Re: [PATCH] Makefile: LTO: have linker check -Wframe-larger-than

[Nick Desaulniers]

On Thu, Mar 11, 2021 at 5:09 PM Nick Desaulniers
<ndesaulniers@xxxxxxxxxx> wrote:
>
> -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@xxxxxxxxxx>
> Reported-by: Candle Sun <candlesea@xxxxxxxxx>
> Suggested-by: Fangrui Song <maskray@xxxxxxxxxx>
> Signed-off-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
> ---
> LTO users might want to `make clean` or `rm -rf .thinlto-cache` to test
> this.
>
>  Makefile | 5 +++++
>  1 file changed, 5 insertions(+)
>
> diff --git a/Makefile b/Makefile
> index f9b54da2fca0..74566b1417b8 100644
> --- a/Makefile
> +++ b/Makefile

Candle sent me a private message that we probably also want coverage
for kernel modules. Let me revise this and test/send a v2.

> @@ -910,6 +910,11 @@ 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
> --
> 2.31.0.rc2.261.g7f71774620-goog
>


-- 
Thanks,
~Nick Desaulniers