[v3,1/2] kretprobe: produce sane stack traces
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Message ID 20181101083551.3805-2-cyphar@cyphar.com
State New
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  • kretprobe: produce sane stack traces
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Commit Message

Aleksa Sarai Nov. 1, 2018, 8:35 a.m. UTC
Historically, kretprobe has always produced unusable stack traces
(kretprobe_trampoline is the only entry in most cases, because of the
funky stack pointer overwriting). This has caused quite a few annoyances
when using tracing to debug problems[1] -- since return values are only
available with kretprobes but stack traces were only usable for kprobes,
users had to probe both and then manually associate them.

With the advent of bpf_trace, users would have been able to do this
association in bpf, but this was less than ideal (because
bpf_get_stackid would still produce rubbish and programs that didn't
know better would get silly results). The main usecase for stack traces
(at least with bpf_trace) is for DTrace-style aggregation on stack
traces (both entry and exit). Therefore we cannot simply correct the
stack trace on exit -- we must stash away the stack trace and return the
entry stack trace when it is requested.

[1]: https://github.com/iovisor/bpftrace/issues/101

Cc: Brendan Gregg <bgregg@netflix.com>
Cc: Christian Brauner <christian@brauner.io>
Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
---
 Documentation/kprobes.txt                     |   6 +-
 include/linux/kprobes.h                       |  27 +++++
 kernel/events/callchain.c                     |   8 +-
 kernel/kprobes.c                              | 101 +++++++++++++++++-
 kernel/trace/trace.c                          |  11 +-
 .../test.d/kprobe/kretprobe_stacktrace.tc     |  25 +++++
 6 files changed, 173 insertions(+), 5 deletions(-)
 create mode 100644 tools/testing/selftests/ftrace/test.d/kprobe/kretprobe_stacktrace.tc

Comments

Masami Hiramatsu Nov. 1, 2018, 3:20 p.m. UTC | #1
Hi Aleksa,

On Thu,  1 Nov 2018 19:35:50 +1100
Aleksa Sarai <cyphar@cyphar.com> wrote:
[...]
> diff --git a/tools/testing/selftests/ftrace/test.d/kprobe/kretprobe_stacktrace.tc b/tools/testing/selftests/ftrace/test.d/kprobe/kretprobe_stacktrace.tc

Please split the test case as an independent patch.

> new file mode 100644
> index 000000000000..03146c6a1a3c
> --- /dev/null
> +++ b/tools/testing/selftests/ftrace/test.d/kprobe/kretprobe_stacktrace.tc
> @@ -0,0 +1,25 @@
> +#!/bin/sh
> +# SPDX-License-Identifier: GPL-2.0+
> +# description: Kretprobe dynamic event with a stacktrace
> +
> +[ -f kprobe_events ] || exit_unsupported # this is configurable
> +
> +echo 0 > events/enable
> +echo 1 > options/stacktrace
> +
> +echo 'r:teststackprobe sched_fork $retval' > kprobe_events
> +grep teststackprobe kprobe_events
> +test -d events/kprobes/teststackprobe

Hmm, what happen if we have 2 or more kretprobes on same stack?
It seems you just save stack in pre_handler, but that stack can already
includes another kretprobe's trampline address...

Thank you,

> +
> +clear_trace
> +echo 1 > events/kprobes/teststackprobe/enable
> +( echo "forked")
> +echo 0 > events/kprobes/teststackprobe/enable
> +
> +# Make sure we don't see kretprobe_trampoline and we see _do_fork.
> +! grep 'kretprobe' trace
> +grep '_do_fork' trace
> +
> +echo '-:teststackprobe' >> kprobe_events
> +clear_trace
> +test -d events/kprobes/teststackprobe && exit_fail || exit_pass
> -- 
> 2.19.1
>
Aleksa Sarai Nov. 1, 2018, 9:13 p.m. UTC | #2
On 2018-11-02, Masami Hiramatsu <mhiramat@kernel.org> wrote:
> Please split the test case as an independent patch.

Will do. Should the Documentation/ change also be a separate patch?

> > new file mode 100644
> > index 000000000000..03146c6a1a3c
> > --- /dev/null
> > +++ b/tools/testing/selftests/ftrace/test.d/kprobe/kretprobe_stacktrace.tc
> > @@ -0,0 +1,25 @@
> > +#!/bin/sh
> > +# SPDX-License-Identifier: GPL-2.0+
> > +# description: Kretprobe dynamic event with a stacktrace
> > +
> > +[ -f kprobe_events ] || exit_unsupported # this is configurable
> > +
> > +echo 0 > events/enable
> > +echo 1 > options/stacktrace
> > +
> > +echo 'r:teststackprobe sched_fork $retval' > kprobe_events
> > +grep teststackprobe kprobe_events
> > +test -d events/kprobes/teststackprobe
> 
> Hmm, what happen if we have 2 or more kretprobes on same stack?
> It seems you just save stack in pre_handler, but that stack can already
> includes another kretprobe's trampline address...

Yeah, you're quite right...

My first instinct was to do something awful like iterate over the set of
"kretprobe_instance"s with ->task == current, and then correct
kretprobe_trampoline entries using ->ret_addr. (I think this would be
correct because each task can only be in one context at once, and in
order to get to a particular kretprobe all of your caller kretprobes
were inserted before you and any sibling or later kretprobe_instances
will have been removed. But I might be insanely wrong on this front.)

However (as I noted in the other thread), there is a problem where
kretprobe_trampoline actually stops the unwinder in its tracks and thus
you only get the first kretprobe_trampoline. This is something I'm going
to look into some more (despite not having made progress on it last
time) since now it's something that actually needs to be fixed (and
as I mentioned in the other thread, show_stack() actually works on x86
in this context unlike the other stack_trace users).
Steven Rostedt Nov. 2, 2018, 12:47 a.m. UTC | #3
On Thu,  1 Nov 2018 19:35:50 +1100
Aleksa Sarai <cyphar@cyphar.com> wrote:

> Historically, kretprobe has always produced unusable stack traces
> (kretprobe_trampoline is the only entry in most cases, because of the
> funky stack pointer overwriting). This has caused quite a few annoyances
> when using tracing to debug problems[1] -- since return values are only
> available with kretprobes but stack traces were only usable for kprobes,
> users had to probe both and then manually associate them.
> 
> With the advent of bpf_trace, users would have been able to do this
> association in bpf, but this was less than ideal (because
> bpf_get_stackid would still produce rubbish and programs that didn't
> know better would get silly results). The main usecase for stack traces
> (at least with bpf_trace) is for DTrace-style aggregation on stack
> traces (both entry and exit). Therefore we cannot simply correct the
> stack trace on exit -- we must stash away the stack trace and return the
> entry stack trace when it is requested.
> 
> [1]: https://github.com/iovisor/bpftrace/issues/101
> 
> Cc: Brendan Gregg <bgregg@netflix.com>
> Cc: Christian Brauner <christian@brauner.io>
> Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
> ---
>  Documentation/kprobes.txt                     |   6 +-
>  include/linux/kprobes.h                       |  27 +++++
>  kernel/events/callchain.c                     |   8 +-
>  kernel/kprobes.c                              | 101 +++++++++++++++++-
>  kernel/trace/trace.c                          |  11 +-
>  .../test.d/kprobe/kretprobe_stacktrace.tc     |  25 +++++
>  6 files changed, 173 insertions(+), 5 deletions(-)
>  create mode 100644 tools/testing/selftests/ftrace/test.d/kprobe/kretprobe_stacktrace.tc
> 
> diff --git a/Documentation/kprobes.txt b/Documentation/kprobes.txt
> index 10f4499e677c..1965585848f4 100644
> --- a/Documentation/kprobes.txt
> +++ b/Documentation/kprobes.txt
> @@ -597,7 +597,11 @@ address with the trampoline's address, stack backtraces and calls
>  to __builtin_return_address() will typically yield the trampoline's
>  address instead of the real return address for kretprobed functions.
>  (As far as we can tell, __builtin_return_address() is used only
> -for instrumentation and error reporting.)
> +for instrumentation and error reporting.) However, since return probes
> +are used extensively in tracing (where stack backtraces are useful),
> +return probes will stash away the stack backtrace during function entry
> +so that return probe handlers can use the entry backtrace instead of
> +having a trace with just kretprobe_trampoline.
>  
>  If the number of times a function is called does not match the number
>  of times it returns, registering a return probe on that function may
> diff --git a/include/linux/kprobes.h b/include/linux/kprobes.h
> index e909413e4e38..1a1629544e56 100644
> --- a/include/linux/kprobes.h
> +++ b/include/linux/kprobes.h
> @@ -40,6 +40,8 @@
>  #include <linux/rcupdate.h>
>  #include <linux/mutex.h>
>  #include <linux/ftrace.h>
> +#include <linux/stacktrace.h>
> +#include <linux/perf_event.h>
>  #include <asm/kprobes.h>
>  
>  #ifdef CONFIG_KPROBES
> @@ -168,11 +170,18 @@ struct kretprobe {
>  	raw_spinlock_t lock;
>  };
>  
> +#define KRETPROBE_TRACE_SIZE 127
> +struct kretprobe_trace {
> +	int nr_entries;
> +	unsigned long entries[KRETPROBE_TRACE_SIZE];
> +};
> +
>  struct kretprobe_instance {
>  	struct hlist_node hlist;
>  	struct kretprobe *rp;
>  	kprobe_opcode_t *ret_addr;
>  	struct task_struct *task;
> +	struct kretprobe_trace entry;
>  	char data[0];
>  };
>  
> @@ -371,6 +380,12 @@ void unregister_kretprobe(struct kretprobe *rp);
>  int register_kretprobes(struct kretprobe **rps, int num);
>  void unregister_kretprobes(struct kretprobe **rps, int num);
>  
> +struct kretprobe_instance *current_kretprobe_instance(void);
> +void kretprobe_save_stack_trace(struct kretprobe_instance *ri,
> +				struct stack_trace *trace);
> +void kretprobe_perf_callchain_kernel(struct kretprobe_instance *ri,
> +				     struct perf_callchain_entry_ctx *ctx);
> +
>  void kprobe_flush_task(struct task_struct *tk);
>  void recycle_rp_inst(struct kretprobe_instance *ri, struct hlist_head *head);
>  
> @@ -397,6 +412,18 @@ static inline struct kprobe *kprobe_running(void)
>  {
>  	return NULL;
>  }
> +static inline struct kretprobe_instance *current_kretprobe_instance(void)
> +{
> +	return NULL;
> +}
> +static inline void kretprobe_save_stack_trace(struct kretprobe_instance *ri,
> +					      struct stack_trace *trace)
> +{
> +}
> +static inline void kretprobe_perf_callchain_kernel(struct kretprobe_instance *ri,
> +						   struct perf_callchain_entry_ctx *ctx)
> +{
> +}
>  static inline int register_kprobe(struct kprobe *p)
>  {
>  	return -ENOSYS;
> diff --git a/kernel/events/callchain.c b/kernel/events/callchain.c
> index 24a77c34e9ad..98edcd8a6987 100644
> --- a/kernel/events/callchain.c
> +++ b/kernel/events/callchain.c
> @@ -12,6 +12,7 @@
>  #include <linux/perf_event.h>
>  #include <linux/slab.h>
>  #include <linux/sched/task_stack.h>
> +#include <linux/kprobes.h>
>  
>  #include "internal.h"
>  
> @@ -197,9 +198,14 @@ get_perf_callchain(struct pt_regs *regs, u32 init_nr, bool kernel, bool user,
>  	ctx.contexts_maxed = false;
>  
>  	if (kernel && !user_mode(regs)) {
> +		struct kretprobe_instance *ri = current_kretprobe_instance();
> +
>  		if (add_mark)
>  			perf_callchain_store_context(&ctx, PERF_CONTEXT_KERNEL);
> -		perf_callchain_kernel(&ctx, regs);
> +		if (ri)
> +			kretprobe_perf_callchain_kernel(ri, &ctx);
> +		else
> +			perf_callchain_kernel(&ctx, regs);
>  	}
>  
>  	if (user) {
> diff --git a/kernel/kprobes.c b/kernel/kprobes.c
> index 90e98e233647..fca3964d18cd 100644
> --- a/kernel/kprobes.c
> +++ b/kernel/kprobes.c
> @@ -1206,6 +1206,16 @@ __releases(hlist_lock)
>  }
>  NOKPROBE_SYMBOL(kretprobe_table_unlock);
>  
> +static bool kretprobe_hash_is_locked(struct task_struct *tsk)
> +{
> +	unsigned long hash = hash_ptr(tsk, KPROBE_HASH_BITS);
> +	raw_spinlock_t *hlist_lock;
> +
> +	hlist_lock = kretprobe_table_lock_ptr(hash);
> +	return raw_spin_is_locked(hlist_lock);
> +}
> +NOKPROBE_SYMBOL(kretprobe_hash_is_locked);
> +
>  /*
>   * This function is called from finish_task_switch when task tk becomes dead,
>   * so that we can recycle any function-return probe instances associated
> @@ -1800,6 +1810,13 @@ unsigned long __weak arch_deref_entry_point(void *entry)
>  	return (unsigned long)entry;
>  }
>  
> +static int pre_handler_kretprobe(struct kprobe *p, struct pt_regs *regs);
> +
> +static inline bool kprobe_is_retprobe(struct kprobe *kp)
> +{
> +	return kp->pre_handler == pre_handler_kretprobe;
> +}
> +
>  #ifdef CONFIG_KRETPROBES
>  /*
>   * This kprobe pre_handler is registered with every kretprobe. When probe
> @@ -1826,6 +1843,8 @@ static int pre_handler_kretprobe(struct kprobe *p, struct pt_regs *regs)
>  	hash = hash_ptr(current, KPROBE_HASH_BITS);
>  	raw_spin_lock_irqsave(&rp->lock, flags);
>  	if (!hlist_empty(&rp->free_instances)) {
> +		struct stack_trace trace = {};
> +
>  		ri = hlist_entry(rp->free_instances.first,
>  				struct kretprobe_instance, hlist);
>  		hlist_del(&ri->hlist);
> @@ -1834,6 +1853,11 @@ static int pre_handler_kretprobe(struct kprobe *p, struct pt_regs *regs)
>  		ri->rp = rp;
>  		ri->task = current;
>  
> +		trace.entries = &ri->entry.entries[0];
> +		trace.max_entries = KRETPROBE_TRACE_SIZE;
> +		save_stack_trace_regs(regs, &trace);
> +		ri->entry.nr_entries = trace.nr_entries;
> +


So basically your saving a copy of the stack trace for each probe, for
something that may not ever be used?

This adds quite a lot of overhead for something not used often.

I think the real answer is to fix kretprobes (and I just checked, the
return call of function graph tracer stack trace doesn't go past the
return_to_handler function either. It's just that a stack trace was
never done on the return side).

The more I look at this patch, the less I like it.

