3..71f7fb625490 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -8847,6 +8847,17 @@  F:	Documentation/kbuild/kconfig*
 F:	scripts/kconfig/
 F:	scripts/Kconfig.include
 
+KCSAN
+M:	Marco Elver <elver@google.com>
+R:	Dmitry Vyukov <dvyukov@google.com>
+L:	kasan-dev@googlegroups.com
+S:	Maintained
+F:	Documentation/dev-tools/kcsan.rst
+F:	include/linux/kcsan*.h
+F:	kernel/kcsan/
+F:	lib/Kconfig.kcsan
+F:	scripts/Makefile.kcsan
+
 KDUMP
 M:	Dave Young <dyoung@redhat.com>
 M:	Baoquan He <bhe@redhat.com>
diff --git a/Makefile b/Makefile
index ffd7a912fc46..ad4729176252 100644
--- a/Makefile
+++ b/Makefile
@@ -478,7 +478,7 @@  export KBUILD_HOSTCXXFLAGS KBUILD_HOSTLDFLAGS KBUILD_HOSTLDLIBS LDFLAGS_MODULE
 
 export KBUILD_CPPFLAGS NOSTDINC_FLAGS LINUXINCLUDE OBJCOPYFLAGS KBUILD_LDFLAGS
 export KBUILD_CFLAGS CFLAGS_KERNEL CFLAGS_MODULE
-export CFLAGS_KASAN CFLAGS_KASAN_NOSANITIZE CFLAGS_UBSAN
+export CFLAGS_KASAN CFLAGS_KASAN_NOSANITIZE CFLAGS_UBSAN CFLAGS_KCSAN
 export KBUILD_AFLAGS AFLAGS_KERNEL AFLAGS_MODULE
 export KBUILD_AFLAGS_MODULE KBUILD_CFLAGS_MODULE KBUILD_LDFLAGS_MODULE
 export KBUILD_AFLAGS_KERNEL KBUILD_CFLAGS_KERNEL
@@ -900,6 +900,7 @@  endif
 include scripts/Makefile.kasan
 include scripts/Makefile.extrawarn
 include scripts/Makefile.ubsan
+include scripts/Makefile.kcsan
 
 # Add user supplied CPPFLAGS, AFLAGS and CFLAGS as the last assignments
 KBUILD_CPPFLAGS += $(KCPPFLAGS)
diff --git a/include/linux/compiler-clang.h b/include/linux/compiler-clang.h
index 333a6695a918..a213eb55e725 100644
--- a/include/linux/compiler-clang.h
+++ b/include/linux/compiler-clang.h
@@ -24,6 +24,15 @@ 
 #define __no_sanitize_address
 #endif
 
+#if __has_feature(thread_sanitizer)
+/* emulate gcc's __SANITIZE_THREAD__ flag */
+#define __SANITIZE_THREAD__
+#define __no_sanitize_thread \
+		__attribute__((no_sanitize("thread")))
+#else
+#define __no_sanitize_thread
+#endif
+
 /*
  * Not all versions of clang implement the the type-generic versions
  * of the builtin overflow checkers. Fortunately, clang implements
diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler-gcc.h
index d7ee4c6bad48..de105ca29282 100644
--- a/include/linux/compiler-gcc.h
+++ b/include/linux/compiler-gcc.h
@@ -145,6 +145,13 @@ 
 #define __no_sanitize_address
 #endif
 
+#if __has_attribute(__no_sanitize_thread__) && defined(__SANITIZE_THREAD__)
+#define __no_sanitize_thread                                                   \
+	__attribute__((__noinline__)) __attribute__((no_sanitize_thread))
+#else
+#define __no_sanitize_thread
+#endif
+
 #if GCC_VERSION >= 50100
 #define COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW 1
 #endif
diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index 5e88e7e33abe..0a7467477f84 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -178,6 +178,7 @@  void ftrace_likely_update(struct ftrace_likely_data *f, int val,
 #endif
 
 #include <uapi/linux/types.h>
+#include <linux/kcsan-checks.h>
 
 #define __READ_ONCE_SIZE						\
 ({									\
@@ -193,12 +194,6 @@  void ftrace_likely_update(struct ftrace_likely_data *f, int val,
 	}								\
 })
 
-static __always_inline
-void __read_once_size(const volatile void *p, void *res, int size)
-{
-	__READ_ONCE_SIZE;
-}
-
 #ifdef CONFIG_KASAN
 /*
  * We can't declare function 'inline' because __no_sanitize_address confilcts
@@ -211,14 +206,38 @@  void __read_once_size(const volatile void *p, void *res, int size)
 # define __no_kasan_or_inline __always_inline
 #endif
 
-static __no_kasan_or_inline
+#ifdef CONFIG_KCSAN
+# define __no_kcsan_or_inline __no_sanitize_thread notrace __maybe_unused
+#else
+# define __no_kcsan_or_inline __always_inline
+#endif
+
+#if defined(CONFIG_KASAN) || defined(CONFIG_KCSAN)
+/* Avoid any instrumentation or inline. */
+#define __no_sanitize_or_inline                                                \
+	__no_sanitize_address __no_sanitize_thread notrace __maybe_unused
+#else
+#define __no_sanitize_or_inline __always_inline
+#endif
+
+static __no_kcsan_or_inline
+void __read_once_size(const volatile void *p, void *res, int size)
+{
+	kcsan_check_atomic((const void *)p, size, false);
+	__READ_ONCE_SIZE;
+}
+
+static __no_sanitize_or_inline
 void __read_once_size_nocheck(const volatile void *p, void *res, int size)
 {
 	__READ_ONCE_SIZE;
 }
 
