[PATCH v9 00/16] Add support for Clang LTO

[Sedat Dilek]

On Fri, Dec 11, 2020 at 7:46 PM 'Sami Tolvanen' via Clang Built Linux
<clang-built-linux at 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
>

Hi Sami,

Thanks for the update.

I am interested in having Clang LTO (Clang-CFI) for x86-64 working and
help with testing.

I tried the Git tree mentioned in [3] <jpoimboe.git#objtool-vmlinux>
(together with changes from <peterz.git#x86/urgent>).

I only see in my build-log...

drivers/gpu/drm/i915/gem/i915_gem_execbuffer.o: warning: objtool:
eb_relocate_parse_slow()+0x3d0: stack state mismatch: cfa1=7+120
cfa2=-1+0
drivers/gpu/drm/i915/gem/i915_gem_execbuffer.o: warning: objtool:
eb_copy_relocations()+0x229: stack state mismatch: cfa1=7+120
cfa2=-1+0

...which was reported and worked on in [1].

This is with Clang-IAS version 11.0.1.

Unfortunately, the recent changes in <samitolvanen.github#clang-cfi>
do not cleanly apply with Josh stuff.
My intention/wish was to report this combination of patchsets "heals"
a lot of objtool-warnings for vmlinux.o I observed with Clang-CFI.

Is it possible to have a Git branch where Josh's objtool-vmlinux is
working together with Clang-LTO?
For testing purposes.

Thanks.

Regards,
- Sedat -

[1] https://github.com/ClangBuiltLinux/linux/issues?q=is%3Aissue+is%3Aopen+sort%3Aupdated-desc

> You can also pull this series from
>
>   https://github.com/samitolvanen/linux.git lto-v9
>
> ---
> Changes in v9:
>
>   - Added HAS_LTO_CLANG dependencies to LLVM=1 and LLVM_IAS=1 to avoid
>     issues with mismatched toolchains.
>
>   - Dropped the .mod patch as Masahiro landed a better solution to
>     the split line issue in commit 7d32358be8ac ("kbuild: avoid split
>     lines in .mod files").
>
>   - Updated CC_FLAGS_LTO to use -fvisibility=hidden to avoid weak symbol
>     visibility issues with ThinLTO on x86.
>
>   - Changed LTO_CLANG_FULL to depend on !COMPILE_TEST to prevent
>     timeouts in automated testing.
>
>   - Added a dependency to CPU_LITTLE_ENDIAN to ARCH_SUPPORTS_LTO_CLANG
>     in arch/arm64/Kconfig.
>
>   - Added a default symbol list to fix an issue with TRIM_UNUSED_KSYMS.
>
>   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 (16):
>   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: add a default list of used symbols
>   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
>   arm64: disable recordmcount with DYNAMIC_FTRACE_WITH_REGS
>   arm64: allow LTO to be selected
>
>  .gitignore                            |   1 +
>  Makefile                              |  45 +++--
>  arch/Kconfig                          |  90 +++++++++
>  arch/arm64/Kconfig                    |   4 +
>  arch/arm64/kernel/vdso/Makefile       |   3 +-
>  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 +-
>  init/Kconfig                          |   1 +
>  kernel/trace/Kconfig                  |  16 ++
>  scripts/Makefile.build                |  48 ++++-
>  scripts/Makefile.lib                  |   6 +-
>  scripts/Makefile.modfinal             |   9 +-
>  scripts/Makefile.modpost              |  25 ++-
>  scripts/generate_initcall_order.pl    | 270 ++++++++++++++++++++++++++
>  scripts/link-vmlinux.sh               |  70 ++++++-
>  scripts/lto-used-symbollist           |   5 +
>  scripts/mod/Makefile                  |   1 +
>  scripts/mod/modpost.c                 |  16 +-
>  scripts/mod/modpost.h                 |   9 +
>  scripts/mod/sumversion.c              |   6 +-
>  scripts/module.lds.S                  |  24 +++
>  24 files changed, 696 insertions(+), 65 deletions(-)
>  create mode 100755 scripts/generate_initcall_order.pl
>  create mode 100644 scripts/lto-used-symbollist
>
>
> base-commit: 33dc9614dc208291d0c4bcdeb5d30d481dcd2c4c
> --
> 2.29.2.576.ga3fc446d84-goog
>
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