-- Steve




>  		if (rp->entry_handler && rp->entry_handler(ri, regs)) {
>  			raw_spin_lock_irqsave(&rp->lock, flags);
>  			hlist_add_head(&ri->hlist, &rp->free_instances);
> @@ -1856,6 +1880,65 @@ static int pre_handler_kretprobe(struct kprobe *p, struct pt_regs *regs)
>  }
>  NOKPROBE_SYMBOL(pre_handler_kretprobe);
>  
> +/*
> + * Return the kretprobe_instance associated with the current_kprobe. Calling
> + * this is only reasonable from within a kretprobe handler context (otherwise
> + * return NULL).
> + *
> + * Must be called within a kretprobe_hash_lock(current, ...) context.
> + */
> +struct kretprobe_instance *current_kretprobe_instance(void)
> +{
> +	struct kprobe *kp;
> +	struct kretprobe *rp;
> +	struct kretprobe_instance *ri;
> +	struct hlist_head *head;
> +	unsigned long hash = hash_ptr(current, KPROBE_HASH_BITS);
> +
> +	kp = kprobe_running();
> +	if (!kp || !kprobe_is_retprobe(kp))
> +		return NULL;
> +	if (WARN_ON(!kretprobe_hash_is_locked(current)))
> +		return NULL;
> +
> +	rp = container_of(kp, struct kretprobe, kp);
> +	head = &kretprobe_inst_table[hash];
> +
> +	hlist_for_each_entry(ri, head, hlist) {
> +		if (ri->task == current && ri->rp == rp)
> +			return ri;
> +	}
> +	return NULL;
> +}
> +EXPORT_SYMBOL_GPL(current_kretprobe_instance);
> +NOKPROBE_SYMBOL(current_kretprobe_instance);
> +
> +void kretprobe_save_stack_trace(struct kretprobe_instance *ri,
> +				struct stack_trace *trace)
> +{
> +	int i;
> +	struct kretprobe_trace *krt = &ri->entry;
> +
> +	for (i = trace->skip; i < krt->nr_entries; i++) {
> +		if (trace->nr_entries >= trace->max_entries)
> +			break;
> +		trace->entries[trace->nr_entries++] = krt->entries[i];
> +	}
> +}
> +
> +void kretprobe_perf_callchain_kernel(struct kretprobe_instance *ri,
> +				     struct perf_callchain_entry_ctx *ctx)
> +{
> +	int i;
> +	struct kretprobe_trace *krt = &ri->entry;
> +
> +	for (i = 0; i < krt->nr_entries; i++) {
> +		if (krt->entries[i] == ULONG_MAX)
> +			break;
> +		perf_callchain_store(ctx, (u64) krt->entries[i]);
> +	}
> +}
> +
>  bool __weak arch_kprobe_on_func_entry(unsigned long offset)
>  {
>  	return !offset;
> @@ -2005,6 +2088,22 @@ static int pre_handler_kretprobe(struct kprobe *p, struct pt_regs *regs)
>  }
>  NOKPROBE_SYMBOL(pre_handler_kretprobe);
>  
> +struct kretprobe_instance *current_kretprobe_instance(void)
> +{
> +	return NULL;
> +}
> +EXPORT_SYMBOL_GPL(current_kretprobe_instance);
> +NOKPROBE_SYMBOL(current_kretprobe_instance);
> +
> +void kretprobe_save_stack_trace(struct kretprobe_instance *ri,
> +				struct stack_trace *trace)
> +{
> +}
> +
> +void kretprobe_perf_callchain_kernel(struct kretprobe_instance *ri,
> +				     struct perf_callchain_entry_ctx *ctx)
> +{
> +}
>  #endif /* CONFIG_KRETPROBES */
>  
>  /* Set the kprobe gone and remove its instruction buffer. */
> @@ -2241,7 +2340,7 @@ static void report_probe(struct seq_file *pi, struct kprobe *p,
>  	char *kprobe_type;
>  	void *addr = p->addr;
>  
> -	if (p->pre_handler == pre_handler_kretprobe)
> +	if (kprobe_is_retprobe(p))
>  		kprobe_type = "r";
>  	else
>  		kprobe_type = "k";
> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index bf6f1d70484d..2210d38a4dbf 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -42,6 +42,7 @@
>  #include <linux/nmi.h>
>  #include <linux/fs.h>
>  #include <linux/trace.h>
> +#include <linux/kprobes.h>
>  #include <linux/sched/clock.h>
>  #include <linux/sched/rt.h>
>  
> @@ -2590,6 +2591,7 @@ static void __ftrace_trace_stack(struct ring_buffer *buffer,
>  	struct ring_buffer_event *event;
>  	struct stack_entry *entry;
>  	struct stack_trace trace;
> +	struct kretprobe_instance *ri = current_kretprobe_instance();
>  	int use_stack;
>  	int size = FTRACE_STACK_ENTRIES;
>  
> @@ -2626,7 +2628,9 @@ static void __ftrace_trace_stack(struct ring_buffer *buffer,
>  		trace.entries		= this_cpu_ptr(ftrace_stack.calls);
>  		trace.max_entries	= FTRACE_STACK_MAX_ENTRIES;
>  
> -		if (regs)
> +		if (ri)
> +			kretprobe_save_stack_trace(ri, &trace);
> +		else if (regs)
>  			save_stack_trace_regs(regs, &trace);
>  		else
>  			save_stack_trace(&trace);
> @@ -2653,7 +2657,10 @@ static void __ftrace_trace_stack(struct ring_buffer *buffer,
>  	else {
>  		trace.max_entries	= FTRACE_STACK_ENTRIES;
>  		trace.entries		= entry->caller;
> -		if (regs)
> +
> +		if (ri)
> +			kretprobe_save_stack_trace(ri, &trace);
> +		else if (regs)
>  			save_stack_trace_regs(regs, &trace);
>  		else
>  			save_stack_trace(&trace);
> diff --git a/tools/testing/selftests/ftrace/test.d/kprobe/kretprobe_stacktrace.tc b/tools/testing/selftests/ftrace/test.d/kprobe/kretprobe_stacktrace.tc
> new file mode 100644
> index 000000000000..03146c6a1a3c
> --- /dev/null
> +++ b/tools/testing/selftests/ftrace/test.d/kprobe/kretprobe_stacktrace.tc
> @@ -0,0 +1,25 @@
> +#!/bin/sh
> +# SPDX-License-Identifier: GPL-2.0+
> +# description: Kretprobe dynamic event with a stacktrace
> +
> +[ -f kprobe_events ] || exit_unsupported # this is configurable
> +
> +echo 0 > events/enable
> +echo 1 > options/stacktrace
> +
> +echo 'r:teststackprobe sched_fork $retval' > kprobe_events
> +grep teststackprobe kprobe_events
> +test -d events/kprobes/teststackprobe
> +
> +clear_trace
> +echo 1 > events/kprobes/teststackprobe/enable
> +( echo "forked")
> +echo 0 > events/kprobes/teststackprobe/enable
> +
> +# Make sure we don't see kretprobe_trampoline and we see _do_fork.
> +! grep 'kretprobe' trace
> +grep '_do_fork' trace
> +
> +echo '-:teststackprobe' >> kprobe_events
> +clear_trace
> +test -d events/kprobes/teststackprobe && exit_fail || exit_pass
Masami Hiramatsu Nov. 2, 2018, 3:04 a.m. UTC | #4
On Fri, 2 Nov 2018 08:13:43 +1100
Aleksa Sarai <cyphar@cyphar.com> wrote:

> On 2018-11-02, Masami Hiramatsu <mhiramat@kernel.org> wrote:
> > Please split the test case as an independent patch.
> 
> Will do. Should the Documentation/ change also be a separate patch?

I think the Documentation change can be coupled with code change
if the change is small. But selftests is different, that can be
backported soon for testing the stable kernels.


> > > new file mode 100644
> > > index 000000000000..03146c6a1a3c
> > > --- /dev/null
> > > +++ b/tools/testing/selftests/ftrace/test.d/kprobe/kretprobe_stacktrace.tc
> > > @@ -0,0 +1,25 @@
> > > +#!/bin/sh
> > > +# SPDX-License-Identifier: GPL-2.0+
> > > +# description: Kretprobe dynamic event with a stacktrace
> > > +
> > > +[ -f kprobe_events ] || exit_unsupported # this is configurable
> > > +
> > > +echo 0 > events/enable
> > > +echo 1 > options/stacktrace
> > > +
> > > +echo 'r:teststackprobe sched_fork $retval' > kprobe_events
> > > +grep teststackprobe kprobe_events
> > > +test -d events/kprobes/teststackprobe
> > 
> > Hmm, what happen if we have 2 or more kretprobes on same stack?
> > It seems you just save stack in pre_handler, but that stack can already
> > includes another kretprobe's trampline address...
> 
> Yeah, you're quite right...
> 
> My first instinct was to do something awful like iterate over the set of
> "kretprobe_instance"s with ->task == current, and then correct
> kretprobe_trampoline entries using ->ret_addr. (I think this would be
> correct because each task can only be in one context at once, and in
> order to get to a particular kretprobe all of your caller kretprobes
> were inserted before you and any sibling or later kretprobe_instances
> will have been removed. But I might be insanely wrong on this front.)

yeah, you are right. 

> 
> However (as I noted in the other thread), there is a problem where
> kretprobe_trampoline actually stops the unwinder in its tracks and thus
> you only get the first kretprobe_trampoline. This is something I'm going
> to look into some more (despite not having made progress on it last
> time) since now it's something that actually needs to be fixed (and
> as I mentioned in the other thread, show_stack() actually works on x86
> in this context unlike the other stack_trace users).

I should read the unwinder code, but anyway, fixing it up in kretprobe
handler context is not hard. Since each instance is on an hlist, so when
we hit the kretprobe_trampoline, we can search it. However, the problem
is the case where the out of kretprobe handler context. In that context
we need to try to lock the hlist and search the list, which will be a
costful operation.

On the other hand, func-graph tracer introduces a shadow return address
stack for each task (thread), and when we hit its trampoline on the stack,
we can easily search the entry from "current" task without locking the
shadow stack (and we already did it). This may need to pay a cost (1 page)
for each task, but smarter than kretprobe, which makes a system-wide 
hash-table and need to search from hlist which has return addresses
of other context coexist.

Thank you,
kbuild test robot Nov. 2, 2018, 4:01 a.m. UTC | #5
Hi Aleksa,

Thank you for the patch! Yet something to improve:

[auto build test ERROR on tip/perf/core]
[also build test ERROR on v4.19 next-20181101]
[if your patch is applied to the wrong git tree, please drop us a note to help improve the system]

url:    https://github.com/0day-ci/linux/commits/Aleksa-Sarai/kretprobe-produce-sane-stack-traces/20181102-034111
config: i386-randconfig-h1-11021006 (attached as .config)
compiler: gcc-4.9 (Debian 4.9.4-2) 4.9.4
reproduce:
        # save the attached .config to linux build tree
        make ARCH=i386 

All errors (new ones prefixed by >>):

   kernel/kprobes.c: In function 'pre_handler_kretprobe':
   kernel/kprobes.c:1846:10: error: variable 'trace' has initializer but incomplete type
      struct stack_trace trace = {};
             ^
   kernel/kprobes.c:1846:22: error: storage size of 'trace' isn't known
      struct stack_trace trace = {};
                         ^
>> kernel/kprobes.c:1858:3: error: implicit declaration of function 'save_stack_trace_regs' [-Werror=implicit-function-declaration]
      save_stack_trace_regs(regs, &trace);
      ^
   kernel/kprobes.c:1846:22: warning: unused variable 'trace' [-Wunused-variable]
      struct stack_trace trace = {};
                         ^
   kernel/kprobes.c: In function 'kretprobe_save_stack_trace':
>> kernel/kprobes.c:1922:16: error: dereferencing pointer to incomplete type
     for (i = trace->skip; i < krt->nr_entries; i++) {
                   ^
   kernel/kprobes.c:1923:12: error: dereferencing pointer to incomplete type
      if (trace->nr_entries >= trace->max_entries)
               ^
   kernel/kprobes.c:1923:33: error: dereferencing pointer to incomplete type
      if (trace->nr_entries >= trace->max_entries)
                                    ^
   kernel/kprobes.c:1925:8: error: dereferencing pointer to incomplete type
      trace->entries[trace->nr_entries++] = krt->entries[i];
           ^
   kernel/kprobes.c:1925:23: error: dereferencing pointer to incomplete type
      trace->entries[trace->nr_entries++] = krt->entries[i];
                          ^
   cc1: some warnings being treated as errors

vim +/save_stack_trace_regs +1858 kernel/kprobes.c

  1819	
  1820	#ifdef CONFIG_KRETPROBES
  1821	/*
  1822	 * This kprobe pre_handler is registered with every kretprobe. When probe
  1823	 * hits it will set up the return probe.
  1824	 */
  1825	static int pre_handler_kretprobe(struct kprobe *p, struct pt_regs *regs)
  1826	{
  1827		struct kretprobe *rp = container_of(p, struct kretprobe, kp);
  1828		unsigned long hash, flags = 0;
  1829		struct kretprobe_instance *ri;
  1830	
  1831		/*
  1832		 * To avoid deadlocks, prohibit return probing in NMI contexts,
  1833		 * just skip the probe and increase the (inexact) 'nmissed'
  1834		 * statistical counter, so that the user is informed that
  1835		 * something happened:
  1836		 */
  1837		if (unlikely(in_nmi())) {
  1838			rp->nmissed++;
  1839			return 0;
  1840		}
  1841	
  1842		/* TODO: consider to only swap the RA after the last pre_handler fired */
  1843		hash = hash_ptr(current, KPROBE_HASH_BITS);
  1844		raw_spin_lock_irqsave(&rp->lock, flags);
  1845		if (!hlist_empty(&rp->free_instances)) {
> 1846			struct stack_trace trace = {};
  1847	
  1848			ri = hlist_entry(rp->free_instances.first,
  1849					struct kretprobe_instance, hlist);
  1850			hlist_del(&ri->hlist);
  1851			raw_spin_unlock_irqrestore(&rp->lock, flags);
  1852	
  1853			ri->rp = rp;
  1854			ri->task = current;
  1855	
  1856			trace.entries = &ri->entry.entries[0];
  1857			trace.max_entries = KRETPROBE_TRACE_SIZE;
> 1858			save_stack_trace_regs(regs, &trace);
  1859			ri->entry.nr_entries = trace.nr_entries;
  1860	
  1861			if (rp->entry_handler && rp->entry_handler(ri, regs)) {
  1862				raw_spin_lock_irqsave(&rp->lock, flags);
  1863				hlist_add_head(&ri->hlist, &rp->free_instances);
  1864				raw_spin_unlock_irqrestore(&rp->lock, flags);
  1865				return 0;
  1866			}
  1867	
  1868			arch_prepare_kretprobe(ri, regs);
  1869	
  1870			/* XXX(hch): why is there no hlist_move_head? */
  1871			INIT_HLIST_NODE(&ri->hlist);
  1872			kretprobe_table_lock(hash, &flags);
  1873			hlist_add_head(&ri->hlist, &kretprobe_inst_table[hash]);
  1874			kretprobe_table_unlock(hash, &flags);
  1875		} else {
  1876			rp->nmissed++;
  1877			raw_spin_unlock_irqrestore(&rp->lock, flags);
  1878		}
  1879		return 0;
  1880	}
  1881	NOKPROBE_SYMBOL(pre_handler_kretprobe);
  1882	
  1883	/*
  1884	 * Return the kretprobe_instance associated with the current_kprobe. Calling
  1885	 * this is only reasonable from within a kretprobe handler context (otherwise
  1886	 * return NULL).
  1887	 *
  1888	 * Must be called within a kretprobe_hash_lock(current, ...) context.
  1889	 */
  1890	struct kretprobe_instance *current_kretprobe_instance(void)
  1891	{
  1892		struct kprobe *kp;
  1893		struct kretprobe *rp;
  1894		struct kretprobe_instance *ri;
  1895		struct hlist_head *head;
  1896		unsigned long hash = hash_ptr(current, KPROBE_HASH_BITS);
  1897	
  1898		kp = kprobe_running();
  1899		if (!kp || !kprobe_is_retprobe(kp))
  1900			return NULL;
  1901		if (WARN_ON(!kretprobe_hash_is_locked(current)))
  1902			return NULL;
  1903	
  1904		rp = container_of(kp, struct kretprobe, kp);
  1905		head = &kretprobe_inst_table[hash];
  1906	
  1907		hlist_for_each_entry(ri, head, hlist) {
  1908			if (ri->task == current && ri->rp == rp)
  1909				return ri;
  1910		}
  1911		return NULL;
  1912	}
  1913	EXPORT_SYMBOL_GPL(current_kretprobe_instance);
  1914	NOKPROBE_SYMBOL(current_kretprobe_instance);
  1915	
  1916	void kretprobe_save_stack_trace(struct kretprobe_instance *ri,
  1917					struct stack_trace *trace)
  1918	{
  1919		int i;
  1920		struct kretprobe_trace *krt = &ri->entry;
  1921	
> 1922		for (i = trace->skip; i < krt->nr_entries; i++) {
  1923			if (trace->nr_entries >= trace->max_entries)
  1924				break;
  1925			trace->entries[trace->nr_entries++] = krt->entries[i];
  1926		}
  1927	}
  1928	

---
0-DAY kernel test infrastructure                Open Source Technology Center
https://lists.01.org/pipermail/kbuild-all                   Intel Corporation
Aleksa Sarai Nov. 2, 2018, 4:37 a.m. UTC | #6
On 2018-11-02, Masami Hiramatsu <mhiramat@kernel.org> wrote:
> On Fri, 2 Nov 2018 08:13:43 +1100
> Aleksa Sarai <cyphar@cyphar.com> wrote:
> 
> > On 2018-11-02, Masami Hiramatsu <mhiramat@kernel.org> wrote:
> > > Please split the test case as an independent patch.
> > 
> > Will do. Should the Documentation/ change also be a separate patch?
> 
> I think the Documentation change can be coupled with code change
> if the change is small. But selftests is different, that can be
> backported soon for testing the stable kernels.
> 
> 
> > > > new file mode 100644
> > > > index 000000000000..03146c6a1a3c
> > > > --- /dev/null
> > > > +++ b/tools/testing/selftests/ftrace/test.d/kprobe/kretprobe_stacktrace.tc
> > > > @@ -0,0 +1,25 @@
> > > > +#!/bin/sh
> > > > +# SPDX-License-Identifier: GPL-2.0+
> > > > +# description: Kretprobe dynamic event with a stacktrace
> > > > +
> > > > +[ -f kprobe_events ] || exit_unsupported # this is configurable
> > > > +
> > > > +echo 0 > events/enable
> > > > +echo 1 > options/stacktrace
> > > > +
> > > > +echo 'r:teststackprobe sched_fork $retval' > kprobe_events
> > > > +grep teststackprobe kprobe_events
> > > > +test -d events/kprobes/teststackprobe
> > > 
> > > Hmm, what happen if we have 2 or more kretprobes on same stack?
> > > It seems you just save stack in pre_handler, but that stack can already
> > > includes another kretprobe's trampline address...
> > 
> > Yeah, you're quite right...
> > 
> > My first instinct was to do something awful like iterate over the set of
> > "kretprobe_instance"s with ->task == current, and then correct
> > kretprobe_trampoline entries using ->ret_addr. (I think this would be
> > correct because each task can only be in one context at once, and in
> > order to get to a particular kretprobe all of your caller kretprobes
> > were inserted before you and any sibling or later kretprobe_instances
> > will have been removed. But I might be insanely wrong on this front.)
> 
> yeah, you are right. 
> 
> > 
> > However (as I noted in the other thread), there is a problem where
> > kretprobe_trampoline actually stops the unwinder in its tracks and thus
> > you only get the first kretprobe_trampoline. This is something I'm going
> > to look into some more (despite not having made progress on it last
> > time) since now it's something that actually needs to be fixed (and
> > as I mentioned in the other thread, show_stack() actually works on x86
> > in this context unlike the other stack_trace users).
> 
> I should read the unwinder code, but anyway, fixing it up in kretprobe
> handler context is not hard. Since each instance is on an hlist, so when
> we hit the kretprobe_trampoline, we can search it.

