mbox series

[v8,00/16] Add support for Clang LTO

Message ID 20201201213707.541432-1-samitolvanen@google.com (mailing list archive)
Headers show
Series Add support for Clang LTO | expand

Message

Sami Tolvanen Dec. 1, 2020, 9:36 p.m. UTC
This patch series adds support for building the kernel with Clang's
Link Time Optimization (LTO). In addition to performance, the primary
motivation for LTO is to allow Clang's Control-Flow Integrity (CFI)
to be used in the kernel. Google has shipped millions of Pixel
devices running three major kernel versions with LTO+CFI since 2018.

Most of the patches are build system changes for handling LLVM
bitcode, which Clang produces with LTO instead of ELF object files,
postponing ELF processing until a later stage, and ensuring initcall
ordering.

Note that arm64 support depends on Will's memory ordering patches
[1]. I will post x86_64 patches separately after we have fixed the
remaining objtool warnings [2][3].

[1] https://git.kernel.org/pub/scm/linux/kernel/git/arm64/linux.git/log/?h=for-next/lto
[2] https://lore.kernel.org/lkml/20201120040424.a3wctajzft4ufoiw@treble/
[3] https://git.kernel.org/pub/scm/linux/kernel/git/jpoimboe/linux.git/log/?h=objtool-vmlinux

You can also pull this series from

  https://github.com/samitolvanen/linux.git lto-v8

---
Changes in v8:

  - Cleaned up the LTO Kconfig options based on suggestions from
    Nick and Kees.

  - Dropped the patch to disable LTO for the arm64 nVHE KVM code as
    David pointed out it's not needed anymore.

Changes in v7:

  - Rebased to master again.

  - Added back arm64 patches as the prerequisites are now staged,
    and dropped x86_64 support until the remaining objtool issues
    are resolved.

  - Dropped ifdefs from module.lds.S.

Changes in v6:

  - Added the missing --mcount flag to patch 5.

  - Dropped the arm64 patches from this series and will repost them
    later.

Changes in v5:

  - Rebased on top of tip/master.

  - Changed the command line for objtool to use --vmlinux --duplicate
    to disable warnings about retpoline thunks and to fix .orc_unwind
    generation for vmlinux.o.

  - Added --noinstr flag to objtool, so we can use --vmlinux without
    also enabling noinstr validation.

  - Disabled objtool's unreachable instruction warnings with LTO to
    disable false positives for the int3 padding in vmlinux.o.

  - Added ANNOTATE_RETPOLINE_SAFE annotations to the indirect jumps
    in x86 assembly code to fix objtool warnings with retpoline.

  - Fixed modpost warnings about missing version information with
    CONFIG_MODVERSIONS.

  - Included Makefile.lib into Makefile.modpost for ld_flags. Thanks
    to Sedat for pointing this out.

  - Updated the help text for ThinLTO to better explain the trade-offs.

  - Updated commit messages with better explanations.

Changes in v4:

  - Fixed a typo in Makefile.lib to correctly pass --no-fp to objtool.

  - Moved ftrace configs related to generating __mcount_loc to Kconfig,
    so they are available also in Makefile.modfinal.

  - Dropped two prerequisite patches that were merged to Linus' tree.

Changes in v3:

  - Added a separate patch to remove the unused DISABLE_LTO treewide,
    as filtering out CC_FLAGS_LTO instead is preferred.

  - Updated the Kconfig help to explain why LTO is behind a choice
    and disabled by default.

  - Dropped CC_FLAGS_LTO_CLANG, compiler-specific LTO flags are now
    appended directly to CC_FLAGS_LTO.

  - Updated $(AR) flags as KBUILD_ARFLAGS was removed earlier.

  - Fixed ThinLTO cache handling for external module builds.

  - Rebased on top of Masahiro's patch for preprocessing modules.lds,
    and moved the contents of module-lto.lds to modules.lds.S.

  - Moved objtool_args to Makefile.lib to avoid duplication of the
    command line parameters in Makefile.modfinal.

  - Clarified in the commit message for the initcall ordering patch
    that the initcall order remains the same as without LTO.

  - Changed link-vmlinux.sh to use jobserver-exec to control the
    number of jobs started by generate_initcall_ordering.pl.

  - Dropped the x86/relocs patch to whitelist L4_PAGE_OFFSET as it's
    no longer needed with ToT kernel.

  - Disabled LTO for arch/x86/power/cpu.c to work around a Clang bug
    with stack protector attributes.

Changes in v2:

  - Fixed -Wmissing-prototypes warnings with W=1.

  - Dropped cc-option from -fsplit-lto-unit and added .thinlto-cache
    scrubbing to make distclean.

  - Added a comment about Clang >=11 being required.

  - Added a patch to disable LTO for the arm64 KVM nVHE code.

  - Disabled objtool's noinstr validation with LTO unless enabled.

  - Included Peter's proposed objtool mcount patch in the series
    and replaced recordmcount with the objtool pass to avoid
    whitelisting relocations that are not calls.

  - Updated several commit messages with better explanations.


Sami Tolvanen (17):
  tracing: move function tracer options to Kconfig
  kbuild: add support for Clang LTO
  kbuild: lto: fix module versioning
  kbuild: lto: limit inlining
  kbuild: lto: merge module sections
  kbuild: lto: remove duplicate dependencies from .mod files
  init: lto: ensure initcall ordering
  init: lto: fix PREL32 relocations
  PCI: Fix PREL32 relocations for LTO
  modpost: lto: strip .lto from module names
  scripts/mod: disable LTO for empty.c
  efi/libstub: disable LTO
  drivers/misc/lkdtm: disable LTO for rodata.o
  arm64: vdso: disable LTO
  KVM: arm64: disable LTO for the nVHE directory
  arm64: disable recordmcount with DYNAMIC_FTRACE_WITH_REGS
  arm64: allow LTO_CLANG and THINLTO to be selected

 .gitignore                            |   1 +
 Makefile                              |  45 +++--
 arch/Kconfig                          |  74 +++++++
 arch/arm64/Kconfig                    |   4 +
 arch/arm64/kernel/vdso/Makefile       |   3 +-
 arch/arm64/kvm/hyp/nvhe/Makefile      |   4 +-
 drivers/firmware/efi/libstub/Makefile |   2 +
 drivers/misc/lkdtm/Makefile           |   1 +
 include/asm-generic/vmlinux.lds.h     |  11 +-
 include/linux/init.h                  |  79 +++++++-
 include/linux/pci.h                   |  19 +-
 kernel/trace/Kconfig                  |  16 ++
 scripts/Makefile.build                |  50 ++++-
 scripts/Makefile.lib                  |   6 +-
 scripts/Makefile.modfinal             |   9 +-
 scripts/Makefile.modpost              |  25 ++-
 scripts/generate_initcall_order.pl    | 270 ++++++++++++++++++++++++++
 scripts/link-vmlinux.sh               |  70 ++++++-
 scripts/mod/Makefile                  |   1 +
 scripts/mod/modpost.c                 |  16 +-
 scripts/mod/modpost.h                 |   9 +
 scripts/mod/sumversion.c              |   6 +-
 scripts/module.lds.S                  |  24 +++
 23 files changed, 677 insertions(+), 68 deletions(-)
 create mode 100755 scripts/generate_initcall_order.pl


base-commit: 0fa8ee0d9ab95c9350b8b84574824d9a384a9f7d

Comments

Nick Desaulniers Dec. 3, 2020, 12:01 a.m. UTC | #1
On Tue, Dec 1, 2020 at 1:37 PM Sami Tolvanen <samitolvanen@google.com> wrote:
>
> This patch series adds support for building the kernel with Clang's
> Link Time Optimization (LTO). In addition to performance, the primary
> motivation for LTO is to allow Clang's Control-Flow Integrity (CFI)
> to be used in the kernel. Google has shipped millions of Pixel
> devices running three major kernel versions with LTO+CFI since 2018.
>
> Most of the patches are build system changes for handling LLVM
> bitcode, which Clang produces with LTO instead of ELF object files,
> postponing ELF processing until a later stage, and ensuring initcall
> ordering.
>
> Note that arm64 support depends on Will's memory ordering patches
> [1]. I will post x86_64 patches separately after we have fixed the
> remaining objtool warnings [2][3].
>
> [1] https://git.kernel.org/pub/scm/linux/kernel/git/arm64/linux.git/log/?h=for-next/lto
> [2] https://lore.kernel.org/lkml/20201120040424.a3wctajzft4ufoiw@treble/
> [3] https://git.kernel.org/pub/scm/linux/kernel/git/jpoimboe/linux.git/log/?h=objtool-vmlinux
>
> You can also pull this series from
>
>   https://github.com/samitolvanen/linux.git lto-v8
>
> ---
> Changes in v8:
>
>   - Cleaned up the LTO Kconfig options based on suggestions from
>     Nick and Kees.

Thanks Sami, for the series:

Tested-by: Nick Desaulniers <ndesaulniers@google.com>

(build and boot tested under emulation with
https://git.kernel.org/pub/scm/linux/kernel/git/arm64/linux.git/log/?h=for-next/lto
additionally rebased on top).

As with v7, if the series changes drastically for v9, please consider
dropping my tested by tag for the individual patches that change and I
will help re-test them.
Will Deacon Dec. 3, 2020, 11:26 a.m. UTC | #2
Hi Sami,

On Tue, Dec 01, 2020 at 01:36:51PM -0800, Sami Tolvanen wrote:
> This patch series adds support for building the kernel with Clang's
> Link Time Optimization (LTO). In addition to performance, the primary
> motivation for LTO is to allow Clang's Control-Flow Integrity (CFI)
> to be used in the kernel. Google has shipped millions of Pixel
> devices running three major kernel versions with LTO+CFI since 2018.
> 
> Most of the patches are build system changes for handling LLVM
> bitcode, which Clang produces with LTO instead of ELF object files,
> postponing ELF processing until a later stage, and ensuring initcall
> ordering.
> 
> Note that arm64 support depends on Will's memory ordering patches
> [1]. I will post x86_64 patches separately after we have fixed the
> remaining objtool warnings [2][3].

