[PATCH v6 00/25] Add support for Clang LTO

Sami Tolvanen
2020-10-13T02:42:10Z

This patch series adds support for building the x86_64 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 this version is based on tip/master to reduce the number
of prerequisite patches, and to make it easier to manage changes to
objtool. Patch 1 is from Masahiro's kbuild tree, and while it's not
directly related to LTO, it makes the module linker script changes
cleaner.

Furthermore, patches 2-6 include Peter's patch for generating
__mcount_loc with objtool, and build system changes to enable it on
x86. With these patches, we no longer need to annotate functions
that have non-call references to __fentry__ with LTO, which greatly
simplifies supporting dynamic ftrace.

You can also pull this series from

https://github.com/samitolvanen/linux.git lto-v6

---
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.

Masahiro Yamada (1):
kbuild: preprocess module linker script

Peter Zijlstra (1):
objtool: Add a pass for generating __mcount_loc

Sami Tolvanen (23):
objtool: Don't autodetect vmlinux.o
tracing: move function tracer options to Kconfig
tracing: add support for objtool mcount
x86, build: use objtool mcount
treewide: remove DISABLE_LTO
kbuild: add support for Clang LTO
kbuild: lto: fix module versioning
objtool: Split noinstr validation from --vmlinux
kbuild: lto: postpone objtool
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
x86/asm: annotate indirect jumps
x86, vdso: disable LTO only for vDSO
x86, cpu: disable LTO for cpu.c
x86, build: allow LTO_CLANG and THINLTO to be selected

.gitignore | 1 +
Makefile | 68 +++--
arch/Kconfig | 74 +++++
arch/arm/Makefile | 4 -
.../module.lds => include/asm/module.lds.h} | 2 +
arch/arm64/Makefile | 4 -
.../module.lds => include/asm/module.lds.h} | 2 +
arch/arm64/kernel/vdso/Makefile | 1 -
arch/ia64/Makefile | 1 -
.../{module.lds => include/asm/module.lds.h} | 0
arch/m68k/Makefile | 1 -
.../module.lds => include/asm/module.lds.h} | 0
arch/powerpc/Makefile | 1 -
.../module.lds => include/asm/module.lds.h} | 0
arch/riscv/Makefile | 3 -
.../module.lds => include/asm/module.lds.h} | 3 +-
arch/sparc/vdso/Makefile | 2 -
arch/um/include/asm/Kbuild | 1 +
arch/x86/Kconfig | 3 +
arch/x86/Makefile | 5 +
arch/x86/entry/vdso/Makefile | 5 +-
arch/x86/kernel/acpi/wakeup_64.S | 2 +
arch/x86/platform/pvh/head.S | 2 +
arch/x86/power/Makefile | 4 +
arch/x86/power/hibernate_asm_64.S | 3 +
drivers/firmware/efi/libstub/Makefile | 2 +
drivers/misc/lkdtm/Makefile | 1 +
include/asm-generic/Kbuild | 1 +
include/asm-generic/module.lds.h | 10 +
include/asm-generic/vmlinux.lds.h | 11 +-
include/linux/init.h | 79 ++++-
include/linux/pci.h | 19 +-
kernel/Makefile | 3 -
kernel/trace/Kconfig | 29 ++
scripts/.gitignore | 1 +
scripts/Makefile | 3 +
scripts/Makefile.build | 69 +++--
scripts/Makefile.lib | 17 +-
scripts/Makefile.modfinal | 29 +-
scripts/Makefile.modpost | 25 +-
scripts/generate_initcall_order.pl | 270 ++++++++++++++++++
scripts/link-vmlinux.sh | 98 ++++++-
scripts/mod/Makefile | 1 +
scripts/mod/modpost.c | 16 +-
scripts/mod/modpost.h | 9 +
scripts/mod/sumversion.c | 6 +-
scripts/{module-common.lds => module.lds.S} | 31 ++
scripts/package/builddeb | 2 +-
tools/objtool/builtin-check.c | 10 +-
tools/objtool/builtin.h | 2 +-
tools/objtool/check.c | 84 +++++-
tools/objtool/check.h | 1 +
tools/objtool/objtool.c | 1 +
tools/objtool/objtool.h | 1 +
54 files changed, 895 insertions(+), 128 deletions(-)
rename arch/arm/{kernel/module.lds => include/asm/module.lds.h} (72%)
rename arch/arm64/{kernel/module.lds => include/asm/module.lds.h} (76%)
rename arch/ia64/{module.lds => include/asm/module.lds.h} (100%)
rename arch/m68k/{kernel/module.lds => include/asm/module.lds.h} (100%)
rename arch/powerpc/{kernel/module.lds => include/asm/module.lds.h} (100%)
rename arch/riscv/{kernel/module.lds => include/asm/module.lds.h} (84%)
create mode 100644 include/asm-generic/module.lds.h
create mode 100755 scripts/generate_initcall_order.pl
rename scripts/{module-common.lds => module.lds.S} (59%)

base-commit: a292570e9f694ed50d3e69afd6d54272fd40deca
--
2.28.0.1011.ga647a8990f-goog