As in, find the stored stack and merge the two? Interesting idea, though
Steven has shot this down because of the associated cost (I was
considering making it a kprobe flag, but that felt far too dirty).

> However, the problem is the case where the out of kretprobe handler
> context. In that context we need to try to lock the hlist and search
> the list, which will be a costful operation.

I think the best solution would be to unify all of the kretprobe-like
things so that we don't need to handle non-kretprobe contexts for
basically the same underlying idea. If we wanted to do it like this.

I think a potentially better option would be to just fix the unwinder to
correctly handle kretprobes (like it handles ftrace today).

> On the other hand, func-graph tracer introduces a shadow return address
> stack for each task (thread), and when we hit its trampoline on the stack,
> we can easily search the entry from "current" task without locking the
> shadow stack (and we already did it). This may need to pay a cost (1 page)
> for each task, but smarter than kretprobe, which makes a system-wide 
> hash-table and need to search from hlist which has return addresses
> of other context coexist.

Probably a silly question (I've been staring at the function_graph code
trying to understand how it handles return addresses -- and I'm really
struggling), is this what ->ret_stack (and ->curr_ret_stack) do?

Can you explain how the .retp handling works, because I'm really missing
how the core arch/ hooks know to pass the correct retp value.
Aleksa Sarai Nov. 2, 2018, 5:05 a.m. UTC | #7
On 2018-11-01, Steven Rostedt <rostedt@goodmis.org> wrote:
> On Thu,  1 Nov 2018 19:35:50 +1100
> Aleksa Sarai <cyphar@cyphar.com> wrote:
> > @@ -1834,6 +1853,11 @@ static int pre_handler_kretprobe(struct kprobe *p, struct pt_regs *regs)
> >  		ri->rp = rp;
> >  		ri->task = current;
> >  
> > +		trace.entries = &ri->entry.entries[0];
> > +		trace.max_entries = KRETPROBE_TRACE_SIZE;
> > +		save_stack_trace_regs(regs, &trace);
> > +		ri->entry.nr_entries = trace.nr_entries;
> > +
> 
> So basically your saving a copy of the stack trace for each probe, for
> something that may not ever be used?
> 
> This adds quite a lot of overhead for something not used often.
> 
> I think the real answer is to fix kretprobes (and I just checked, the
> return call of function graph tracer stack trace doesn't go past the
> return_to_handler function either. It's just that a stack trace was
> never done on the return side).
> 
> The more I look at this patch, the less I like it.

That's more than fair.

There's also an issue that Masami noted -- nested kretprobes don't give
you the full stack trace with this solution since the stack was already
overwritten. I think that the only real option is to fix the unwinder to
work in a kretprobe context -- which is what I'm looking at now.

Unfortunately, I'm having a lot of trouble understanding how the current
ftrace hooking works -- ORC has a couple of ftrace hooks that seem
reasonable on the surface but I don't understand (for instance) how
HAVE_FUNCTION_GRAPH_RET_ADDR_PTR *actually* works. Though your comment
appears to indicate that it doesn't work for stack traces?

For kretprobes I think it would be fairly easy to reconstruct what
landed you into a kretprobe_trampoline by walking the set of
kretprobe_instances (since all new ones are added to the head, you can
get the real return address in-order).

But I still have to figure out what is actually stopping the
save_stack_trace() unwinder that isn't stopping the show_stacks()
unwinder (though the show_stacks() code is more ... liberal with the
degree of certainty it has about the unwind).

Am I barking up the wrong tree?
Aleksa Sarai Nov. 2, 2018, 6:59 a.m. UTC | #8
On 2018-11-02, Aleksa Sarai <cyphar@cyphar.com> wrote:
> For kretprobes I think it would be fairly easy to reconstruct what
> landed you into a kretprobe_trampoline by walking the set of
> kretprobe_instances (since all new ones are added to the head, you can
> get the real return address in-order).
> 
> But I still have to figure out what is actually stopping the
> save_stack_trace() unwinder that isn't stopping the show_stacks()
> unwinder (though the show_stacks() code is more ... liberal with the
> degree of certainty it has about the unwind).

As an aside, I just tested with the frame unwinder and it isn't thrown
off-course by kretprobe_trampoline (though obviously the stack is still
wrong). So I think we just need to hook into the ORC unwinder to get it
to continue skipping up the stack, as well as add the rewriting code for
the stack traces (for all unwinders I guess -- though ideally we should
do this without having to add the same code to every architecture).
Aleksa Sarai Nov. 2, 2018, 7:58 a.m. UTC | #9
On 2018-11-02, Aleksa Sarai <cyphar@cyphar.com> wrote:
> Unfortunately, I'm having a lot of trouble understanding how the current
> ftrace hooking works -- ORC has a couple of ftrace hooks that seem
> reasonable on the surface but I don't understand (for instance) how
> HAVE_FUNCTION_GRAPH_RET_ADDR_PTR *actually* works. Though your comment
> appears to indicate that it doesn't work for stack traces?

Sorry, I just figured out how it works! (To make sure I actually
understand -- retp is a pointer to the place in the stack where the
return address is stored, so it uniquely specifies what each trampoline
actually points to -- this trick could also be used for kretprobes).
Steven Rostedt Nov. 2, 2018, 1:16 p.m. UTC | #10
On Fri, 2 Nov 2018 17:59:32 +1100
Aleksa Sarai <cyphar@cyphar.com> wrote:

> As an aside, I just tested with the frame unwinder and it isn't thrown
> off-course by kretprobe_trampoline (though obviously the stack is still
> wrong). So I think we just need to hook into the ORC unwinder to get it
> to continue skipping up the stack, as well as add the rewriting code for
> the stack traces (for all unwinders I guess -- though ideally we should

I agree that this is the right solution.

> do this without having to add the same code to every architecture).

True, and there's an art to consolidating the code between
architectures.

I'm currently looking at function graph and seeing if I can consolidate
it too. And I'm also trying to get multiple uses to hook into its
infrastructure. I think I finally figured out a way to do so.

The reason it is difficult, is that you need to maintain state between
the entry of a function and the exit for each task and callback that is
registered. Hence, it's a 3x tuple (function stack, task, callbacks).
And this must be maintained with preemption. A task may sleep for
minutes, and the state needs to be retained.

The only state that must be retained is the function stack with the
task, because if that gets out of sync, the system crashes. But the
callback state can be removed.

Here's what is there now:

 When something is registered with the function graph tracer, every
 task gets a shadowed stack. A hook is added to fork to add shadow
 stacks to new tasks. Once a shadow stack is added to a task, that
 shadow stack is never removed until the task exits.

 When the function is entered, the real return code is stored in the
 shadow stack and the trampoline address is put in its place.

 On return, the trampoline is called, and it will pop off the return
 code from the shadow stack and return to that.

The issue with multiple users, is that different users may want to
trace different functions. On entry, the user could say it doesn't want
to trace the current function, and the return part must not be called
on exit. Keeping track of which user needs the return called is the
tricky part.

Here's what I plan on implementing:

 Along with a shadow stack, I was going to add a 4096 byte (one page)
 array that holds 64 8 byte masks to every task as well. This will allow
 64 simultaneous users (which is rather extreme). If we need to support
 more, we could allocate another page for all tasks. The 8 byte mask
 will represent each depth (allowing to do this for 64 function call
 stack depth, which should also be enough).

 Each user will be assigned one of the masks. Each bit in the mask
 represents the depth of the shadow stack. When a function is called,
 each user registered with the function graph tracer will get called
 (if they asked to be called for this function, via the ftrace_ops
 hashes) and if they want to trace the function, then the bit is set in
 the mask for that stack depth.

 When the function exits the function and we pop off the return code
 from the shadow stack, we then look at all the bits set for the
 corresponding users, and call their return callbacks, and ignore
 anything that is not set.


When a user is unregistered, it the corresponding bits that represent
it are cleared, and it the return callback will not be called. But the
tasks being traced will still have their shadow stack to allow it to
get back to normal.

I'll hopefully have a prototype ready by plumbers.

And this too will require each architecture to probably change. As a
side project to this, I'm going to try to consolidate the function
graph code among all the architectures as well. Not an easy task.

-- Steve
Josh Poimboeuf Nov. 2, 2018, 3:43 p.m. UTC | #11
On Fri, Nov 02, 2018 at 09:16:58AM -0400, Steven Rostedt wrote:
> On Fri, 2 Nov 2018 17:59:32 +1100
> Aleksa Sarai <cyphar@cyphar.com> wrote:
> 
> > As an aside, I just tested with the frame unwinder and it isn't thrown
> > off-course by kretprobe_trampoline (though obviously the stack is still
> > wrong). So I think we just need to hook into the ORC unwinder to get it
> > to continue skipping up the stack, as well as add the rewriting code for
> > the stack traces (for all unwinders I guess -- though ideally we should
> 
> I agree that this is the right solution.

Sounds good to me.

However, it would be *really* nice if function graph and kretprobes
shared the same infrastructure, like they do for function entry.
There's a lot of duplicated effort there.

> > do this without having to add the same code to every architecture).
> 
> True, and there's an art to consolidating the code between
> architectures.
> 
> I'm currently looking at function graph and seeing if I can consolidate
> it too. And I'm also trying to get multiple uses to hook into its
> infrastructure. I think I finally figured out a way to do so.
> 
> The reason it is difficult, is that you need to maintain state between
> the entry of a function and the exit for each task and callback that is
> registered. Hence, it's a 3x tuple (function stack, task, callbacks).
> And this must be maintained with preemption. A task may sleep for
> minutes, and the state needs to be retained.
> 
> The only state that must be retained is the function stack with the
> task, because if that gets out of sync, the system crashes. But the
> callback state can be removed.
> 
> Here's what is there now:
> 
>  When something is registered with the function graph tracer, every
>  task gets a shadowed stack. A hook is added to fork to add shadow
>  stacks to new tasks. Once a shadow stack is added to a task, that
>  shadow stack is never removed until the task exits.
> 
>  When the function is entered, the real return code is stored in the
>  shadow stack and the trampoline address is put in its place.
> 
>  On return, the trampoline is called, and it will pop off the return
>  code from the shadow stack and return to that.
> 
> The issue with multiple users, is that different users may want to
> trace different functions. On entry, the user could say it doesn't want
> to trace the current function, and the return part must not be called
> on exit. Keeping track of which user needs the return called is the
> tricky part.
> 
> Here's what I plan on implementing:
> 
>  Along with a shadow stack, I was going to add a 4096 byte (one page)
>  array that holds 64 8 byte masks to every task as well. This will allow
>  64 simultaneous users (which is rather extreme). If we need to support
>  more, we could allocate another page for all tasks. The 8 byte mask
>  will represent each depth (allowing to do this for 64 function call
>  stack depth, which should also be enough).
> 
>  Each user will be assigned one of the masks. Each bit in the mask
>  represents the depth of the shadow stack. When a function is called,
>  each user registered with the function graph tracer will get called
>  (if they asked to be called for this function, via the ftrace_ops
>  hashes) and if they want to trace the function, then the bit is set in
>  the mask for that stack depth.
> 
>  When the function exits the function and we pop off the return code
>  from the shadow stack, we then look at all the bits set for the
>  corresponding users, and call their return callbacks, and ignore
>  anything that is not set.
> 
> 
> When a user is unregistered, it the corresponding bits that represent
> it are cleared, and it the return callback will not be called. But the
> tasks being traced will still have their shadow stack to allow it to
> get back to normal.
> 
> I'll hopefully have a prototype ready by plumbers.

Why do we need multiple users?  It would be a lot simpler if we could
just enforce a single user per fgraphed/kretprobed function (and return
-EBUSY if it's already being traced/probed).

> And this too will require each architecture to probably change. As a
> side project to this, I'm going to try to consolidate the function
> graph code among all the architectures as well. Not an easy task.

Do you mean implementing HAVE_FUNCTION_GRAPH_RET_ADDR_PTR for all the
arches?  If so, I think have an old crusty patch which attempted to
that.  I could try to dig it up if you're interested.
Steven Rostedt Nov. 2, 2018, 4:13 p.m. UTC | #12
On Fri, 2 Nov 2018 10:43:26 -0500
Josh Poimboeuf <jpoimboe@redhat.com> wrote:

> > I'll hopefully have a prototype ready by plumbers.  
> 
> Why do we need multiple users?  It would be a lot simpler if we could
> just enforce a single user per fgraphed/kretprobed function (and return
> -EBUSY if it's already being traced/probed).

Because that means if function graph tracer is active, then you can't
do a kretprobe, and vice versa. I'd really like to have it working for
multiple users, then we could trace different graph functions and store
them in different buffers. It would also allow for perf to use function
graph tracer too.

> 
> > And this too will require each architecture to probably change. As a
> > side project to this, I'm going to try to consolidate the function
> > graph code among all the architectures as well. Not an easy task.  
> 
> Do you mean implementing HAVE_FUNCTION_GRAPH_RET_ADDR_PTR for all the
> arches?  If so, I think have an old crusty patch which attempted to
> that.  I could try to dig it up if you're interested.
> 

I'd like to have that, but it still requires some work. But I'd just
the truly architecture dependent code be in the architecture (basically
the asm code), and have the ability to move most of the duplicate code
out of the archs.

-- Steve
Aleksa Sarai Nov. 3, 2018, 7:02 a.m. UTC | #13
On 2018-11-02, Steven Rostedt <rostedt@goodmis.org> wrote:
> > As an aside, I just tested with the frame unwinder and it isn't thrown
> > off-course by kretprobe_trampoline (though obviously the stack is still
> > wrong). So I think we just need to hook into the ORC unwinder to get it
> > to continue skipping up the stack, as well as add the rewriting code for
> > the stack traces (for all unwinders I guess -- though ideally we should
> 
> I agree that this is the right solution.
> 
> > do this without having to add the same code to every architecture).
> 
> True, and there's an art to consolidating the code between
> architectures.
> 
> I'm currently looking at function graph and seeing if I can consolidate
> it too. And I'm also trying to get multiple uses to hook into its
> infrastructure. I think I finally figured out a way to do so.
> 
> The reason it is difficult, is that you need to maintain state between
> the entry of a function and the exit for each task and callback that is
> registered. Hence, it's a 3x tuple (function stack, task, callbacks).
> And this must be maintained with preemption. A task may sleep for
> minutes, and the state needs to be retained.
> 
> The only state that must be retained is the function stack with the
> task, because if that gets out of sync, the system crashes. But the
> callback state can be removed.
> 
> Here's what is there now:
> 
>  When something is registered with the function graph tracer, every
>  task gets a shadowed stack. A hook is added to fork to add shadow
>  stacks to new tasks. Once a shadow stack is added to a task, that
>  shadow stack is never removed until the task exits.
> 
>  When the function is entered, the real return code is stored in the
>  shadow stack and the trampoline address is put in its place.
> 
>  On return, the trampoline is called, and it will pop off the return
>  code from the shadow stack and return to that.