I took this series for a spin, with my for-next/lto branch merged in but
I see a failure during the LTO stage with clang 11.0.5 because it doesn't
understand the '.arch_extension rcpc' directive we throw out in READ_ONCE().

We actually check that this extension is available before using it in
the arm64 Kconfig:

	config AS_HAS_LDAPR
		def_bool $(as-instr,.arch_extension rcpc)

so this shouldn't happen. I then realised, I wasn't passing LLVM_IAS=1
on my Make command line; with that, then the detection works correctly
and the LTO step succeeds.

Why is it necessary to pass LLVM_IAS=1 if LTO is enabled? I think it
would be _much_ better if this was implicit (or if LTO depended on it).

Cheers,

Will
Sami Tolvanen Dec. 3, 2020, 5:07 p.m. UTC | #3
On Thu, Dec 3, 2020 at 3:26 AM Will Deacon <will@kernel.org> wrote:
>
> Hi Sami,
>
> On Tue, Dec 01, 2020 at 01:36:51PM -0800, Sami Tolvanen wrote:
> > This patch series adds support for building the kernel with Clang's
> > Link Time Optimization (LTO). In addition to performance, the primary
> > motivation for LTO is to allow Clang's Control-Flow Integrity (CFI)
> > to be used in the kernel. Google has shipped millions of Pixel
> > devices running three major kernel versions with LTO+CFI since 2018.
> >
> > Most of the patches are build system changes for handling LLVM
> > bitcode, which Clang produces with LTO instead of ELF object files,
> > postponing ELF processing until a later stage, and ensuring initcall
> > ordering.
> >
> > Note that arm64 support depends on Will's memory ordering patches
> > [1]. I will post x86_64 patches separately after we have fixed the
> > remaining objtool warnings [2][3].
>
> I took this series for a spin, with my for-next/lto branch merged in but
> I see a failure during the LTO stage with clang 11.0.5 because it doesn't
> understand the '.arch_extension rcpc' directive we throw out in READ_ONCE().

I just tested this with Clang 11.0.0, which I believe is the latest
11.x version, and the current Clang 12 development branch, and both
work for me. Godbolt confirms that '.arch_extension rcpc' is supported
by the integrated assembler starting with Clang 11 (the example fails
with 10.0.1):

https://godbolt.org/z/1csGcT

What does running clang --version and ld.lld --version tell you?

> We actually check that this extension is available before using it in
> the arm64 Kconfig:
>
>         config AS_HAS_LDAPR
>                 def_bool $(as-instr,.arch_extension rcpc)
>
> so this shouldn't happen. I then realised, I wasn't passing LLVM_IAS=1
> on my Make command line; with that, then the detection works correctly
> and the LTO step succeeds.
>
> Why is it necessary to pass LLVM_IAS=1 if LTO is enabled? I think it
> would be _much_ better if this was implicit (or if LTO depended on it).

Without LLVM_IAS=1, Clang uses two different assemblers when LTO is
enabled: the external GNU assembler for stand-alone assembly, and
LLVM's integrated assembler for inline assembly. as-instr tests the
external assembler and makes an admittedly reasonable assumption that
the test is also valid for inline assembly.

I agree that it would reduce confusion in future if we just always
enabled IAS with LTO. Nick, Nathan, any thoughts about this?

Sami
Nathan Chancellor Dec. 3, 2020, 6:21 p.m. UTC | #4
On Thu, Dec 03, 2020 at 09:07:30AM -0800, Sami Tolvanen wrote:
> On Thu, Dec 3, 2020 at 3:26 AM Will Deacon <will@kernel.org> wrote:
> >
> > Hi Sami,
> >
> > On Tue, Dec 01, 2020 at 01:36:51PM -0800, Sami Tolvanen wrote:
> > > This patch series adds support for building the kernel with Clang's
> > > Link Time Optimization (LTO). In addition to performance, the primary
> > > motivation for LTO is to allow Clang's Control-Flow Integrity (CFI)
> > > to be used in the kernel. Google has shipped millions of Pixel
> > > devices running three major kernel versions with LTO+CFI since 2018.
> > >
> > > Most of the patches are build system changes for handling LLVM
> > > bitcode, which Clang produces with LTO instead of ELF object files,
> > > postponing ELF processing until a later stage, and ensuring initcall
> > > ordering.
> > >
> > > Note that arm64 support depends on Will's memory ordering patches
> > > [1]. I will post x86_64 patches separately after we have fixed the
> > > remaining objtool warnings [2][3].
> >
> > I took this series for a spin, with my for-next/lto branch merged in but
> > I see a failure during the LTO stage with clang 11.0.5 because it doesn't
> > understand the '.arch_extension rcpc' directive we throw out in READ_ONCE().
> 
> I just tested this with Clang 11.0.0, which I believe is the latest
> 11.x version, and the current Clang 12 development branch, and both
> work for me. Godbolt confirms that '.arch_extension rcpc' is supported
> by the integrated assembler starting with Clang 11 (the example fails
> with 10.0.1):
> 
> https://godbolt.org/z/1csGcT
> 
> What does running clang --version and ld.lld --version tell you?

11.0.5 is AOSP's clang, which is behind the upstream 11.0.0 release so
it is most likely the case that it is missing the patch that added rcpc.
I think that a version based on the development branch (12.0.0) is in
the works but I am not sure.

> > We actually check that this extension is available before using it in
> > the arm64 Kconfig:
> >
> >         config AS_HAS_LDAPR
> >                 def_bool $(as-instr,.arch_extension rcpc)
> >
> > so this shouldn't happen. I then realised, I wasn't passing LLVM_IAS=1
> > on my Make command line; with that, then the detection works correctly
> > and the LTO step succeeds.
> >
> > Why is it necessary to pass LLVM_IAS=1 if LTO is enabled? I think it
> > would be _much_ better if this was implicit (or if LTO depended on it).
> 
> Without LLVM_IAS=1, Clang uses two different assemblers when LTO is
> enabled: the external GNU assembler for stand-alone assembly, and
> LLVM's integrated assembler for inline assembly. as-instr tests the
> external assembler and makes an admittedly reasonable assumption that
> the test is also valid for inline assembly.
> 
> I agree that it would reduce confusion in future if we just always
> enabled IAS with LTO. Nick, Nathan, any thoughts about this?

I am personally fine with that. As far as I am aware, we are in a fairly
good spot on arm64 and x86_64 when it comes to the integrated assembler.
Should we make it so that the user has to pass LLVM_IAS=1 explicitly or
we just make adding the no integrated assembler flag to CLANG_FLAGS
depend on not LTO (although that will require extra handling because
Kconfig is not available at that stage I think)?

Cheers,
Nathan
Will Deacon Dec. 3, 2020, 6:22 p.m. UTC | #5
On Thu, Dec 03, 2020 at 09:07:30AM -0800, Sami Tolvanen wrote:
> On Thu, Dec 3, 2020 at 3:26 AM Will Deacon <will@kernel.org> wrote:
> > On Tue, Dec 01, 2020 at 01:36:51PM -0800, Sami Tolvanen wrote:
> > > This patch series adds support for building the kernel with Clang's
> > > Link Time Optimization (LTO). In addition to performance, the primary
> > > motivation for LTO is to allow Clang's Control-Flow Integrity (CFI)
> > > to be used in the kernel. Google has shipped millions of Pixel
> > > devices running three major kernel versions with LTO+CFI since 2018.
> > >
> > > Most of the patches are build system changes for handling LLVM
> > > bitcode, which Clang produces with LTO instead of ELF object files,
> > > postponing ELF processing until a later stage, and ensuring initcall
> > > ordering.
> > >
> > > Note that arm64 support depends on Will's memory ordering patches
> > > [1]. I will post x86_64 patches separately after we have fixed the
> > > remaining objtool warnings [2][3].
> >
> > I took this series for a spin, with my for-next/lto branch merged in but
> > I see a failure during the LTO stage with clang 11.0.5 because it doesn't
> > understand the '.arch_extension rcpc' directive we throw out in READ_ONCE().
> 
> I just tested this with Clang 11.0.0, which I believe is the latest
> 11.x version, and the current Clang 12 development branch, and both
> work for me. Godbolt confirms that '.arch_extension rcpc' is supported
> by the integrated assembler starting with Clang 11 (the example fails
> with 10.0.1):
> 
> https://godbolt.org/z/1csGcT
> 
> What does running clang --version and ld.lld --version tell you?

I'm using some Android prebuilts I had kicking around:

Android (6875598, based on r399163b) clang version 11.0.5 (https://android.googlesource.com/toolchain/llvm-project 87f1315dfbea7c137aa2e6d362dbb457e388158d)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /usr/local/google/home/willdeacon/work/android/repo/android-kernel/prebuilts-master/clang/host/linux-x86/clang-r399163b/bin

and:

LLD 11.0.5 (/buildbot/tmp/tmpx1DlI_ 87f1315dfbea7c137aa2e6d362dbb457e388158d) (compatible with GNU linkers)

> > We actually check that this extension is available before using it in
> > the arm64 Kconfig:
> >
> >         config AS_HAS_LDAPR
> >                 def_bool $(as-instr,.arch_extension rcpc)
> >
> > so this shouldn't happen. I then realised, I wasn't passing LLVM_IAS=1
> > on my Make command line; with that, then the detection works correctly
> > and the LTO step succeeds.
> >
> > Why is it necessary to pass LLVM_IAS=1 if LTO is enabled? I think it
> > would be _much_ better if this was implicit (or if LTO depended on it).
> 
> Without LLVM_IAS=1, Clang uses two different assemblers when LTO is
> enabled: the external GNU assembler for stand-alone assembly, and
> LLVM's integrated assembler for inline assembly. as-instr tests the
> external assembler and makes an admittedly reasonable assumption that
> the test is also valid for inline assembly.
> 
> I agree that it would reduce confusion in future if we just always
> enabled IAS with LTO. Nick, Nathan, any thoughts about this?

That works for me, although I'm happy with anything which means that the
assembler checks via as-instr apply to the assembler which will ultimately
be used.

