Re: [PATCH] selftests: harness: Mark test fixture objects __maybe_unused

[Nathan Chancellor <nathan@kernel.org>]

On Tue, Jun 30, 2026 at 04:52:44PM -0600, Alex Williamson wrote:
> On Tue, 30 Jun 2026 21:33:41 +0000
> David Matlack <dmatlack@google.com> wrote:
> 
> > Mark test fixture objects __maybe_unused since they will not be used
> > when there are fixture variants (the variants will be used instead).
> > 
> > This fixes the VFIO selftest build with clang and -Werror:
> > 
> > vfio_dma_mapping_mmio_test.c:106:1: error: variable '_vfio_dma_mapping_mmio_test_map_full_bar_object' set but not used [-Werror,-Wunused-but-set-global]
> >   106 | TEST_F(vfio_dma_mapping_mmio_test, map_full_bar)
> >       | ^
> > ./../kselftest_harness.h:398:2: note: expanded from macro 'TEST_F'
> >   398 |         __TEST_F_IMPL(fixture_name, test_name, -1, TEST_TIMEOUT_DEFAULT)
> >       |         ^
> > ./../kselftest_harness.h:470:33: note: expanded from macro '__TEST_F_IMPL'
> >   470 |         static struct __test_metadata *_##fixture_name##_##test_name##_object; \
> >       |
> > 
> > Fixes: ff556bd98348 ("vfio: selftests: Add -Wall and -Werror to the Makefile")
> > Reported-by: Aaron Lewis <aaronlewis@google.com>
> > Signed-off-by: David Matlack <dmatlack@google.com>
> > ---
> >  tools/testing/selftests/kselftest_harness.h | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/tools/testing/selftests/kselftest_harness.h b/tools/testing/selftests/kselftest_harness.h
> > index 261e4df94d9d..29a19bc87084 100644
> > --- a/tools/testing/selftests/kselftest_harness.h
> > +++ b/tools/testing/selftests/kselftest_harness.h
> > @@ -467,7 +467,7 @@ static inline void __kselftest_memset_safe(void *s, int c, size_t n)
> >  				!__atomic_test_and_set(_metadata->no_teardown, __ATOMIC_RELAXED)) \
> >  			fixture_name##_teardown(_metadata, self, variant); \
> >  	} \
> > -	static struct __test_metadata *_##fixture_name##_##test_name##_object; \
> > +	static struct __test_metadata *_##fixture_name##_##test_name##_object __maybe_unused; \
> >  	static void __attribute__((constructor(KSELFTEST_PRIO_TEST))) \
> >  			_register_##fixture_name##_##test_name(void) \
> >  	{ \
> > 
> > base-commit: dc59e4fea9d83f03bad6bddf3fa2e52491777482
> 
> Reviewed-by: Alex Williamson <alex@shazbot.org>
> 
> Kees/Shuah, I can take this through the vfio tree if you prefer, it
> fixes a commit that entered through vfio selftests.  Thanks,

Not that it is a big deal or matters all too much but I do not think
this fixes tag is entirely accurate. The blamed commit may have
introduced the build warning/error in the VFIO code but that same
warning could occur in any selftest code, as evidenced by the exact same
patch submitted a month ago:

  https://lore.kernel.org/20260530054722.271157-1-kuniyu@google.com/

clang made a change to -Wunused-but-set-variable (split out into its own
subwarning, -Wunused-but-set-global) that exposes this. Personally, I
find that change explains the problem better but this one makes it clear
that it can become fatal with -Werror.

-- 
Cheers,
Nathan