I was playing with a toy version of this using the existing kretprobe
architecture (by borrowing the shadow stack concept of storing the
stack_addr() -- though it's actually stored inside the existing
kretprobe_instance and thus doesn't need separate push-pop code).

The rewriting of kretprobe_trampoline in the stack unwinder *works* on
x86 now except for the regs handling (the current function is still
incorrectly shown as kretprobe_trampoline). This is actually a general
bug in ftrace as well, because (for instance) while the unwinder calls
into ftrace_graph_ret_addr() while walking up the stack this isn't used
to correct regs->ip in perf_callchain_kernel(). I think this is the
cause of the bug in ftrace-graph (if you switch to the "frame" unwinder
you might be able to see it more clearly) -- but when trying to fix it
I'm having trouble figuring out what retp should be (stack_addr(regs)
doesn't give the right result for some reason).

I will try to fix it up and attach it, but if you're already working on
a prototype the unifies all the users that works too. The patch I have
at the moment duplicates each of the key ftrace_graph_return_addr
invocations with a matching kretprobe_return_addr (though there's only
three of these).
Masami Hiramatsu Nov. 3, 2018, 12:47 p.m. UTC | #14
On Fri, 2 Nov 2018 15:37:33 +1100
Aleksa Sarai <cyphar@cyphar.com> wrote:

> On 2018-11-02, Masami Hiramatsu <mhiramat@kernel.org> wrote:
> > On Fri, 2 Nov 2018 08:13:43 +1100
> > Aleksa Sarai <cyphar@cyphar.com> wrote:
> > 
> > > On 2018-11-02, Masami Hiramatsu <mhiramat@kernel.org> wrote:
> > > > Please split the test case as an independent patch.
> > > 
> > > Will do. Should the Documentation/ change also be a separate patch?
> > 
> > I think the Documentation change can be coupled with code change
> > if the change is small. But selftests is different, that can be
> > backported soon for testing the stable kernels.
> > 
> > 
> > > > > new file mode 100644
> > > > > index 000000000000..03146c6a1a3c
> > > > > --- /dev/null
> > > > > +++ b/tools/testing/selftests/ftrace/test.d/kprobe/kretprobe_stacktrace.tc
> > > > > @@ -0,0 +1,25 @@
> > > > > +#!/bin/sh
> > > > > +# SPDX-License-Identifier: GPL-2.0+
> > > > > +# description: Kretprobe dynamic event with a stacktrace
> > > > > +
> > > > > +[ -f kprobe_events ] || exit_unsupported # this is configurable
> > > > > +
> > > > > +echo 0 > events/enable
> > > > > +echo 1 > options/stacktrace
> > > > > +
> > > > > +echo 'r:teststackprobe sched_fork $retval' > kprobe_events
> > > > > +grep teststackprobe kprobe_events
> > > > > +test -d events/kprobes/teststackprobe
> > > > 
> > > > Hmm, what happen if we have 2 or more kretprobes on same stack?
> > > > It seems you just save stack in pre_handler, but that stack can already
> > > > includes another kretprobe's trampline address...
> > > 
> > > Yeah, you're quite right...
> > > 
> > > My first instinct was to do something awful like iterate over the set of
> > > "kretprobe_instance"s with ->task == current, and then correct
> > > kretprobe_trampoline entries using ->ret_addr. (I think this would be
> > > correct because each task can only be in one context at once, and in
> > > order to get to a particular kretprobe all of your caller kretprobes
> > > were inserted before you and any sibling or later kretprobe_instances
> > > will have been removed. But I might be insanely wrong on this front.)
> > 
> > yeah, you are right. 
> > 
> > > 
> > > However (as I noted in the other thread), there is a problem where
> > > kretprobe_trampoline actually stops the unwinder in its tracks and thus
> > > you only get the first kretprobe_trampoline. This is something I'm going
> > > to look into some more (despite not having made progress on it last
> > > time) since now it's something that actually needs to be fixed (and
> > > as I mentioned in the other thread, show_stack() actually works on x86
> > > in this context unlike the other stack_trace users).
> > 
> > I should read the unwinder code, but anyway, fixing it up in kretprobe
> > handler context is not hard. Since each instance is on an hlist, so when
> > we hit the kretprobe_trampoline, we can search it.
> 
> As in, find the stored stack and merge the two? Interesting idea, though
> Steven has shot this down because of the associated cost (I was
> considering making it a kprobe flag, but that felt far too dirty).

I think we don't need the flag, we just need to register a trampoline and
the trampoline must restore(just pop) the original address from shadow
stack.
Also the stack unwinder query restored address from the stack if it
hits the unfamilier address (or registered address).

> > However, the problem is the case where the out of kretprobe handler
> > context. In that context we need to try to lock the hlist and search
> > the list, which will be a costful operation.
> 
> I think the best solution would be to unify all of the kretprobe-like
> things so that we don't need to handle non-kretprobe contexts for
> basically the same underlying idea. If we wanted to do it like this.

Yes, anyway, current kretprobe instance idea is not good, it requires
locks, and user must specify "enough amount of instances" for each
kretprobe.

> I think a potentially better option would be to just fix the unwinder to
> correctly handle kretprobes (like it handles ftrace today).

I think both can be fixed by same way.

> 
> > On the other hand, func-graph tracer introduces a shadow return address
> > stack for each task (thread), and when we hit its trampoline on the stack,
> > we can easily search the entry from "current" task without locking the
> > shadow stack (and we already did it). This may need to pay a cost (1 page)
> > for each task, but smarter than kretprobe, which makes a system-wide 
> > hash-table and need to search from hlist which has return addresses
> > of other context coexist.
> 
> Probably a silly question (I've been staring at the function_graph code
> trying to understand how it handles return addresses -- and I'm really
> struggling), is this what ->ret_stack (and ->curr_ret_stack) do?

Yes.

> 
> Can you explain how the .retp handling works, because I'm really missing
> how the core arch/ hooks know to pass the correct retp value.

Sure, the current->ret_stack can be an hidden stack array, it will save the original
return address, function address and the modified address for each entry.
(Kretprobe can be searched by function address.)

Stack unwinder will check the unwinded stack entry on comparing with
current->ret_stack[i].retp and if it is same, it can find the original return
address from ret_stack[i].ret.

This should make things simpler and solve in generic way.

Thank you,
Masami Hiramatsu Nov. 3, 2018, 1 p.m. UTC | #15
On Fri, 2 Nov 2018 12:13:07 -0400
Steven Rostedt <rostedt@goodmis.org> wrote:

> On Fri, 2 Nov 2018 10:43:26 -0500
> Josh Poimboeuf <jpoimboe@redhat.com> wrote:
> 
> > > I'll hopefully have a prototype ready by plumbers.  
> > 
> > Why do we need multiple users?  It would be a lot simpler if we could
> > just enforce a single user per fgraphed/kretprobed function (and return
> > -EBUSY if it's already being traced/probed).
> 
> Because that means if function graph tracer is active, then you can't
> do a kretprobe, and vice versa. I'd really like to have it working for
> multiple users, then we could trace different graph functions and store
> them in different buffers. It would also allow for perf to use function
> graph tracer too.

Steve, how woul you allow multiple users on it? Something like this?

ret_trampoline_multiple(){
   list_for_each(handler, &shadow_entry[i].handlers, list)
	handler(shadow_entry[i]);
   restore_retval_and_jump_to(shadow_entry[i].orig);
}


> > > And this too will require each architecture to probably change. As a
> > > side project to this, I'm going to try to consolidate the function
> > > graph code among all the architectures as well. Not an easy task.  
> > 
> > Do you mean implementing HAVE_FUNCTION_GRAPH_RET_ADDR_PTR for all the
> > arches?  If so, I think have an old crusty patch which attempted to
> > that.  I could try to dig it up if you're interested.
> > 
> 
> I'd like to have that, but it still requires some work. But I'd just
> the truly architecture dependent code be in the architecture (basically
> the asm code), and have the ability to move most of the duplicate code
> out of the archs.

I will also do that for kretprobe handlers.

Thank you,
Steven Rostedt Nov. 3, 2018, 1:13 p.m. UTC | #16
On Sat, 3 Nov 2018 22:00:12 +0900
Masami Hiramatsu <mhiramat@kernel.org> wrote:

> On Fri, 2 Nov 2018 12:13:07 -0400
> Steven Rostedt <rostedt@goodmis.org> wrote:
> 

> > Because that means if function graph tracer is active, then you can't
> > do a kretprobe, and vice versa. I'd really like to have it working for
> > multiple users, then we could trace different graph functions and store
> > them in different buffers. It would also allow for perf to use function
> > graph tracer too.  
> 
> Steve, how woul you allow multiple users on it? Something like this?
> 
> ret_trampoline_multiple(){
>    list_for_each(handler, &shadow_entry[i].handlers, list)
> 	handler(shadow_entry[i]);
>    restore_retval_and_jump_to(shadow_entry[i].orig);
> }
>

Something like that. But since it's not a single mapping between shadow
entries and handlers, that is we have multiple tasks with multiple
shadow entries and multiple handlers, we can't use a link list (two
different parents per handler).

I was thinking of a bitmask that represents the handlers, and use that
to map which handler gets called for which shadow entry for a
particular task.

-- Steve
Masami Hiramatsu Nov. 3, 2018, 1:23 p.m. UTC | #17
On Fri, 2 Nov 2018 09:16:58 -0400
Steven Rostedt <rostedt@goodmis.org> wrote:

> On Fri, 2 Nov 2018 17:59:32 +1100
> Aleksa Sarai <cyphar@cyphar.com> wrote:
> 
> > As an aside, I just tested with the frame unwinder and it isn't thrown
> > off-course by kretprobe_trampoline (though obviously the stack is still
> > wrong). So I think we just need to hook into the ORC unwinder to get it
> > to continue skipping up the stack, as well as add the rewriting code for
> > the stack traces (for all unwinders I guess -- though ideally we should
> 
> I agree that this is the right solution.
> 
> > do this without having to add the same code to every architecture).
> 
> True, and there's an art to consolidating the code between
> architectures.
> 
> I'm currently looking at function graph and seeing if I can consolidate
> it too. And I'm also trying to get multiple uses to hook into its
> infrastructure. I think I finally figured out a way to do so.

For supporting multiple users without any memory allocation, I think
each user should consume the shadow stack and store on it.
My old generic retstack implementation did that.

https://github.com/mhiramat/linux/commit/8804f76580cd863d555854b41b9c6df719f8087e

I hope this may give you any insites.
My idea is to generalize shadow stack, not func graph tracer, since
I don't like making kretprobe depends on func graph tracer, but only
the shadow stack.

> 
> The reason it is difficult, is that you need to maintain state between
> the entry of a function and the exit for each task and callback that is
> registered. Hence, it's a 3x tuple (function stack, task, callbacks).
> And this must be maintained with preemption. A task may sleep for
> minutes, and the state needs to be retained.

Would you mean preeempt_disable()? Anyway, we just need to increment index
atomically, don't we?

> The only state that must be retained is the function stack with the
> task, because if that gets out of sync, the system crashes. But the
> callback state can be removed.
> 
> Here's what is there now:
> 
>  When something is registered with the function graph tracer, every
>  task gets a shadowed stack. A hook is added to fork to add shadow
>  stacks to new tasks. Once a shadow stack is added to a task, that
>  shadow stack is never removed until the task exits.
> 
>  When the function is entered, the real return code is stored in the
>  shadow stack and the trampoline address is put in its place.
> 
>  On return, the trampoline is called, and it will pop off the return
>  code from the shadow stack and return to that.
> 
> The issue with multiple users, is that different users may want to
> trace different functions. On entry, the user could say it doesn't want
> to trace the current function, and the return part must not be called
> on exit. Keeping track of which user needs the return called is the
> tricky part.

So that I think only the "shadow stack" part should be generalized.

> 
> Here's what I plan on implementing:
> 
>  Along with a shadow stack, I was going to add a 4096 byte (one page)
>  array that holds 64 8 byte masks to every task as well. This will allow
>  64 simultaneous users (which is rather extreme). If we need to support
>  more, we could allocate another page for all tasks. The 8 byte mask
>  will represent each depth (allowing to do this for 64 function call
>  stack depth, which should also be enough).
> 
>  Each user will be assigned one of the masks. Each bit in the mask
>  represents the depth of the shadow stack. When a function is called,
>  each user registered with the function graph tracer will get called
>  (if they asked to be called for this function, via the ftrace_ops
>  hashes) and if they want to trace the function, then the bit is set in
>  the mask for that stack depth.
> 
>  When the function exits the function and we pop off the return code
>  from the shadow stack, we then look at all the bits set for the
>  corresponding users, and call their return callbacks, and ignore
>  anything that is not set.
> 

It sounds too complicated... why we don't just open the shadow stack for
each user? Of course it may requires a bit "repeat" unwind on the shadow
stack, but it is simple.

Thank you,

> When a user is unregistered, it the corresponding bits that represent
> it are cleared, and it the return callback will not be called. But the
> tasks being traced will still have their shadow stack to allow it to
> get back to normal.
> 
> I'll hopefully have a prototype ready by plumbers.
> 
> And this too will require each architecture to probably change. As a
> side project to this, I'm going to try to consolidate the function
> graph code among all the architectures as well. Not an easy task.
> 
> -- Steve
Masami Hiramatsu Nov. 3, 2018, 4:34 p.m. UTC | #18
On Sat, 3 Nov 2018 09:13:41 -0400
Steven Rostedt <rostedt@goodmis.org> wrote:

> On Sat, 3 Nov 2018 22:00:12 +0900
> Masami Hiramatsu <mhiramat@kernel.org> wrote:
> 
> > On Fri, 2 Nov 2018 12:13:07 -0400
> > Steven Rostedt <rostedt@goodmis.org> wrote:
> > 
> 
> > > Because that means if function graph tracer is active, then you can't
> > > do a kretprobe, and vice versa. I'd really like to have it working for
> > > multiple users, then we could trace different graph functions and store
> > > them in different buffers. It would also allow for perf to use function
> > > graph tracer too.  
> > 
> > Steve, how woul you allow multiple users on it? Something like this?
> > 
> > ret_trampoline_multiple(){
> >    list_for_each(handler, &shadow_entry[i].handlers, list)
> > 	handler(shadow_entry[i]);
> >    restore_retval_and_jump_to(shadow_entry[i].orig);
> > }
> >
> 
> Something like that. But since it's not a single mapping between shadow
> entries and handlers, that is we have multiple tasks with multiple
> shadow entries and multiple handlers, we can't use a link list (two
> different parents per handler).

Yes, I understand it.

> I was thinking of a bitmask that represents the handlers, and use that
> to map which handler gets called for which shadow entry for a
> particular task.

Hmm, I doubt that is too complicated and not scalable. I rather like to see
the open shadow entry...

entry: [[original_retaddr][function][modified_retaddr]]

So if there are many users on same function, the entries will be like this 

[[original_return_address][function][trampoline_A]]
[[trampline_A][function][trampoline_B]]
[[trampline_B][function][trampoline_C]]

And on the top of the stack, there is trampline_C instead of original_return_address.
In this case, return to trampoline_C(), it jumps back to trampline_B() and then
it jumps back to trampline_A(). And eventually it jumps back to
original_return_address.

This way, we don't need allocate another bitmap/pages for the shadow stack.
We only need a shadow stack for each task.
Also, unwinder can easily find the trampline_C from the shadow stack and restores
original_return_address. (of course trampline_A,B,C must be registered so that
search function can skip it.)

