Re: [PATCH] selftests/powerpc: Suppress false positive -Wmaybe-uninitialized with GCC 15

[Amit Machhiwal <amachhiw@linux.ibm.com>]

Hi Christhophe,

Thanks for looking at the patch. Please find my comments inline:j

On 2026/03/10 11:54 AM, Christophe Leroy (CS GROUP) wrote:
> 
> 
> Le 10/03/2026 à 11:15, Amit Machhiwal a écrit :
> > GCC 15 reports the below false positive '-Wmaybe-uninitialized' warning
> > in vphn_unpack_associativity() when building the powerpc selftests.
> > 
> >    # make -C tools/testing/selftests TARGETS="powerpc"
> >    [...]
> >      CC       test-vphn
> >    In file included from test-vphn.c:3:
> >    In function ‘vphn_unpack_associativity’,
> >        inlined from ‘test_one’ at test-vphn.c:371:2,
> >        inlined from ‘test_vphn’ at test-vphn.c:399:9:
> >    test-vphn.c:10:33: error: ‘be_packed’ may be used uninitialized [-Werror=maybe-uninitialized]
> >       10 | #define be16_to_cpup(x)         bswap_16(*x)
> >          |                                 ^~~~~~~~
> >    vphn.c:42:27: note: in expansion of macro ‘be16_to_cpup’
> >       42 |                 u16 new = be16_to_cpup(field++);
> >          |                           ^~~~~~~~~~~~
> >    In file included from test-vphn.c:19:
> >    vphn.c: In function ‘test_vphn’:
> >    vphn.c:27:16: note: ‘be_packed’ declared here
> >       27 |         __be64 be_packed[VPHN_REGISTER_COUNT];
> >          |                ^~~~~~~~~
> >    cc1: all warnings being treated as errors
> > 
> > When vphn_unpack_associativity() is called from hcall_vphn(), this error
> > is not seen during compilation because GCC 15 seems to consider 'retbuf'
> > always populated from the hypervisor which is eventually referred by
> > 'be_packed'. However, GCC 15's dataflow analysis can’t prove the same
> > before the first dereference when vphn_unpack_associativity() is called
> > from test_one() with pre-initialized array of 'struct test'. This
> > results in a false positive warning which is promoted to an error under
> > '-Werror'. This problem is not seen when the compilation is performed
> > with GCC 13 and 14.
> > 
> > Suppress the warning locally around the offending statement when
> > building with GCC 15 using a diagnostic pragma. This keeps the build
> > working while limiting the scope of the suppression to the specific
> > statement that triggers the false positive. An issue [1] has also been
> > created on GCC bugzilla.
> 
> Usually when we get this kind of warning this is because the code is too
> complex. We should try to make it more obvious instead of just hiding the
> warning.

The real issue here is that GCC 15 emits '-Wmaybe-uninitialized' due to
type punning between __be64[] and __b16* when accessing the buffer via
be16_to_cpup(). The underlying object is fully initialized but GCC 15
fails to track the aliasing due to the strict aliasing violation here.
Please refer [1] and [2].

The selftest compiles fine with '-fno-strict-aliasing'. I see that when
we build vphn.c while compiling the kernel, the top level Makefile
includes '-fno-strict-aliasing' flag always.

So, I believe the same flag should be used to build vphn tests when
compiling vphn.c via the selftests. I'll send the v2 to achieve this
thus avoiding the compilation failure.

Please let me know you have different thoughts.

[1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=124427 
[2] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99768

~Amit

> 
> Here the for loop is a bit misleading.
> 
> > 
> > [1] https://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgcc.gnu.org%2Fbugzilla%2Fshow_bug.cgi%3Fid%3D124427&data=05%7C02%7Cchristophe.leroy%40csgroup.eu%7C06a4d55b55f24c5cf00208de7e8e3676%7C8b87af7d86474dc78df45f69a2011bb5%7C0%7C0%7C639087346428583316%7CUnknown%7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ%3D%3D%7C0%7C%7C%7C&sdata=xEfO94N6IfGYhmmapNFduv3OrMarxpjTpZR6B38uR1s%3D&reserved=0
> > 
> > Fixes: 58dae82843f5 ("selftests/powerpc: Add test for VPHN")
> > Reviewed-by: Vaibhav Jain <vaibhav@linux.ibm.com>
> > Signed-off-by: Amit Machhiwal <amachhiw@linux.ibm.com>
> > ---
> >   arch/powerpc/platforms/pseries/vphn.c | 15 +++++++++++++++
> >   1 file changed, 15 insertions(+)
> > 
> > diff --git a/arch/powerpc/platforms/pseries/vphn.c b/arch/powerpc/platforms/pseries/vphn.c
> > index 3f85ece3c872..9bc891143fec 100644
> > --- a/arch/powerpc/platforms/pseries/vphn.c
> > +++ b/arch/powerpc/platforms/pseries/vphn.c
> > @@ -39,7 +39,22 @@ static int vphn_unpack_associativity(const long *packed, __be32 *unpacked)
> >   		be_packed[i] = cpu_to_be64(packed[i]);
> >   	for (i = 1; i < VPHN_ASSOC_BUFSIZE; i++) {
> > +/*
> > + * When this function is called from hcall_vphn(), GCC 15 seems to consider
> > + * 'retbuf' always populated from the hypervisor which is eventually referred by
> > + * 'be_packed'. However, GCC 15's dataflow analysis can’t prove the same before
> > + * the first dereference when this function is called from test_one() with
> > + * pre-initialized array of 'struct test'. This results in a false positive
> > + * '-Wmaybe-uninitialized' warning which is promoted to an error under
> > + * '-Werror'. This problem is not seen when the compilation is performed with
> > + * older GCC versions.
> > + */
> > +#pragma GCC diagnostic push
> > +#if defined(__GNUC__) && __GNUC__ >= 15
> > +#pragma GCC diagnostic ignored "-Wmaybe-uninitialized"
> > +#endif
> >   		u16 new = be16_to_cpup(field++);
> > +#pragma GCC diagnostic pop
> >   		if (is_32bit) {
> >   			/*
> > 
> > base-commit: 1f318b96cc84d7c2ab792fcc0bfd42a7ca890681
>