-static __always_inline void __write_once_size(volatile void *p, void *res, int size)
+static __no_kcsan_or_inline
+void __write_once_size(volatile void *p, void *res, int size)
 {
+	kcsan_check_atomic((const void *)p, size, true);
+
 	switch (size) {
 	case 1: *(volatile __u8 *)p = *(__u8 *)res; break;
 	case 2: *(volatile __u16 *)p = *(__u16 *)res; break;
diff --git a/include/linux/kcsan-checks.h b/include/linux/kcsan-checks.h
new file mode 100644
index 000000000000..bee619b66e1c
--- /dev/null
+++ b/include/linux/kcsan-checks.h
@@ -0,0 +1,116 @@ 
+/* SPDX-License-Identifier: GPL-2.0 */
+
+#ifndef _LINUX_KCSAN_CHECKS_H
+#define _LINUX_KCSAN_CHECKS_H
+
+#include <linux/types.h>
+
+/*
+ * __kcsan_*: Always available when KCSAN is enabled. This may be used
+ * even in compilation units that selectively disable KCSAN, but must use KCSAN
+ * to validate access to an address.   Never use these in header files!
+ */
+#ifdef CONFIG_KCSAN
+/**
+ * __kcsan_check_watchpoint - check if a watchpoint exists
+ *
+ * Returns true if no race was detected, and we may then proceed to set up a
+ * watchpoint after. Returns false if either KCSAN is disabled or a race was
+ * encountered, and we may not set up a watchpoint after.
+ *
+ * @ptr address of access
+ * @size size of access
+ * @is_write is access a write
+ * @return true if no race was detected, false otherwise.
+ */
+bool __kcsan_check_watchpoint(const volatile void *ptr, size_t size,
+			      bool is_write);
+
+/**
+ * __kcsan_setup_watchpoint - set up watchpoint and report data-races
+ *
+ * Sets up a watchpoint (if sampled), and if a racing access was observed,
+ * reports the data-race.
+ *
+ * @ptr address of access
+ * @size size of access
+ * @is_write is access a write
+ */
+void __kcsan_setup_watchpoint(const volatile void *ptr, size_t size,
+			      bool is_write);
+#else
+static inline bool __kcsan_check_watchpoint(const volatile void *ptr,
+					    size_t size, bool is_write)
+{
+	return true;
+}
+static inline void __kcsan_setup_watchpoint(const volatile void *ptr,
+					    size_t size, bool is_write)
+{
+}
+#endif
+
+/*
+ * kcsan_*: Only available when the particular compilation unit has KCSAN
+ * instrumentation enabled. May be used in header files.
+ */
+#ifdef __SANITIZE_THREAD__
+#define kcsan_check_watchpoint __kcsan_check_watchpoint
+#define kcsan_setup_watchpoint __kcsan_setup_watchpoint
+#else
+static inline bool kcsan_check_watchpoint(const volatile void *ptr, size_t size,
+					  bool is_write)
+{
+	return true;
+}
+static inline void kcsan_setup_watchpoint(const volatile void *ptr, size_t size,
+					  bool is_write)
+{
+}
+#endif
+
+/**
+ * __kcsan_check_access - check regular access for data-races
+ *
+ * Full access that checks watchpoint and sets up a watchpoint if this access is
+ * sampled.
+ *
+ * @ptr address of access
+ * @size size of access
+ * @is_write is access a write
+ */
+#define __kcsan_check_access(ptr, size, is_write)                              \
+	do {                                                                   \
+		if (__kcsan_check_watchpoint(ptr, size, is_write) &&           \
+		    !(IS_ENABLED(CONFIG_KCSAN_PLAIN_WRITE_PRETEND_ONCE) &&     \
+		      is_write))                                               \
+			__kcsan_setup_watchpoint(ptr, size, is_write);         \
+	} while (0)
+/**
+ * kcsan_check_access - check regular access for data-races
+ *
+ * @ptr address of access
+ * @size size of access
+ * @is_write is access a write
+ */
+#define kcsan_check_access(ptr, size, is_write)                                \
+	do {                                                                   \
+		if (kcsan_check_watchpoint(ptr, size, is_write) &&             \
+		    !(IS_ENABLED(CONFIG_KCSAN_PLAIN_WRITE_PRETEND_ONCE) &&     \
+		      is_write))                                               \
+			kcsan_setup_watchpoint(ptr, size, is_write);           \
+	} while (0)
+
+/*
+ * Check for atomic accesses: if atomics are not ignored, this simply aliases to
+ * kcsan_check_watchpoint, otherwise becomes a no-op.
+ */
+#ifdef CONFIG_KCSAN_IGNORE_ATOMICS
+#define kcsan_check_atomic(...)                                                \
+	do {                                                                   \
+	} while (0)
+#else
+#define kcsan_check_atomic kcsan_check_watchpoint
+#endif
+
+#endif /* _LINUX_KCSAN_CHECKS_H */
diff --git a/include/linux/kcsan.h b/include/linux/kcsan.h
new file mode 100644
index 000000000000..18c660628376
--- /dev/null
+++ b/include/linux/kcsan.h
@@ -0,0 +1,85 @@ 
+/* SPDX-License-Identifier: GPL-2.0 */
+
+#ifndef _LINUX_KCSAN_H
+#define _LINUX_KCSAN_H
+
+#include <linux/types.h>
+#include <linux/kcsan-checks.h>
+
+#ifdef CONFIG_KCSAN
+
+/**
+ * kcsan_init - initialize KCSAN runtime
+ */
+void kcsan_init(void);
+
+/**
+ * kcsan_disable_current - disable KCSAN for the current context
+ *
+ * Supports nesting.
+ */
+void kcsan_disable_current(void);
+
+/**
+ * kcsan_enable_current - re-enable KCSAN for the current context
+ *
+ * Supports nesting.
+ */
+void kcsan_enable_current(void);
+
+/**
+ * kcsan_begin_atomic - use to denote an atomic region
+ *
+ * Accesses within the atomic region may appear to race with other accesses but
+ * should be considered atomic.
+ *
+ * @nest true if regions may be nested, or false for flat region
+ */
+void kcsan_begin_atomic(bool nest);
+
+/**
+ * kcsan_end_atomic - end atomic region
+ *
+ * @nest must match argument to kcsan_begin_atomic().
+ */
+void kcsan_end_atomic(bool nest);
+
+/**
+ * kcsan_atomic_next - consider following accesses as atomic
+ *
+ * Force treating the next n memory accesses for the current context as atomic
+ * operations.
+ *
+ * @n number of following memory accesses to treat as atomic.
+ */
+void kcsan_atomic_next(int n);
+
+#else /* CONFIG_KCSAN */
+
+static inline void kcsan_init(void)
+{
+}
+
+static inline void kcsan_disable_current(void)
+{
+}
+
+static inline void kcsan_enable_current(void)
+{
+}
+
+static inline void kcsan_begin_atomic(bool nest)
+{
+}
+
+static inline void kcsan_end_atomic(bool nest)
+{
+}
+
+static inline void kcsan_atomic_next(int n)
+{
+}
+
+#endif /* CONFIG_KCSAN */
+
+#endif /* _LINUX_KCSAN_H */
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 2c2e56bd8913..34a1d9310304 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1171,6 +1171,13 @@  struct task_struct {
 #ifdef CONFIG_KASAN
 	unsigned int			kasan_depth;
 #endif
+#ifdef CONFIG_KCSAN
+	/* See comments at kernel/kcsan/core.c: struct cpu_state. */
+	int				kcsan_disable;
+	int				kcsan_atomic_next;
+	int				kcsan_atomic_region;
+	bool				kcsan_atomic_region_flat;
+#endif
 
 #ifdef CONFIG_FUNCTION_GRAPH_TRACER
 	/* Index of current stored address in ret_stack: */
diff --git a/init/init_task.c b/init/init_task.c
index 9e5cbe5eab7b..f98fc4c9f635 100644
--- a/init/init_task.c
+++ b/init/init_task.c
@@ -161,6 +161,12 @@  struct task_struct init_task
 #ifdef CONFIG_KASAN
 	.kasan_depth	= 1,
 #endif
+#ifdef CONFIG_KCSAN
+	.kcsan_disable			= 1,
+	.kcsan_atomic_next		= 0,
+	.kcsan_atomic_region		= 0,
+	.kcsan_atomic_region_flat	= 0,
+#endif
 #ifdef CONFIG_TRACE_IRQFLAGS
 	.softirqs_enabled = 1,
 #endif
diff --git a/init/main.c b/init/main.c
index 91f6ebb30ef0..4d814de017ee 100644
--- a/init/main.c
+++ b/init/main.c
@@ -93,6 +93,7 @@ 
 #include <linux/rodata_test.h>
 #include <linux/jump_label.h>
 #include <linux/mem_encrypt.h>
+#include <linux/kcsan.h>
 
 #include <asm/io.h>
 #include <asm/bugs.h>
@@ -779,6 +780,7 @@  asmlinkage __visible void __init start_kernel(void)
 	acpi_subsystem_init();
 	arch_post_acpi_subsys_init();
 	sfi_init_late();
+	kcsan_init();
 
 	/* Do the rest non-__init'ed, we're now alive */
 	arch_call_rest_init();
diff --git a/kernel/Makefile b/kernel/Makefile
index daad787fb795..74ab46e2ebd1 100644
--- a/kernel/Makefile
+++ b/kernel/Makefile
@@ -102,6 +102,7 @@  obj-$(CONFIG_TRACEPOINTS) += trace/
 obj-$(CONFIG_IRQ_WORK) += irq_work.o
 obj-$(CONFIG_CPU_PM) += cpu_pm.o
 obj-$(CONFIG_BPF) += bpf/
+obj-$(CONFIG_KCSAN) += kcsan/
 
