Show the rejected function name when attaching tracing programs to
functions in deny list.

With this change, we know why tracing programs can't attach to functions
like migrate_disable() from log.

$ ./fentry
libbpf: prog 'migrate_disable': BPF program load failed: -EINVAL
libbpf: prog 'migrate_disable': -- BEGIN PROG LOAD LOG --
Attaching tracing programs to function 'migrate_disable' is rejected.

Suggested-by: Leon Hwang <leon.hwang@linux.dev>
Signed-off-by: KaFai Wan <mannkafai@gmail.com>
---
 kernel/bpf/verifier.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 00d287814f12..c24c0d57e595 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -23942,6 +23942,8 @@ static int check_attach_btf_id(struct bpf_verifier_env *env)
 			return ret;
 	} else if (prog->type == BPF_PROG_TYPE_TRACING &&
 		   btf_id_set_contains(&btf_id_deny, btf_id)) {
+		verbose(env, "Attaching tracing programs to function '%s' is rejected.\n",
+			tgt_info.tgt_name);
 		return -EINVAL;
 	} else if ((prog->expected_attach_type == BPF_TRACE_FEXIT ||
 		   prog->expected_attach_type == BPF_MODIFY_RETURN) &&
-- 
2.43.0