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

[Nick Desaulniers]

On Wed, Aug 28, 2019 at 10:53 AM Nathan Chancellor
<natechancellor@xxxxxxxxx> wrote:
>
> On Wed, Aug 28, 2019 at 11:43:53PM +1000, Michael Ellerman wrote:
> > Nathan Chancellor <natechancellor@xxxxxxxxx> 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@xxxxxxxxxxxxxxx # 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@xxxxxxxxx>
> > > ---
> > >
> > > 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.

The `-f` family of flags have dire implications for codegen, I'd
really prefer we think long and hard before adding/removing them to
suppress warnings.  I don't think it's a solution for this particular
problem.

>
> > 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



-- 
Thanks,
~Nick Desaulniers