 obj-$(CONFIG_PERF_EVENTS) += events/
 
diff --git a/kernel/kcsan/Makefile b/kernel/kcsan/Makefile
new file mode 100644
index 000000000000..c25f07062d26
--- /dev/null
+++ b/kernel/kcsan/Makefile
@@ -0,0 +1,14 @@ 
+# SPDX-License-Identifier: GPL-2.0
+KCSAN_SANITIZE := n
+KCOV_INSTRUMENT := n
+
+CFLAGS_REMOVE_kcsan.o = $(CC_FLAGS_FTRACE)
+CFLAGS_REMOVE_core.o = $(CC_FLAGS_FTRACE)
+CFLAGS_REMOVE_atomic.o = $(CC_FLAGS_FTRACE)
+
+CFLAGS_kcsan.o = $(call cc-option, -fno-conserve-stack -fno-stack-protector)
+CFLAGS_core.o = $(call cc-option, -fno-conserve-stack -fno-stack-protector)
+CFLAGS_atomic.o = $(call cc-option, -fno-conserve-stack -fno-stack-protector)
+
+obj-y := kcsan.o core.o atomic.o debugfs.o report.o
+obj-$(CONFIG_KCSAN_SELFTEST) += test.o
diff --git a/kernel/kcsan/atomic.c b/kernel/kcsan/atomic.c
new file mode 100644
index 000000000000..dd44f7d9e491
--- /dev/null
+++ b/kernel/kcsan/atomic.c
@@ -0,0 +1,21 @@ 
+// SPDX-License-Identifier: GPL-2.0
+
+#include <linux/jiffies.h>
+
+#include "kcsan.h"
+
+/*
+ * List all volatile globals that have been observed in races, to suppress
+ * data-race reports between accesses to these variables.
+ *
+ * For now, we assume that volatile accesses of globals are as strong as atomic
+ * accesses (READ_ONCE, WRITE_ONCE cast to volatile). The situation is still not
+ * entirely clear, as on some architectures (Alpha) READ_ONCE/WRITE_ONCE do more
+ * than cast to volatile. Eventually, we hope to be able to remove this
+ * function.
+ */
+bool kcsan_is_atomic(const volatile void *ptr)
+{
+	/* only jiffies for now */
+	return ptr == &jiffies;
+}
diff --git a/kernel/kcsan/core.c b/kernel/kcsan/core.c
new file mode 100644
index 000000000000..e8c3823bf7c4
--- /dev/null
+++ b/kernel/kcsan/core.c
@@ -0,0 +1,458 @@ 
+// SPDX-License-Identifier: GPL-2.0
+
+#include <linux/atomic.h>
+#include <linux/bug.h>
+#include <linux/delay.h>
+#include <linux/export.h>
+#include <linux/init.h>
+#include <linux/percpu.h>
+#include <linux/preempt.h>
+#include <linux/random.h>
+#include <linux/sched.h>
+#include <linux/uaccess.h>
+
+#include "kcsan.h"
+#include "encoding.h"
+
+/*
+ * Helper macros to iterate slots, starting from address slot itself, followed
+ * by the right and left slots.
+ */
+#define CHECK_NUM_SLOTS (1 + 2 * KCSAN_CHECK_ADJACENT)
+#define SLOT_IDX(slot, i)                                                      \
+	((slot + (((i + KCSAN_CHECK_ADJACENT) % CHECK_NUM_SLOTS) -             \
+		  KCSAN_CHECK_ADJACENT)) %                                     \
+	 KCSAN_NUM_WATCHPOINTS)
+
+bool kcsan_enabled;
+
+/*
+ * Per-CPU state that should be used instead of 'current' if we are not in a
+ * task.
+ */
+struct cpu_state {
+	int disable; /* disable counter */
+	int atomic_next; /* number of following atomic ops */
+
+	/*
+	 * We use separate variables to store if we are in a nestable or flat
+	 * atomic region. This helps make sure that an atomic region with
+	 * nesting support is not suddenly aborted when a flat region is
+	 * contained within. Effectively this allows supporting nesting flat
+	 * atomic regions within an outer nestable atomic region. Support for
+	 * this is required as there are cases where a seqlock reader critical
+	 * section (flat atomic region) is contained within a seqlock writer
+	 * critical section (nestable atomic region), and the "mismatching
+	 * kcsan_end_atomic()" warning would trigger otherwise.
+	 */
+	int atomic_region;
+	bool atomic_region_flat;
+};
+static DEFINE_PER_CPU(struct cpu_state, this_state) = {
+	.disable = 0,
+	.atomic_next = 0,
+	.atomic_region = 0,
+	.atomic_region_flat = 0,
+};
+
+/*
+ * Watchpoints, with each entry encoded as defined in encoding.h: in order to be
+ * able to safely update and access a watchpoint without introducing locking
+ * overhead, we encode each watchpoint as a single atomic long. The initial
+ * zero-initialized state matches INVALID_WATCHPOINT.
+ */
+static atomic_long_t watchpoints[KCSAN_NUM_WATCHPOINTS];
+
+/*
+ * Instructions skipped counter; see should_watch().
+ */
+static DEFINE_PER_CPU(unsigned long, kcsan_skip);
+
+static inline atomic_long_t *find_watchpoint(unsigned long addr, size_t size,
+					     bool expect_write,
+					     long *encoded_watchpoint)
+{
+	const int slot = watchpoint_slot(addr);
+	const unsigned long addr_masked = addr & WATCHPOINT_ADDR_MASK;
+	atomic_long_t *watchpoint;
+	unsigned long wp_addr_masked;
+	size_t wp_size;
+	bool is_write;
+	int i;
+
+	for (i = 0; i < CHECK_NUM_SLOTS; ++i) {
+		watchpoint = &watchpoints[SLOT_IDX(slot, i)];
+		*encoded_watchpoint = atomic_long_read(watchpoint);
+		if (!decode_watchpoint(*encoded_watchpoint, &wp_addr_masked,
+				       &wp_size, &is_write))
+			continue;
+
+		if (expect_write && !is_write)
+			continue;
+
+		/* Check if the watchpoint matches the access. */
+		if (matching_access(wp_addr_masked, wp_size, addr_masked, size))
+			return watchpoint;
+	}
+
+	return NULL;
+}
+
+static inline atomic_long_t *insert_watchpoint(unsigned long addr, size_t size,
+					       bool is_write)
+{
+	const int slot = watchpoint_slot(addr);
+	const long encoded_watchpoint = encode_watchpoint(addr, size, is_write);
+	atomic_long_t *watchpoint;
+	int i;
+
+	for (i = 0; i < CHECK_NUM_SLOTS; ++i) {
+		long expect_val = INVALID_WATCHPOINT;
+
+		/* Try to acquire this slot. */
+		watchpoint = &watchpoints[SLOT_IDX(slot, i)];
+		if (atomic_long_try_cmpxchg_relaxed(watchpoint, &expect_val,
+						    encoded_watchpoint))
+			return watchpoint;
+	}
+
+	return NULL;
+}
+
+/*
+ * Return true if watchpoint was successfully consumed, false otherwise.
+ *
+ * This may return false if:
+ *
+ *	1. another thread already consumed the watchpoint;
+ *	2. the thread that set up the watchpoint already removed it;
+ *	3. the watchpoint was removed and then re-used.
+ */
+static inline bool try_consume_watchpoint(atomic_long_t *watchpoint,
+					  long encoded_watchpoint)
+{
+	return atomic_long_try_cmpxchg_relaxed(watchpoint, &encoded_watchpoint,
+					       CONSUMED_WATCHPOINT);
+}
+
+/*
+ * Return true if watchpoint was not touched, false if consumed.
+ */
+static inline bool remove_watchpoint(atomic_long_t *watchpoint)
+{
+	return atomic_long_xchg_relaxed(watchpoint, INVALID_WATCHPOINT) !=
+	       CONSUMED_WATCHPOINT;
+}
+
+static inline bool is_atomic(const volatile void *ptr)
+{
+	if (in_task()) {
+		if (unlikely(current->kcsan_atomic_next > 0)) {
+			--current->kcsan_atomic_next;
+			return true;
+		}
+		if (unlikely(current->kcsan_atomic_region > 0 ||
+			     current->kcsan_atomic_region_flat))
+			return true;
+	} else { /* interrupt */
+		if (unlikely(this_cpu_read(this_state.atomic_next) > 0)) {
+			this_cpu_dec(this_state.atomic_next);
+			return true;
+		}
+		if (unlikely(this_cpu_read(this_state.atomic_region) > 0 ||
+			     this_cpu_read(this_state.atomic_region_flat)))
+			return true;
+	}
+
+	return kcsan_is_atomic(ptr);
+}
+
+static inline bool should_watch(const volatile void *ptr)
+{
+	/*
+	 * Never set up watchpoints when memory operations are atomic.
+	 *
+	 * We need to check this first, because: 1) atomics should not count
+	 * towards skipped instructions below, and 2) to actually decrement
+	 * kcsan_atomic_next for each atomic.
+	 */
+	if (is_atomic(ptr))
+		return false;
+
+	/*
+	 * We use a per-CPU counter, to avoid excessive contention; there is
+	 * still enough non-determinism for the precise instructions that end up
+	 * being watched to be mostly unpredictable. Using a PRNG like
+	 * prandom_u32() turned out to be too slow.
+	 */
+	return (this_cpu_inc_return(kcsan_skip) %
+		CONFIG_KCSAN_WATCH_SKIP_INST) == 0;
+}
+
+static inline bool is_enabled(void)
+{
+	return READ_ONCE(kcsan_enabled) &&
+	       (in_task() ? current->kcsan_disable :
+			    this_cpu_read(this_state.disable)) == 0;
+}
+
+static inline unsigned int get_delay(void)
+{
+	unsigned int max_delay = in_task() ? CONFIG_KCSAN_UDELAY_MAX_TASK :
+					     CONFIG_KCSAN_UDELAY_MAX_INTERRUPT;
+	return IS_ENABLED(CONFIG_KCSAN_DELAY_RANDOMIZE) ?
+		       ((prandom_u32() % max_delay) + 1) :
+		       max_delay;
+}
+
+/* === Public interface ===================================================== */
+
+void __init kcsan_init(void)
+{
+	BUG_ON(!in_task());
+
+	kcsan_debugfs_init();
+	kcsan_enable_current();
+#ifdef CONFIG_KCSAN_EARLY_ENABLE
+	/*
+	 * We are in the init task, and no other tasks should be running.
+	 */
+	WRITE_ONCE(kcsan_enabled, true);
+#endif
+}
+
+/* === Exported interface =================================================== */
+
+void kcsan_disable_current(void)
+{
+	if (in_task())
+		++current->kcsan_disable;
+	else
+		this_cpu_inc(this_state.disable);
+}
+EXPORT_SYMBOL(kcsan_disable_current);
+
