[PATCH bpf v5 2/2] selftests/bpf: strengthen bpf_kfunc_implicit_arg to verify aux injection

[chenyuan_fl]

From: Yuan Chen <chenyuan@xxxxxxxxxx>

Verify that the KF_IMPLICIT_ARGS injection path correctly passes
the bpf_prog_aux pointer by checking aux->name in
bpf_kfunc_implicit_arg() for the expected program name prefix.
If the verifier incorrectly skipped injection (as could happen
with pahole 1.30's BTF mismatch), the stale register would not
contain a valid aux pointer and the name check would fail.

This is a positive test exercised by the existing kfunc_implicit_args
selftest, which calls bpf_kfunc_implicit_arg(5) and expects a return
value of 5.

Signed-off-by: Yuan Chen <chenyuan@xxxxxxxxxx>
---
 tools/testing/selftests/bpf/test_kmods/bpf_testmod.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/tools/testing/selftests/bpf/test_kmods/bpf_testmod.c b/tools/testing/selftests/bpf/test_kmods/bpf_testmod.c
index 30f1cd23093c..624d57a5c79a 100644
--- a/tools/testing/selftests/bpf/test_kmods/bpf_testmod.c
+++ b/tools/testing/selftests/bpf/test_kmods/bpf_testmod.c
@@ -1906,7 +1906,11 @@ int bpf_kfunc_multi_st_ops_test_1_assoc(struct st_ops_args *args, struct bpf_pro
 
 int bpf_kfunc_implicit_arg(int a, struct bpf_prog_aux *aux)
 {
-	if (aux && a > 0)
+	/* Verify the kernel injected the correct bpf_prog_aux pointer
+	 * rather than leaving a stale register value. */
+	if (!aux || strncmp(aux->name, "test_kfunc", sizeof("test_kfunc") - 1))
+		return -EINVAL;
+	if (a > 0)
 		return a;
 	return -EINVAL;
 }
-- 
2.54.0