linux-kernel - Re: [PATCH] Revert "Revert "powerpc/xmon: Relax frame size for clang""

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Date:   Mon, 28 Aug 2023 10:46:31 -0700
From:   Nathan Chancellor <nathan@...nel.org>
To:     ndesaulniers@...gle.com
Cc:     Michael Ellerman <mpe@...erman.id.au>,
        Nicholas Piggin <npiggin@...il.com>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Tom Rix <trix@...hat.com>, linuxppc-dev@...ts.ozlabs.org,
        linux-kernel@...r.kernel.org, llvm@...ts.linux.dev,
        kernel test robot <lkp@...el.com>
Subject: Re: [PATCH] Revert "Revert "powerpc/xmon: Relax frame size for
 clang""

On Mon, Aug 28, 2023 at 10:35:26AM -0700, ndesaulniers@...gle.com wrote:
> This reverts commit 7f3c5d099b6f8452dc4dcfe4179ea48e6a13d0eb.
> 
> Turns out that this is reproducible still under specific compiler
> versions (mea culpa: I did not test every supported version of clang),
> and even a few randconfigs bots found.
> 
> We'll have to revisit this again in the future, for now back this out.
> 
> Reported-by: Nathan Chancellor <nathan@...nel.org>
> Closes: https://github.com/ClangBuiltLinux/linux/issues/252#issuecomment-1690371256
> Reported-by: kernel test robot <lkp@...el.com>
> Closes: https://lore.kernel.org/llvm/202308260344.Vc4Giuk7-lkp@intel.com/
> Signed-off-by: Nick Desaulniers <ndesaulniers@...gle.com>

I know this is just a straight reapplication of the original workaround
but could we use ccflags here instead of KBUILD_CFLAGS (with it placed
after the NO_MIMINAL_TOC assignment)?

  # clang stores addresses on the stack causing the frame size to blow
  # out. See https://github.com/ClangBuiltLinux/linux/issues/252
  ccflags-$(CONFIG_CC_IS_CLANG) += -Wframe-larger-than=4096

The addition to KBUILD_CFLAGS makes me think this flags will get applied
to the rest of the kernel but I have not actually verified.

Regardless:

Reviewed-by: Nathan Chancellor <nathan@...nel.org>

Side note, seems like b4 is still doing the thing with "From:".

> ---
>  arch/powerpc/xmon/Makefile | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/arch/powerpc/xmon/Makefile b/arch/powerpc/xmon/Makefile
> index 7705aa74a24d..d334de392e6c 100644
> --- a/arch/powerpc/xmon/Makefile
> +++ b/arch/powerpc/xmon/Makefile
> @@ -10,6 +10,12 @@ KCSAN_SANITIZE := n
>  # Disable ftrace for the entire directory
>  ccflags-remove-$(CONFIG_FUNCTION_TRACER) += $(CC_FLAGS_FTRACE)
>  
> +ifdef CONFIG_CC_IS_CLANG
> +# clang stores addresses on the stack causing the frame size to blow
> +# out. See https://github.com/ClangBuiltLinux/linux/issues/252
> +KBUILD_CFLAGS += -Wframe-larger-than=4096
> +endif
> +
>  ccflags-$(CONFIG_PPC64) := $(NO_MINIMAL_TOC)
>  
>  obj-y			+= xmon.o nonstdio.o spr_access.o xmon_bpts.o
> 
> ---
> base-commit: 2ee82481c392eec06a7ef28df61b7f0d8e45be2e
> change-id: 20230828-ppc_rerevert-647427f04ce1
> 
> Best regards,
> -- 
> Nick Desaulniers <ndesaulniers@...gle.com>
>