Thank you,
Steven Rostedt Nov. 3, 2018, 5:30 p.m. UTC | #19
On Sun, 4 Nov 2018 01:34:30 +0900
Masami Hiramatsu <mhiramat@kernel.org> wrote:
> 
> > I was thinking of a bitmask that represents the handlers, and use that
> > to map which handler gets called for which shadow entry for a
> > particular task.  
> 
> Hmm, I doubt that is too complicated and not scalable. I rather like to see
> the open shadow entry...

It can scale and not too complex (I already played a little with it).
But that said, I'm not committed to it, and using the shadow stack is
also an interesting idea.

> 
> entry: [[original_retaddr][function][modified_retaddr]]
> 
> So if there are many users on same function, the entries will be like this 
> 
> [[original_return_address][function][trampoline_A]]
> [[trampline_A][function][trampoline_B]]
> [[trampline_B][function][trampoline_C]]
> 
> And on the top of the stack, there is trampline_C instead of original_return_address.
> In this case, return to trampoline_C(), it jumps back to trampline_B() and then
> it jumps back to trampline_A(). And eventually it jumps back to
> original_return_address.

Where are trampolines A, B, and C made? Do we also need to dynamically
create them? If I register multiple function tracing ones, each one
will need its own trampoline?

> 
> This way, we don't need allocate another bitmap/pages for the shadow stack.
> We only need a shadow stack for each task.
> Also, unwinder can easily find the trampline_C from the shadow stack and restores
> original_return_address. (of course trampline_A,B,C must be registered so that
> search function can skip it.)

What I was thinking was to store a count and the functions to be called:


	[original_return_address]
	[function_A]
	[function_B]
	[function_C]
	[ 3 ]

Then the trampoline that processes the return codes for ftrace (and
kretprobes and everyone else) can simply do:

	count = pop_shadow_stack();
	for (i = 0; i < count; i++) {
		func = pop_shadow_stack();
		func(...);
	}
	return_address = pop_shadow_stack();

That way we only need to register a function to the return handler and
it will be called, without worrying about making trampolines. There
will just be a single trampoline that handles all the work.

-- Steve
Steven Rostedt Nov. 3, 2018, 5:33 p.m. UTC | #20
On Sat, 3 Nov 2018 13:30:21 -0400
Steven Rostedt <rostedt@goodmis.org> wrote:

> What I was thinking was to store a count and the functions to be called:
> 
> 
> 	[original_return_address]
> 	[function_A]
> 	[function_B]
> 	[function_C]
> 	[ 3 ]
> 
> Then the trampoline that processes the return codes for ftrace (and
> kretprobes and everyone else) can simply do:
> 
> 	count = pop_shadow_stack();
> 	for (i = 0; i < count; i++) {
> 		func = pop_shadow_stack();
> 		func(...);
> 	}
> 	return_address = pop_shadow_stack();
> 
> That way we only need to register a function to the return handler and
> it will be called, without worrying about making trampolines. There
> will just be a single trampoline that handles all the work.

And since the most common case is a single function to call, instead of
using a count, we can take advantage that kernel functions are negative
numbers and do:

	[original_return_address]
	[function_A]

	----

	long count;

	count = pop_shadow_stack();
	if (count < 0) {
		func = (void *)count;
		func();
	} else {
		for (i = 0; i < count; i++) {
			[...]

The unwinder will just need to know how to handle all this :-)

-- Steve
Masami Hiramatsu Nov. 4, 2018, 2:25 a.m. UTC | #21
On Sat, 3 Nov 2018 13:30:21 -0400
Steven Rostedt <rostedt@goodmis.org> wrote:

> On Sun, 4 Nov 2018 01:34:30 +0900
> Masami Hiramatsu <mhiramat@kernel.org> wrote:
> > 
> > > I was thinking of a bitmask that represents the handlers, and use that
> > > to map which handler gets called for which shadow entry for a
> > > particular task.  
> > 
> > Hmm, I doubt that is too complicated and not scalable. I rather like to see
> > the open shadow entry...
> 
> It can scale and not too complex (I already played a little with it).
> But that said, I'm not committed to it, and using the shadow stack is
> also an interesting idea.
> 
> > 
> > entry: [[original_retaddr][function][modified_retaddr]]
> > 
> > So if there are many users on same function, the entries will be like this 
> > 
> > [[original_return_address][function][trampoline_A]]
> > [[trampline_A][function][trampoline_B]]
> > [[trampline_B][function][trampoline_C]]
> > 
> > And on the top of the stack, there is trampline_C instead of original_return_address.
> > In this case, return to trampoline_C(), it jumps back to trampline_B() and then
> > it jumps back to trampline_A(). And eventually it jumps back to
> > original_return_address.
> 
> Where are trampolines A, B, and C made? Do we also need to dynamically
> create them? If I register multiple function tracing ones, each one
> will need its own trampoline?
> 

No, I think tramplines are very limited. currently we will only have ftrace
and kretprobe trampolines.


> > This way, we don't need allocate another bitmap/pages for the shadow stack.
> > We only need a shadow stack for each task.
> > Also, unwinder can easily find the trampline_C from the shadow stack and restores
> > original_return_address. (of course trampline_A,B,C must be registered so that
> > search function can skip it.)
> 
> What I was thinking was to store a count and the functions to be called:
> 
> 
> 	[original_return_address]
> 	[function_A]
> 	[function_B]
> 	[function_C]
> 	[ 3 ]
> 
> Then the trampoline that processes the return codes for ftrace (and
> kretprobes and everyone else) can simply do:
> 
> 	count = pop_shadow_stack();
> 	for (i = 0; i < count; i++) {
> 		func = pop_shadow_stack();
> 		func(...);
> 	}
> 	return_address = pop_shadow_stack();

Ah, that's a good idea. I think we also have to store the called function
entry address with the number header, but basically I agree with you.

If we have a space to store a data with the function address, that is also
good to kretprobe. systemtap heavily uses "entry data" for saving some data
at function entry for exit handler.

> That way we only need to register a function to the return handler and
> it will be called, without worrying about making trampolines. There
> will just be a single trampoline that handles all the work.

OK, and could you make it independent from func graph tracer, so that
CONFIG_KPROBES=y but CONFIG_FUNCTION_GRAPH_TRACER=n kernel can support
kretprobes too.

Thank you,
Aleksa Sarai Nov. 4, 2018, 11:59 a.m. UTC | #22
On 2018-11-03, Aleksa Sarai <cyphar@cyphar.com> wrote:
> This is actually a general bug in ftrace as well, because (for
> instance) while the unwinder calls into ftrace_graph_ret_addr() while
> walking up the stack this isn't used to correct regs->ip in
> perf_callchain_kernel(). I think this is the cause of the bug in
> ftrace-graph (if you switch to the "frame" unwinder you might be able
> to see it more clearly) -- but when trying to fix it I'm having
> trouble figuring out what retp should be (stack_addr(regs) doesn't
> give the right result for some reason).

@Steven, @Masami: Can you verify whether this is an actual bug? The
snippet is like (arch/x86/events/core.c):

  void
  perf_callchain_kernel(struct perf_callchain_entry_ctx *entry, struct pt_regs *regs)
  {
    /* ... */
	if (perf_callchain_store(entry, regs->ip))
		return;
	/* rest of unwind code */
  }

And it seems to me that there's an ftrace_graph_ret_addr missing here
(though I was having trouble figuring out what the right retp would be
in this context -- both stack_addr(regs) and &regs->sp appear to be
wrong when I was trying to get this to work with kretprobes -- while my
patch worked with all other kretprobe_trampoline cases in the unwinder).

The same issue is present in __save_stack_trace
(arch/x86/kernel/stacktrace.c). This is likely the only reason that --
as Steven said -- stacktraces wouldn't work with ftrace-graph (and thus
with the refactor both of you are discussing).

Thanks.
Steven Rostedt Nov. 6, 2018, 10:15 p.m. UTC | #23
On Sun, 4 Nov 2018 22:59:13 +1100
Aleksa Sarai <cyphar@cyphar.com> wrote:

> The same issue is present in __save_stack_trace
> (arch/x86/kernel/stacktrace.c). This is likely the only reason that --
> as Steven said -- stacktraces wouldn't work with ftrace-graph (and thus
> with the refactor both of you are discussing).

By the way, I was playing with the the orc unwinder and stack traces
from the function graph tracer return code, and got it working with the
below patch. Caution, that patch also has a stack trace hardcoded in
the return path of the function graph tracer, so you don't want to run
function graph tracing without filtering.

You can apply the patch and do:

 # cd /sys/kernel/debug/tracing
 # echo schedule > set_ftrace_filter
 # echo function_graph > current_tracer
 # cat trace

 3)               |  schedule() {
     rcu_preempt-10    [003] ....    91.160297: <stack trace>
 => ftrace_return_to_handler
 => return_to_handler
 => schedule_timeout
 => rcu_gp_kthread
 => kthread
 => ret_from_fork
 3) # 4009.085 us |  }
 3)               |  schedule() {
     kworker/1:0-17    [001] ....    91.163288: <stack trace>
 => ftrace_return_to_handler
 => return_to_handler
 => worker_thread
 => kthread
 => ret_from_fork
 1) # 7000.070 us |  }
 1)               |  schedule() {
     rcu_preempt-10    [003] ....    91.164311: <stack trace>
 => ftrace_return_to_handler
 => return_to_handler
 => schedule_timeout
 => rcu_gp_kthread
 => kthread
 => ret_from_fork
 3) # 4006.540 us |  }


Where just adding the stack trace without the other code, these traces
ended at "return_to_handler".

This patch is not for inclusion, it was just a test to see how to make
this work.

-- Steve

diff --git a/arch/x86/kernel/ftrace_64.S b/arch/x86/kernel/ftrace_64.S
index 91b2cff4b79a..4bcd646ae1f4 100644
--- a/arch/x86/kernel/ftrace_64.S
+++ b/arch/x86/kernel/ftrace_64.S
@@ -320,13 +320,15 @@ ENTRY(ftrace_graph_caller)
 ENDPROC(ftrace_graph_caller)
 
 ENTRY(return_to_handler)
-	UNWIND_HINT_EMPTY
-	subq  $24, %rsp
+	subq $8, %rsp
+	UNWIND_HINT_FUNC
+	subq  $16, %rsp
 
 	/* Save the return values */
 	movq %rax, (%rsp)
 	movq %rdx, 8(%rsp)
 	movq %rbp, %rdi
+	leaq 16(%rsp), %rsi
 
 	call ftrace_return_to_handler
 
diff --git a/kernel/trace/trace_functions_graph.c b/kernel/trace/trace_functions_graph.c
index 169b3c44ee97..aaeca73218cc 100644
--- a/kernel/trace/trace_functions_graph.c
+++ b/kernel/trace/trace_functions_graph.c
@@ -242,13 +242,16 @@ ftrace_pop_return_trace(struct ftrace_graph_ret *trace, unsigned long *ret,
 	trace->calltime = current->ret_stack[index].calltime;
 	trace->overrun = atomic_read(&current->trace_overrun);
 	trace->depth = index;
+
+	trace_dump_stack(0);
 }
 
 /*
  * Send the trace to the ring-buffer.
  * @return the original return address.
  */
-unsigned long ftrace_return_to_handler(unsigned long frame_pointer)
+unsigned long ftrace_return_to_handler(unsigned long frame_pointer,
+	unsigned long *ptr)
 {
 	struct ftrace_graph_ret trace;
 	unsigned long ret;
@@ -257,6 +260,8 @@ unsigned long ftrace_return_to_handler(unsigned long frame_pointer)
 	trace.rettime = trace_clock_local();
 	barrier();
 	current->curr_ret_stack--;
+	*ptr = ret;
+
 	/*
 	 * The curr_ret_stack can be less than -1 only if it was
 	 * filtered out and it's about to return from the function.
Aleksa Sarai Nov. 8, 2018, 7:46 a.m. UTC | #24
On 2018-11-06, Steven Rostedt <rostedt@goodmis.org> wrote:
> On Sun, 4 Nov 2018 22:59:13 +1100
> Aleksa Sarai <cyphar@cyphar.com> wrote:
> 
> > The same issue is present in __save_stack_trace
> > (arch/x86/kernel/stacktrace.c). This is likely the only reason that --
> > as Steven said -- stacktraces wouldn't work with ftrace-graph (and thus
> > with the refactor both of you are discussing).
> 
> By the way, I was playing with the the orc unwinder and stack traces
> from the function graph tracer return code, and got it working with the
> below patch. Caution, that patch also has a stack trace hardcoded in
> the return path of the function graph tracer, so you don't want to run
> function graph tracing without filtering.

Neat!

> diff --git a/kernel/trace/trace_functions_graph.c b/kernel/trace/trace_functions_graph.c
> index 169b3c44ee97..aaeca73218cc 100644
> --- a/kernel/trace/trace_functions_graph.c
> +++ b/kernel/trace/trace_functions_graph.c
> @@ -242,13 +242,16 @@ ftrace_pop_return_trace(struct ftrace_graph_ret *trace, unsigned long *ret,
>  	trace->calltime = current->ret_stack[index].calltime;
>  	trace->overrun = atomic_read(&current->trace_overrun);
>  	trace->depth = index;
> +
> +	trace_dump_stack(0);

Right, this works because save_stack is not being passed a pt_regs. But if
you pass a pt_regs (as happens with bpf_getstackid -- which is what
spawned this discussion) then the top-most entry of the stack will still
be a trampoline because there is no ftrace_graph_ret_addr call.

(I'm struggling with how to fix this -- I can't figure out what retp
should be if you have a pt_regs. ->sp doesn't appear to work -- it's off
by a few bytes.)

I will attach what I have at the moment to hopefully explain what the
issue I've found is (re-using the kretprobe architecture but with the
shadow-stack idea).
Aleksa Sarai Nov. 8, 2018, 8:04 a.m. UTC | #25
On 2018-11-08, Aleksa Sarai <cyphar@cyphar.com> wrote:
> I will attach what I have at the moment to hopefully explain what the
> issue I've found is (re-using the kretprobe architecture but with the
> shadow-stack idea).

Here is the patch I have at the moment (it works, except for the
question I have about how to handle the top-level pt_regs -- I've marked
that code with XXX).
Josh Poimboeuf Nov. 8, 2018, 2:44 p.m. UTC | #26
On Thu, Nov 08, 2018 at 07:04:48PM +1100, Aleksa Sarai wrote:
> On 2018-11-08, Aleksa Sarai <cyphar@cyphar.com> wrote:
> > I will attach what I have at the moment to hopefully explain what the
> > issue I've found is (re-using the kretprobe architecture but with the
> > shadow-stack idea).
> 
> Here is the patch I have at the moment (it works, except for the
> question I have about how to handle the top-level pt_regs -- I've marked
> that code with XXX).
> 
> -- 
> Aleksa Sarai
> Senior Software Engineer (Containers)
> SUSE Linux GmbH
> <https://www.cyphar.com/>
> 
> --8<---------------------------------------------------------------------
> 
> Since the return address is modified by kretprobe, the various unwinders
> can produce invalid and confusing stack traces. ftrace mostly solved
> this problem by teaching each unwinder how to find the original return
> address for stack trace purposes. This same technique can be applied to
> kretprobes by simply adding a pointer to where the return address was
> replaced in the stack, and then looking up the relevant
> kretprobe_instance when a stack trace is requested.
> 
> [WIP: This is currently broken because the *first entry* will not be
>       overwritten since it looks like the stack pointer is different
>       when we are provided pt_regs. All other addresses are correctly
>       handled.]

