Re: [PATCH v5 00/29] Add support for Clang LTO

Sedat Dilek
2020-10-09T16:30:40Z

On Fri, Oct 9, 2020 at 6:13 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 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-v5
>
> ---
> Changes in v5:
>
> - Rebased on top of tip/master.
>

What are the plans to get this into mainline?
Linux v5.10 :-) too early - needs more review/testing?

Will clang-cfi be based on this, too?

> - 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.
>

That was a long way to detect this as I had very big Debian Linux
debug packages generated with CONFIG_DEBUG_INFO_COMPRESSED=y.

Thanks for v5 of clang-lto.

- Sedat -

[1] https://github.com/ClangBuiltLinux/linux/issues/1086#issuecomment-705754002

> - 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 (27):
> 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
> arm64: vdso: disable LTO
> KVM: arm64: disable LTO for the nVHE directory
> arm64: disable recordmcount with DYNAMIC_FTRACE_WITH_REGS
> arm64: allow LTO_CLANG and THINLTO to be selected
> 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/Kconfig | 4 +
> arch/arm64/Makefile | 4 -
> .../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/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/check.c | 84 +++++-
> tools/objtool/include/objtool/builtin.h | 2 +-
> tools/objtool/include/objtool/check.h | 1 +
> tools/objtool/include/objtool/objtool.h | 1 +
> tools/objtool/objtool.c | 1 +
> 56 files changed, 903 insertions(+), 131 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: 80396d76da65fc8b82581c0260c25a6aa0a495a3
> --
> 2.28.0.1011.ga647a8990f-goog
>
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