Will
Nick Desaulniers Dec. 3, 2020, 10:32 p.m. UTC | #6
On Thu, Dec 3, 2020 at 10:23 AM Will Deacon <will@kernel.org> wrote:
>
> On Thu, Dec 03, 2020 at 09:07:30AM -0800, Sami Tolvanen wrote:
> > On Thu, Dec 3, 2020 at 3:26 AM Will Deacon <will@kernel.org> wrote:
> > > I took this series for a spin, with my for-next/lto branch merged in but
> > > I see a failure during the LTO stage with clang 11.0.5 because it doesn't
> > > understand the '.arch_extension rcpc' directive we throw out in READ_ONCE().
> >
> > I just tested this with Clang 11.0.0, which I believe is the latest
> > 11.x version, and the current Clang 12 development branch, and both
> > work for me. Godbolt confirms that '.arch_extension rcpc' is supported
> > by the integrated assembler starting with Clang 11 (the example fails
> > with 10.0.1):
> >
> > https://godbolt.org/z/1csGcT
> >
> > What does running clang --version and ld.lld --version tell you?
>
> I'm using some Android prebuilts I had kicking around:
>
> Android (6875598, based on r399163b) clang version 11.0.5 (https://android.googlesource.com/toolchain/llvm-project 87f1315dfbea7c137aa2e6d362dbb457e388158d)
> Target: x86_64-unknown-linux-gnu
> Thread model: posix
> InstalledDir: /usr/local/google/home/willdeacon/work/android/repo/android-kernel/prebuilts-master/clang/host/linux-x86/clang-r399163b/bin
>
> and:
>
> LLD 11.0.5 (/buildbot/tmp/tmpx1DlI_ 87f1315dfbea7c137aa2e6d362dbb457e388158d) (compatible with GNU linkers)

On Thu, Dec 3, 2020 at 10:22 AM Nathan Chancellor
<natechancellor@gmail.com> wrote:
>
> 11.0.5 is AOSP's clang, which is behind the upstream 11.0.0 release so
> it is most likely the case that it is missing the patch that added rcpc.
> I think that a version based on the development branch (12.0.0) is in
> the works but I am not sure.

Yep, I have a lot of thoughts on the AOSP LLVM versioning scheme, but
they're not for LKML.  That's yet another reason to prefer feature
detection as opposed to brittle version checks.  Of course, as Will
points out, if your feature detection is broken, that helps no
one...more thoughts below.

> > > We actually check that this extension is available before using it in
> > > the arm64 Kconfig:
> > >
> > >         config AS_HAS_LDAPR
> > >                 def_bool $(as-instr,.arch_extension rcpc)
> > >
> > > so this shouldn't happen. I then realised, I wasn't passing LLVM_IAS=1
> > > on my Make command line; with that, then the detection works correctly
> > > and the LTO step succeeds.
> > >
> > > Why is it necessary to pass LLVM_IAS=1 if LTO is enabled? I think it
> > > would be _much_ better if this was implicit (or if LTO depended on it).
> >
> > Without LLVM_IAS=1, Clang uses two different assemblers when LTO is
> > enabled: the external GNU assembler for stand-alone assembly, and
> > LLVM's integrated assembler for inline assembly. as-instr tests the
> > external assembler and makes an admittedly reasonable assumption that
> > the test is also valid for inline assembly.
> >
> > I agree that it would reduce confusion in future if we just always
> > enabled IAS with LTO. Nick, Nathan, any thoughts about this?
>
> That works for me, although I'm happy with anything which means that the
> assembler checks via as-instr apply to the assembler which will ultimately
> be used.

I agree with Will.

I think interoperability of tools is important.  We should be able to
mix tools from GNU and LLVM and produce working images. Specifically,
combinations like gcc+llvm-nm+as+llvm-objcopy, or clang+nm+as+objcopy
as two examples.  There's a combinatorial explosion of combinations to
test/validate, which we're not doing today, but if for some reason
someone wants to use some varied combination and it doesn't work, it's
worthwhile to understand the differences and issues and try to fix
them.  That is a win for optionality and loose coupling.

That's not what's going on here though.

While I think it's ok to select a compiler and assembler and linker
etc from ecosystem or another, I think trying to support a build that
mixes or uses different assemblers (or linkers, compilers, etc) from
both for the same build is something we should draw a line in the sand
and explicitly not support (except for the compat vdso's*...).  ie. if
I say `make LD=ld.bfd` and ld.lld gets invoked somehow (or vice
versa); I consider that a bug in KBUILD.

That is what's happening here, it's why as-instr feature detection is
broken; because two different assemblers were used in the same build.
One for inline asm, a different one for out of line asm.  At the very
least, it violates the Principle of Least Surprise (or is it the Law
of Equivalent Exchange, I forget).

In fact, lots of the work we've been doing to enable LLVM tools to
build the kernel have been identifying places throughout KBUILD where
tools were hardcoded rather than using what make was told to use, and
we've been making progress fixing those.  The ultimate test of Linux
kernel build hermiticity IMO is that I should be able to build a
kernel in an environment that only has one version of either
GCC/binutils or LLVM, and the kernel should build without failure.
That's not the case today for all arch's; cross compiling compat vdsos
again are a major pain point*, but we're making progress.  In that
sense, the mixing of an individual GNU and LLVM utility is what I
would consider a bug in KBUILD.  I want to emphasize that's distinct
from mixing and matching tools when invoking make, which I consider
OK, if under-tested.

Ok (mixes GNU and LLVM tools; gcc is the only compiler invoked, ld.lld
is the only linker invoked):
$ make CC=gcc LD=ld.lld

Not ok (if ld.bfd or both are invoked)
$ make LD=ld.lld

Not ok (if ld.lld or both are invoked)
$ make LD=ld.bfd

Not ok (if clang's integrated assembler and GAS are invoked)
$ ./scripts/config -e LTO_CLANG
$ make LLVM=1 LLVM_IAS=1

The mixing of GAS and clang's integrated assembler for kernel LTO
builds is a relic of a time when this series was first written when
Clang's integrated assembler was in no form ready to assemble the
entire Linux kernel, but could handle the inline asm for aarch64.
Fortunately, ARM's LLVM team has done great work to ensure the latest
extensions like RCpc are supported and compatible, and Jian has done
the hard work ironing out the last mile issues in clang's assembler to
get the ball in the end zone.  Removing mixing GAS and clang's IA here
ups the ante and removes a fallback/pressure relief valve, but I'm
fine with that.  Requiring clang's integrated assembler here aligns
incentives to keep this working and to continue investing here.

Just because it's possible to mix the use of clang's integrated
assembler with GNU assembler for LTO (for some combination of versions
of these tools) doesn't mean we should support it, or encourage it,
for all of the reasons above.  We should make this config depend on
clang's integrated assembler, and not support the mixing of assemblers
in one build.

Thou shalt not support invoking of different tools than what's
specified*.  Do not pass go, do not collect $200. Full stop.

* The compat vdso's are again a special case; when cross compiling
using GNU tools, a separate binary with a different target triple
prefix will typically get invoked than what's used to build the rest
of the kernel image.  This still doesn't cross the GNU/LLVM boundary
though, and most importantly doesn't involve linking together object
files that were built with distinct assemblers (for example).

So I'd recommend to Sami to simply make the Kconfig also depend on
clang's integrated assembler (not just llvm-nm and llvm-ar).  If
someone cares about LTO with Clang as the compiler but GAS as the
assembler, then we can revisit supporting that combination (and the
changes to KCONFIG), but it shouldn't be something we consider Tier 1
supported or a combination that need be supported in a minimum viable
product. And at that point we should make it avoid clang's integrated
assembler entirely (I suspect LTO won't work at all in that case, so
maybe even considering it is a waste of time).

One question I have to Will; if for aarch64 LTO will depend on RCpc,
but RCpc is an ARMv8.3 extension, what are the implications for LTO on
pre-ARMv8.3 aarch64 processors?
Will Deacon Dec. 4, 2020, 9:35 a.m. UTC | #7
On Thu, Dec 03, 2020 at 02:32:13PM -0800, Nick Desaulniers wrote:
> On Thu, Dec 3, 2020 at 10:23 AM Will Deacon <will@kernel.org> wrote:
> > On Thu, Dec 03, 2020 at 09:07:30AM -0800, Sami Tolvanen wrote:
> > > Without LLVM_IAS=1, Clang uses two different assemblers when LTO is
> > > enabled: the external GNU assembler for stand-alone assembly, and
> > > LLVM's integrated assembler for inline assembly. as-instr tests the
> > > external assembler and makes an admittedly reasonable assumption that
> > > the test is also valid for inline assembly.
> > >
> > > I agree that it would reduce confusion in future if we just always
> > > enabled IAS with LTO. Nick, Nathan, any thoughts about this?
> >
> > That works for me, although I'm happy with anything which means that the
> > assembler checks via as-instr apply to the assembler which will ultimately
> > be used.
> 
> I agree with Will.

[...]

> So I'd recommend to Sami to simply make the Kconfig also depend on
> clang's integrated assembler (not just llvm-nm and llvm-ar).  If
> someone cares about LTO with Clang as the compiler but GAS as the
> assembler, then we can revisit supporting that combination (and the
> changes to KCONFIG), but it shouldn't be something we consider Tier 1
> supported or a combination that need be supported in a minimum viable
> product. And at that point we should make it avoid clang's integrated
> assembler entirely (I suspect LTO won't work at all in that case, so
> maybe even considering it is a waste of time).
> 
> One question I have to Will; if for aarch64 LTO will depend on RCpc,
> but RCpc is an ARMv8.3 extension, what are the implications for LTO on
> pre-ARMv8.3 aarch64 processors?

It doesn't depend on RCpc -- we just emit a more expensive instruction
(an RCsc acquire) if the RCpc one is not supported by both the toolchain
and the CPU. So the implication for those processors is that READ_ONCE()
may be more expensive.

Will
Sami Tolvanen Dec. 4, 2020, 10:52 p.m. UTC | #8
On Thu, Dec 3, 2020 at 2:32 PM Nick Desaulniers <ndesaulniers@google.com> wrote:
>
> So I'd recommend to Sami to simply make the Kconfig also depend on
> clang's integrated assembler (not just llvm-nm and llvm-ar).

