This patch series adds support for building x86_64 and arm64 kernels
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 patches 1-5 are not directly related to LTO, but are
needed to compile LTO kernels with ToT Clang, so I'm including them
in the series for your convenience:
- Patches 1-3 fix build issues with LLVM and they are already in
linux-next.
- Patch 4 fixes x86 builds with LLVM IAS, but it hasn't yet been
picked up by maintainers.
- Patch 5 is from Masahiro's kbuild tree and makes the LTO linker
script changes much cleaner.
---
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.
Arvind Sankar (2):
x86/boot/compressed: Disable relocation relaxation
x86/asm: Replace __force_order with memory clobber
Luca Stefani (1):
RAS/CEC: Fix cec_init() prototype
Masahiro Yamada (1):
kbuild: preprocess module linker script
Nick Desaulniers (1):
lib/string.c: implement stpcpy
Peter Zijlstra (1):
objtool: Add a pass for generating __mcount_loc
Sami Tolvanen (24):
objtool: Don't autodetect vmlinux.o
kbuild: 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
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
arm64: export CC_USING_PATCHABLE_FUNCTION_ENTRY
arm64: vdso: disable LTO
KVM: arm64: disable LTO for the nVHE directory
arm64: allow LTO_CLANG and THINLTO to be selected
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 | 74 ++++-
arch/Kconfig | 68 +++++
arch/arm/Makefile | 4 -
.../module.lds => include/asm/module.lds.h} | 2 +
arch/arm64/Kconfig | 2 +
arch/arm64/Makefile | 5 +-
.../module.lds => include/asm/module.lds.h} | 2 +
arch/arm64/kernel/vdso/Makefile | 4 +-
arch/arm64/kvm/hyp/nvhe/Makefile | 4 +-
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/boot/compressed/Makefile | 2 +
arch/x86/boot/compressed/pgtable_64.c | 9 -
arch/x86/entry/vdso/Makefile | 5 +-
arch/x86/include/asm/special_insns.h | 28 +-
arch/x86/kernel/cpu/common.c | 4 +-
arch/x86/power/Makefile | 4 +
drivers/firmware/efi/libstub/Makefile | 2 +
drivers/misc/lkdtm/Makefile | 1 +
drivers/ras/cec.c | 9 +-
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 | 5 +
lib/string.c | 24 ++
scripts/.gitignore | 1 +
scripts/Makefile | 3 +
scripts/Makefile.build | 69 +++--
scripts/Makefile.lib | 17 +-
scripts/Makefile.modfinal | 29 +-
scripts/Makefile.modpost | 22 +-
scripts/generate_initcall_order.pl | 270 ++++++++++++++++++
scripts/link-vmlinux.sh | 95 +++++-
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 | 13 +-
tools/objtool/builtin.h | 2 +-
tools/objtool/check.c | 83 ++++++
tools/objtool/check.h | 1 +
tools/objtool/objtool.h | 1 +
58 files changed, 919 insertions(+), 154 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: 92ab97adeefccf375de7ebaad9d5b75d4125fe8b
--
2.28.0.681.g6f77f65b4e-goog
