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

Sami Tolvanen
2020-12-11T20:16:34Z

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-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