tcp: bug fix Fast Open client retransmission

If SYN-ACK partially acks SYN-data, the client retransmits the
remaining data by tcp_retransmit_skb(). This increments lost recovery
state variables like tp->retrans_out in Open state. If loss recovery
happens before the retransmission is acked, it triggers the WARN_ON
check in tcp_fastretrans_alert(). For example: the client sends
SYN-data, gets SYN-ACK acking only ISN, retransmits data, sends
another 4 data packets and get 3 dupacks.

Since the retransmission is not caused by network drop it should not
update the recovery state variables. Further the server may return a
smaller MSS than the cached MSS used for SYN-data, so the retranmission
needs a loop. Otherwise some data will not be retransmitted until timeout
or other loss recovery events.

Signed-off-by: Yuchung Cheng <ycheng@google.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

2 files changed, 15 insertions, 6 deletions

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 609ff98aeb47..181fc8234a52 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -5645,7 +5645,11 @@ static bool tcp_rcv_fastopen_synack(struct sock *sk, struct sk_buff *synack,
         tcp_fastopen_cache_set(sk, mss, cookie, syn_drop);
 
         if (data) { /* Retransmit unacked data in SYN */
-                tcp_retransmit_skb(sk, data);
+                tcp_for_write_queue_from(data, sk) {
+                        if (data == tcp_send_head(sk) ||
+                            __tcp_retransmit_skb(sk, data))
+                                break;
+                }
                 tcp_rearm_rto(sk);
                 return true;
         }
diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 2798706cb063..948ac275b9b5 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -2309,12 +2309,11 @@ static void tcp_retrans_try_collapse(struct sock *sk, struct sk_buff *to,
  * state updates are done by the caller.  Returns non-zero if an
  * error occurred which prevented the send.
  */
-int tcp_retransmit_skb(struct sock *sk, struct sk_buff *skb)
+int __tcp_retransmit_skb(struct sock *sk, struct sk_buff *skb)
 {
         struct tcp_sock *tp = tcp_sk(sk);
         struct inet_connection_sock *icsk = inet_csk(sk);
         unsigned int cur_mss;
-        int err;
 
         /* Inconslusive MTU probe */
         if (icsk->icsk_mtup.probe_size) {
@@ -2387,11 +2386,17 @@ int tcp_retransmit_skb(struct sock *sk, struct sk_buff *skb)
         if (unlikely(NET_IP_ALIGN && ((unsigned long)skb->data & 3))) {
                 struct sk_buff *nskb = __pskb_copy(skb, MAX_TCP_HEADER,
                                                    GFP_ATOMIC);
-                err = nskb ? tcp_transmit_skb(sk, nskb, 0, GFP_ATOMIC) :
-                             -ENOBUFS;
+                return nskb ? tcp_transmit_skb(sk, nskb, 0, GFP_ATOMIC) :
+                              -ENOBUFS;
         } else {
-                err = tcp_transmit_skb(sk, skb, 1, GFP_ATOMIC);
+                return tcp_transmit_skb(sk, skb, 1, GFP_ATOMIC);
         }
+}
+
+int tcp_retransmit_skb(struct sock *sk, struct sk_buff *skb)
+{
+        struct tcp_sock *tp = tcp_sk(sk);
+        int err = __tcp_retransmit_skb(sk, skb);
 
         if (err == 0) {
                 /* Update global TCP statistics. */