linux-kernel - Re: [PATCH] powerpc/lib/xor_vmx: Relax frame size for clang

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Date:   Thu, 08 Sep 2022 10:27:19 +1000
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Christophe Leroy <christophe.leroy@...roup.eu>,
        Mathieu Malaterre <malat@...ian.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     linux-kernel@...r.kernel.org, Paul Mackerras <paulus@...ba.org>,
        Joel Stanley <joel@....id.au>, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH] powerpc/lib/xor_vmx: Relax frame size for clang

Christophe Leroy <christophe.leroy@...roup.eu> writes:
> Le 21/06/2019 à 10:58, Mathieu Malaterre a écrit :
>> When building with clang-8 the frame size limit is hit:
>> 
>>    ../arch/powerpc/lib/xor_vmx.c:119:6: error: stack frame size of 1200 bytes in function '__xor_altivec_5' [-Werror,-Wframe-larger-than=]
>> 
>> Follow the same approach as commit 9c87156cce5a ("powerpc/xmon: Relax
>> frame size for clang") until a proper fix is implemented upstream in
>> clang and relax requirement for clang.
>
> With Clang 14 I get the following errors, but only with KASAN selected.
>
>    CC      arch/powerpc/lib/xor_vmx.o
> arch/powerpc/lib/xor_vmx.c:95:6: error: stack frame size (1040) exceeds 
> limit (1024) in '__xor_altivec_4' [-Werror,-Wframe-larger-than]
> void __xor_altivec_4(unsigned long bytes,
>       ^
> arch/powerpc/lib/xor_vmx.c:124:6: error: stack frame size (1312) exceeds 
> limit (1024) in '__xor_altivec_5' [-Werror,-Wframe-larger-than]
> void __xor_altivec_5(unsigned long bytes,
>       ^

That's a 32-bit build?

> Is this patch still relevant ?

The clang issue was closed because a different change fixed the issue:

  https://github.com/ClangBuiltLinux/linux/issues/563

> Or should frame size be relaxed when KASAN is selected ? After all the 
> stack size is multiplied by 2 when we have KASAN, so maybe the warning 
> limit should be increased as well ?

Yeah that would make some sense.

On 64-bit the largest frame in that file is 1424, which is below the
default 2048 byte limit.

So maybe just increase it for 32-bit && KASAN.

What would be nice is if the FRAME_WARN value could be calculated as a
percentage of the THREAD_SHIFT, but that's not easily doable with the
way things are structured in Kconfig.

cheers