[PATCH bpf-next v2 2/2] selftests/bpf: Adjust fexit_bpf2bpf ctx layout for llvm23 true signature

[Yonghong Song <yonghong.song@linux.dev>]

test_pkt_access_subprog2() is defined in C as

  int test_pkt_access_subprog2(int val, volatile struct __sk_buff *skb)

but llvm optimizes away the unused 'int val' argument. Before llvm23 the
BTF signature did not match the optimized assembly, so the verifier set
attach_func_proto to NULL and fell back to MAX_BPF_FUNC_REG_ARGS (5) u64
arguments (see btf_ctx_access()). The fexit ctx struct therefore placed
the return value after args[5].

With llvm23 the 'true' signature

  int test_pkt_access_subprog2(volatile struct __sk_buff *skb)

is recorded in BTF, so nr_args becomes 1 and the return value moves to
the slot right after args[1]. Select the matching args_subprog2 layout
based on __clang_major__ so the test works with both old and new llvm.

Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
---
 tools/testing/selftests/bpf/progs/fexit_bpf2bpf.c | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/tools/testing/selftests/bpf/progs/fexit_bpf2bpf.c b/tools/testing/selftests/bpf/progs/fexit_bpf2bpf.c
index 983b7c233382..f4bbf87b82dd 100644
--- a/tools/testing/selftests/bpf/progs/fexit_bpf2bpf.c
+++ b/tools/testing/selftests/bpf/progs/fexit_bpf2bpf.c
@@ -53,14 +53,23 @@ int BPF_PROG(test_subprog1, struct sk_buff *skb, int ret)
  *   r0 = *(u32 *)(r1 + 0)
  *   w0 <<= 1
  *   exit
- * In such case the verifier falls back to conservative and
+ * Before llvm23, in such case the verifier falls back to conservative and
  * tracing program can access arguments and return value as u64
- * instead of accurate types.
+ * instead of accurate types. With llvm23, the true signature
+ *   int test_pkt_access_subprog2(volatile struct __sk_buff *skb)
+ * is available in btf.
  */
+#if __clang_major__ >= 23
+struct args_subprog2 {
+	__u64 args[1];
+	__u64 ret;
+};
+#else
 struct args_subprog2 {
 	__u64 args[5];
 	__u64 ret;
 };
+#endif
 __u64 test_result_subprog2 = 0;
 SEC("fexit/test_pkt_access_subprog2")
 int test_subprog2(struct args_subprog2 *ctx)
-- 
2.53.0-Meta