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 <[email protected]>
Signed-off-by: Amit Machhiwal <[email protected]>
---
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