+void kcsan_enable_current(void)
+{
+	int prev = in_task() ? current->kcsan_disable-- :
+			       (this_cpu_dec_return(this_state.disable) + 1);
+	if (prev == 0) {
+		kcsan_disable_current(); /* restore to 0 */
+		kcsan_disable_current();
+		WARN(1, "mismatching %s", __func__);
+		kcsan_enable_current();
+	}
+}
+EXPORT_SYMBOL(kcsan_enable_current);
+
+void kcsan_begin_atomic(bool nest)
+{
+	if (nest) {
+		if (in_task())
+			++current->kcsan_atomic_region;
+		else
+			this_cpu_inc(this_state.atomic_region);
+	} else {
+		if (in_task())
+			current->kcsan_atomic_region_flat = true;
+		else
+			this_cpu_write(this_state.atomic_region_flat, true);
+	}
+}
+EXPORT_SYMBOL(kcsan_begin_atomic);
+
+void kcsan_end_atomic(bool nest)
+{
+	if (nest) {
+		int prev =
+			in_task() ?
+				current->kcsan_atomic_region-- :
+				(this_cpu_dec_return(this_state.atomic_region) +
+				 1);
+		if (prev == 0) {
+			kcsan_begin_atomic(true); /* restore to 0 */
+			kcsan_disable_current();
+			WARN(1, "mismatching %s", __func__);
+			kcsan_enable_current();
+		}
+	} else {
+		if (in_task())
+			current->kcsan_atomic_region_flat = false;
+		else
+			this_cpu_write(this_state.atomic_region_flat, false);
+	}
+}
+EXPORT_SYMBOL(kcsan_end_atomic);
+
+void kcsan_atomic_next(int n)
+{
+	if (in_task())
+		current->kcsan_atomic_next = n;
+	else
+		this_cpu_write(this_state.atomic_next, n);
+}
+EXPORT_SYMBOL(kcsan_atomic_next);
+
+bool __kcsan_check_watchpoint(const volatile void *ptr, size_t size,
+			      bool is_write)
+{
+	atomic_long_t *watchpoint;
+	long encoded_watchpoint;
+	unsigned long flags;
+	enum kcsan_report_type report_type;
+
+	if (unlikely(!is_enabled()))
+		return false;
+
+	watchpoint = find_watchpoint((unsigned long)ptr, size, !is_write,
+				     &encoded_watchpoint);
+	if (watchpoint == NULL)
+		return true;
+
+	flags = user_access_save();
+	if (!try_consume_watchpoint(watchpoint, encoded_watchpoint)) {
+		/*
+		 * The other thread may not print any diagnostics, as it has
+		 * already removed the watchpoint, or another thread consumed
+		 * the watchpoint before this thread.
+		 */
+		kcsan_counter_inc(kcsan_counter_report_races);
+		report_type = kcsan_report_race_check_race;
+	} else {
+		report_type = kcsan_report_race_check;
+	}
+
+	/* Encountered a data-race. */
+	kcsan_counter_inc(kcsan_counter_data_races);
+	kcsan_report(ptr, size, is_write, raw_smp_processor_id(), report_type);
+
+	user_access_restore(flags);
+	return false;
+}
+EXPORT_SYMBOL(__kcsan_check_watchpoint);
+
+void __kcsan_setup_watchpoint(const volatile void *ptr, size_t size,
+			      bool is_write)
+{
+	atomic_long_t *watchpoint;
+	union {
+		u8 _1;
+		u16 _2;
+		u32 _4;
+		u64 _8;
+	} expect_value;
+	bool is_expected = true;
+	unsigned long ua_flags = user_access_save();
+	unsigned long irq_flags;
+
+	if (!should_watch(ptr))
+		goto out;
+
+	if (!check_encodable((unsigned long)ptr, size)) {
+		kcsan_counter_inc(kcsan_counter_unencodable_accesses);
+		goto out;
+	}
+
+	/*
+	 * Disable interrupts & preemptions, to ignore races due to accesses in
+	 * threads running on the same CPU.
+	 */
+	local_irq_save(irq_flags);
+	preempt_disable();
+
+	watchpoint = insert_watchpoint((unsigned long)ptr, size, is_write);
+	if (watchpoint == NULL) {
+		/*
+		 * Out of capacity: the size of `watchpoints`, and the frequency
+		 * with which `should_watch()` returns true should be tweaked so
+		 * that this case happens very rarely.
+		 */
+		kcsan_counter_inc(kcsan_counter_no_capacity);
+		goto out_unlock;
+	}
+
+	kcsan_counter_inc(kcsan_counter_setup_watchpoints);
+	kcsan_counter_inc(kcsan_counter_used_watchpoints);
+
+	/*
+	 * Read the current value, to later check and infer a race if the data
+	 * was modified via a non-instrumented access, e.g. from a device.
+	 */
+	switch (size) {
+	case 1:
+		expect_value._1 = READ_ONCE(*(const u8 *)ptr);
+		break;
+	case 2:
+		expect_value._2 = READ_ONCE(*(const u16 *)ptr);
+		break;
+	case 4:
+		expect_value._4 = READ_ONCE(*(const u32 *)ptr);
+		break;
+	case 8:
+		expect_value._8 = READ_ONCE(*(const u64 *)ptr);
+		break;
+	default:
+		break; /* ignore; we do not diff the values */
+	}
+
+#ifdef CONFIG_KCSAN_DEBUG
+	kcsan_disable_current();
+	pr_err("KCSAN: watching %s, size: %zu, addr: %px [slot: %d, encoded: %lx]\n",
+	       is_write ? "write" : "read", size, ptr,
+	       watchpoint_slot((unsigned long)ptr),
+	       encode_watchpoint((unsigned long)ptr, size, is_write));
+	kcsan_enable_current();
+#endif
+
+	/*
+	 * Delay this thread, to increase probability of observing a racy
+	 * conflicting access.
+	 */
+	udelay(get_delay());
+
+	/*
+	 * Re-read value, and check if it is as expected; if not, we infer a
+	 * racy access.
+	 */
+	switch (size) {
+	case 1:
+		is_expected = expect_value._1 == READ_ONCE(*(const u8 *)ptr);
+		break;
+	case 2:
+		is_expected = expect_value._2 == READ_ONCE(*(const u16 *)ptr);
+		break;
+	case 4:
+		is_expected = expect_value._4 == READ_ONCE(*(const u32 *)ptr);
+		break;
+	case 8:
+		is_expected = expect_value._8 == READ_ONCE(*(const u64 *)ptr);
+		break;
+	default:
+		break; /* ignore; we do not diff the values */
+	}
+
+	/* Check if this access raced with another. */
+	if (!remove_watchpoint(watchpoint)) {
+		/*
+		 * No need to increment 'race' counter, as the racing thread
+		 * already did.
+		 */
+		kcsan_report(ptr, size, is_write, smp_processor_id(),
+			     kcsan_report_race_setup);
+	} else if (!is_expected) {
+		/* Inferring a race, since the value should not have changed. */
+		kcsan_counter_inc(kcsan_counter_races_unknown_origin);
+#ifdef CONFIG_KCSAN_REPORT_RACE_UNKNOWN_ORIGIN
+		kcsan_report(ptr, size, is_write, smp_processor_id(),
+			     kcsan_report_race_unknown_origin);
+#endif
+	}
+
+	kcsan_counter_dec(kcsan_counter_used_watchpoints);
+out_unlock:
+	preempt_enable();
+	local_irq_restore(irq_flags);
+out:
+	user_access_restore(ua_flags);
+}
+EXPORT_SYMBOL(__kcsan_setup_watchpoint);
diff --git a/kernel/kcsan/debugfs.c b/kernel/kcsan/debugfs.c
new file mode 100644
index 000000000000..6ddcbd185f3a
--- /dev/null
+++ b/kernel/kcsan/debugfs.c
@@ -0,0 +1,225 @@ 
+// SPDX-License-Identifier: GPL-2.0
+
+#include <linux/atomic.h>
+#include <linux/bsearch.h>
+#include <linux/bug.h>
+#include <linux/debugfs.h>
+#include <linux/init.h>
+#include <linux/kallsyms.h>
+#include <linux/mm.h>
+#include <linux/seq_file.h>
+#include <linux/sort.h>
+#include <linux/string.h>
+#include <linux/uaccess.h>
+
+#include "kcsan.h"
+
+/*
+ * Statistics counters.
+ */
+static atomic_long_t counters[kcsan_counter_count];
+
+/*
+ * Addresses for filtering functions from reporting. This list can be used as a
+ * whitelist or blacklist.
+ */
+static struct {
+	unsigned long *addrs; /* array of addresses */
+	size_t size; /* current size */
+	int used; /* number of elements used */
+	bool sorted; /* if elements are sorted */
+	bool whitelist; /* if list is a blacklist or whitelist */
+} report_filterlist = {
+	.addrs = NULL,
+	.size = 8, /* small initial size */
+	.used = 0,
+	.sorted = false,
+	.whitelist = false, /* default is blacklist */
+};
+static DEFINE_SPINLOCK(report_filterlist_lock);
+
+static const char *counter_to_name(enum kcsan_counter_id id)
+{
+	switch (id) {
+	case kcsan_counter_used_watchpoints:
+		return "used_watchpoints";
+	case kcsan_counter_setup_watchpoints:
+		return "setup_watchpoints";
+	case kcsan_counter_data_races:
+		return "data_races";
+	case kcsan_counter_no_capacity:
+		return "no_capacity";
+	case kcsan_counter_report_races:
+		return "report_races";
+	case kcsan_counter_races_unknown_origin:
+		return "races_unknown_origin";
+	case kcsan_counter_unencodable_accesses:
+		return "unencodable_accesses";
+	case kcsan_counter_encoding_false_positives:
+		return "encoding_false_positives";
+	case kcsan_counter_count:
+		BUG();
+	}
+	return NULL;
+}
+
+void kcsan_counter_inc(enum kcsan_counter_id id)
+{
+	atomic_long_inc(&counters[id]);
+}
+
+void kcsan_counter_dec(enum kcsan_counter_id id)
+{
+	atomic_long_dec(&counters[id]);
+}
+
+static int cmp_filterlist_addrs(const void *rhs, const void *lhs)
+{
+	const unsigned long a = *(const unsigned long *)rhs;
+	const unsigned long b = *(const unsigned long *)lhs;
+
+	return a < b ? -1 : a == b ? 0 : 1;
+}
+
+bool kcsan_skip_report(unsigned long func_addr)
+{
+	unsigned long symbolsize, offset;
+	unsigned long flags;
+	bool ret = false;
+
+	if (!kallsyms_lookup_size_offset(func_addr, &symbolsize, &offset))
+		return false;
+	func_addr -= offset; /* get function start */
+
+	spin_lock_irqsave(&report_filterlist_lock, flags);
+	if (report_filterlist.used == 0)
+		goto out;
+
+	/* Sort array if it is unsorted, and then do a binary search. */
+	if (!report_filterlist.sorted) {
