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

Sami Tolvanen samitolvanen at google.com
Fri Dec 11 13:46:17 EST 2020

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

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-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.

    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

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

  - 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

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