Sure, sounds good to me. What's the preferred way to test for this in Kconfig?

It looks like actually trying to test if we have an LLVM assembler
(e.g. using $(as-instr,.section
".linker-options","e",@llvm_linker_options)) doesn't work as Kconfig
doesn't pass -no-integrated-as to clang here. I could do something
simple like $(success,echo $(LLVM) $(LLVM_IAS) | grep -q "1 1").

Thoughts?

Sami
Nathan Chancellor Dec. 6, 2020, 6:50 a.m. UTC | #9
On Fri, Dec 04, 2020 at 02:52:41PM -0800, Sami Tolvanen wrote:
> On Thu, Dec 3, 2020 at 2:32 PM Nick Desaulniers <ndesaulniers@google.com> wrote:
> >
> > So I'd recommend to Sami to simply make the Kconfig also depend on
> > clang's integrated assembler (not just llvm-nm and llvm-ar).
> 
> Sure, sounds good to me. What's the preferred way to test for this in Kconfig?
> 
> It looks like actually trying to test if we have an LLVM assembler
> (e.g. using $(as-instr,.section
> ".linker-options","e",@llvm_linker_options)) doesn't work as Kconfig
> doesn't pass -no-integrated-as to clang here. I could do something
> simple like $(success,echo $(LLVM) $(LLVM_IAS) | grep -q "1 1").
> 
> Thoughts?
> 
> Sami

I think

    depends on $(success,test $(LLVM_IAS) -eq 1)

should work, at least according to my brief test.

Cheers,
Nathan
Sami Tolvanen Dec. 6, 2020, 8:09 p.m. UTC | #10
On Sat, Dec 5, 2020 at 10:50 PM Nathan Chancellor
<natechancellor@gmail.com> wrote:
>
> On Fri, Dec 04, 2020 at 02:52:41PM -0800, Sami Tolvanen wrote:
> > On Thu, Dec 3, 2020 at 2:32 PM Nick Desaulniers <ndesaulniers@google.com> wrote:
> > >
> > > So I'd recommend to Sami to simply make the Kconfig also depend on
> > > clang's integrated assembler (not just llvm-nm and llvm-ar).
> >
> > Sure, sounds good to me. What's the preferred way to test for this in Kconfig?
> >
> > It looks like actually trying to test if we have an LLVM assembler
> > (e.g. using $(as-instr,.section
> > ".linker-options","e",@llvm_linker_options)) doesn't work as Kconfig
> > doesn't pass -no-integrated-as to clang here.

After a closer look, that's actually not correct, this seems to work
with Clang+LLD no matter which assembler is used. I suppose we could
test for .gasversion. to detect GNU as, but that's hardly ideal.

> >I could do something
> > simple like $(success,echo $(LLVM) $(LLVM_IAS) | grep -q "1 1").
> >
> > Thoughts?
> >
> > Sami
>
> I think
>
>     depends on $(success,test $(LLVM_IAS) -eq 1)
>
> should work, at least according to my brief test.

Sure, looks good to me. However, I think we should also test for
LLVM=1 to avoid possible further issues with mismatched toolchains
instead of only checking for llvm-nm and llvm-ar.

Sami
Nathan Chancellor Dec. 8, 2020, 12:46 a.m. UTC | #11
On Sun, Dec 06, 2020 at 12:09:31PM -0800, Sami Tolvanen wrote:
> Sure, looks good to me. However, I think we should also test for
> LLVM=1 to avoid possible further issues with mismatched toolchains
> instead of only checking for llvm-nm and llvm-ar.

It might still be worth testing for $(AR) and $(NM) because in theory, a
user could say 'make AR=ar LLVM=1'. Highly unlikely I suppose but worth
considering.

Cheers,
Nathan
Arnd Bergmann Dec. 8, 2020, 12:15 p.m. UTC | #12
On Tue, Dec 1, 2020 at 10:37 PM 'Sami Tolvanen' via Clang Built Linux
<clang-built-linux@googlegroups.com> wrote:
>
> This patch series adds support for building the kernel with Clang's
> Link Time Optimization (LTO). In addition to performance, the primary
> motivation for LTO is to allow Clang's Control-Flow Integrity (CFI)
> to be used in the kernel. Google has shipped millions of Pixel
> devices running three major kernel versions with LTO+CFI since 2018.
>
> Most of the patches are build system changes for handling LLVM
> bitcode, which Clang produces with LTO instead of ELF object files,
> postponing ELF processing until a later stage, and ensuring initcall
> ordering.
>
> Note that arm64 support depends on Will's memory ordering patches
> [1]. I will post x86_64 patches separately after we have fixed the
> remaining objtool warnings [2][3].
>
> [1] https://git.kernel.org/pub/scm/linux/kernel/git/arm64/linux.git/log/?h=for-next/lto
> [2] https://lore.kernel.org/lkml/20201120040424.a3wctajzft4ufoiw@treble/
> [3] https://git.kernel.org/pub/scm/linux/kernel/git/jpoimboe/linux.git/log/?h=objtool-vmlinux
>
> You can also pull this series from
>
>   https://github.com/samitolvanen/linux.git lto-v8

I've tried pull this into my randconfig test tree to give it a spin.
So far I have
not managed to get a working build out of it, the main problem so far being
that it is really slow to build because the link stage only uses one CPU.
These are the other issues I've seen so far:

- one build seems to take even longer to link. It's currently at 35GB RAM
  usage and 40 minutes into the final link, but I'm worried it might
not complete
  before it runs out of memory.  I only have 128GB installed, and google-chrome
  uses another 30GB of that, and I'm also doing some other builds in parallel.
  Is there a minimum recommended amount of memory for doing LTO builds?

- One build failed with
 ld.lld -EL -maarch64elf -mllvm -import-instr-limit=5 -r -o vmlinux.o
-T .tmp_initcalls.lds --whole-archive arch/arm64/kernel/head.o
init/built-in.a usr/built-in.a arch/arm64/built-in.a kernel/built-in.a
certs/built-in.a mm/built-in.a fs/built-in.a ipc/built-in.a
security/built-in.a crypto/built-in.a block/built-in.a
arch/arm64/lib/built-in.a lib/built-in.a drivers/built-in.a
sound/built-in.a net/built-in.a virt/built-in.a --no-whole-archive
--start-group arch/arm64/lib/lib.a lib/lib.a
./drivers/firmware/efi/libstub/lib.a --end-group
  "ld.lld: error: arch/arm64/kernel/head.o: invalid symbol index"
  after about 30 minutes

- CONFIG_CPU_BIG_ENDIAN doesn't seem to work with lld, and LTO
  doesn't work with ld.bfd.
  I've added a CPU_LITTLE_ENDIAN dependency to
  ARCH_SUPPORTS_LTO_CLANG{,THIN}

- one build failed with
  "ld.lld: error: Never resolved function from blockaddress (Producer:
'LLVM12.0.0' Reader: 'LLVM 12.0.0')"
  Not sure how to debug this

- one build seems to have dropped all symbols the string operations
from vmlinux,
  so while the link goes through, modules cannot be loaded:
 ERROR: modpost: "memmove" [drivers/media/rc/rc-core.ko] undefined!
 ERROR: modpost: "memcpy" [net/wireless/cfg80211.ko] undefined!
 ERROR: modpost: "memcpy" [net/8021q/8021q.ko] undefined!
 ERROR: modpost: "memset" [net/8021q/8021q.ko] undefined!
 ERROR: modpost: "memcpy" [net/unix/unix.ko] undefined!
 ERROR: modpost: "memset" [net/sched/cls_u32.ko] undefined!
 ERROR: modpost: "memcpy" [net/sched/cls_u32.ko] undefined!
 ERROR: modpost: "memset" [net/sched/sch_skbprio.ko] undefined!
 ERROR: modpost: "memcpy" [net/802/garp.ko] undefined!
 I first thought this was related to a clang-12 bug I saw the other day, but
 this also happens with clang-11

- many builds complain about thousands of duplicate symbols in the kernel, e.g.
  ld.lld: error: duplicate symbol: qrtr_endpoint_post
 >>> defined in net/qrtr/qrtr.lto.o
 >>> defined in net/qrtr/qrtr.o
 ld.lld: error: duplicate symbol: init_module
 >>> defined in crypto/842.lto.o
 >>> defined in crypto/842.o
 ld.lld: error: duplicate symbol: init_module
 >>> defined in net/netfilter/nfnetlink_log.lto.o
 >>> defined in net/netfilter/nfnetlink_log.o
 ld.lld: error: duplicate symbol: vli_from_be64
 >>> defined in crypto/ecc.lto.o
 >>> defined in crypto/ecc.o
 ld.lld: error: duplicate symbol: __mod_of__plldig_clk_id_device_table
 >>> defined in drivers/clk/clk-plldig.lto.o
 >>> defined in drivers/clk/clk-plldig.o

Not sure if these are all known issues. If there is one you'd like me try
take a closer look at for finding which config options break it, I can try

     Arnd
Arnd Bergmann Dec. 8, 2020, 1:54 p.m. UTC | #13
On Tue, Dec 8, 2020 at 1:15 PM Arnd Bergmann <arnd@kernel.org> wrote:
> On Tue, Dec 1, 2020 at 10:37 PM 'Sami Tolvanen' via Clang Built Linux <clang-built-linux@googlegroups.com> wrote:
>
> - many builds complain about thousands of duplicate symbols in the kernel, e.g.
>   ld.lld: error: duplicate symbol: qrtr_endpoint_post
>  >>> defined in net/qrtr/qrtr.lto.o
>  >>> defined in net/qrtr/qrtr.o
>  ld.lld: error: duplicate symbol: init_module
>  >>> defined in crypto/842.lto.o
>  >>> defined in crypto/842.o
>  ld.lld: error: duplicate symbol: init_module
>  >>> defined in net/netfilter/nfnetlink_log.lto.o
>  >>> defined in net/netfilter/nfnetlink_log.o
>  ld.lld: error: duplicate symbol: vli_from_be64
>  >>> defined in crypto/ecc.lto.o
>  >>> defined in crypto/ecc.o
>  ld.lld: error: duplicate symbol: __mod_of__plldig_clk_id_device_table
>  >>> defined in drivers/clk/clk-plldig.lto.o
>  >>> defined in drivers/clk/clk-plldig.o

A small update here: I see this behavior with every single module
build, including 'tinyconfig' with one module enabled, and 'defconfig'.

