[PATCH AUTOSEL 6.8 28/30] bpf: avoid uninitialized warnings in verifier_global_subprogs.c

[Sasha Levin <sashal@kernel.org>]

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 67dddd9418911..27f4b2da131b1 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