When you see this problem, what does regs->ip point to?  If it's
pointing to generated code, then we don't _currently_ have a way of
dealing with that.  If it's pointing to a real function, we can fix that
with unwind hints.
Masami Hiramatsu Nov. 9, 2018, 7:15 a.m. UTC | #27
On Thu, 8 Nov 2018 19:04:48 +1100
Aleksa Sarai <cyphar@cyphar.com> wrote:

> On 2018-11-08, Aleksa Sarai <cyphar@cyphar.com> wrote:
> > I will attach what I have at the moment to hopefully explain what the
> > issue I've found is (re-using the kretprobe architecture but with the
> > shadow-stack idea).
> 
> Here is the patch I have at the moment (it works, except for the
> question I have about how to handle the top-level pt_regs -- I've marked
> that code with XXX).
> 
> -- 
> Aleksa Sarai
> Senior Software Engineer (Containers)
> SUSE Linux GmbH
> <https://www.cyphar.com/>
> 
> --8<---------------------------------------------------------------------
> 
> Since the return address is modified by kretprobe, the various unwinders
> can produce invalid and confusing stack traces. ftrace mostly solved
> this problem by teaching each unwinder how to find the original return
> address for stack trace purposes. This same technique can be applied to
> kretprobes by simply adding a pointer to where the return address was
> replaced in the stack, and then looking up the relevant
> kretprobe_instance when a stack trace is requested.
> 
> [WIP: This is currently broken because the *first entry* will not be
>       overwritten since it looks like the stack pointer is different
>       when we are provided pt_regs. All other addresses are correctly
>       handled.]
> 
> Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
> ---
>  arch/x86/events/core.c         |  6 +++-
>  arch/x86/include/asm/ptrace.h  |  1 +
>  arch/x86/kernel/kprobes/core.c |  5 ++--
>  arch/x86/kernel/stacktrace.c   | 10 +++++--
>  arch/x86/kernel/unwind_frame.c |  2 ++
>  arch/x86/kernel/unwind_guess.c |  5 +++-
>  arch/x86/kernel/unwind_orc.c   |  2 ++
>  include/linux/kprobes.h        | 15 +++++++++-
>  kernel/kprobes.c               | 55 ++++++++++++++++++++++++++++++++++
>  9 files changed, 93 insertions(+), 8 deletions(-)
> 
> diff --git a/arch/x86/events/core.c b/arch/x86/events/core.c
> index de32741d041a..d71062702179 100644
> --- a/arch/x86/events/core.c
> +++ b/arch/x86/events/core.c
> @@ -2371,7 +2371,11 @@ perf_callchain_kernel(struct perf_callchain_entry_ctx *entry, struct pt_regs *re
>  		return;
>  	}
>  
> -	if (perf_callchain_store(entry, regs->ip))
> +	/* XXX: Currently broken -- stack_addr(regs) doesn't match entry. */
> +	addr = regs->ip;
> +	//addr = ftrace_graph_ret_addr(current, &state.graph_idx, addr, stack_addr(regs));
> +	addr = kretprobe_ret_addr(current, addr, stack_addr(regs));
> +	if (perf_callchain_store(entry, addr))
>  		return;
>  
>  	for (unwind_start(&state, current, regs, NULL); !unwind_done(&state);
> diff --git a/arch/x86/include/asm/ptrace.h b/arch/x86/include/asm/ptrace.h
> index ee696efec99f..c4dfafd43e11 100644
> --- a/arch/x86/include/asm/ptrace.h
> +++ b/arch/x86/include/asm/ptrace.h
> @@ -172,6 +172,7 @@ static inline unsigned long kernel_stack_pointer(struct pt_regs *regs)
>  	return regs->sp;
>  }
>  #endif
> +#define stack_addr(regs) ((unsigned long *) kernel_stack_pointer(regs))

No, you should use kernel_stack_pointer(regs) itself instead of stack_addr().

> 
>  #define GET_IP(regs) ((regs)->ip)
>  #define GET_FP(regs) ((regs)->bp)
> diff --git a/arch/x86/kernel/kprobes/core.c b/arch/x86/kernel/kprobes/core.c
> index b0d1e81c96bb..eb4da885020c 100644
> --- a/arch/x86/kernel/kprobes/core.c
> +++ b/arch/x86/kernel/kprobes/core.c
> @@ -69,8 +69,6 @@
>  DEFINE_PER_CPU(struct kprobe *, current_kprobe) = NULL;
>  DEFINE_PER_CPU(struct kprobe_ctlblk, kprobe_ctlblk);
>  
> -#define stack_addr(regs) ((unsigned long *)kernel_stack_pointer(regs))

I don't like keeping this meaningless macro... this should be replaced with generic
kernel_stack_pointer() macro.

> -
>  #define W(row, b0, b1, b2, b3, b4, b5, b6, b7, b8, b9, ba, bb, bc, bd, be, bf)\
>  	(((b0##UL << 0x0)|(b1##UL << 0x1)|(b2##UL << 0x2)|(b3##UL << 0x3) |   \
>  	  (b4##UL << 0x4)|(b5##UL << 0x5)|(b6##UL << 0x6)|(b7##UL << 0x7) |   \
> @@ -568,7 +566,8 @@ void arch_prepare_kretprobe(struct kretprobe_instance *ri, struct pt_regs *regs)
>  {
>  	unsigned long *sara = stack_addr(regs);
>  
> -	ri->ret_addr = (kprobe_opcode_t *) *sara;
> +	ri->ret_addrp = (kprobe_opcode_t **) sara;
> +	ri->ret_addr = *ri->ret_addrp;
>  
>  	/* Replace the return addr with trampoline addr */
>  	*sara = (unsigned long) &kretprobe_trampoline;
> diff --git a/arch/x86/kernel/stacktrace.c b/arch/x86/kernel/stacktrace.c
> index 7627455047c2..8a4fb3109d6b 100644
> --- a/arch/x86/kernel/stacktrace.c
> +++ b/arch/x86/kernel/stacktrace.c
> @@ -8,6 +8,7 @@
>  #include <linux/sched/task_stack.h>
>  #include <linux/stacktrace.h>
>  #include <linux/export.h>
> +#include <linux/kprobes.h>
>  #include <linux/uaccess.h>
>  #include <asm/stacktrace.h>
>  #include <asm/unwind.h>
> @@ -37,8 +38,13 @@ static void noinline __save_stack_trace(struct stack_trace *trace,
>  	struct unwind_state state;
>  	unsigned long addr;
>  
> -	if (regs)
> -		save_stack_address(trace, regs->ip, nosched);
> +	if (regs) {
> +		/* XXX: Currently broken -- stack_addr(regs) doesn't match entry. */
> +		addr = regs->ip;

Since this part is for storing regs->ip as a top of call-stack, this
seems correct code. Stack unwind will be done next block.

> +		//addr = ftrace_graph_ret_addr(current, &state.graph_idx, addr, stack_addr(regs));

so func graph return trampoline address will be shown only when unwinding stack entries.
I mean func-graph tracer is not used as an event, so it never kicks stackdump.

> +		addr = kretprobe_ret_addr(current, addr, stack_addr(regs));

But since kretprobe will be an event, which can kick the stackdump.
BTW, from kretprobe, regs->ip should always be the trampoline handler, 
see arch/x86/kernel/kprobes/core.c:772 :-)
So it must be fixed always.

> +		save_stack_address(trace, addr, nosched);
> +	}
>  
>  	for (unwind_start(&state, task, regs, NULL); !unwind_done(&state);
>  	     unwind_next_frame(&state)) {
> diff --git a/arch/x86/kernel/unwind_frame.c b/arch/x86/kernel/unwind_frame.c
> index 3dc26f95d46e..47062427e9a3 100644
> --- a/arch/x86/kernel/unwind_frame.c
> +++ b/arch/x86/kernel/unwind_frame.c
> @@ -1,4 +1,5 @@
>  #include <linux/sched.h>
> +#include <linux/kprobes.h>
>  #include <linux/sched/task.h>
>  #include <linux/sched/task_stack.h>
>  #include <linux/interrupt.h>
> @@ -270,6 +271,7 @@ static bool update_stack_state(struct unwind_state *state,
>  		addr = READ_ONCE_TASK_STACK(state->task, *addr_p);
>  		state->ip = ftrace_graph_ret_addr(state->task, &state->graph_idx,
>  						  addr, addr_p);
> +		state->ip = kretprobe_ret_addr(state->task, state->ip, addr_p);
>  	}
>  
>  	/* Save the original stack pointer for unwind_dump(): */
> diff --git a/arch/x86/kernel/unwind_guess.c b/arch/x86/kernel/unwind_guess.c
> index 4f0e17b90463..554fd7c5c331 100644
> --- a/arch/x86/kernel/unwind_guess.c
> +++ b/arch/x86/kernel/unwind_guess.c
> @@ -1,5 +1,6 @@
>  #include <linux/sched.h>
>  #include <linux/ftrace.h>
> +#include <linux/kprobes.h>
>  #include <asm/ptrace.h>
>  #include <asm/bitops.h>
>  #include <asm/stacktrace.h>
> @@ -14,8 +15,10 @@ unsigned long unwind_get_return_address(struct unwind_state *state)
>  
>  	addr = READ_ONCE_NOCHECK(*state->sp);
>  
> -	return ftrace_graph_ret_addr(state->task, &state->graph_idx,
> +	addr = ftrace_graph_ret_addr(state->task, &state->graph_idx,
>  				     addr, state->sp);
> +	addr = kretprobe_ret_addr(state->task, addr, state->sp);
> +	return addr;
>  }
>  EXPORT_SYMBOL_GPL(unwind_get_return_address);
>  
> diff --git a/arch/x86/kernel/unwind_orc.c b/arch/x86/kernel/unwind_orc.c
> index 26038eacf74a..b6393500d505 100644
> --- a/arch/x86/kernel/unwind_orc.c
> +++ b/arch/x86/kernel/unwind_orc.c
> @@ -1,5 +1,6 @@
>  #include <linux/module.h>
>  #include <linux/sort.h>
> +#include <linux/kprobes.h>
>  #include <asm/ptrace.h>
>  #include <asm/stacktrace.h>
>  #include <asm/unwind.h>
> @@ -462,6 +463,7 @@ bool unwind_next_frame(struct unwind_state *state)
>  
>  		state->ip = ftrace_graph_ret_addr(state->task, &state->graph_idx,
>  						  state->ip, (void *)ip_p);
> +		state->ip = kretprobe_ret_addr(state->task, state->ip, (void *)ip_p);
>  
>  		state->sp = sp;
>  		state->regs = NULL;
> diff --git a/include/linux/kprobes.h b/include/linux/kprobes.h
> index e909413e4e38..3a01f9998064 100644
> --- a/include/linux/kprobes.h
> +++ b/include/linux/kprobes.h
> @@ -172,6 +172,7 @@ struct kretprobe_instance {
>  	struct hlist_node hlist;
>  	struct kretprobe *rp;
>  	kprobe_opcode_t *ret_addr;
> +	kprobe_opcode_t **ret_addrp;
>  	struct task_struct *task;
>  	char data[0];
>  };
> @@ -203,6 +204,7 @@ static inline int kprobes_built_in(void)
>  extern void arch_prepare_kretprobe(struct kretprobe_instance *ri,
>  				   struct pt_regs *regs);
>  extern int arch_trampoline_kprobe(struct kprobe *p);
> +extern void kretprobe_trampoline(void);
>  #else /* CONFIG_KRETPROBES */
>  static inline void arch_prepare_kretprobe(struct kretprobe *rp,
>  					struct pt_regs *regs)
> @@ -212,6 +214,9 @@ static inline int arch_trampoline_kprobe(struct kprobe *p)
>  {
>  	return 0;
>  }
> +static inline void kretprobe_trampoline(void)
> +{
> +}
>  #endif /* CONFIG_KRETPROBES */
>  
>  extern struct kretprobe_blackpoint kretprobe_blacklist[];
> @@ -341,7 +346,7 @@ struct kprobe *get_kprobe(void *addr);
>  void kretprobe_hash_lock(struct task_struct *tsk,
>  			 struct hlist_head **head, unsigned long *flags);
>  void kretprobe_hash_unlock(struct task_struct *tsk, unsigned long *flags);
> -struct hlist_head * kretprobe_inst_table_head(struct task_struct *tsk);
> +struct hlist_head *kretprobe_inst_table_head(struct task_struct *tsk);
>  
>  /* kprobe_running() will just return the current_kprobe on this CPU */
>  static inline struct kprobe *kprobe_running(void)
> @@ -371,6 +376,9 @@ void unregister_kretprobe(struct kretprobe *rp);
>  int register_kretprobes(struct kretprobe **rps, int num);
>  void unregister_kretprobes(struct kretprobe **rps, int num);
>  
> +unsigned long kretprobe_ret_addr(struct task_struct *tsk, unsigned long ret,
> +				 unsigned long *retp);
> +
>  void kprobe_flush_task(struct task_struct *tk);
>  void recycle_rp_inst(struct kretprobe_instance *ri, struct hlist_head *head);
>  
> @@ -425,6 +433,11 @@ static inline void unregister_kretprobe(struct kretprobe *rp)
>  static inline void unregister_kretprobes(struct kretprobe **rps, int num)
>  {
>  }
> +unsigned long kretprobe_ret_addr(struct task_struct *task, unsigned long ret,
> +				 unsigned long *retp)
> +{
> +	return ret;
> +}
>  static inline void kprobe_flush_task(struct task_struct *tk)
>  {
>  }
> diff --git a/kernel/kprobes.c b/kernel/kprobes.c
> index 90e98e233647..ed78141664ec 100644
> --- a/kernel/kprobes.c
> +++ b/kernel/kprobes.c
> @@ -83,6 +83,11 @@ static raw_spinlock_t *kretprobe_table_lock_ptr(unsigned long hash)
>  	return &(kretprobe_table_locks[hash].lock);
>  }
>  
> +struct hlist_head *kretprobe_inst_table_head(struct task_struct *tsk)
> +{
> +	return &kretprobe_inst_table[hash_ptr(tsk, KPROBE_HASH_BITS)];
> +}
> +
>  /* Blacklist -- list of struct kprobe_blacklist_entry */
>  static LIST_HEAD(kprobe_blacklist);
>  
> @@ -1206,6 +1211,15 @@ __releases(hlist_lock)
>  }
>  NOKPROBE_SYMBOL(kretprobe_table_unlock);
>  
> +static bool kretprobe_hash_is_locked(struct task_struct *tsk)
> +{
> +	unsigned long hash = hash_ptr(tsk, KPROBE_HASH_BITS);
> +	raw_spinlock_t *hlist_lock = kretprobe_table_lock_ptr(hash);
> +
> +	return raw_spin_is_locked(hlist_lock);
> +}
> +NOKPROBE_SYMBOL(kretprobe_hash_is_locked);
> +
>  /*
>   * This function is called from finish_task_switch when task tk becomes dead,
>   * so that we can recycle any function-return probe instances associated
> @@ -1856,6 +1870,41 @@ static int pre_handler_kretprobe(struct kprobe *p, struct pt_regs *regs)
>  }
>  NOKPROBE_SYMBOL(pre_handler_kretprobe);
>  
> +unsigned long kretprobe_ret_addr(struct task_struct *tsk, unsigned long ret,
> +				 unsigned long *retp)
> +{
> +	struct kretprobe_instance *ri;
> +	unsigned long flags = 0;
> +	struct hlist_head *head;
> +	bool need_lock;
> +
> +	if (likely(ret != (unsigned long) &kretprobe_trampoline))
> +		return ret;
> +
> +	need_lock = !kretprobe_hash_is_locked(tsk);
> +	if (WARN_ON(need_lock))
> +		kretprobe_hash_lock(tsk, &head, &flags);
> +	else
> +		head = kretprobe_inst_table_head(tsk);

This may not work unless this is called from the kretprobe handler context,
since if we are out of kretprobe handler context, another CPU can lock the
hash table and it can be detected by kretprobe_hash_is_locked();.