I tuned the randconfig setting using KCONFIG_PROBABILITY=2:2:1
now, which only enables a few symbols. With this I see faster build
times (obvioulsy), aroudn 30 seconds per kernel, and all small builds
with CONFIG_MODULES disabled so far succeed.
It appears that the problems I saw originally only happen for larger
configurations, or possibly a combination of Kconfig options that don't
happen that often on randconfig builds with low
KCONFIG_PROBABILITY.

      Arnd
Sami Tolvanen Dec. 8, 2020, 4:43 p.m. UTC | #14
On Tue, Dec 8, 2020 at 4:15 AM Arnd Bergmann <arnd@kernel.org> wrote:
>
> On Tue, Dec 1, 2020 at 10:37 PM 'Sami Tolvanen' via Clang Built Linux
> <clang-built-linux@googlegroups.com> wrote:
> >
> > This patch series adds support for building the kernel with Clang's
> > Link Time Optimization (LTO). In addition to performance, the primary
> > motivation for LTO is to allow Clang's Control-Flow Integrity (CFI)
> > to be used in the kernel. Google has shipped millions of Pixel
> > devices running three major kernel versions with LTO+CFI since 2018.
> >
> > Most of the patches are build system changes for handling LLVM
> > bitcode, which Clang produces with LTO instead of ELF object files,
> > postponing ELF processing until a later stage, and ensuring initcall
> > ordering.
> >
> > Note that arm64 support depends on Will's memory ordering patches
> > [1]. I will post x86_64 patches separately after we have fixed the
> > remaining objtool warnings [2][3].
> >
> > [1] https://git.kernel.org/pub/scm/linux/kernel/git/arm64/linux.git/log/?h=for-next/lto
> > [2] https://lore.kernel.org/lkml/20201120040424.a3wctajzft4ufoiw@treble/
> > [3] https://git.kernel.org/pub/scm/linux/kernel/git/jpoimboe/linux.git/log/?h=objtool-vmlinux
> >
> > You can also pull this series from
> >
> >   https://github.com/samitolvanen/linux.git lto-v8
>
> I've tried pull this into my randconfig test tree to give it a spin.

Great, thank you for testing this!

> So far I have
> not managed to get a working build out of it, the main problem so far being
> that it is really slow to build because the link stage only uses one CPU.
> These are the other issues I've seen so far:

You may want to limit your testing only to ThinLTO at first, because
full LTO is going to be extremely slow with larger configs, especially
when building arm64 kernels.

> - one build seems to take even longer to link. It's currently at 35GB RAM
>   usage and 40 minutes into the final link, but I'm worried it might
> not complete
>   before it runs out of memory.  I only have 128GB installed, and google-chrome
>   uses another 30GB of that, and I'm also doing some other builds in parallel.
>   Is there a minimum recommended amount of memory for doing LTO builds?

When building arm64 defconfig, the maximum memory usage I measured
with ThinLTO was 3.5 GB, and with full LTO 20.3 GB. I haven't measured
larger configurations, but I believe LLD can easily consume 3-4x that
much with full LTO allyesconfig.

> - One build failed with
>  ld.lld -EL -maarch64elf -mllvm -import-instr-limit=5 -r -o vmlinux.o
> -T .tmp_initcalls.lds --whole-archive arch/arm64/kernel/head.o
> init/built-in.a usr/built-in.a arch/arm64/built-in.a kernel/built-in.a
> certs/built-in.a mm/built-in.a fs/built-in.a ipc/built-in.a
> security/built-in.a crypto/built-in.a block/built-in.a
> arch/arm64/lib/built-in.a lib/built-in.a drivers/built-in.a
> sound/built-in.a net/built-in.a virt/built-in.a --no-whole-archive
> --start-group arch/arm64/lib/lib.a lib/lib.a
> ./drivers/firmware/efi/libstub/lib.a --end-group
>   "ld.lld: error: arch/arm64/kernel/head.o: invalid symbol index"
>   after about 30 minutes

That's interesting. Did you use LLVM_IAS=1?

> - CONFIG_CPU_BIG_ENDIAN doesn't seem to work with lld, and LTO
>   doesn't work with ld.bfd.
>   I've added a CPU_LITTLE_ENDIAN dependency to
>   ARCH_SUPPORTS_LTO_CLANG{,THIN}

Ah, good point. I'll fix this in v9.

[...]
> Not sure if these are all known issues. If there is one you'd like me try
> take a closer look at for finding which config options break it, I can try

No, none of these are known issues. I would be happy to take a closer
look if you can share configs that reproduce these.

Sami
Sami Tolvanen Dec. 8, 2020, 4:53 p.m. UTC | #15
On Tue, Dec 8, 2020 at 5:55 AM Arnd Bergmann <arnd@kernel.org> wrote:
>
> On Tue, Dec 8, 2020 at 1:15 PM Arnd Bergmann <arnd@kernel.org> wrote:
> > On Tue, Dec 1, 2020 at 10:37 PM 'Sami Tolvanen' via Clang Built Linux <clang-built-linux@googlegroups.com> wrote:
> >
> > - many builds complain about thousands of duplicate symbols in the kernel, e.g.
> >   ld.lld: error: duplicate symbol: qrtr_endpoint_post
> >  >>> defined in net/qrtr/qrtr.lto.o
> >  >>> defined in net/qrtr/qrtr.o
> >  ld.lld: error: duplicate symbol: init_module
> >  >>> defined in crypto/842.lto.o
> >  >>> defined in crypto/842.o
> >  ld.lld: error: duplicate symbol: init_module
> >  >>> defined in net/netfilter/nfnetlink_log.lto.o
> >  >>> defined in net/netfilter/nfnetlink_log.o
> >  ld.lld: error: duplicate symbol: vli_from_be64
> >  >>> defined in crypto/ecc.lto.o
> >  >>> defined in crypto/ecc.o
> >  ld.lld: error: duplicate symbol: __mod_of__plldig_clk_id_device_table
> >  >>> defined in drivers/clk/clk-plldig.lto.o
> >  >>> defined in drivers/clk/clk-plldig.o
>
> A small update here: I see this behavior with every single module
> build, including 'tinyconfig' with one module enabled, and 'defconfig'.

The .o file here is a thin archive of the bitcode files for the
module. We compile .lto.o from that before modpost, because we need an
ELF binary to process, and then reuse the .lto.o file when linking the
final module.

At no point should we link the .o file again, especially not with
.lto.o, because that would clearly cause every symbol to be
duplicated, so I'm not sure what goes wrong here. Here's the relevant
part of scripts/Makefile.modfinal:

ifdef CONFIG_LTO_CLANG
# With CONFIG_LTO_CLANG, reuse the object file we compiled for modpost to
# avoid a second slow LTO link
prelink-ext := .lto
...
$(modules): %.ko: %$(prelink-ext).o %.mod.o scripts/module.lds FORCE
        +$(call if_changed,ld_ko_o)

Sami
Arnd Bergmann Dec. 8, 2020, 4:56 p.m. UTC | #16
On Tue, Dec 8, 2020 at 5:53 PM 'Sami Tolvanen' via Clang Built Linux
<clang-built-linux@googlegroups.com> wrote:
>
> > A small update here: I see this behavior with every single module
> > build, including 'tinyconfig' with one module enabled, and 'defconfig'.
>
> The .o file here is a thin archive of the bitcode files for the
> module. We compile .lto.o from that before modpost, because we need an
> ELF binary to process, and then reuse the .lto.o file when linking the
> final module.
>
> At no point should we link the .o file again, especially not with
> .lto.o, because that would clearly cause every symbol to be
> duplicated, so I'm not sure what goes wrong here. Here's the relevant
> part of scripts/Makefile.modfinal:
>
> ifdef CONFIG_LTO_CLANG
> # With CONFIG_LTO_CLANG, reuse the object file we compiled for modpost to
> # avoid a second slow LTO link
> prelink-ext := .lto
> ...
> $(modules): %.ko: %$(prelink-ext).o %.mod.o scripts/module.lds FORCE
>         +$(call if_changed,ld_ko_o)

Ah, it's probably a local problem now, as I had a merge conflict against
linux-next in this Makefile and I must have resolved the conflict incorrectly.

        Arnd
Arnd Bergmann Dec. 8, 2020, 8:59 p.m. UTC | #17
On Tue, Dec 8, 2020 at 5:43 PM 'Sami Tolvanen' via Clang Built Linux
<clang-built-linux@googlegroups.com> wrote:
>
> On Tue, Dec 8, 2020 at 4:15 AM Arnd Bergmann <arnd@kernel.org> wrote:
> >
> > On Tue, Dec 1, 2020 at 10:37 PM 'Sami Tolvanen' via Clang Built Linux
> > <clang-built-linux@googlegroups.com> wrote:
> > >
> > > This patch series adds support for building the kernel with Clang's
> > > Link Time Optimization (LTO). In addition to performance, the primary
> > > motivation for LTO is to allow Clang's Control-Flow Integrity (CFI)
> > > to be used in the kernel. Google has shipped millions of Pixel
> > > devices running three major kernel versions with LTO+CFI since 2018.
> > >
> > > Most of the patches are build system changes for handling LLVM
> > > bitcode, which Clang produces with LTO instead of ELF object files,
> > > postponing ELF processing until a later stage, and ensuring initcall
> > > ordering.
> > >
> > > Note that arm64 support depends on Will's memory ordering patches
> > > [1]. I will post x86_64 patches separately after we have fixed the
> > > remaining objtool warnings [2][3].
> > >
> > > [1] https://git.kernel.org/pub/scm/linux/kernel/git/arm64/linux.git/log/?h=for-next/lto
> > > [2] https://lore.kernel.org/lkml/20201120040424.a3wctajzft4ufoiw@treble/
> > > [3] https://git.kernel.org/pub/scm/linux/kernel/git/jpoimboe/linux.git/log/?h=objtool-vmlinux
> > >
> > > You can also pull this series from
> > >
> > >   https://github.com/samitolvanen/linux.git lto-v8
> >
> > I've tried pull this into my randconfig test tree to give it a spin.
>
> Great, thank you for testing this!
>
> > So far I have
> > not managed to get a working build out of it, the main problem so far being
> > that it is really slow to build because the link stage only uses one CPU.
> > These are the other issues I've seen so far:
>
> You may want to limit your testing only to ThinLTO at first, because
> full LTO is going to be extremely slow with larger configs, especially
> when building arm64 kernels.

