bpf: clarify a misleading verifier error message

The current verifier error message states that tail_calls are not
allowed in non-JITed programs with BPF-to-BPF calls. While this is
accurate, it is not the only scenario where this restriction applies.
Some architectures do not support this feature combination even when
programs are JITed. This update improves the error message to better
reflect these limitations.

Suggested-by: Shung-Hsi Yu <[email protected]>
Signed-off-by: Andrea Terzolo <[email protected]>
Acked-by: Shung-Hsi Yu <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Alexei Starovoitov <[email protected]>

Author: Andreagit97

kernel/bpf/verifier.c

@@ -9887,7 +9887,7 @@ static int check_map_func_compatibility(struct bpf_verifier_env *env,
 		if (map->map_type != BPF_MAP_TYPE_PROG_ARRAY)
 			goto error;
 		if (env->subprog_cnt > 1 && !allow_tail_call_in_subprogs(env)) {
-			verbose(env, "tail_calls are not allowed in non-JITed programs with bpf-to-bpf calls\n");
+			verbose(env, "mixing of tail_calls and bpf-to-bpf calls is not supported\n");
 			return -EINVAL;
 		}
 		break;