Hi Masahiro,

On Mon, Jun 29, 2020 at 01:56:19AM +0900, Masahiro Yamada wrote:

On Thu, Jun 25, 2020 at 5:32 AM 'Sami Tolvanen' via Clang Built Linux
[off-list ref] wrote:

quoted

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's
Pixel devices have shipped with LTO+CFI kernels 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 first objtool patch in the series is already in linux-next,
but as it's needed with LTO, I'm including it also here to make testing
easier.

I put this series on a testing branch,
and 0-day bot started reporting some issues.

Yes, I'll fix those issues in v2.

(but 0-day bot is quieter than I expected.
Perhaps, 0-day bot does not turn on LLVM=1 ?)

In order for it to test an LTO build, it would need to enable LTO_CLANG
explicitly though, in addition to LLVM=1.

I also got an error for
ARCH=arm64 allyesconfig + CONFIG_LTO_CLANG=y



$ make ARCH=arm64 LLVM=1 LLVM_IAS=1
CROSS_COMPILE=~/tools/aarch64-linaro-7.5/bin/aarch64-linux-gnu-
-j24

  ...

  GEN     .version
  CHK     include/generated/compile.h
  UPD     include/generated/compile.h
  CC      init/version.o
  AR      init/built-in.a
  GEN     .tmp_initcalls.lds
  GEN     .tmp_symversions.lds
  LTO     vmlinux.o
  MODPOST vmlinux.symvers
  MODINFO modules.builtin.modinfo
  GEN     modules.builtin
  LD      .tmp_vmlinux.kallsyms1
ld.lld: error: undefined symbol: __compiletime_assert_905

quoted

quoted

quoted

referenced by irqbypass.c
              vmlinux.o:(jeq_imm)

make: *** [Makefile:1161: vmlinux] Error 1

I can reproduce this with ToT LLVM and it's BUILD_BUG_ON_MSG(..., "value
too large for the field") in drivers/net/ethernet/netronome/nfp/bpf/jit.c.
Specifically, the FIELD_FIT / __BF_FIELD_CHECK macro in ur_load_imm_any.

This compiles just fine with an earlier LLVM revision, so it could be a
relatively recent regression. I'll take a look. Thanks for catching this!

Sami

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