[PATCH v2 00/28] Add support for Clang LTO

Sami Tolvanen
2020-09-03T20:31:14Z

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 1-4 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 are required for building the kernel with ToT Clang,
and IAS, and patch 4 is needed to build allmodconfig with LTO.

- Patches 3-4 are already in linux-next, but not yet in 5.9-rc.

---
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

Nick Desaulniers (1):
lib/string.c: implement stpcpy

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

Sami Tolvanen (23):
objtool: Don't autodetect vmlinux.o
kbuild: add support for objtool mcount
x86, build: use objtool mcount
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, relocs: Ignore L4_PAGE_OFFSET relocations
x86, build: allow LTO_CLANG and THINLTO to be selected

.gitignore | 1 +
Makefile | 65 ++++++-
arch/Kconfig | 67 +++++++
arch/arm64/Kconfig | 2 +
arch/arm64/Makefile | 1 +
arch/arm64/kernel/vdso/Makefile | 4 +-
arch/arm64/kvm/hyp/nvhe/Makefile | 4 +-
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/tools/relocs.c | 1 +
drivers/firmware/efi/libstub/Makefile | 2 +
drivers/misc/lkdtm/Makefile | 1 +
drivers/ras/cec.c | 9 +-
include/asm-generic/vmlinux.lds.h | 11 +-
include/linux/init.h | 79 +++++++-
include/linux/pci.h | 19 +-
kernel/trace/Kconfig | 5 +
lib/string.c | 24 +++
scripts/Makefile.build | 55 +++++-
scripts/Makefile.lib | 6 +-
scripts/Makefile.modfinal | 31 ++-
scripts/Makefile.modpost | 26 ++-
scripts/generate_initcall_order.pl | 270 ++++++++++++++++++++++++++
scripts/link-vmlinux.sh | 94 ++++++++-
scripts/mod/Makefile | 1 +
scripts/mod/modpost.c | 16 +-
scripts/mod/modpost.h | 9 +
scripts/mod/sumversion.c | 6 +-
scripts/module-lto.lds | 26 +++
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 +
39 files changed, 883 insertions(+), 108 deletions(-)
create mode 100755 scripts/generate_initcall_order.pl
create mode 100644 scripts/module-lto.lds

base-commit: e28f0104343d0c132fa37f479870c9e43355fee4
--
2.28.0.402.g5ffc5be6b7-goog