So, we should check we are in the kretprobe handler context if tsk == current,
if not, we definately can lock the hash lock without any warning. This can
be something like;

if (is_kretprobe_handler_context()) {
  // kretprobe_hash_lock(current == tsk) has been locked by caller  
  if (tsk != current && kretprobe_hash(tsk) != kretprobe_hash(current))
    // the hash of tsk and current can be same.
    need_lock = true;
} else
  // we should take a lock for tsk.
  need_lock = true;


Thank you,

> +
> +	hlist_for_each_entry(ri, head, hlist) {
> +		if (ri->task != current)
> +			continue;
> +		if (ri->ret_addr == (kprobe_opcode_t *) &kretprobe_trampoline)
> +			continue;
> +		if (ri->ret_addrp == (kprobe_opcode_t **) retp) {
> +			ret = (unsigned long) ri->ret_addr;
> +			goto out;
> +		}
> +	}
> +
> +out:
> +	if (need_lock)
> +		kretprobe_hash_unlock(tsk, &flags);
> +	return ret;
> +}
> +NOKPROBE_SYMBOL(kretprobe_ret_addr);
> +
>  bool __weak arch_kprobe_on_func_entry(unsigned long offset)
>  {
>  	return !offset;
> @@ -2005,6 +2054,12 @@ static int pre_handler_kretprobe(struct kprobe *p, struct pt_regs *regs)
>  }
>  NOKPROBE_SYMBOL(pre_handler_kretprobe);
>  
> +unsigned long kretprobe_ret_addr(struct task_struct *tsk, unsigned long ret,
> +				 unsigned long *retp)
> +{
> +	return ret;
> +}
> +NOKPROBE_SYMBOL(kretprobe_ret_addr);
>  #endif /* CONFIG_KRETPROBES */
>  
>  /* Set the kprobe gone and remove its instruction buffer. */
> -- 
> 2.19.1
Masami Hiramatsu Nov. 9, 2018, 7:26 a.m. UTC | #28
On Thu, 8 Nov 2018 08:44:37 -0600
Josh Poimboeuf <jpoimboe@redhat.com> wrote:

> On Thu, Nov 08, 2018 at 07:04:48PM +1100, Aleksa Sarai wrote:
> > On 2018-11-08, Aleksa Sarai <cyphar@cyphar.com> wrote:
> > > I will attach what I have at the moment to hopefully explain what the
> > > issue I've found is (re-using the kretprobe architecture but with the
> > > shadow-stack idea).
> > 
> > Here is the patch I have at the moment (it works, except for the
> > question I have about how to handle the top-level pt_regs -- I've marked
> > that code with XXX).
> > 
> > -- 
> > Aleksa Sarai
> > Senior Software Engineer (Containers)
> > SUSE Linux GmbH
> > <https://www.cyphar.com/>
> > 
> > --8<---------------------------------------------------------------------
> > 
> > Since the return address is modified by kretprobe, the various unwinders
> > can produce invalid and confusing stack traces. ftrace mostly solved
> > this problem by teaching each unwinder how to find the original return
> > address for stack trace purposes. This same technique can be applied to
> > kretprobes by simply adding a pointer to where the return address was
> > replaced in the stack, and then looking up the relevant
> > kretprobe_instance when a stack trace is requested.
> > 
> > [WIP: This is currently broken because the *first entry* will not be
> >       overwritten since it looks like the stack pointer is different
> >       when we are provided pt_regs. All other addresses are correctly
> >       handled.]
> 
> When you see this problem, what does regs->ip point to?  If it's
> pointing to generated code, then we don't _currently_ have a way of
> dealing with that.  If it's pointing to a real function, we can fix that
> with unwind hints.

As I replied, If the stackdump is called from kretprobe event, regs->ip
always points trampoline function. Otherwise (maybe from kprobe event,
or panic, BUG etc.) it always be the address which the event occurs.

So fixing regs->ip is correct.

Thank you,
Aleksa Sarai Nov. 9, 2018, 3:06 p.m. UTC | #29
On 2018-11-09, Masami Hiramatsu <mhiramat@kernel.org> wrote:
> > diff --git a/arch/x86/include/asm/ptrace.h b/arch/x86/include/asm/ptrace.h
> > index ee696efec99f..c4dfafd43e11 100644
> > --- a/arch/x86/include/asm/ptrace.h
> > +++ b/arch/x86/include/asm/ptrace.h
> > @@ -172,6 +172,7 @@ static inline unsigned long kernel_stack_pointer(struct pt_regs *regs)
> >  	return regs->sp;
> >  }
> >  #endif
> > +#define stack_addr(regs) ((unsigned long *) kernel_stack_pointer(regs))
> 
> No, you should use kernel_stack_pointer(regs) itself instead of stack_addr().
> 
> > 
> >  #define GET_IP(regs) ((regs)->ip)
> >  #define GET_FP(regs) ((regs)->bp)
> > diff --git a/arch/x86/kernel/kprobes/core.c b/arch/x86/kernel/kprobes/core.c
> > index b0d1e81c96bb..eb4da885020c 100644
> > --- a/arch/x86/kernel/kprobes/core.c
> > +++ b/arch/x86/kernel/kprobes/core.c
> > @@ -69,8 +69,6 @@
> >  DEFINE_PER_CPU(struct kprobe *, current_kprobe) = NULL;
> >  DEFINE_PER_CPU(struct kprobe_ctlblk, kprobe_ctlblk);
> >  
> > -#define stack_addr(regs) ((unsigned long *)kernel_stack_pointer(regs))
> 
> I don't like keeping this meaningless macro... this should be replaced with generic
> kernel_stack_pointer() macro.

Sure. This patch was just an example -- I can remove stack_addr() all
over.

> > -	if (regs)
> > -		save_stack_address(trace, regs->ip, nosched);
> > +	if (regs) {
> > +		/* XXX: Currently broken -- stack_addr(regs) doesn't match entry. */
> > +		addr = regs->ip;
> 
> Since this part is for storing regs->ip as a top of call-stack, this
> seems correct code. Stack unwind will be done next block.

This comment was referring to the usage of stack_addr(). stack_addr()
doesn't give you the right result (it isn't the address of the return
address -- it's slightly wrong). This is the main issue I was having --
am I doing something wrong here?

> > +		//addr = ftrace_graph_ret_addr(current, &state.graph_idx, addr, stack_addr(regs));
> 
> so func graph return trampoline address will be shown only when unwinding stack entries.
> I mean func-graph tracer is not used as an event, so it never kicks stackdump.

Just to make sure I understand what you're saying -- func-graph trace
will never actually call __ftrace_stack_trace? Because if it does, then
this code will be necessary (and then I'm a bit confused why the
unwinder has func-graph trace code -- if stack traces are never taken
under func-graph then the code in the unwinder is not necessary)

My reason for commenting this out is because at this point "state" isn't
initialised and thus .graph_idx would not be correctly handled during
unwind (and it's the same reason I commented it out later).

> > +		addr = kretprobe_ret_addr(current, addr, stack_addr(regs));
> 
> But since kretprobe will be an event, which can kick the stackdump.
> BTW, from kretprobe, regs->ip should always be the trampoline handler, 
> see arch/x86/kernel/kprobes/core.c:772 :-)
> So it must be fixed always.

Right, but kretprobe_ret_addr() is returning the *original* return
address (and we need to do an (addr == kretprobe_trampoline)). The
real problem is that stack_addr(regs) isn't the same as it is during
kretprobe setup (but kretprobe_ret_addr() works everywhere else).

> > @@ -1856,6 +1870,41 @@ static int pre_handler_kretprobe(struct kprobe *p, struct pt_regs *regs)
> >  }
> >  NOKPROBE_SYMBOL(pre_handler_kretprobe);
> >  
> > +unsigned long kretprobe_ret_addr(struct task_struct *tsk, unsigned long ret,
> > +				 unsigned long *retp)
> > +{
> > +	struct kretprobe_instance *ri;
> > +	unsigned long flags = 0;
> > +	struct hlist_head *head;
> > +	bool need_lock;
> > +
> > +	if (likely(ret != (unsigned long) &kretprobe_trampoline))
> > +		return ret;
> > +
> > +	need_lock = !kretprobe_hash_is_locked(tsk);
> > +	if (WARN_ON(need_lock))
> > +		kretprobe_hash_lock(tsk, &head, &flags);
> > +	else
> > +		head = kretprobe_inst_table_head(tsk);
> 
> This may not work unless this is called from the kretprobe handler context,
> since if we are out of kretprobe handler context, another CPU can lock the
> hash table and it can be detected by kretprobe_hash_is_locked();.

Yeah, I noticed this as well when writing it (but needed a quick impl
that I could test). I will fix this, thanks!

By is_kretprobe_handler_context() I imagine you are referring to
checking is_kretprobe(current_kprobe())?

> So, we should check we are in the kretprobe handler context if tsk == current,
> if not, we definately can lock the hash lock without any warning. This can
> be something like;
> 
> if (is_kretprobe_handler_context()) {
>   // kretprobe_hash_lock(current == tsk) has been locked by caller  
>   if (tsk != current && kretprobe_hash(tsk) != kretprobe_hash(current))
>     // the hash of tsk and current can be same.
>     need_lock = true;
> } else
>   // we should take a lock for tsk.
>   need_lock = true;
Aleksa Sarai Nov. 9, 2018, 3:10 p.m. UTC | #30
On 2018-11-09, Masami Hiramatsu <mhiramat@kernel.org> wrote:
> On Thu, 8 Nov 2018 08:44:37 -0600
> Josh Poimboeuf <jpoimboe@redhat.com> wrote:
> 
> > On Thu, Nov 08, 2018 at 07:04:48PM +1100, Aleksa Sarai wrote:
> > > On 2018-11-08, Aleksa Sarai <cyphar@cyphar.com> wrote:
> > > > I will attach what I have at the moment to hopefully explain what the
> > > > issue I've found is (re-using the kretprobe architecture but with the
> > > > shadow-stack idea).
> > > 
> > > Here is the patch I have at the moment (it works, except for the
> > > question I have about how to handle the top-level pt_regs -- I've marked
> > > that code with XXX).
> > > 
> > > -- 
> > > Aleksa Sarai
> > > Senior Software Engineer (Containers)
> > > SUSE Linux GmbH
> > > <https://www.cyphar.com/>
> > > 
> > > --8<---------------------------------------------------------------------
> > > 
> > > Since the return address is modified by kretprobe, the various unwinders
> > > can produce invalid and confusing stack traces. ftrace mostly solved
> > > this problem by teaching each unwinder how to find the original return
> > > address for stack trace purposes. This same technique can be applied to
> > > kretprobes by simply adding a pointer to where the return address was
> > > replaced in the stack, and then looking up the relevant
> > > kretprobe_instance when a stack trace is requested.
> > > 
> > > [WIP: This is currently broken because the *first entry* will not be
> > >       overwritten since it looks like the stack pointer is different
> > >       when we are provided pt_regs. All other addresses are correctly
> > >       handled.]
> > 
> > When you see this problem, what does regs->ip point to?  If it's
> > pointing to generated code, then we don't _currently_ have a way of
> > dealing with that.  If it's pointing to a real function, we can fix that
> > with unwind hints.
> 
> As I replied, If the stackdump is called from kretprobe event, regs->ip
> always points trampoline function. Otherwise (maybe from kprobe event,
> or panic, BUG etc.) it always be the address which the event occurs.
> 
> So fixing regs->ip is correct.

The problem is that the pointer to the *return address* is wrong
(kernel_stack_pointer() gives you a different result than the function
entry), it's not that regs->ip is wrong. And I'm sure that it's "wrong"
because it's not possible for "regs->ip == kretprobe_trampoline" unless
you are in a stack frame that has been modified by the kretprobe core.

I will take a closer look at this over the weekend -- I posted the patch
to try to help explain what the underlying issue I was trying to solve
with this patch series is (and why I don't think the ftrace changes
proposed in the thread will completely fix them).
Masami Hiramatsu Nov. 10, 2018, 3:31 p.m. UTC | #31
On Sat, 10 Nov 2018 02:06:29 +1100
Aleksa Sarai <asarai@suse.de> wrote:

> On 2018-11-09, Masami Hiramatsu <mhiramat@kernel.org> wrote:
> > > diff --git a/arch/x86/include/asm/ptrace.h b/arch/x86/include/asm/ptrace.h
> > > index ee696efec99f..c4dfafd43e11 100644
> > > --- a/arch/x86/include/asm/ptrace.h
> > > +++ b/arch/x86/include/asm/ptrace.h
> > > @@ -172,6 +172,7 @@ static inline unsigned long kernel_stack_pointer(struct pt_regs *regs)
> > >  	return regs->sp;
> > >  }
> > >  #endif
> > > +#define stack_addr(regs) ((unsigned long *) kernel_stack_pointer(regs))
> > 
> > No, you should use kernel_stack_pointer(regs) itself instead of stack_addr().
> > 
> > > 
> > >  #define GET_IP(regs) ((regs)->ip)
> > >  #define GET_FP(regs) ((regs)->bp)
> > > diff --git a/arch/x86/kernel/kprobes/core.c b/arch/x86/kernel/kprobes/core.c
> > > index b0d1e81c96bb..eb4da885020c 100644
> > > --- a/arch/x86/kernel/kprobes/core.c
> > > +++ b/arch/x86/kernel/kprobes/core.c
> > > @@ -69,8 +69,6 @@
> > >  DEFINE_PER_CPU(struct kprobe *, current_kprobe) = NULL;
> > >  DEFINE_PER_CPU(struct kprobe_ctlblk, kprobe_ctlblk);
> > >  
> > > -#define stack_addr(regs) ((unsigned long *)kernel_stack_pointer(regs))
> > 
> > I don't like keeping this meaningless macro... this should be replaced with generic
> > kernel_stack_pointer() macro.
> 
> Sure. This patch was just an example -- I can remove stack_addr() all
> over.
> 
> > > -	if (regs)
> > > -		save_stack_address(trace, regs->ip, nosched);
> > > +	if (regs) {
> > > +		/* XXX: Currently broken -- stack_addr(regs) doesn't match entry. */
> > > +		addr = regs->ip;
> > 
> > Since this part is for storing regs->ip as a top of call-stack, this
> > seems correct code. Stack unwind will be done next block.
> 
> This comment was referring to the usage of stack_addr(). stack_addr()
> doesn't give you the right result (it isn't the address of the return
> address -- it's slightly wrong). This is the main issue I was having --
> am I doing something wrong here?

Of course stack_addr() actually just returns where the stack is. It should
not return address, but maybe a return address from this event happens.
Note that the "regs != NULL" means you will be in the interrupt handler
and it will be returned to the regs->ip.


> > > +		//addr = ftrace_graph_ret_addr(current, &state.graph_idx, addr, stack_addr(regs));
> > 
> > so func graph return trampoline address will be shown only when unwinding stack entries.
> > I mean func-graph tracer is not used as an event, so it never kicks stackdump.
> 
> Just to make sure I understand what you're saying -- func-graph trace
> will never actually call __ftrace_stack_trace? Because if it does, then
> this code will be necessary (and then I'm a bit confused why the
> unwinder has func-graph trace code -- if stack traces are never taken
> under func-graph then the code in the unwinder is not necessary)

You seems misunderstanding. Even if this is not called from func-graph
tracer, the stack entries are already replaced with func-graph trampoline.
However, regs->ip (IRQ return address) is never replaced by the func-graph
trampoline.

> My reason for commenting this out is because at this point "state" isn't
> initialised and thus .graph_idx would not be correctly handled during
> unwind (and it's the same reason I commented it out later).

OK, but anyway, I think we don't need it.

> > > +		addr = kretprobe_ret_addr(current, addr, stack_addr(regs));
> > 
> > But since kretprobe will be an event, which can kick the stackdump.
> > BTW, from kretprobe, regs->ip should always be the trampoline handler, 
> > see arch/x86/kernel/kprobes/core.c:772 :-)
> > So it must be fixed always.
> 
> Right, but kretprobe_ret_addr() is returning the *original* return
> address (and we need to do an (addr == kretprobe_trampoline)). The
> real problem is that stack_addr(regs) isn't the same as it is during
> kretprobe setup (but kretprobe_ret_addr() works everywhere else).

I think stack_addr(regs) should be same when this is called from kretprobe
handler context. Otherwise, yes, it is not same, but in that case, regs->ip
is not kretprobe_trampoline too.

If you find kretprobe_trampoline on the "stack", of course it's address should be
same as it is during kretprobe setup, but if you find kretprobe_trampoline on the
regs->ip, that should always happen on kretprobe handler context. Otherwise,
some critical violation happens on kretprobe_trampoline. In that case, we should
dump the kretprobe_trampoline address itself, should not recover it.

