udp: preserve skb->dst if required for IP options processing

Paolo Abeni · davem330 · commit 0ddf3fb2c43d · 2017-07-18T12:00:13.000-07:00
Eric noticed that in udp_recvmsg() we still need to access skb->dst while processing the IP options. Since commit 0a463c7 ("udp: avoid a cache miss on dequeue") skb->dst is no more available at recvmsg() time and bad things will happen if we enter the relevant code path. This commit address the issue, avoid clearing skb->dst if any IP options are present into the relevant skb. Since the IP CB is contained in the first skb cacheline, we can test it to decide to leverage the consume_stateless_skb() optimization, without measurable additional cost in the faster path. v1 -> v2: updated commit message tags Fixes: 0a463c7 ("udp: avoid a cache miss on dequeue") Reported-by: Andrey Konovalov <andreyknvl@google.com> Reported-by: Eric Dumazet <edumazet@google.com> Signed-off-by: Paolo Abeni <pabeni@redhat.com> Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
@@ -1388,6 +1388,11 @@ void skb_consume_udp(struct sock *sk, struct sk_buff *skb, int len)
 		unlock_sock_fast(sk, slow);
 	}
 
+	/* we cleared the head states previously only if the skb lacks any IP
+	 * options, see __udp_queue_rcv_skb().
+	 */
+	if (unlikely(IPCB(skb)->opt.optlen > 0))
+		skb_release_head_state(skb);
 	consume_stateless_skb(skb);
 }
 EXPORT_SYMBOL_GPL(skb_consume_udp);
@@ -1779,8 +1784,12 @@ static int __udp_queue_rcv_skb(struct sock *sk, struct sk_buff *skb)
 		sk_mark_napi_id_once(sk, skb);
 	}
 
-	/* clear all pending head states while they are hot in the cache */
-	skb_release_head_state(skb);
+	/* At recvmsg() time we need skb->dst to process IP options-related
+	 * cmsg, elsewhere can we clear all pending head states while they are
+	 * hot in the cache
+	 */
+	if (likely(IPCB(skb)->opt.optlen == 0))
+		skb_release_head_state(skb);
 
 	rc = __udp_enqueue_schedule_skb(sk, skb);
 	if (rc < 0) {
