bpf: allow invoking bpf_for_each_map_elem with different maps

Taking different maps within a single bpf_for_each_map_elem call is not
allowed before, because from the second map,
bpf_insn_aux_data->map_ptr_state will be marked as *poison*. In fact
both map_ptr and state are needed to support this use case: map_ptr is
used by set_map_elem_callback_state() while poison state is needed to
determine whether to use direct call.

Signed-off-by: Philo Lu <lulie@linux.alibaba.com>

kernel/bpf/verifier.c

@@ -9651,11 +9651,7 @@ static int set_map_elem_callback_state(struct bpf_verifier_env *env,
 	struct bpf_map *map;
 	int err;
 
-	if (bpf_map_ptr_poisoned(insn_aux)) {
-		verbose(env, "tail_call abusing map_ptr\n");
-		return -EINVAL;
-	}
-
+	/* valid map_ptr and poison value does not matter */
 	map = insn_aux->map_ptr_state.map_ptr;
 	if (!map->ops->map_set_for_each_callback_args ||
 	    !map->ops->map_for_each_callback) {