[bpf-next,V2] bpf: avoid uninitialized warnings in verifier_global_subprogs.c

Commit Message

Jose E. Marchesi May 7, 2024, 6:47 p.m. UTC

[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 <[e-mail]>
Cc: [e-mail]
Cc: [e-mail]
Cc: Yonghong Song <[e-mail]>
Cc: Eduard Zingerman <[e-mail]>
Acked-by: Yonghong Song <[e-mail]>
---
 .../testing/selftests/bpf/progs/verifier_global_subprogs.c | 7 +++++++
 1 file changed, 7 insertions(+)

Comments

[e-mail] May 8, 2024, 5 p.m. UTC | #1

Hello:

This patch was applied to bpf/bpf-next.git (master)
by Alexei Starovoitov <[e-mail]>:

On Tue,  7 May 2024 20:47:56 +0200 you wrote:
> [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:
> 
> [...]

Here is the summary with links:
  - [bpf-next,V2] bpf: avoid uninitialized warnings in verifier_global_subprogs.c
    https://git.kernel.org/bpf/bpf-next/c/cd3fc3b97821

You are awesome, thank you!

Patch

diff --git a/tools/testing/selftests/bpf/progs/verifier_global_subprogs.c b/tools/testing/selftests/bpf/progs/verifier_global_subprogs.c
index baff5ffe9405..a9fc30ed4d73 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;