bpf: Add log for attaching tracing programs to functions in deny list

JIRA: https://issues.redhat.com/browse/RHEL-78204

commit 863aab3
Author: KaFai Wan <[email protected]>
Date:   Thu Jul 24 23:14:52 2025 +0800

    bpf: Add log for attaching tracing programs to functions in deny list
    
    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 __rcu_read_lock() from log.
    
    $ ./fentry
    libbpf: prog '__rcu_read_lock': BPF program load failed: -EINVAL
    libbpf: prog '__rcu_read_lock': -- BEGIN PROG LOAD LOG --
    Attaching tracing programs to function '__rcu_read_lock' is rejected.
    
    Suggested-by: Leon Hwang <[email protected]>
    Signed-off-by: KaFai Wan <[email protected]>
    Acked-by: Yafang Shao <[email protected]>
    Acked-by: Yonghong Song <[email protected]>
    Link: https://lore.kernel.org/r/[email protected]
    Signed-off-by: Alexei Starovoitov <[email protected]>

Signed-off-by: Jerome Marchand <[email protected]>

Author: jeromemarchand

kernel/bpf/verifier.c

@@ -23985,6 +23985,8 @@ 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) &&