+		sort(report_filterlist.addrs, report_filterlist.used,
+		     sizeof(unsigned long), cmp_filterlist_addrs, NULL);
+		report_filterlist.sorted = true;
+	}
+	ret = !!bsearch(&func_addr, report_filterlist.addrs,
+			report_filterlist.used, sizeof(unsigned long),
+			cmp_filterlist_addrs);
+	if (report_filterlist.whitelist)
+		ret = !ret;
+
+out:
+	spin_unlock_irqrestore(&report_filterlist_lock, flags);
+	return ret;
+}
+
+static void set_report_filterlist_whitelist(bool whitelist)
+{
+	unsigned long flags;
+
+	spin_lock_irqsave(&report_filterlist_lock, flags);
+	report_filterlist.whitelist = whitelist;
+	spin_unlock_irqrestore(&report_filterlist_lock, flags);
+}
+
+static void insert_report_filterlist(const char *func)
+{
+	unsigned long flags;
+	unsigned long addr = kallsyms_lookup_name(func);
+
+	if (!addr) {
+		pr_err("KCSAN: could not find function: '%s'\n", func);
+		return;
+	}
+
+	spin_lock_irqsave(&report_filterlist_lock, flags);
+
+	if (report_filterlist.addrs == NULL)
+		report_filterlist.addrs = /* initial allocation */
+			kvmalloc_array(report_filterlist.size,
+				       sizeof(unsigned long), GFP_KERNEL);
+	else if (report_filterlist.used == report_filterlist.size) {
+		/* resize filterlist */
+		unsigned long *new_addrs;
+
+		report_filterlist.size *= 2;
+		new_addrs = kvmalloc_array(report_filterlist.size,
+					   sizeof(unsigned long), GFP_KERNEL);
+		memcpy(new_addrs, report_filterlist.addrs,
+		       report_filterlist.used * sizeof(unsigned long));
+		kvfree(report_filterlist.addrs);
+		report_filterlist.addrs = new_addrs;
+	}
+
+	/* Note: deduplicating should be done in userspace. */
+	report_filterlist.addrs[report_filterlist.used++] =
+		kallsyms_lookup_name(func);
+	report_filterlist.sorted = false;
+
+	spin_unlock_irqrestore(&report_filterlist_lock, flags);
+}
+
+static int show_info(struct seq_file *file, void *v)
+{
+	int i;
+	unsigned long flags;
+
+	/* show stats */
+	seq_printf(file, "enabled: %i\n", READ_ONCE(kcsan_enabled));
+	for (i = 0; i < kcsan_counter_count; ++i)
+		seq_printf(file, "%s: %ld\n", counter_to_name(i),
+			   atomic_long_read(&counters[i]));
+
+	/* show filter functions, and filter type */
+	spin_lock_irqsave(&report_filterlist_lock, flags);
+	seq_printf(file, "\n%s functions: %s\n",
+		   report_filterlist.whitelist ? "whitelisted" : "blacklisted",
+		   report_filterlist.used == 0 ? "none" : "");
+	for (i = 0; i < report_filterlist.used; ++i)
+		seq_printf(file, " %ps\n", (void *)report_filterlist.addrs[i]);
+	spin_unlock_irqrestore(&report_filterlist_lock, flags);
+
+	return 0;
+}
+
+static int debugfs_open(struct inode *inode, struct file *file)
+{
+	return single_open(file, show_info, NULL);
+}
+
+static ssize_t debugfs_write(struct file *file, const char __user *buf,
+			     size_t count, loff_t *off)
+{
+	char kbuf[KSYM_NAME_LEN];
+	char *arg;
+	int read_len = count < (sizeof(kbuf) - 1) ? count : (sizeof(kbuf) - 1);
+
+	if (copy_from_user(kbuf, buf, read_len))
+		return -EINVAL;
+	kbuf[read_len] = '\0';
+	arg = strstrip(kbuf);
+
+	if (!strncmp(arg, "on", sizeof("on") - 1))
+		WRITE_ONCE(kcsan_enabled, true);
+	else if (!strncmp(arg, "off", sizeof("off") - 1))
+		WRITE_ONCE(kcsan_enabled, false);
+	else if (!strncmp(arg, "whitelist", sizeof("whitelist") - 1))
+		set_report_filterlist_whitelist(true);
+	else if (!strncmp(arg, "blacklist", sizeof("blacklist") - 1))
+		set_report_filterlist_whitelist(false);
+	else if (arg[0] == '!')
+		insert_report_filterlist(&arg[1]);
+	else
+		return -EINVAL;
+
+	return count;
+}
+
+static const struct file_operations debugfs_ops = { .read = seq_read,
+						    .open = debugfs_open,
+						    .write = debugfs_write,
+						    .release = single_release };
+
+void __init kcsan_debugfs_init(void)
+{
+	debugfs_create_file("kcsan", 0644, NULL, NULL, &debugfs_ops);
+}
diff --git a/kernel/kcsan/encoding.h b/kernel/kcsan/encoding.h
new file mode 100644
index 000000000000..8f9b1ce0e59f
--- /dev/null
+++ b/kernel/kcsan/encoding.h
@@ -0,0 +1,94 @@ 
+/* SPDX-License-Identifier: GPL-2.0 */
+
+#ifndef _MM_KCSAN_ENCODING_H
+#define _MM_KCSAN_ENCODING_H
+
+#include <linux/bits.h>
+#include <linux/log2.h>
+#include <linux/mm.h>
+
+#include "kcsan.h"
+
+#define SLOT_RANGE PAGE_SIZE
+#define INVALID_WATCHPOINT 0
+#define CONSUMED_WATCHPOINT 1
+
+/*
+ * The maximum useful size of accesses for which we set up watchpoints is the
+ * max range of slots we check on an access.
+ */
+#define MAX_ENCODABLE_SIZE (SLOT_RANGE * (1 + KCSAN_CHECK_ADJACENT))
+
+/*
+ * Number of bits we use to store size info.
+ */
+#define WATCHPOINT_SIZE_BITS bits_per(MAX_ENCODABLE_SIZE)
+/*
+ * This encoding for addresses discards the upper (1 for is-write + SIZE_BITS);
+ * however, most 64-bit architectures do not use the full 64-bit address space.
+ * Also, in order for a false positive to be observable 2 things need to happen:
+ *
+ *	1. different addresses but with the same encoded address race;
+ *	2. and both map onto the same watchpoint slots;
+ *
+ * Both these are assumed to be very unlikely. However, in case it still happens
+ * happens, the report logic will filter out the false positive (see report.c).
+ */
+#define WATCHPOINT_ADDR_BITS (BITS_PER_LONG - 1 - WATCHPOINT_SIZE_BITS)
+
+/*
+ * Masks to set/retrieve the encoded data.
+ */
+#define WATCHPOINT_WRITE_MASK BIT(BITS_PER_LONG - 1)
+#define WATCHPOINT_SIZE_MASK                                                   \
+	GENMASK(BITS_PER_LONG - 2, BITS_PER_LONG - 2 - WATCHPOINT_SIZE_BITS)
+#define WATCHPOINT_ADDR_MASK                                                   \
+	GENMASK(BITS_PER_LONG - 3 - WATCHPOINT_SIZE_BITS, 0)
+
+static inline bool check_encodable(unsigned long addr, size_t size)
+{
+	return size <= MAX_ENCODABLE_SIZE;
+}
+
+static inline long encode_watchpoint(unsigned long addr, size_t size,
+				     bool is_write)
+{
+	return (long)((is_write ? WATCHPOINT_WRITE_MASK : 0) |
+		      (size << WATCHPOINT_ADDR_BITS) |
+		      (addr & WATCHPOINT_ADDR_MASK));
+}
+
+static inline bool decode_watchpoint(long watchpoint,
+				     unsigned long *addr_masked, size_t *size,
+				     bool *is_write)
+{
+	if (watchpoint == INVALID_WATCHPOINT ||
+	    watchpoint == CONSUMED_WATCHPOINT)
+		return false;
+
+	*addr_masked = (unsigned long)watchpoint & WATCHPOINT_ADDR_MASK;
+	*size = ((unsigned long)watchpoint & WATCHPOINT_SIZE_MASK) >>
+		WATCHPOINT_ADDR_BITS;
+	*is_write = !!((unsigned long)watchpoint & WATCHPOINT_WRITE_MASK);
+
+	return true;
+}
+
+/*
+ * Return watchpoint slot for an address.
+ */
+static inline int watchpoint_slot(unsigned long addr)
+{
+	return (addr / PAGE_SIZE) % KCSAN_NUM_WATCHPOINTS;
+}
+
+static inline bool matching_access(unsigned long addr1, size_t size1,
+				   unsigned long addr2, size_t size2)
+{
+	unsigned long end_range1 = addr1 + size1 - 1;
+	unsigned long end_range2 = addr2 + size2 - 1;
+
+	return addr1 <= end_range2 && addr2 <= end_range1;
+}
+
+#endif /* _MM_KCSAN_ENCODING_H */
diff --git a/kernel/kcsan/kcsan.c b/kernel/kcsan/kcsan.c
new file mode 100644
index 000000000000..ce13e0b38ba2
--- /dev/null
+++ b/kernel/kcsan/kcsan.c
@@ -0,0 +1,81 @@ 
+// SPDX-License-Identifier: GPL-2.0
+
+/*
+ * The Kernel Concurrency Sanitizer (KCSAN) infrastructure. For more info please
+ * see Documentation/dev-tools/kcsan.rst.
+ */
+
+#include <linux/export.h>
+
+#include "kcsan.h"
+
+/*
+ * Concurrency Sanitizer uses the same instrumentation as Thread Sanitizer.
+ */
+
+#define DEFINE_TSAN_READ_WRITE(size)                                           \
+	void __tsan_read##size(void *ptr)                                      \
+	{                                                                      \
+		__kcsan_check_access(ptr, size, false);                        \
+	}                                                                      \
+	EXPORT_SYMBOL(__tsan_read##size);                                      \
+	void __tsan_write##size(void *ptr)                                     \
+	{                                                                      \
+		__kcsan_check_access(ptr, size, true);                         \
+	}                                                                      \
+	EXPORT_SYMBOL(__tsan_write##size)
+
+DEFINE_TSAN_READ_WRITE(1);
+DEFINE_TSAN_READ_WRITE(2);
+DEFINE_TSAN_READ_WRITE(4);
+DEFINE_TSAN_READ_WRITE(8);
+DEFINE_TSAN_READ_WRITE(16);
+
+/*
+ * Not all supported compiler versions distinguish aligned/unaligned accesses,
+ * but e.g. recent versions of Clang do.
+ */
+#define DEFINE_TSAN_UNALIGNED_READ_WRITE(size)                                 \
+	void __tsan_unaligned_read##size(void *ptr)                            \
+	{                                                                      \
