The BPF selftest test_global_func9.c performs type punning and breaks
srict-aliasing rules.
In particular, given:
int global_func9(struct __sk_buff *skb)
{
int result = 0;
[...]
{
const struct C c = {.x = skb->len, .y = skb->family };
result |= foo((const struct S *)&c);
}
}
When building with strict-aliasing enabled (the default) the
initialization of `c' gets optimized away in its entirely:
[... no initialization of `c' ...]
r1 = r10
r1 += -40
call foo
w0 |= w6
Since GCC knows that `foo' accesses s->x, we get a "maybe
uninitialized" warning.
On the other hand, when strict-aliasing is disabled GCC only optimizes
away the store to `.y':
r1 = *(u32 *) (r6+0)
*(u32 *) (r10+-40) = r1 ; This is .x = skb->len in `c'
r1 = r10
r1 += -40
call foo
w0 |= w6
In this case the warning is not emitted, because s-> is initialized.
This patch disables strict aliasing in this test when building with
GCC. clang seems to not optimize this particular code even when
strict aliasing is enabled.
Tested in bpf-next master.
Signed-off-by: Jose E. Marchesi <jose.marchesi@xxxxxxxxxx>
Cc: david.faust@xxxxxxxxxx
Cc: cupertino.miranda@xxxxxxxxxx
Cc: Yonghong Song <yonghong.song@xxxxxxxxx>
Cc: Eduard Zingerman <eddyz87@xxxxxxxxx>
---
tools/testing/selftests/bpf/Makefile | 1 +
1 file changed, 1 insertion(+)
diff --git a/tools/testing/selftests/bpf/Makefile b/tools/testing/selftests/bpf/Makefile
index 135023a357b3..979838c5a495 100644
--- a/tools/testing/selftests/bpf/Makefile
+++ b/tools/testing/selftests/bpf/Makefile
@@ -53,6 +53,7 @@ progs/syscall.c-CFLAGS := -fno-strict-aliasing
progs/test_pkt_md_access.c-CFLAGS := -fno-strict-aliasing
progs/test_sk_lookup.c-CFLAGS := -fno-strict-aliasing
progs/timer_crash.c-CFLAGS := -fno-strict-aliasing
+progs/test_global_func9.c-CFLAGS := -fno-strict-aliasing
ifneq ($(LLVM),)
# Silence some warnings when compiled with clang
--
2.30.2