Ok, that seems to solve most of the remaining problems after I fixed
the module linking bug I introduced.

> > - one build seems to take even longer to link. It's currently at 35GB RAM
> >   usage and 40 minutes into the final link, but I'm worried it might
> > not complete
> >   before it runs out of memory.  I only have 128GB installed, and google-chrome
> >   uses another 30GB of that, and I'm also doing some other builds in parallel.
> >   Is there a minimum recommended amount of memory for doing LTO builds?
>
> When building arm64 defconfig, the maximum memory usage I measured
> with ThinLTO was 3.5 GB, and with full LTO 20.3 GB. I haven't measured
> larger configurations, but I believe LLD can easily consume 3-4x that
> much with full LTO allyesconfig.

Ok, that's not too bad then. Is there actually a reason to still
support full-lto
in your series? As I understand it, full LTO was the initial approach and
used to work better, but thin LTO is actually what we want to use in the
long run. Perhaps dropping the full LTO option from your series now
that thin LTO works well enough and uses less resources would help
avoid some of the problems.

> > - One build failed with
> >  ld.lld -EL -maarch64elf -mllvm -import-instr-limit=5 -r -o vmlinux.o
> > -T .tmp_initcalls.lds --whole-archive arch/arm64/kernel/head.o
> > init/built-in.a usr/built-in.a arch/arm64/built-in.a kernel/built-in.a
> > certs/built-in.a mm/built-in.a fs/built-in.a ipc/built-in.a
> > security/built-in.a crypto/built-in.a block/built-in.a
> > arch/arm64/lib/built-in.a lib/built-in.a drivers/built-in.a
> > sound/built-in.a net/built-in.a virt/built-in.a --no-whole-archive
> > --start-group arch/arm64/lib/lib.a lib/lib.a
> > ./drivers/firmware/efi/libstub/lib.a --end-group
> >   "ld.lld: error: arch/arm64/kernel/head.o: invalid symbol index"
> >   after about 30 minutes
>
> That's interesting. Did you use LLVM_IAS=1?

I think I did, but it's possible that one of my build scripts didn't pass
that along correctly. This one seems to be gone with thin LTO.

> [...]
> > Not sure if these are all known issues. If there is one you'd like me try
> > take a closer look at for finding which config options break it, I can try
>
> No, none of these are known issues. I would be happy to take a closer
> look if you can share configs that reproduce these.

Attaching the config for "ld.lld: error: Never resolved function from
  blockaddress (Producer: 'LLVM12.0.0' Reader: 'LLVM 12.0.0')"

     Arnd
Arnd Bergmann Dec. 8, 2020, 9:02 p.m. UTC | #18
On Tue, Dec 8, 2020 at 9:59 PM Arnd Bergmann <arnd@kernel.org> wrote:
>
> Attaching the config for "ld.lld: error: Never resolved function from
>   blockaddress (Producer: 'LLVM12.0.0' Reader: 'LLVM 12.0.0')"

And here is a new one: "ld.lld: error: assignment to symbol
init_pg_end does not converge"

      Arnd
Nick Desaulniers Dec. 8, 2020, 9:09 p.m. UTC | #19
On Tue, Dec 8, 2020 at 1:00 PM Arnd Bergmann <arnd@kernel.org> wrote:
>
> On Tue, Dec 8, 2020 at 5:43 PM 'Sami Tolvanen' via Clang Built Linux
> <clang-built-linux@googlegroups.com> wrote:
> >
> > On Tue, Dec 8, 2020 at 4:15 AM Arnd Bergmann <arnd@kernel.org> wrote:
> > >
> > > - one build seems to take even longer to link. It's currently at 35GB RAM
> > >   usage and 40 minutes into the final link, but I'm worried it might
> > > not complete
> > >   before it runs out of memory.  I only have 128GB installed, and google-chrome
> > >   uses another 30GB of that, and I'm also doing some other builds in parallel.
> > >   Is there a minimum recommended amount of memory for doing LTO builds?
> >
> > When building arm64 defconfig, the maximum memory usage I measured
> > with ThinLTO was 3.5 GB, and with full LTO 20.3 GB. I haven't measured
> > larger configurations, but I believe LLD can easily consume 3-4x that
> > much with full LTO allyesconfig.
>
> Ok, that's not too bad then. Is there actually a reason to still
> support full-lto
> in your series? As I understand it, full LTO was the initial approach and
> used to work better, but thin LTO is actually what we want to use in the
> long run. Perhaps dropping the full LTO option from your series now
> that thin LTO works well enough and uses less resources would help
> avoid some of the problems.

While all developers agree that ThinLTO is a much more palatable
experience than full LTO; our product teams prefer the excessive build
time and memory high water mark (at build time) costs in exchange for
slightly better performance than ThinLTO in <benchmarks that I've been
told are important>.  Keeping support for full LTO in tree would help
our product teams reduce the amount of out of tree code they have.  As
long as <benchmarks that I've been told are important> help
sell/differentiate phones, I suspect our product teams will continue
to ship full LTO in production.
Arnd Bergmann Dec. 8, 2020, 10:20 p.m. UTC | #20
On Tue, Dec 8, 2020 at 10:10 PM 'Nick Desaulniers' via Clang Built
Linux <clang-built-linux@googlegroups.com> wrote:
>
> On Tue, Dec 8, 2020 at 1:00 PM Arnd Bergmann <arnd@kernel.org> wrote:
> >
> > On Tue, Dec 8, 2020 at 5:43 PM 'Sami Tolvanen' via Clang Built Linux
> > <clang-built-linux@googlegroups.com> wrote:
> > >
> > > On Tue, Dec 8, 2020 at 4:15 AM Arnd Bergmann <arnd@kernel.org> wrote:
> > > >
> > > > - one build seems to take even longer to link. It's currently at 35GB RAM
> > > >   usage and 40 minutes into the final link, but I'm worried it might
> > > > not complete
> > > >   before it runs out of memory.  I only have 128GB installed, and google-chrome
> > > >   uses another 30GB of that, and I'm also doing some other builds in parallel.
> > > >   Is there a minimum recommended amount of memory for doing LTO builds?
> > >
> > > When building arm64 defconfig, the maximum memory usage I measured
> > > with ThinLTO was 3.5 GB, and with full LTO 20.3 GB. I haven't measured
> > > larger configurations, but I believe LLD can easily consume 3-4x that
> > > much with full LTO allyesconfig.
> >
> > Ok, that's not too bad then. Is there actually a reason to still
> > support full-lto
> > in your series? As I understand it, full LTO was the initial approach and
> > used to work better, but thin LTO is actually what we want to use in the
> > long run. Perhaps dropping the full LTO option from your series now
> > that thin LTO works well enough and uses less resources would help
> > avoid some of the problems.
>
> While all developers agree that ThinLTO is a much more palatable
> experience than full LTO; our product teams prefer the excessive build
> time and memory high water mark (at build time) costs in exchange for
> slightly better performance than ThinLTO in <benchmarks that I've been
> told are important>.  Keeping support for full LTO in tree would help
> our product teams reduce the amount of out of tree code they have.  As
> long as <benchmarks that I've been told are important> help
> sell/differentiate phones, I suspect our product teams will continue
> to ship full LTO in production.

Ok, fair enough. How about marking FULL_LTO as 'depends on
!COMPILE_TEST' then? I'll do that locally for my randconfig tests,
but it would help the other build bots that also force-enable
COMPILE_TEST.

       Arnd
Fāng-ruì Sòng Dec. 9, 2020, 4:55 a.m. UTC | #21
On 2020-12-08, 'Sami Tolvanen' via Clang Built Linux wrote:
>On Tue, Dec 8, 2020 at 4:15 AM Arnd Bergmann <arnd@kernel.org> wrote:
>>
>> On Tue, Dec 1, 2020 at 10:37 PM 'Sami Tolvanen' via Clang Built Linux
>> <clang-built-linux@googlegroups.com> wrote:
>> >
>> > This patch series adds support for building the kernel with Clang's
>> > Link Time Optimization (LTO). In addition to performance, the primary
>> > motivation for LTO is to allow Clang's Control-Flow Integrity (CFI)
>> > to be used in the kernel. Google has shipped millions of Pixel
>> > devices running three major kernel versions with LTO+CFI since 2018.
>> >
>> > Most of the patches are build system changes for handling LLVM
>> > bitcode, which Clang produces with LTO instead of ELF object files,
>> > postponing ELF processing until a later stage, and ensuring initcall
>> > ordering.
>> >
>> > Note that arm64 support depends on Will's memory ordering patches
>> > [1]. I will post x86_64 patches separately after we have fixed the
>> > remaining objtool warnings [2][3].
>> >
>> > [1] https://git.kernel.org/pub/scm/linux/kernel/git/arm64/linux.git/log/?h=for-next/lto
>> > [2] https://lore.kernel.org/lkml/20201120040424.a3wctajzft4ufoiw@treble/
>> > [3] https://git.kernel.org/pub/scm/linux/kernel/git/jpoimboe/linux.git/log/?h=objtool-vmlinux
>> >
>> > You can also pull this series from
>> >
>> >   https://github.com/samitolvanen/linux.git lto-v8
>>
>> I've tried pull this into my randconfig test tree to give it a spin.
>
>Great, thank you for testing this!
>
>> So far I have
>> not managed to get a working build out of it, the main problem so far being
>> that it is really slow to build because the link stage only uses one CPU.
>> These are the other issues I've seen so far:

ld.lld ThinLTO uses the number of (physical cores enabled by affinity) by default.