+		__kcsan_check_access(ptr, size, false);                        \
+	}                                                                      \
+	EXPORT_SYMBOL(__tsan_unaligned_read##size);                            \
+	void __tsan_unaligned_write##size(void *ptr)                           \
+	{                                                                      \
+		__kcsan_check_access(ptr, size, true);                         \
+	}                                                                      \
+	EXPORT_SYMBOL(__tsan_unaligned_write##size)
+
+DEFINE_TSAN_UNALIGNED_READ_WRITE(2);
+DEFINE_TSAN_UNALIGNED_READ_WRITE(4);
+DEFINE_TSAN_UNALIGNED_READ_WRITE(8);
+DEFINE_TSAN_UNALIGNED_READ_WRITE(16);
+
+void __tsan_read_range(void *ptr, size_t size)
+{
+	__kcsan_check_access(ptr, size, false);
+}
+EXPORT_SYMBOL(__tsan_read_range);
+
+void __tsan_write_range(void *ptr, size_t size)
+{
+	__kcsan_check_access(ptr, size, true);
+}
+EXPORT_SYMBOL(__tsan_write_range);
+
+/*
+ * The below are not required KCSAN, but can still be emitted by the compiler.
+ */
+void __tsan_func_entry(void *call_pc)
+{
+}
+EXPORT_SYMBOL(__tsan_func_entry);
+void __tsan_func_exit(void)
+{
+}
+EXPORT_SYMBOL(__tsan_func_exit);
+void __tsan_init(void)
+{
+}
+EXPORT_SYMBOL(__tsan_init);
diff --git a/kernel/kcsan/kcsan.h b/kernel/kcsan/kcsan.h
new file mode 100644
index 000000000000..429479b3041d
--- /dev/null
+++ b/kernel/kcsan/kcsan.h
@@ -0,0 +1,140 @@ 
+/* SPDX-License-Identifier: GPL-2.0 */
+
+#ifndef _MM_KCSAN_KCSAN_H
+#define _MM_KCSAN_KCSAN_H
+
+#include <linux/kcsan.h>
+
+/*
+ * Total number of watchpoints. An address range maps into a specific slot as
+ * specified in `encoding.h`. Although larger number of watchpoints may not even
+ * be usable due to limited thread count, a larger value will improve
+ * performance due to reducing cache-line contention.
+ */
+#define KCSAN_NUM_WATCHPOINTS 64
+
+/*
+ * The number of adjacent watchpoints to check; the purpose is 2-fold:
+ *
+ *	1. the address slot is already occupied, check if any adjacent slots are
+ *	   free;
+ *	2. accesses that straddle a slot boundary due to size that exceeds a
+ *	   slot's range may check adjacent slots if any watchpoint matches.
+ *
+ * Note that accesses with very large size may still miss a watchpoint; however,
+ * given this should be rare, this is a reasonable trade-off to make, since this
+ * will avoid:
+ *
+ *	1. excessive contention between watchpoint checks and setup;
+ *	2. larger number of simultaneous watchpoints without sacrificing
+ *	   performance.
+ */
+#define KCSAN_CHECK_ADJACENT 1
+
+/*
+ * Globally enable and disable KCSAN.
+ */
+extern bool kcsan_enabled;
+
+/*
+ * Helper that returns true if access to ptr should be considered as an atomic
+ * access, even though it is not explicitly atomic.
+ */
+bool kcsan_is_atomic(const volatile void *ptr);
+
+/*
+ * Initialize debugfs file.
+ */
+void kcsan_debugfs_init(void);
+
+enum kcsan_counter_id {
+	/*
+	 * Number of watchpoints currently in use.
+	 */
+	kcsan_counter_used_watchpoints,
+
+	/*
+	 * Total number of watchpoints set up.
+	 */
+	kcsan_counter_setup_watchpoints,
+
+	/*
+	 * Total number of data-races.
+	 */
+	kcsan_counter_data_races,
+
+	/*
+	 * Number of times no watchpoints were available.
+	 */
+	kcsan_counter_no_capacity,
+
+	/*
+	 * A thread checking a watchpoint raced with another checking thread;
+	 * only one will be reported.
+	 */
+	kcsan_counter_report_races,
+
+	/*
+	 * Observed data value change, but writer thread unknown.
+	 */
+	kcsan_counter_races_unknown_origin,
+
+	/*
+	 * The access cannot be encoded to a valid watchpoint.
+	 */
+	kcsan_counter_unencodable_accesses,
+
+	/*
+	 * Watchpoint encoding caused a watchpoint to fire on mismatching
+	 * accesses.
+	 */
+	kcsan_counter_encoding_false_positives,
+
+	kcsan_counter_count, /* number of counters */
+};
+
+/*
+ * Increment/decrement counter with given id; avoid calling these in fast-path.
+ */
+void kcsan_counter_inc(enum kcsan_counter_id id);
+void kcsan_counter_dec(enum kcsan_counter_id id);
+
+/*
+ * Returns true if data-races in the function symbol that maps to addr (offsets
+ * are ignored) should *not* be reported.
+ */
+bool kcsan_skip_report(unsigned long func_addr);
+
+enum kcsan_report_type {
+	/*
+	 * The thread that set up the watchpoint and briefly stalled was
+	 * signalled that another thread triggered the watchpoint, and thus a
+	 * race was encountered.
+	 */
+	kcsan_report_race_setup,
+
+	/*
+	 * A thread encountered a watchpoint for the access, therefore a race
+	 * was encountered.
+	 */
+	kcsan_report_race_check,
+
+	/*
+	 * A thread encountered a watchpoint for the access, but the other
+	 * racing thread can no longer be signaled that a race occurred.
+	 */
+	kcsan_report_race_check_race,
+
+	/*
+	 * No other thread was observed to race with the access, but the data
+	 * value before and after the stall differs.
+	 */
+	kcsan_report_race_unknown_origin,
+};
+/*
+ * Print a race report from thread that encountered the race.
+ */
+void kcsan_report(const volatile void *ptr, size_t size, bool is_write,
+		  int cpu_id, enum kcsan_report_type type);
+
+#endif /* _MM_KCSAN_KCSAN_H */
diff --git a/kernel/kcsan/report.c b/kernel/kcsan/report.c
new file mode 100644
index 000000000000..1a0f34b623bf
--- /dev/null
+++ b/kernel/kcsan/report.c
@@ -0,0 +1,307 @@ 
+// SPDX-License-Identifier: GPL-2.0
+
+#include <linux/kernel.h>
+#include <linux/preempt.h>
+#include <linux/printk.h>
+#include <linux/sched.h>
+#include <linux/spinlock.h>
+#include <linux/stacktrace.h>
+
+#include "kcsan.h"
+#include "encoding.h"
+
+/*
+ * Max. number of stack entries to show in the report.
+ */
+#define NUM_STACK_ENTRIES 16
+
+/*
+ * Other thread info: communicated from other racing thread to thread that set
+ * up the watchpoint, which then prints the complete report atomically. Only
+ * need one struct, as all threads should to be serialized regardless to print
+ * the reports, with reporting being in the slow-path.
+ */
+static struct {
+	const volatile void *ptr;
+	size_t size;
+	bool is_write;
+	int task_pid;
+	int cpu_id;
+	unsigned long stack_entries[NUM_STACK_ENTRIES];
+	int num_stack_entries;
+} other_info = { .ptr = NULL };
+
+static DEFINE_SPINLOCK(other_info_lock);
+static DEFINE_SPINLOCK(report_lock);
+
+static bool set_or_lock_other_info(unsigned long *flags,
+				   const volatile void *ptr, size_t size,
+				   bool is_write, int cpu_id,
+				   enum kcsan_report_type type)
+{
+	if (type != kcsan_report_race_check && type != kcsan_report_race_setup)
+		return true;
+
+	for (;;) {
+		spin_lock_irqsave(&other_info_lock, *flags);
+
+		switch (type) {
+		case kcsan_report_race_check:
+			if (other_info.ptr != NULL) {
+				/* still in use, retry */
+				break;
+			}
+			other_info.ptr = ptr;
+			other_info.size = size;
+			other_info.is_write = is_write;
+			other_info.task_pid =
+				in_task() ? task_pid_nr(current) : -1;
+			other_info.cpu_id = cpu_id;
+			other_info.num_stack_entries = stack_trace_save(
+				other_info.stack_entries, NUM_STACK_ENTRIES, 1);
+			/*
+			 * other_info may now be consumed by thread we raced
+			 * with.
+			 */
+			spin_unlock_irqrestore(&other_info_lock, *flags);
+			return false;
+
+		case kcsan_report_race_setup:
+			if (other_info.ptr == NULL)
+				break; /* no data available yet, retry */
+
+			/*
+			 * First check if matching based on how watchpoint was
+			 * encoded.
+			 */
+			if (!matching_access((unsigned long)other_info.ptr &
+						     WATCHPOINT_ADDR_MASK,
+					     other_info.size,
+					     (unsigned long)ptr &
+						     WATCHPOINT_ADDR_MASK,
+					     size))
+				break; /* mismatching access, retry */
+
+			if (!matching_access((unsigned long)other_info.ptr,
+					     other_info.size,
+					     (unsigned long)ptr, size)) {
+				/*
+				 * If the actual accesses to not match, this was
+				 * a false positive due to watchpoint encoding.
+				 */
+				other_info.ptr = NULL; /* mark for reuse */
+				kcsan_counter_inc(
+					kcsan_counter_encoding_false_positives);
+				spin_unlock_irqrestore(&other_info_lock,
+						       *flags);
+				return false;
+			}
+
+			/*
+			 * Matching access: keep other_info locked, as this
+			 * thread uses it to print the full report; unlocked in
+			 * end_report.
+			 */
+			return true;
+
+		default:
+			BUG();
+		}
+
+		spin_unlock_irqrestore(&other_info_lock, *flags);
+	}
+}
+
+static void start_report(unsigned long *flags, enum kcsan_report_type type)
+{
+	switch (type) {
+	case kcsan_report_race_setup:
+		/* irqsaved already via other_info_lock */
+		spin_lock(&report_lock);
+		break;
+
+	case kcsan_report_race_unknown_origin:
+		spin_lock_irqsave(&report_lock, *flags);
+		break;
