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

["Christophe Leroy (CS GROUP)" <chleroy@kernel.org>]

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.

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