>You may want to limit your testing only to ThinLTO at first, because
>full LTO is going to be extremely slow with larger configs, especially
>when building arm64 kernels.
>
>> - one build seems to take even longer to link. It's currently at 35GB RAM
>>   usage and 40 minutes into the final link, but I'm worried it might
>> not complete
>>   before it runs out of memory.  I only have 128GB installed, and google-chrome
>>   uses another 30GB of that, and I'm also doing some other builds in parallel.
>>   Is there a minimum recommended amount of memory for doing LTO builds?
>
>When building arm64 defconfig, the maximum memory usage I measured
>with ThinLTO was 3.5 GB, and with full LTO 20.3 GB. I haven't measured
>larger configurations, but I believe LLD can easily consume 3-4x that
>much with full LTO allyesconfig.
>
>> - One build failed with
>>  ld.lld -EL -maarch64elf -mllvm -import-instr-limit=5 -r -o vmlinux.o
>> -T .tmp_initcalls.lds --whole-archive arch/arm64/kernel/head.o
>> init/built-in.a usr/built-in.a arch/arm64/built-in.a kernel/built-in.a
>> certs/built-in.a mm/built-in.a fs/built-in.a ipc/built-in.a
>> security/built-in.a crypto/built-in.a block/built-in.a
>> arch/arm64/lib/built-in.a lib/built-in.a drivers/built-in.a
>> sound/built-in.a net/built-in.a virt/built-in.a --no-whole-archive
>> --start-group arch/arm64/lib/lib.a lib/lib.a
>> ./drivers/firmware/efi/libstub/lib.a --end-group
>>   "ld.lld: error: arch/arm64/kernel/head.o: invalid symbol index"
>>   after about 30 minutes
>
>That's interesting. Did you use LLVM_IAS=1?

May be worth checking which relocation or (SHT_GROUP section's sh_info) in arch/arm64/kernel/head.o is incorrect.

>> - CONFIG_CPU_BIG_ENDIAN doesn't seem to work with lld, and LTO
>>   doesn't work with ld.bfd.
>>   I've added a CPU_LITTLE_ENDIAN dependency to
>>   ARCH_SUPPORTS_LTO_CLANG{,THIN}
>
>Ah, good point. I'll fix this in v9.

Full/Thin LTO should work with GNU ld and gold with LLVMgold.so built from
llvm-project (https://llvm.org/docs/GoldPlugin.html ). You'll need to make sure
that LLVMgold.so is newer than clang. (Newer clang may introduce bitcode
attributes which are unrecognizable by older LLVMgold.so/ld.lld)

>[...]
>> Not sure if these are all known issues. If there is one you'd like me try
>> take a closer look at for finding which config options break it, I can try
>
>No, none of these are known issues. I would be happy to take a closer
>look if you can share configs that reproduce these.
>
>Sami
>
>-- 
>You received this message because you are subscribed to the Google Groups "Clang Built Linux" group.
>To unsubscribe from this group and stop receiving emails from it, send an email to clang-built-linux+unsubscribe@googlegroups.com.
>To view this discussion on the web visit https://groups.google.com/d/msgid/clang-built-linux/CABCJKueCHo2RYfx_A21m%2B%3Dd1gQLR9QsOOxCsHFeicCqyHkb-Kg%40mail.gmail.com.
Fāng-ruì Sòng Dec. 9, 2020, 5:23 a.m. UTC | #22
On Tue, Dec 8, 2020 at 1:02 PM Arnd Bergmann <arnd@kernel.org> wrote:
>
> On Tue, Dec 8, 2020 at 9:59 PM Arnd Bergmann <arnd@kernel.org> wrote:
> >
> > Attaching the config for "ld.lld: error: Never resolved function from
> >   blockaddress (Producer: 'LLVM12.0.0' Reader: 'LLVM 12.0.0')"
>
> And here is a new one: "ld.lld: error: assignment to symbol
> init_pg_end does not converge"
>
>       Arnd
>

This is interesting. I changed the symbol assignment to a separate
loop in https://reviews.llvm.org/D66279
Does raising the limit help? Sometimes the kernel linker script can be
rewritten to be more friendly to the linker...
Arnd Bergmann Dec. 9, 2020, 9:07 a.m. UTC | #23
On Wed, Dec 9, 2020 at 6:23 AM 'Fāng-ruì Sòng' via Clang Built Linux
<clang-built-linux@googlegroups.com> wrote:
>
> On Tue, Dec 8, 2020 at 1:02 PM Arnd Bergmann <arnd@kernel.org> wrote:
> >
> > On Tue, Dec 8, 2020 at 9:59 PM Arnd Bergmann <arnd@kernel.org> wrote:
> > >
> > > Attaching the config for "ld.lld: error: Never resolved function from
> > >   blockaddress (Producer: 'LLVM12.0.0' Reader: 'LLVM 12.0.0')"
> >
> > And here is a new one: "ld.lld: error: assignment to symbol
> > init_pg_end does not converge"
>
> This is interesting. I changed the symbol assignment to a separate
> loop in https://reviews.llvm.org/D66279
> Does raising the limit help? Sometimes the kernel linker script can be
> rewritten to be more friendly to the linker...

If that requires rebuilding lld, testing it is beyond what I can help with
right now. Hopefully someone can reproduce it with my .config.

       Arnd
Arnd Bergmann Dec. 9, 2020, 9:19 a.m. UTC | #24
On Wed, Dec 9, 2020 at 5:56 AM 'Fangrui Song' via Clang Built Linux
<clang-built-linux@googlegroups.com> wrote:
> On 2020-12-08, 'Sami Tolvanen' via Clang Built Linux wrote:
> >On Tue, Dec 8, 2020 at 4:15 AM Arnd Bergmann <arnd@kernel.org> wrote:
> >> So far I have
> >> not managed to get a working build out of it, the main problem so far being
> >> that it is really slow to build because the link stage only uses one CPU.
> >> These are the other issues I've seen so far:
>
> ld.lld ThinLTO uses the number of (physical cores enabled by affinity) by default.

Ah, I see.  Do you know if it's also possible to do something like
-flto=jobserver
to integrate better with the kernel build system?

I tend to run multiple builds under a top-level makefile with 'make
-j30' in order
to use 30 of the 32 threads and leave the scheduling to jobserver instead of
the kernel. If the linker itself is multithreaded but the jobserver
thinks it is a
single thread, could end up with 30 concurrent linkers each trying to use
16 cores.

> >> - CONFIG_CPU_BIG_ENDIAN doesn't seem to work with lld, and LTO
> >>   doesn't work with ld.bfd.
> >>   I've added a CPU_LITTLE_ENDIAN dependency to
> >>   ARCH_SUPPORTS_LTO_CLANG{,THIN}
> >
> >Ah, good point. I'll fix this in v9.
>
> Full/Thin LTO should work with GNU ld and gold with LLVMgold.so built from
> llvm-project (https://llvm.org/docs/GoldPlugin.html ). You'll need to make sure
> that LLVMgold.so is newer than clang. (Newer clang may introduce bitcode
> attributes which are unrecognizable by older LLVMgold.so/ld.lld)

The current patch series requires LLD:

config HAS_LTO_CLANG
       def_bool y
       depends on CC_IS_CLANG && CLANG_VERSION >= 110000 && LD_IS_LLD

Is this something we should change then, or try to keep it simple with the
current approach, leaving LTO disabled for big-endian builds and hosts without
a working lld?

       Arnd
Arnd Bergmann Dec. 9, 2020, 9:59 a.m. UTC | #25
On Tue, Dec 8, 2020 at 10:02 PM Arnd Bergmann <arnd@kernel.org> wrote:
>
> On Tue, Dec 8, 2020 at 9:59 PM Arnd Bergmann <arnd@kernel.org> wrote:
> >
> > Attaching the config for "ld.lld: error: Never resolved function from
> >   blockaddress (Producer: 'LLVM12.0.0' Reader: 'LLVM 12.0.0')"

After rerunning this one with thinlto, it disappeared.

> And here is a new one: "ld.lld: error: assignment to symbol
> init_pg_end does not converge"

this one is still there.

and I reproduced another one with thinlto now:

mm/highmem.o: no symbols
lib/nmi_backtrace.o: no symbols
lib/bitrev.o: no symbols
mm/highmem.o: no symbols
lib/nmi_backtrace.o: no symbols
lib/bitrev.o: no symbols
mm/highmem.o: no symbols
lib/nmi_backtrace.o: no symbols
lib/bitrev.o: no symbols
ERROR: modpost: "memset" [drivers/most/most_cdev.ko] undefined!
ERROR: modpost: "__stack_chk_guard" [drivers/most/most_cdev.ko] undefined!
ERROR: modpost: "__stack_chk_fail" [drivers/most/most_cdev.ko] undefined!
ERROR: modpost: "memset" [drivers/most/most_usb.ko] undefined!
ERROR: modpost: "memmove" [drivers/most/most_usb.ko] undefined!
ERROR: modpost: "__stack_chk_guard" [drivers/most/most_usb.ko] undefined!
ERROR: modpost: "__stack_chk_fail" [drivers/most/most_usb.ko] undefined!
ERROR: modpost: "__stack_chk_guard" [drivers/most/most_core.ko] undefined!
ERROR: modpost: "__stack_chk_fail" [drivers/most/most_core.ko] undefined!
ERROR: modpost: "memset" [drivers/ntb/ntb_transport.ko] undefined!
ERROR: modpost: "memcpy" [drivers/ntb/ntb_transport.ko] undefined!
ERROR: modpost: "__stack_chk_guard" [drivers/ntb/ntb_transport.ko] undefined!
ERROR: modpost: "__stack_chk_fail" [drivers/ntb/ntb_transport.ko] undefined!
ERROR: modpost: "__stack_chk_guard" [drivers/ntb/test/ntb_perf.ko] undefined!
ERROR: modpost: "__stack_chk_fail" [drivers/ntb/test/ntb_perf.ko] undefined!
...

