On Fri, Dec 11, 2020 at 10:46:17AM -0800, Sami Tolvanen 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 > > 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. This continues to look good to me. I'd like to see it in -next ASAP so we can continue to find any little needed tweaks. Since it works in the general case, we can add on top of this series once it's in -next. Masahiro, let me know if you'd rather I not carry this in -next. AFAICT, the bulk of the changes in the build system continue to be confined to the LTO-only cases, so it should not be disruptive to anyone doing non-LTO builds. -- Kees Cook
