This patch series adds support for building the 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 arm64 support depends on Will's memory ordering patches [1]. I will post x86_64 patches separately after we have fixed the remaining objtool warnings [2][3]. [1] https://git.kernel.org/pub/scm/linux/kernel/git/arm64/linux.git/log/?h=for-next/lto [2] https://lore.kernel.org/lkml/20201120040424.a3wctajzft4ufoiw@treble/ [3] https://git.kernel.org/pub/scm/linux/kernel/git/jpoimboe/linux.git/log/?h=objtool-vmlinux You can also pull this series from https://github.com/samitolvanen/linux.git lto-v8 --- Changes in v8: - Cleaned up the LTO Kconfig options based on suggestions from Nick and Kees. - Dropped the patch to disable LTO for the arm64 nVHE KVM code as David pointed out it's not needed anymore. Changes in v7: - Rebased to master again. - Added back arm64 patches as the prerequisites are now staged, and dropped x86_64 support until the remaining objtool issues are resolved. - Dropped ifdefs from module.lds.S. 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. Sami Tolvanen (17): tracing: move function tracer options to Kconfig kbuild: add support for Clang LTO kbuild: lto: fix module versioning 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 .gitignore | 1 + Makefile | 45 +++-- arch/Kconfig | 74 +++++++ arch/arm64/Kconfig | 4 + arch/arm64/kernel/vdso/Makefile | 3 +- arch/arm64/kvm/hyp/nvhe/Makefile | 4 +- drivers/firmware/efi/libstub/Makefile | 2 + drivers/misc/lkdtm/Makefile | 1 + include/asm-generic/vmlinux.lds.h | 11 +- include/linux/init.h | 79 +++++++- include/linux/pci.h | 19 +- kernel/trace/Kconfig | 16 ++ scripts/Makefile.build | 50 ++++- scripts/Makefile.lib | 6 +- scripts/Makefile.modfinal | 9 +- scripts/Makefile.modpost | 25 ++- scripts/generate_initcall_order.pl | 270 ++++++++++++++++++++++++++ scripts/link-vmlinux.sh | 70 ++++++- scripts/mod/Makefile | 1 + scripts/mod/modpost.c | 16 +- scripts/mod/modpost.h | 9 + scripts/mod/sumversion.c | 6 +- scripts/module.lds.S | 24 +++ 23 files changed, 677 insertions(+), 68 deletions(-) create mode 100755 scripts/generate_initcall_order.pl base-commit: 0fa8ee0d9ab95c9350b8b84574824d9a384a9f7d -- 2.29.2.299.gdc1121823c-goog *** BLURB HERE *** Sami Tolvanen (16): tracing: move function tracer options to Kconfig kbuild: add support for Clang LTO kbuild: lto: fix module versioning 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 arm64: disable recordmcount with DYNAMIC_FTRACE_WITH_REGS arm64: allow LTO to be selected .gitignore | 1 + Makefile | 45 +++-- arch/Kconfig | 87 +++++++++ arch/arm64/Kconfig | 4 + arch/arm64/kernel/vdso/Makefile | 3 +- drivers/firmware/efi/libstub/Makefile | 2 + drivers/misc/lkdtm/Makefile | 1 + include/asm-generic/vmlinux.lds.h | 11 +- include/linux/init.h | 79 +++++++- include/linux/pci.h | 19 +- kernel/trace/Kconfig | 16 ++ scripts/Makefile.build | 50 ++++- scripts/Makefile.lib | 6 +- scripts/Makefile.modfinal | 9 +- scripts/Makefile.modpost | 25 ++- scripts/generate_initcall_order.pl | 270 ++++++++++++++++++++++++++ scripts/link-vmlinux.sh | 70 ++++++- scripts/mod/Makefile | 1 + scripts/mod/modpost.c | 16 +- scripts/mod/modpost.h | 9 + scripts/mod/sumversion.c | 6 +- scripts/module.lds.S | 24 +++ 22 files changed, 688 insertions(+), 66 deletions(-) create mode 100755 scripts/generate_initcall_order.pl base-commit: b65054597872ce3aefbc6a666385eabdf9e288da -- 2.29.2.576.ga3fc446d84-goog