> > > @@ -1856,6 +1870,41 @@ static int pre_handler_kretprobe(struct kprobe *p, struct pt_regs *regs)
> > >  }
> > >  NOKPROBE_SYMBOL(pre_handler_kretprobe);
> > >  
> > > +unsigned long kretprobe_ret_addr(struct task_struct *tsk, unsigned long ret,
> > > +				 unsigned long *retp)
> > > +{
> > > +	struct kretprobe_instance *ri;
> > > +	unsigned long flags = 0;
> > > +	struct hlist_head *head;
> > > +	bool need_lock;
> > > +
> > > +	if (likely(ret != (unsigned long) &kretprobe_trampoline))
> > > +		return ret;
> > > +
> > > +	need_lock = !kretprobe_hash_is_locked(tsk);
> > > +	if (WARN_ON(need_lock))
> > > +		kretprobe_hash_lock(tsk, &head, &flags);
> > > +	else
> > > +		head = kretprobe_inst_table_head(tsk);
> > 
> > This may not work unless this is called from the kretprobe handler context,
> > since if we are out of kretprobe handler context, another CPU can lock the
> > hash table and it can be detected by kretprobe_hash_is_locked();.
> 
> Yeah, I noticed this as well when writing it (but needed a quick impl
> that I could test). I will fix this, thanks!
> 
> By is_kretprobe_handler_context() I imagine you are referring to
> checking is_kretprobe(current_kprobe())?

yes, that's correct :)

Thank you,

> 
> > So, we should check we are in the kretprobe handler context if tsk == current,
> > if not, we definately can lock the hash lock without any warning. This can
> > be something like;
> > 
> > if (is_kretprobe_handler_context()) {
> >   // kretprobe_hash_lock(current == tsk) has been locked by caller  
> >   if (tsk != current && kretprobe_hash(tsk) != kretprobe_hash(current))
> >     // the hash of tsk and current can be same.
> >     need_lock = true;
> > } else
> >   // we should take a lock for tsk.
> >   need_lock = true;
> 
> -- 
> Aleksa Sarai
> Senior Software Engineer (Containers)
> SUSE Linux GmbH
> <https://www.cyphar.com/>
Aleksa Sarai Nov. 12, 2018, 10:38 a.m. UTC | #32
On 2018-11-11, Masami Hiramatsu <mhiramat@kernel.org> wrote:
> > > > +		addr = kretprobe_ret_addr(current, addr, stack_addr(regs));
> > > 
> > > But since kretprobe will be an event, which can kick the stackdump.
> > > BTW, from kretprobe, regs->ip should always be the trampoline handler, 
> > > see arch/x86/kernel/kprobes/core.c:772 :-)
> > > So it must be fixed always.
> > 
> > Right, but kretprobe_ret_addr() is returning the *original* return
> > address (and we need to do an (addr == kretprobe_trampoline)). The
> > real problem is that stack_addr(regs) isn't the same as it is during
> > kretprobe setup (but kretprobe_ret_addr() works everywhere else).
> 
> I think stack_addr(regs) should be same when this is called from kretprobe
> handler context. Otherwise, yes, it is not same, but in that case, regs->ip
> is not kretprobe_trampoline too.

I figured it out.

It should be (regs->sp - 1) (just like it is inside the relevant
unwinder function for ORC). I now have a prototype which works under the
frame unwinder[*] -- however under ORC you can only see the top-most
function (the unwinder doesn't see the other function calls). I'm
playing with ORC hints with kretprobe_trampoline to try to improve
things but it's still a bit screwy.


[*]: However, I've noticed that the stack traces between the two traces
	 no longer match. On kprobe you get function_name+1, but on
	 kretprobe you get function_caller+foo. Obviously it's working but
	 the return address results in slightly different stack traces. This
	 means that stack trace aggregation between kprobe and kretprobe
	 won't work anymore -- at least not like it did in my original
	 patch. So I'm really not sure where to go from here.

I can send around another patchset to illustrate the problem if you like
(as well as show how the current unwinding code works).

Patch
diff mbox series

diff --git a/Documentation/kprobes.txt b/Documentation/kprobes.txt
index 10f4499e677c..1965585848f4 100644
--- a/Documentation/kprobes.txt
+++ b/Documentation/kprobes.txt
@@ -597,7 +597,11 @@  address with the trampoline's address, stack backtraces and calls
 to __builtin_return_address() will typically yield the trampoline's
 address instead of the real return address for kretprobed functions.
 (As far as we can tell, __builtin_return_address() is used only
-for instrumentation and error reporting.)
+for instrumentation and error reporting.) However, since return probes
+are used extensively in tracing (where stack backtraces are useful),
+return probes will stash away the stack backtrace during function entry
+so that return probe handlers can use the entry backtrace instead of
+having a trace with just kretprobe_trampoline.
 
 If the number of times a function is called does not match the number
 of times it returns, registering a return probe on that function may
diff --git a/include/linux/kprobes.h b/include/linux/kprobes.h
index e909413e4e38..1a1629544e56 100644
--- a/include/linux/kprobes.h
+++ b/include/linux/kprobes.h
@@ -40,6 +40,8 @@ 
 #include <linux/rcupdate.h>
 #include <linux/mutex.h>
 #include <linux/ftrace.h>
+#include <linux/stacktrace.h>
+#include <linux/perf_event.h>
 #include <asm/kprobes.h>
 
 #ifdef CONFIG_KPROBES
@@ -168,11 +170,18 @@  struct kretprobe {
 	raw_spinlock_t lock;
 };
 
+#define KRETPROBE_TRACE_SIZE 127
+struct kretprobe_trace {
+	int nr_entries;
+	unsigned long entries[KRETPROBE_TRACE_SIZE];
+};
+
 struct kretprobe_instance {
 	struct hlist_node hlist;
 	struct kretprobe *rp;
 	kprobe_opcode_t *ret_addr;
 	struct task_struct *task;
+	struct kretprobe_trace entry;
 	char data[0];
 };
 
@@ -371,6 +380,12 @@  void unregister_kretprobe(struct kretprobe *rp);
 int register_kretprobes(struct kretprobe **rps, int num);
 void unregister_kretprobes(struct kretprobe **rps, int num);
 
+struct kretprobe_instance *current_kretprobe_instance(void);
+void kretprobe_save_stack_trace(struct kretprobe_instance *ri,
+				struct stack_trace *trace);
+void kretprobe_perf_callchain_kernel(struct kretprobe_instance *ri,
+				     struct perf_callchain_entry_ctx *ctx);
+
 void kprobe_flush_task(struct task_struct *tk);
 void recycle_rp_inst(struct kretprobe_instance *ri, struct hlist_head *head);
 
@@ -397,6 +412,18 @@  static inline struct kprobe *kprobe_running(void)
 {
 	return NULL;
 }
+static inline struct kretprobe_instance *current_kretprobe_instance(void)
+{
+	return NULL;
+}
+static inline void kretprobe_save_stack_trace(struct kretprobe_instance *ri,
+					      struct stack_trace *trace)
+{
+}
+static inline void kretprobe_perf_callchain_kernel(struct kretprobe_instance *ri,
+						   struct perf_callchain_entry_ctx *ctx)
+{
+}
 static inline int register_kprobe(struct kprobe *p)
 {
 	return -ENOSYS;
diff --git a/kernel/events/callchain.c b/kernel/events/callchain.c
index 24a77c34e9ad..98edcd8a6987 100644
--- a/kernel/events/callchain.c
+++ b/kernel/events/callchain.c
@@ -12,6 +12,7 @@ 
 #include <linux/perf_event.h>
 #include <linux/slab.h>
 #include <linux/sched/task_stack.h>
+#include <linux/kprobes.h>
 
 #include "internal.h"
 
@@ -197,9 +198,14 @@  get_perf_callchain(struct pt_regs *regs, u32 init_nr, bool kernel, bool user,
 	ctx.contexts_maxed = false;
 
 	if (kernel && !user_mode(regs)) {
+		struct kretprobe_instance *ri = current_kretprobe_instance();
+
 		if (add_mark)
 			perf_callchain_store_context(&ctx, PERF_CONTEXT_KERNEL);
-		perf_callchain_kernel(&ctx, regs);
+		if (ri)
+			kretprobe_perf_callchain_kernel(ri, &ctx);
+		else
+			perf_callchain_kernel(&ctx, regs);
 	}
 
 	if (user) {
diff --git a/kernel/kprobes.c b/kernel/kprobes.c
index 90e98e233647..fca3964d18cd 100644
--- a/kernel/kprobes.c
+++ b/kernel/kprobes.c
@@ -1206,6 +1206,16 @@  __releases(hlist_lock)
 }
 NOKPROBE_SYMBOL(kretprobe_table_unlock);
 
+static bool kretprobe_hash_is_locked(struct task_struct *tsk)
+{
+	unsigned long hash = hash_ptr(tsk, KPROBE_HASH_BITS);
+	raw_spinlock_t *hlist_lock;
+
+	hlist_lock = kretprobe_table_lock_ptr(hash);
+	return raw_spin_is_locked(hlist_lock);
+}
+NOKPROBE_SYMBOL(kretprobe_hash_is_locked);
+
 /*
  * This function is called from finish_task_switch when task tk becomes dead,
  * so that we can recycle any function-return probe instances associated
@@ -1800,6 +1810,13 @@  unsigned long __weak arch_deref_entry_point(void *entry)
 	return (unsigned long)entry;
 }
 
+static int pre_handler_kretprobe(struct kprobe *p, struct pt_regs *regs);
+
+static inline bool kprobe_is_retprobe(struct kprobe *kp)
+{
+	return kp->pre_handler == pre_handler_kretprobe;
+}
+
 #ifdef CONFIG_KRETPROBES
 /*
  * This kprobe pre_handler is registered with every kretprobe. When probe
@@ -1826,6 +1843,8 @@  static int pre_handler_kretprobe(struct kprobe *p, struct pt_regs *regs)
 	hash = hash_ptr(current, KPROBE_HASH_BITS);
 	raw_spin_lock_irqsave(&rp->lock, flags);
 	if (!hlist_empty(&rp->free_instances)) {
+		struct stack_trace trace = {};
+
 		ri = hlist_entry(rp->free_instances.first,
 				struct kretprobe_instance, hlist);
 		hlist_del(&ri->hlist);
@@ -1834,6 +1853,11 @@  static int pre_handler_kretprobe(struct kprobe *p, struct pt_regs *regs)
 		ri->rp = rp;
 		ri->task = current;
 
+		trace.entries = &ri->entry.entries[0];
+		trace.max_entries = KRETPROBE_TRACE_SIZE;
+		save_stack_trace_regs(regs, &trace);
+		ri->entry.nr_entries = trace.nr_entries;
+
 		if (rp->entry_handler && rp->entry_handler(ri, regs)) {
 			raw_spin_lock_irqsave(&rp->lock, flags);
 			hlist_add_head(&ri->hlist, &rp->free_instances);
@@ -1856,6 +1880,65 @@  static int pre_handler_kretprobe(struct kprobe *p, struct pt_regs *regs)
 }
 NOKPROBE_SYMBOL(pre_handler_kretprobe);
 
+/*
+ * Return the kretprobe_instance associated with the current_kprobe. Calling
+ * this is only reasonable from within a kretprobe handler context (otherwise
+ * return NULL).
+ *
+ * Must be called within a kretprobe_hash_lock(current, ...) context.
+ */
+struct kretprobe_instance *current_kretprobe_instance(void)
+{
+	struct kprobe *kp;
+	struct kretprobe *rp;
+	struct kretprobe_instance *ri;
+	struct hlist_head *head;
+	unsigned long hash = hash_ptr(current, KPROBE_HASH_BITS);
+
+	kp = kprobe_running();
+	if (!kp || !kprobe_is_retprobe(kp))
+		return NULL;
+	if (WARN_ON(!kretprobe_hash_is_locked(current)))
+		return NULL;
+
+	rp = container_of(kp, struct kretprobe, kp);
+	head = &kretprobe_inst_table[hash];
+
+	hlist_for_each_entry(ri, head, hlist) {
+		if (ri->task == current && ri->rp == rp)
+			return ri;
+	}
+	return NULL;
+}
+EXPORT_SYMBOL_GPL(current_kretprobe_instance);
+NOKPROBE_SYMBOL(current_kretprobe_instance);
+
+void kretprobe_save_stack_trace(struct kretprobe_instance *ri,
+				struct stack_trace *trace)
+{
+	int i;
+	struct kretprobe_trace *krt = &ri->entry;
+
+	for (i = trace->skip; i < krt->nr_entries; i++) {
+		if (trace->nr_entries >= trace->max_entries)
+			break;
+		trace->entries[trace->nr_entries++] = krt->entries[i];
+	}
+}
+
+void kretprobe_perf_callchain_kernel(struct kretprobe_instance *ri,
+				     struct perf_callchain_entry_ctx *ctx)
+{
+	int i;
+	struct kretprobe_trace *krt = &ri->entry;
+
+	for (i = 0; i < krt->nr_entries; i++) {
+		if (krt->entries[i] == ULONG_MAX)
+			break;
+		perf_callchain_store(ctx, (u64) krt->entries[i]);
+	}
+}
+
 bool __weak arch_kprobe_on_func_entry(unsigned long offset)
 {
 	return !offset;
@@ -2005,6 +2088,22 @@  static int pre_handler_kretprobe(struct kprobe *p, struct pt_regs *regs)
 }
 NOKPROBE_SYMBOL(pre_handler_kretprobe);
 
+struct kretprobe_instance *current_kretprobe_instance(void)
+{
+	return NULL;
+}
+EXPORT_SYMBOL_GPL(current_kretprobe_instance);
+NOKPROBE_SYMBOL(current_kretprobe_instance);
+
+void kretprobe_save_stack_trace(struct kretprobe_instance *ri,
+				struct stack_trace *trace)
+{
+}
+
+void kretprobe_perf_callchain_kernel(struct kretprobe_instance *ri,
+				     struct perf_callchain_entry_ctx *ctx)
+{
+}
 #endif /* CONFIG_KRETPROBES */
 
 /* Set the kprobe gone and remove its instruction buffer. */
@@ -2241,7 +2340,7 @@  static void report_probe(struct seq_file *pi, struct kprobe *p,
 	char *kprobe_type;
 	void *addr = p->addr;
 
-	if (p->pre_handler == pre_handler_kretprobe)
+	if (kprobe_is_retprobe(p))
 		kprobe_type = "r";
 	else
 		kprobe_type = "k";
diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index bf6f1d70484d..2210d38a4dbf 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -42,6 +42,7 @@ 
 #include <linux/nmi.h>
 #include <linux/fs.h>
 #include <linux/trace.h>
+#include <linux/kprobes.h>
 #include <linux/sched/clock.h>
 #include <linux/sched/rt.h>
 
@@ -2590,6 +2591,7 @@  static void __ftrace_trace_stack(struct ring_buffer *buffer,
 	struct ring_buffer_event *event;
 	struct stack_entry *entry;
 	struct stack_trace trace;
+	struct kretprobe_instance *ri = current_kretprobe_instance();
 	int use_stack;
 	int size = FTRACE_STACK_ENTRIES;
 
@@ -2626,7 +2628,9 @@  static void __ftrace_trace_stack(struct ring_buffer *buffer,
 		trace.entries		= this_cpu_ptr(ftrace_stack.calls);
 		trace.max_entries	= FTRACE_STACK_MAX_ENTRIES;
 
-		if (regs)
+		if (ri)
+			kretprobe_save_stack_trace(ri, &trace);
+		else if (regs)
 			save_stack_trace_regs(regs, &trace);
 		else
 			save_stack_trace(&trace);
@@ -2653,7 +2657,10 @@  static void __ftrace_trace_stack(struct ring_buffer *buffer,
 	else {
 		trace.max_entries	= FTRACE_STACK_ENTRIES;
 		trace.entries		= entry->caller;
-		if (regs)
+
+		if (ri)
+			kretprobe_save_stack_trace(ri, &trace);
+		else if (regs)
 			save_stack_trace_regs(regs, &trace);
 		else
 			save_stack_trace(&trace);
diff --git a/tools/testing/selftests/ftrace/test.d/kprobe/kretprobe_stacktrace.tc b/tools/testing/selftests/ftrace/test.d/kprobe/kretprobe_stacktrace.tc
new file mode 100644
index 000000000000..03146c6a1a3c
--- /dev/null
+++ b/tools/testing/selftests/ftrace/test.d/kprobe/kretprobe_stacktrace.tc
@@ -0,0 +1,25 @@ 
+#!/bin/sh
+# SPDX-License-Identifier: GPL-2.0+
+# description: Kretprobe dynamic event with a stacktrace
+
+[ -f kprobe_events ] || exit_unsupported # this is configurable
+
+echo 0 > events/enable
+echo 1 > options/stacktrace
+
+echo 'r:teststackprobe sched_fork $retval' > kprobe_events
+grep teststackprobe kprobe_events
+test -d events/kprobes/teststackprobe
+
+clear_trace
+echo 1 > events/kprobes/teststackprobe/enable
+( echo "forked")
+echo 0 > events/kprobes/teststackprobe/enable
+
+# Make sure we don't see kretprobe_trampoline and we see _do_fork.
+! grep 'kretprobe' trace
+grep '_do_fork' trace
+
+echo '-:teststackprobe' >> kprobe_events
+clear_trace
+test -d events/kprobes/teststackprobe && exit_fail || exit_pass