Re: [PATCH v7 00/17] Add support for Clang LTO

Nick Desaulniers
2020-11-18T23:42:40Z

On Wed, Nov 18, 2020 at 2:07 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 v7 brings back arm64 support as Will has now staged the
> prerequisite memory ordering patches [1], and drops x86_64 while we work
> on fixing the remaining objtool warnings [2].
>
> [1] https://git.kernel.org/pub/scm/linux/kernel/git/arm64/linux.git/log/?h=for-next/lto
> [2] https://lore.kernel.org/lkml/20201114004911.aip52eimk6c2uxd4@treble/
>
> You can also pull this series from
>
> https://github.com/samitolvanen/linux.git lto-v7

Thanks for continuing to drive this series Sami. For the series,

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

I did virtualized boot tests with the series applied to aarch64
defconfig without CONFIG_LTO, with CONFIG_LTO_CLANG, and a third time
with CONFIG_THINLTO. If you make changes to the series in follow ups,
please drop my tested by tag from the modified patches and I'll help
re-test. Some minor feedback on the Kconfig change, but I'll post it
off of that patch.

>
> ---
> 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
> --
> 2.29.2.299.gdc1121823c-goog
>

--
Thanks,
~Nick Desaulniers