inet: reqsk_alloc() needs to take care of dead listeners

We'll soon no longer take a refcount on listeners,
so reqsk_alloc() can not assume a listener refcount is not
zero. We need to use atomic_inc_not_zero()

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Author: edumazet

include/net/request_sock.h

@@ -85,24 +85,23 @@ reqsk_alloc(const struct request_sock_ops *ops, struct sock *sk_listener,
 	struct request_sock *req;
 
 	req = kmem_cache_alloc(ops->slab, GFP_ATOMIC | __GFP_NOWARN);
-
-	if (req) {
-		req->rsk_ops = ops;
-		if (attach_listener) {
-			sock_hold(sk_listener);
-			req->rsk_listener = sk_listener;
-		} else {
-			req->rsk_listener = NULL;
+	if (!req)
+		return NULL;
+	req->rsk_listener = NULL;
+	if (attach_listener) {
+		if (unlikely(!atomic_inc_not_zero(&sk_listener->sk_refcnt))) {
+			kmem_cache_free(ops->slab, req);
+			return NULL;
 		}
-		req_to_sk(req)->sk_prot = sk_listener->sk_prot;
-		sk_node_init(&req_to_sk(req)->sk_node);
-		sk_tx_queue_clear(req_to_sk(req));
-		req->saved_syn = NULL;
-		/* Following is temporary. It is coupled with debugging
-		 * helpers in reqsk_put() & reqsk_free()
-		 */
-		atomic_set(&req->rsk_refcnt, 0);
+		req->rsk_listener = sk_listener;
 	}
+	req->rsk_ops = ops;
+	req_to_sk(req)->sk_prot = sk_listener->sk_prot;
+	sk_node_init(&req_to_sk(req)->sk_node);
+	sk_tx_queue_clear(req_to_sk(req));
+	req->saved_syn = NULL;
+	atomic_set(&req->rsk_refcnt, 0);
+
 	return req;
 }