    Arnd
Arnd Bergmann Dec. 9, 2020, 12:35 p.m. UTC | #26
On Tue, Dec 8, 2020 at 1:15 PM Arnd Bergmann <arnd@kernel.org> wrote:
>
> - one build seems to have dropped all symbols the string operations
> from vmlinux,
>   so while the link goes through, modules cannot be loaded:
>  ERROR: modpost: "memmove" [drivers/media/rc/rc-core.ko] undefined!
>  ERROR: modpost: "memcpy" [net/wireless/cfg80211.ko] undefined!
>  ERROR: modpost: "memcpy" [net/8021q/8021q.ko] undefined!
>  ERROR: modpost: "memset" [net/8021q/8021q.ko] undefined!
>  ERROR: modpost: "memcpy" [net/unix/unix.ko] undefined!
>  ERROR: modpost: "memset" [net/sched/cls_u32.ko] undefined!
>  ERROR: modpost: "memcpy" [net/sched/cls_u32.ko] undefined!
>  ERROR: modpost: "memset" [net/sched/sch_skbprio.ko] undefined!
>  ERROR: modpost: "memcpy" [net/802/garp.ko] undefined!
>  I first thought this was related to a clang-12 bug I saw the other day, but
>  this also happens with clang-11

It seems to happen because of CONFIG_TRIM_UNUSED_KSYMS,
which is a shame, since I think that is an option we'd always want to
have enabled with LTO, to allow more dead code to be eliminated.

       Arnd
Sami Tolvanen Dec. 9, 2020, 4:09 p.m. UTC | #27
On Tue, Dec 8, 2020 at 1:02 PM Arnd Bergmann <arnd@kernel.org> wrote:
>
> On Tue, Dec 8, 2020 at 9:59 PM Arnd Bergmann <arnd@kernel.org> wrote:
> >
> > Attaching the config for "ld.lld: error: Never resolved function from
> >   blockaddress (Producer: 'LLVM12.0.0' Reader: 'LLVM 12.0.0')"
>
> And here is a new one: "ld.lld: error: assignment to symbol
> init_pg_end does not converge"

Thanks for these. I can reproduce the "Never resolved function from
blockaddress" issue with full LTO, but I couldn't reproduce this one
with ToT Clang, and the config doesn't have LTO enabled:

$ grep LTO 0x2824F594_defconfig
CONFIG_ARCH_SUPPORTS_LTO_CLANG_THIN=y

Is this the correct config file?

Sami
Sami Tolvanen Dec. 9, 2020, 4:11 p.m. UTC | #28
On Tue, Dec 8, 2020 at 2:20 PM Arnd Bergmann <arnd@kernel.org> wrote:
>
> On Tue, Dec 8, 2020 at 10:10 PM 'Nick Desaulniers' via Clang Built
> Linux <clang-built-linux@googlegroups.com> wrote:
> >
> > On Tue, Dec 8, 2020 at 1:00 PM Arnd Bergmann <arnd@kernel.org> wrote:
> > >
> > > On Tue, Dec 8, 2020 at 5:43 PM 'Sami Tolvanen' via Clang Built Linux
> > > <clang-built-linux@googlegroups.com> wrote:
> > > >
> > > > On Tue, Dec 8, 2020 at 4:15 AM Arnd Bergmann <arnd@kernel.org> wrote:
> > > > >
> > > > > - one build seems to take even longer to link. It's currently at 35GB RAM
> > > > >   usage and 40 minutes into the final link, but I'm worried it might
> > > > > not complete
> > > > >   before it runs out of memory.  I only have 128GB installed, and google-chrome
> > > > >   uses another 30GB of that, and I'm also doing some other builds in parallel.
> > > > >   Is there a minimum recommended amount of memory for doing LTO builds?
> > > >
> > > > When building arm64 defconfig, the maximum memory usage I measured
> > > > with ThinLTO was 3.5 GB, and with full LTO 20.3 GB. I haven't measured
> > > > larger configurations, but I believe LLD can easily consume 3-4x that
> > > > much with full LTO allyesconfig.
> > >
> > > Ok, that's not too bad then. Is there actually a reason to still
> > > support full-lto
> > > in your series? As I understand it, full LTO was the initial approach and
> > > used to work better, but thin LTO is actually what we want to use in the
> > > long run. Perhaps dropping the full LTO option from your series now
> > > that thin LTO works well enough and uses less resources would help
> > > avoid some of the problems.
> >
> > While all developers agree that ThinLTO is a much more palatable
> > experience than full LTO; our product teams prefer the excessive build
> > time and memory high water mark (at build time) costs in exchange for
> > slightly better performance than ThinLTO in <benchmarks that I've been
> > told are important>.  Keeping support for full LTO in tree would help
> > our product teams reduce the amount of out of tree code they have.  As
> > long as <benchmarks that I've been told are important> help
> > sell/differentiate phones, I suspect our product teams will continue
> > to ship full LTO in production.
>
> Ok, fair enough. How about marking FULL_LTO as 'depends on
> !COMPILE_TEST' then? I'll do that locally for my randconfig tests,
> but it would help the other build bots that also force-enable
> COMPILE_TEST.

Sure, that sounds reasonable to me. I'll add it in v9.

Sami
Sami Tolvanen Dec. 9, 2020, 4:25 p.m. UTC | #29
On Wed, Dec 9, 2020 at 4:36 AM Arnd Bergmann <arnd@kernel.org> wrote:
>
> On Tue, Dec 8, 2020 at 1:15 PM Arnd Bergmann <arnd@kernel.org> wrote:
> >
> > - one build seems to have dropped all symbols the string operations
> > from vmlinux,
> >   so while the link goes through, modules cannot be loaded:
> >  ERROR: modpost: "memmove" [drivers/media/rc/rc-core.ko] undefined!
> >  ERROR: modpost: "memcpy" [net/wireless/cfg80211.ko] undefined!
> >  ERROR: modpost: "memcpy" [net/8021q/8021q.ko] undefined!
> >  ERROR: modpost: "memset" [net/8021q/8021q.ko] undefined!
> >  ERROR: modpost: "memcpy" [net/unix/unix.ko] undefined!
> >  ERROR: modpost: "memset" [net/sched/cls_u32.ko] undefined!
> >  ERROR: modpost: "memcpy" [net/sched/cls_u32.ko] undefined!
> >  ERROR: modpost: "memset" [net/sched/sch_skbprio.ko] undefined!
> >  ERROR: modpost: "memcpy" [net/802/garp.ko] undefined!
> >  I first thought this was related to a clang-12 bug I saw the other day, but
> >  this also happens with clang-11
>
> It seems to happen because of CONFIG_TRIM_UNUSED_KSYMS,
> which is a shame, since I think that is an option we'd always want to
> have enabled with LTO, to allow more dead code to be eliminated.

Ah yes, this is a known issue. We use TRIM_UNUSED_KSYMS with LTO in
Android's Generic Kernel Image and the problem is that bitcode doesn't
yet contain calls to these functions, so autoksyms won't see them. The
solution is to use a symbol whitelist with LTO to prevent these from
being trimmed. I suspect we would need a default whitelist for LTO
builds.

Sami
Arnd Bergmann Dec. 9, 2020, 5:51 p.m. UTC | #30
On Wed, Dec 9, 2020 at 5:25 PM 'Sami Tolvanen' via Clang Built Linux
<clang-built-linux@googlegroups.com> wrote:
>
> On Wed, Dec 9, 2020 at 4:36 AM Arnd Bergmann <arnd@kernel.org> wrote:
> >
> > On Tue, Dec 8, 2020 at 1:15 PM Arnd Bergmann <arnd@kernel.org> wrote:
> >
> >
> > It seems to happen because of CONFIG_TRIM_UNUSED_KSYMS,
> > which is a shame, since I think that is an option we'd always want to
> > have enabled with LTO, to allow more dead code to be eliminated.
>
> Ah yes, this is a known issue. We use TRIM_UNUSED_KSYMS with LTO in
> Android's Generic Kernel Image and the problem is that bitcode doesn't
> yet contain calls to these functions, so autoksyms won't see them. The
> solution is to use a symbol whitelist with LTO to prevent these from
> being trimmed. I suspect we would need a default whitelist for LTO
> builds.

A built-in allowlist sounds good to me. FWIW, in the randconfigs so far, I only
saw five symbols that would need to be on it:

memcpy(), memmove(), memset(), __stack_chk_fail() and __stack_chk_guard

       Arnd
Arnd Bergmann Dec. 9, 2020, 7:24 p.m. UTC | #31
On Wed, Dec 9, 2020 at 5:09 PM 'Sami Tolvanen' via Clang Built Linux
<clang-built-linux@googlegroups.com> wrote:
> On Tue, Dec 8, 2020 at 1:02 PM Arnd Bergmann <arnd@kernel.org> wrote:
> > On Tue, Dec 8, 2020 at 9:59 PM Arnd Bergmann <arnd@kernel.org> wrote:
> > >
> > > Attaching the config for "ld.lld: error: Never resolved function from
> > >   blockaddress (Producer: 'LLVM12.0.0' Reader: 'LLVM 12.0.0')"
> >
> > And here is a new one: "ld.lld: error: assignment to symbol
> > init_pg_end does not converge"
>
> Thanks for these. I can reproduce the "Never resolved function from
> blockaddress" issue with full LTO, but I couldn't reproduce this one
> with ToT Clang, and the config doesn't have LTO enabled:
>
> $ grep LTO 0x2824F594_defconfig
> CONFIG_ARCH_SUPPORTS_LTO_CLANG_THIN=y
>
> Is this the correct config file?

It is the right file, and so far this is the only defconfig on which I
see the "does not converge" error, so I don't have any other one.

I suspect this might be an issue in the version of lld that I have here
and unrelated to LTO, and I can confirm that I see the error
with LTO still disabled.

It seems to be completely random. I do see the bug on next-20201203
but not on a later one. I also tried bisecting through linux-next and
arrived at "lib: stackdepot: add support to configure STACK_HASH_SIZE",
which is almost certainly not related, other than just changing a few
symbols around.

      Arnd