Makefile: LTO: have linker check -Wframe-larger-than

-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: NFangrui Song <maskray@google.com>
Signed-off-by: NNick Desaulniers <ndesaulniers@google.com>
Signed-off-by: NKees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20210312010942.1546679-1-ndesaulniers@google.com

Makefile

@@ -928,6 +928,11 @@ CC_FLAGS_LTO	+= -fvisibility=hidden
 
 # Limit inlining across translation units to reduce binary size
 KBUILD_LDFLAGS += -mllvm -import-instr-limit=5
+
+# Check for frame size exceeding threshold during prolog/epilog insertion.
+ifneq ($(CONFIG_FRAME_WARN),0)
+KBUILD_LDFLAGS	+= -plugin-opt=-warn-stack-size=$(CONFIG_FRAME_WARN)
+endif
 endif
 
 ifdef CONFIG_LTO