+
+	default:
+		BUG();
+	}
+}
+
+static void end_report(unsigned long *flags, enum kcsan_report_type type)
+{
+	switch (type) {
+	case kcsan_report_race_setup:
+		other_info.ptr = NULL; /* mark for reuse */
+		spin_unlock(&report_lock);
+		spin_unlock_irqrestore(&other_info_lock, *flags);
+		break;
+
+	case kcsan_report_race_unknown_origin:
+		spin_unlock_irqrestore(&report_lock, *flags);
+		break;
+
+	default:
+		BUG();
+	}
+}
+
+static const char *get_access_type(bool is_write)
+{
+	return is_write ? "write" : "read";
+}
+
+/* Return thread description: in task or interrupt. */
+static const char *get_thread_desc(int task_id)
+{
+	if (task_id != -1) {
+		static char buf[32]; /* safe: protected by report_lock */
+
+		snprintf(buf, sizeof(buf), "task %i", task_id);
+		return buf;
+	}
+	return in_nmi() ? "NMI" : "interrupt";
+}
+
+/* Helper to skip KCSAN-related functions in stack-trace. */
+static int get_stack_skipnr(unsigned long stack_entries[], int num_entries)
+{
+	char buf[64];
+	int skip = 0;
+
+	for (; skip < num_entries; ++skip) {
+		snprintf(buf, sizeof(buf), "%ps", (void *)stack_entries[skip]);
+		if (!strnstr(buf, "csan_", sizeof(buf)) &&
+		    !strnstr(buf, "tsan_", sizeof(buf)) &&
+		    !strnstr(buf, "_once_size", sizeof(buf))) {
+			break;
+		}
+	}
+	return skip;
+}
+
+/* Compares symbolized strings of addr1 and addr2. */
+static int sym_strcmp(void *addr1, void *addr2)
+{
+	char buf1[64];
+	char buf2[64];
+
+	snprintf(buf1, sizeof(buf1), "%pS", addr1);
+	snprintf(buf2, sizeof(buf2), "%pS", addr2);
+	return strncmp(buf1, buf2, sizeof(buf1));
+}
+
+/*
+ * Returns true if a report was generated, false otherwise.
+ */
+static bool print_summary(const volatile void *ptr, size_t size, bool is_write,
+			  int cpu_id, enum kcsan_report_type type)
+{
+	unsigned long stack_entries[NUM_STACK_ENTRIES] = { 0 };
+	int num_stack_entries =
+		stack_trace_save(stack_entries, NUM_STACK_ENTRIES, 1);
+	int skipnr = get_stack_skipnr(stack_entries, num_stack_entries);
+	int other_skipnr;
+
+	/* Check if the top stackframe is in a blacklisted function. */
+	if (kcsan_skip_report(stack_entries[skipnr]))
+		return false;
+	if (type == kcsan_report_race_setup) {
+		other_skipnr = get_stack_skipnr(other_info.stack_entries,
+						other_info.num_stack_entries);
+		if (kcsan_skip_report(other_info.stack_entries[other_skipnr]))
+			return false;
+	}
+
+	/* Print report header. */
+	pr_err("==================================================================\n");
+	switch (type) {
+	case kcsan_report_race_setup: {
+		void *this_fn = (void *)stack_entries[skipnr];
+		void *other_fn = (void *)other_info.stack_entries[other_skipnr];
+		int cmp;
+
+		/*
+		 * Order functions lexographically for consistent bug titles.
+		 * Do not print offset of functions to keep title short.
+		 */
+		cmp = sym_strcmp(other_fn, this_fn);
+		pr_err("BUG: KCSAN: data-race in %ps / %ps\n",
+		       cmp < 0 ? other_fn : this_fn,
+		       cmp < 0 ? this_fn : other_fn);
+	} break;
+
+	case kcsan_report_race_unknown_origin:
+		pr_err("BUG: KCSAN: racing %s in %pS\n",
+		       get_access_type(is_write),
+		       (void *)stack_entries[skipnr]);
+		break;
+
+	default:
+		BUG();
+	}
+
+	pr_err("\n");
+
+	/* Print information about the racing accesses. */
+	switch (type) {
+	case kcsan_report_race_setup:
+		pr_err("%s to 0x%px of %zu bytes by %s on cpu %i:\n",
+		       get_access_type(other_info.is_write), other_info.ptr,
+		       other_info.size, get_thread_desc(other_info.task_pid),
+		       other_info.cpu_id);
+
+		/* Print the other thread's stack trace. */
+		stack_trace_print(other_info.stack_entries + other_skipnr,
+				  other_info.num_stack_entries - other_skipnr,
+				  0);
+
+		pr_err("\n");
+		pr_err("%s to 0x%px of %zu bytes by %s on cpu %i:\n",
+		       get_access_type(is_write), ptr, size,
+		       get_thread_desc(in_task() ? task_pid_nr(current) : -1),
+		       cpu_id);
+		break;
+
+	case kcsan_report_race_unknown_origin:
+		pr_err("race at unknown origin, with %s to 0x%px of %zu bytes by %s on cpu %i:\n",
+		       get_access_type(is_write), ptr, size,
+		       get_thread_desc(in_task() ? task_pid_nr(current) : -1),
+		       cpu_id);
+		break;
+
+	default:
+		BUG();
+	}
+	/* Print stack trace of this thread. */
+	stack_trace_print(stack_entries + skipnr, num_stack_entries - skipnr,
+			  0);
+
+	/* Print report footer. */
+	pr_err("\n");
+	pr_err("Reported by Kernel Concurrency Sanitizer on:\n");
+	dump_stack_print_info(KERN_DEFAULT);
+	pr_err("==================================================================\n");
+
+	return true;
+}
+
+void kcsan_report(const volatile void *ptr, size_t size, bool is_write,
+		  int cpu_id, enum kcsan_report_type type)
+{
+	unsigned long flags = 0;
+
+	if (type == kcsan_report_race_check_race)
+		return;
+
+	kcsan_disable_current();
+	if (set_or_lock_other_info(&flags, ptr, size, is_write, cpu_id, type)) {
+		start_report(&flags, type);
+		if (print_summary(ptr, size, is_write, cpu_id, type) &&
+		    panic_on_warn)
+			panic("panic_on_warn set ...\n");
+		end_report(&flags, type);
+	}
+	kcsan_enable_current();
+}
diff --git a/kernel/kcsan/test.c b/kernel/kcsan/test.c
new file mode 100644
index 000000000000..68c896a24529
--- /dev/null
+++ b/kernel/kcsan/test.c
@@ -0,0 +1,117 @@ 
+// SPDX-License-Identifier: GPL-2.0
+
+#include <linux/init.h>
+#include <linux/kernel.h>
+#include <linux/printk.h>
+#include <linux/random.h>
+#include <linux/types.h>
+
+#include "encoding.h"
+
+#define ITERS_PER_TEST 2000
+
+/* Test requirements. */
+static bool test_requires(void)
+{
+	/* random should be initialized */
+	return prandom_u32() + prandom_u32() != 0;
+}
+
+/* Test watchpoint encode and decode. */
+static bool test_encode_decode(void)
+{
+	int i;
+
+	for (i = 0; i < ITERS_PER_TEST; ++i) {
+		size_t size = prandom_u32() % MAX_ENCODABLE_SIZE + 1;
+		bool is_write = prandom_u32() % 2;
+		unsigned long addr;
+
+		prandom_bytes(&addr, sizeof(addr));
+		if (WARN_ON(!check_encodable(addr, size)))
+			return false;
+
+		/* encode and decode */
+		{
+			const long encoded_watchpoint =
+				encode_watchpoint(addr, size, is_write);
+			unsigned long verif_masked_addr;
+			size_t verif_size;
+			bool verif_is_write;
+
+			/* check special watchpoints */
+			if (WARN_ON(decode_watchpoint(
+				    INVALID_WATCHPOINT, &verif_masked_addr,
+				    &verif_size, &verif_is_write)))
+				return false;
+			if (WARN_ON(decode_watchpoint(
+				    CONSUMED_WATCHPOINT, &verif_masked_addr,
+				    &verif_size, &verif_is_write)))
+				return false;
+
+			/* check decoding watchpoint returns same data */
+			if (WARN_ON(!decode_watchpoint(
+				    encoded_watchpoint, &verif_masked_addr,
+				    &verif_size, &verif_is_write)))
+				return false;
+			if (WARN_ON(verif_masked_addr !=
+				    (addr & WATCHPOINT_ADDR_MASK)))
+				goto fail;
+			if (WARN_ON(verif_size != size))
+				goto fail;
+			if (WARN_ON(is_write != verif_is_write))
+				goto fail;
+
+			continue;
+fail:
+			pr_err("%s fail: %s %zu bytes @ %lx -> encoded: %lx -> %s %zu bytes @ %lx\n",
+			       __func__, is_write ? "write" : "read", size,
+			       addr, encoded_watchpoint,
+			       verif_is_write ? "write" : "read", verif_size,
+			       verif_masked_addr);
+			return false;
+		}
+	}
+
+	return true;
+}
+
+static bool test_matching_access(void)
+{
+	if (WARN_ON(!matching_access(10, 1, 10, 1)))
+		return false;
+	if (WARN_ON(!matching_access(10, 2, 11, 1)))
+		return false;
+	if (WARN_ON(!matching_access(10, 1, 9, 2)))
+		return false;
+	if (WARN_ON(matching_access(10, 1, 11, 1)))
+		return false;
+	if (WARN_ON(matching_access(9, 1, 10, 1)))
+		return false;
+	return true;
+}
+
+static int __init kcsan_selftest(void)
+{
+	int passed = 0;
+	int total = 0;
+
+#define RUN_TEST(do_test)                                                      \
+	do {                                                                   \
+		++total;                                                       \
+		if (do_test())                                                 \
+			++passed;                                              \
+		else                                                           \
+			pr_err("KCSAN selftest: " #do_test " failed");         \
+	} while (0)
+
+	RUN_TEST(test_requires);
+	RUN_TEST(test_encode_decode);
+	RUN_TEST(test_matching_access);
+
+	pr_info("KCSAN selftest: %d/%d tests passed\n", passed, total);
+	if (passed != total)
+		panic("KCSAN selftests failed");
+	return 0;
+}
+postcore_initcall(kcsan_selftest);
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index 93d97f9b0157..35accd1d93de 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -2086,6 +2086,8 @@  source "lib/Kconfig.kgdb"
 
