[PATCH AUTOSEL 6.9 32/35] bpf: avoid uninitialized warnings in verifier_global_subprogs.c

Sasha Levin
2024-05-27T14:13:28Z

From: "Jose E. Marchesi" <jose.marchesi@oracle.com>

[ Upstream commit cd3fc3b9782130a5bc1dc3dfccffbc1657637a93 ]

[Changes from V1:
- The warning to disable is -Wmaybe-uninitialized, not -Wuninitialized.
- This warning is only supported in GCC.]

The BPF selftest verifier_global_subprogs.c contains code that
purposedly performs out of bounds access to memory, to check whether
the kernel verifier is able to catch them. For example:

__noinline int global_unsupp(const int *mem)
{
if (!mem)
return 0;
return mem[100]; /* BOOM */
}

With -O1 and higher and no inlining, GCC notices this fact and emits a
"maybe uninitialized" warning. This is by design. Note that the
emission of these warnings is highly dependent on the precise
optimizations that are performed.

This patch adds a compiler pragma to verifier_global_subprogs.c to
ignore these warnings.

Tested in bpf-next master.
No regressions.

Signed-off-by: Jose E. Marchesi <jose.marchesi@oracle.com>
Cc: david.faust@oracle.com
Cc: cupertino.miranda@oracle.com
Cc: Yonghong Song <yonghong.song@linux.dev>
Cc: Eduard Zingerman <eddyz87@gmail.com>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20240507184756.1772-1-jose.marchesi@oracle.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
.../testing/selftests/bpf/progs/verifier_global_subprogs.c | 7 +++++++
1 file changed, 7 insertions(+)

diff --git a/tools/testing/selftests/bpf/progs/verifier_global_subprogs.c b/tools/testing/selftests/bpf/progs/verifier_global_subprogs.c
index baff5ffe94051..a9fc30ed4d732 100644
--- a/tools/testing/selftests/bpf/progs/verifier_global_subprogs.c
+++ b/tools/testing/selftests/bpf/progs/verifier_global_subprogs.c
@@ -8,6 +8,13 @@
#include "xdp_metadata.h"
#include "bpf_kfuncs.h"

+/* The compiler may be able to detect the access to uninitialized
+ memory in the routines performing out of bound memory accesses and
+ emit warnings about it. This is the case of GCC. */
+#if !defined(__clang__)
+#pragma GCC diagnostic ignored "-Wmaybe-uninitialized"
+#endif
+
int arr[1];
int unkn_idx;
const volatile bool call_dead_subprog = false;
--
2.43.0