linux-kernel - Re: [PATCH] powerpc: Avoid clang warnings around setjmp and longjmp

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Message-ID: <878srdv206.fsf@mpe.ellerman.id.au>
Date:   Wed, 28 Aug 2019 23:43:53 +1000
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Nathan Chancellor <natechancellor@...il.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
        clang-built-linux@...glegroups.com,
        Nathan Chancellor <natechancellor@...il.com>,
        stable@...r.kernel.org
Subject: Re: [PATCH] powerpc: Avoid clang warnings around setjmp and longjmp

Nathan Chancellor <natechancellor@...il.com> writes:

> Commit aea447141c7e ("powerpc: Disable -Wbuiltin-requires-header when
> setjmp is used") disabled -Wbuiltin-requires-header because of a warning
> about the setjmp and longjmp declarations.
>
> r367387 in clang added another diagnostic around this, complaining that
> there is no jmp_buf declaration.
>
> In file included from ../arch/powerpc/xmon/xmon.c:47:
> ../arch/powerpc/include/asm/setjmp.h:10:13: error: declaration of
> built-in function 'setjmp' requires the declaration of the 'jmp_buf'
> type, commonly provided in the header <setjmp.h>.
> [-Werror,-Wincomplete-setjmp-declaration]
> extern long setjmp(long *);
>             ^
> ../arch/powerpc/include/asm/setjmp.h:11:13: error: declaration of
> built-in function 'longjmp' requires the declaration of the 'jmp_buf'
> type, commonly provided in the header <setjmp.h>.
> [-Werror,-Wincomplete-setjmp-declaration]
> extern void longjmp(long *, long);
>             ^
> 2 errors generated.
>
> Take the same approach as the above commit by disabling the warning for
> the same reason, we provide our own longjmp/setjmp function.
>
> Cc: stable@...r.kernel.org # 4.19+
> Link: https://github.com/ClangBuiltLinux/linux/issues/625
> Link: https://github.com/llvm/llvm-project/commit/3be25e79477db2d31ac46493d97eca8c20592b07
> Signed-off-by: Nathan Chancellor <natechancellor@...il.com>
> ---
>
> It may be worth using -fno-builtin-setjmp and -fno-builtin-longjmp
> instead as it makes it clear to clang that we are not using the builtin
> longjmp and setjmp functions, which I think is why these warnings are
> appearing (at least according to the commit that introduced this waring).
>
> Sample patch:
> https://github.com/ClangBuiltLinux/linux/issues/625#issuecomment-519251372

Couldn't we just add those flags to CFLAGS for the whole kernel? Rather
than making them per-file.

I mean there's no kernel code that wants to use clang's builtin
setjmp/longjmp implementation at all right?

cheers