 source "lib/Kconfig.ubsan"
 
+source "lib/Kconfig.kcsan"
+
 config ARCH_HAS_DEVMEM_IS_ALLOWED
 	bool
 
diff --git a/lib/Kconfig.kcsan b/lib/Kconfig.kcsan
new file mode 100644
index 000000000000..b532d0d98f7a
--- /dev/null
+++ b/lib/Kconfig.kcsan
@@ -0,0 +1,88 @@ 
+# SPDX-License-Identifier: GPL-2.0-only
+
+config HAVE_ARCH_KCSAN
+	bool
+
+menuconfig KCSAN
+	bool "KCSAN: watchpoint-based dynamic data-race detector"
+	depends on HAVE_ARCH_KCSAN && !KASAN && STACKTRACE
+	default n
+	help
+	  Kernel Concurrency Sanitizer is a dynamic data-race detector, which
+	  uses a watchpoint-based sampling approach to detect races.
+
+if KCSAN
+
+config KCSAN_SELFTEST
+	bool "KCSAN: perform short selftests on boot"
+	default y
+	help
+	  Run KCSAN selftests on boot. On test failure, causes kernel to panic.
+
+config KCSAN_EARLY_ENABLE
+	bool "KCSAN: early enable"
+	default y
+	help
+	  If KCSAN should be enabled globally as soon as possible. KCSAN can
+	  later be enabled/disabled via debugfs.
+
+config KCSAN_UDELAY_MAX_TASK
+	int "KCSAN: maximum delay in microseconds (for tasks)"
+	default 80
+	help
+	  For tasks, the max. microsecond delay after setting up a watchpoint.
+
+config KCSAN_UDELAY_MAX_INTERRUPT
+	int "KCSAN: maximum delay in microseconds (for interrupts)"
+	default 20
+	help
+	  For interrupts, the max. microsecond delay after setting up a watchpoint.
+
+config KCSAN_DELAY_RANDOMIZE
+	bool "KCSAN: randomize delays"
+	default y
+	help
+	  If delays should be randomized; if false, the chosen delay is simply
+	  the maximum values defined above.
+
+config KCSAN_WATCH_SKIP_INST
+	int "KCSAN: watchpoint instruction skip"
+	default 2000
+	help
+	  The number of per-CPU memory operations to skip watching, before
+	  another watchpoint is set up; in other words, 1 in
+	  KCSAN_WATCH_SKIP_INST per-CPU memory operations are used to set up a
+	  watchpoint. A smaller value results in more aggressive race
+	  detection, whereas a larger value improves system performance at the
+	  cost of missing some races.
+
+config KCSAN_REPORT_RACE_UNKNOWN_ORIGIN
+	bool "KCSAN: report races of unknown origin"
+	default y
+	help
+	  If KCSAN should report races where only one access is known, and the
+	  conflicting access is of unknown origin. This type of race is
+	  reported if it was only possible to infer a race due to a data-value
+	  change while an access is being delayed on a watchpoint.
+
+config KCSAN_IGNORE_ATOMICS
+	bool "KCSAN: do not instrument atomic accesses"
+	default n
+	help
+	  If enabled, never instruments atomic accesses. This results in not
+	  reporting data-races where one access is atomic and the other is a
+	  plain access.
+
+config KCSAN_PLAIN_WRITE_PRETEND_ONCE
+	bool "KCSAN: pretend plain writes are WRITE_ONCE"
+	default n
+	help
+	  This option makes KCSAN pretend that all plain writes are WRITE_ONCE.
+	  This option should only be used to prune initial data-races found in
+	  existing code.
+
+config KCSAN_DEBUG
+	bool "Debugging of KCSAN internals"
+	default n
+
+endif # KCSAN
diff --git a/lib/Makefile b/lib/Makefile
index c5892807e06f..778ab704e3ad 100644
--- a/lib/Makefile
+++ b/lib/Makefile
@@ -24,6 +24,9 @@  KASAN_SANITIZE_string.o := n
 CFLAGS_string.o := $(call cc-option, -fno-stack-protector)
 endif
 
+# Used by KCSAN while enabled, avoid recursion.
+KCSAN_SANITIZE_random32.o := n
+
 lib-y := ctype.o string.o vsprintf.o cmdline.o \
 	 rbtree.o radix-tree.o timerqueue.o xarray.o \
 	 idr.o extable.o \
diff --git a/scripts/Makefile.kcsan b/scripts/Makefile.kcsan
new file mode 100644
index 000000000000..caf1111a28ae
--- /dev/null
+++ b/scripts/Makefile.kcsan
@@ -0,0 +1,6 @@ 
+# SPDX-License-Identifier: GPL-2.0
+ifdef CONFIG_KCSAN
+
+CFLAGS_KCSAN := -fsanitize=thread
+
+endif # CONFIG_KCSAN
diff --git a/scripts/Makefile.lib b/scripts/Makefile.lib
index 179d55af5852..0e78abab7d83 100644
--- a/scripts/Makefile.lib
+++ b/scripts/Makefile.lib
@@ -152,6 +152,16 @@  _c_flags += $(if $(patsubst n%,, \
 	$(CFLAGS_KCOV))
 endif
 
+#
+# Enable ConcurrencySanitizer flags for kernel except some files or directories
+# we don't want to check (depends on variables KCSAN_SANITIZE_obj.o, KCSAN_SANITIZE)
+#
+ifeq ($(CONFIG_KCSAN),y)
+_c_flags += $(if $(patsubst n%,, \
+	$(KCSAN_SANITIZE_$(basetarget).o)$(KCSAN_SANITIZE)y), \
+	$(CFLAGS_KCSAN))
+endif
+
 # $(srctree)/$(src) for including checkin headers from generated source files
 # $(objtree)/$(obj) for including generated headers from checkin source files
 ifeq ($(KBUILD_EXTMOD),)