Re: [PATCH v3 00/30] Add support for Clang LTO

Sedat Dilek
2020-09-18T20:29:22Z

On Fri, Sep 18, 2020 at 10:14 PM 'Sami Tolvanen' via Clang Built Linux
<clang-built-linux@googlegroups.com> wrote:
>
> 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.
>

Hi Sami,

might be good to point to your GitHub tree and corresponding
release-tag for easy fetching.

Thanks.

Regards,
- Sedat -

> ---
> 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
>
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