On Fri, Dec 11, 2020 at 7:46 PM 'Sami Tolvanen' via Clang Built Linux <clang-built-linux@xxxxxxxxxxxxxxxx> wrote: > > 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 > Hi Sami, Thanks for the update. I am interested in having Clang LTO (Clang-CFI) for x86-64 working and help with testing. I tried the Git tree mentioned in [3] <jpoimboe.git#objtool-vmlinux> (together with changes from <peterz.git#x86/urgent>). I only see in my build-log... drivers/gpu/drm/i915/gem/i915_gem_execbuffer.o: warning: objtool: eb_relocate_parse_slow()+0x3d0: stack state mismatch: cfa1=7+120 cfa2=-1+0 drivers/gpu/drm/i915/gem/i915_gem_execbuffer.o: warning: objtool: eb_copy_relocations()+0x229: stack state mismatch: cfa1=7+120 cfa2=-1+0 ...which was reported and worked on in [1]. This is with Clang-IAS version 11.0.1. Unfortunately, the recent changes in <samitolvanen.github#clang-cfi> do not cleanly apply with Josh stuff. My intention/wish was to report this combination of patchsets "heals" a lot of objtool-warnings for vmlinux.o I observed with Clang-CFI. Is it possible to have a Git branch where Josh's objtool-vmlinux is working together with Clang-LTO? For testing purposes. Thanks. Regards, - Sedat - [1] https://github.com/ClangBuiltLinux/linux/issues?q=is%3Aissue+is%3Aopen+sort%3Aupdated-desc > You can also pull this series from > > https://github.com/samitolvanen/linux.git lto-v9 > > --- > Changes in v9: > > - Added HAS_LTO_CLANG dependencies to LLVM=1 and LLVM_IAS=1 to avoid > issues with mismatched toolchains. > > - Dropped the .mod patch as Masahiro landed a better solution to > the split line issue in commit 7d32358be8ac ("kbuild: avoid split > lines in .mod files"). > > - Updated CC_FLAGS_LTO to use -fvisibility=hidden to avoid weak symbol > visibility issues with ThinLTO on x86. > > - Changed LTO_CLANG_FULL to depend on !COMPILE_TEST to prevent > timeouts in automated testing. > > - Added a dependency to CPU_LITTLE_ENDIAN to ARCH_SUPPORTS_LTO_CLANG > in arch/arm64/Kconfig. > > - Added a default symbol list to fix an issue with TRIM_UNUSED_KSYMS. > > 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 (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: add a default list of used symbols > 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 | 90 +++++++++ > 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 +- > init/Kconfig | 1 + > kernel/trace/Kconfig | 16 ++ > scripts/Makefile.build | 48 ++++- > scripts/Makefile.lib | 6 +- > scripts/Makefile.modfinal | 9 +- > scripts/Makefile.modpost | 25 ++- > scripts/generate_initcall_order.pl | 270 ++++++++++++++++++++++++++ > scripts/link-vmlinux.sh | 70 ++++++- > scripts/lto-used-symbollist | 5 + > scripts/mod/Makefile | 1 + > scripts/mod/modpost.c | 16 +- > scripts/mod/modpost.h | 9 + > scripts/mod/sumversion.c | 6 +- > scripts/module.lds.S | 24 +++ > 24 files changed, 696 insertions(+), 65 deletions(-) > create mode 100755 scripts/generate_initcall_order.pl > create mode 100644 scripts/lto-used-symbollist > > > base-commit: 33dc9614dc208291d0c4bcdeb5d30d481dcd2c4c > -- > 2.29.2.576.ga3fc446d84-goog > > -- > You received this message because you are subscribed to the Google Groups "Clang Built Linux" group. > To unsubscribe from this group and stop receiving emails from it, send an email to clang-built-linux+unsubscribe@xxxxxxxxxxxxxxxx. > To view this discussion on the web visit https://groups.google.com/d/msgid/clang-built-linux/20201211184633.3213045-1-samitolvanen%40google.com.
