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

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Date:   Wed, 28 Aug 2019 10:53:22 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Michael Ellerman <mpe@...erman.id.au>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
        clang-built-linux@...glegroups.com, stable@...r.kernel.org
Subject: Re: [PATCH] powerpc: Avoid clang warnings around setjmp and longjmp

On Wed, Aug 28, 2019 at 11:43:53PM +1000, Michael Ellerman wrote:
> 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.

Yes, I don't think this would be unreasonable. Are you referring to the
cc-disable-warning flags or the -fno-builtin flags? I personally think
the -fno-builtin flags convey to clang what the kernel is intending to
do better than disabling the warnings outright.

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

I did a quick search of the tree and it looks like powerpc and x86/um
are the only architectures that do anything with setjmp/longjmp. x86/um
avoids this by using a define flag to change setjmp to kernel_setjmp:

arch/um/Makefile: -Dlongjmp=kernel_longjmp -Dsetjmp=kernel_setjmp \

Seems like adding those flags should be safe.

Cheers,
Nathan