On Wed, Aug 28, 2019 at 10:53 AM Nathan Chancellor
[off-list ref] wrote:

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

quoted

Nathan Chancellor [off-list ref] writes:

quoted

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@vger.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 <redacted>
---

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.

quoted

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