Pull clang LTO fix from Kees Cook:
 "It seems Clang has been scrubbing through the missing LTO IR flags for
  Clang 13, and the last of these 'only with LTO' flags is fixed now.

  I've asked that they please consider making these changes in a less
  'break all the Clang kernel builds' kind of way in the future. :P

  Summary:

   - The '-warn-stack-size' option under LTO has moved in Clang 13 (Tor
     Vic)"

* tag 'clang-features-v5.13-rc7' of git://git.kernel.org/pub/scm/linux/kernel/git/kees/linux:
  Makefile: lto: Pass -warn-stack-size only on LLD < 13.0.0

Since LLVM commit fc018eb, the '-warn-stack-size' flag has been dropped
[1], leading to the following error message when building with Clang-13
and LLD-13:

    ld.lld: error: -plugin-opt=-: ld.lld: Unknown command line argument
    '-warn-stack-size=2048'.  Try: 'ld.lld --help'
    ld.lld: Did you mean '--asan-stack=2048'?

In the same way as with commit 2398ce80152a ("x86, lto: Pass
-stack-alignment only on LLD < 13.0.0") , make '-warn-stack-size'
conditional on LLD < 13.0.0.

[1] https://reviews.llvm.org/D103928

Fixes: 24845dcb170e ("Makefile: LTO: have linker check -Wframe-larger-than")
Cc: stable@vger.kernel.org
Link: https://github.com/ClangBuiltLinux/linux/issues/1377
Signed-off-by: Tor Vic <torvic9@mailbox.org>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/7631bab7-a8ab-f884-ab54-f4198976125c@mailbox.org

Pull clang feature fixes from Kees Cook:

 - Correctly pass stack frame size checking under LTO (Nick Desaulniers)

 - Avoid CFI mismatches by checking initcall_t types (Marco Elver)

* tag 'clang-features-v5.13-rc4' of git://git.kernel.org/pub/scm/linux/kernel/git/kees/linux:
  Makefile: LTO: have linker check -Wframe-larger-than
  init: verify that function is initcall_t at compile-time

-Wframe-larger-than= requires stack frame information, which the
frontend cannot provide. This diagnostic is emitted late during
compilation once stack frame size is available.

When building with LTO, the frontend simply lowers C to LLVM IR and does
not have stack frame information, so it cannot emit this diagnostic.
When the linker drives LTO, it restarts optimizations and lowers LLVM IR
to object code. At that point, it has stack frame information but
doesn't know to check for a specific max stack frame size.

I consider this a bug in LLVM that we need to fix. There are some
details we're working out related to LTO such as which value to use when
there are multiple different values specified per TU, or how to
propagate these to compiler synthesized routines properly, if at all.

Until it's fixed, ensure we don't miss these. At that point we can wrap
this in a compiler version guard or revert this based on the minimum
support version of Clang.

The error message is not generated during link:
  LTO     vmlinux.o
ld.lld: warning: stack size limit exceeded (8224) in foobarbaz

Cc: Sami Tolvanen <samitolvanen@google.com>
Reported-by: Candle Sun <candlesea@gmail.com>
Suggested-by: Fangrui Song <maskray@google.com>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20210312010942.1546679-1-ndesaulniers@google.com