net-tcp: Fast Open client - receiving SYN-ACK

On receiving the SYN-ACK after SYN-data, the client needs to
a) update the cached MSS and cookie (if included in SYN-ACK)
b) retransmit the data not yet acknowledged by the SYN-ACK in the final ACK of
   the handshake.

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

1 files changed, 35 insertions, 5 deletions

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index a06bb8959e7e..38b6a811edfc 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -5646,6 +5646,34 @@ void tcp_finish_connect(struct sock *sk, struct sk_buff *skb)
         }
 }
 
+static bool tcp_rcv_fastopen_synack(struct sock *sk, struct sk_buff *synack,
+                                    struct tcp_fastopen_cookie *cookie)
+{
+        struct tcp_sock *tp = tcp_sk(sk);
+        struct sk_buff *data = tcp_write_queue_head(sk);
+        u16 mss = tp->rx_opt.mss_clamp;
+
+        if (mss == tp->rx_opt.user_mss) {
+                struct tcp_options_received opt;
+                const u8 *hash_location;
+
+                /* Get original SYNACK MSS value if user MSS sets mss_clamp */
+                tcp_clear_options(&opt);
+                opt.user_mss = opt.mss_clamp = 0;
+                tcp_parse_options(synack, &opt, &hash_location, 0, NULL);
+                mss = opt.mss_clamp;
+        }
+
+        tcp_fastopen_cache_set(sk, mss, cookie);
+
+        if (data) { /* Retransmit unacked data in SYN */
+                tcp_retransmit_skb(sk, data);
+                tcp_rearm_rto(sk);
+                return true;
+        }
+        return false;
+}
+
 static int tcp_rcv_synsent_state_process(struct sock *sk, struct sk_buff *skb,
                                          const struct tcphdr *th, unsigned int len)
 {
@@ -5653,9 +5681,10 @@ static int tcp_rcv_synsent_state_process(struct sock *sk, struct sk_buff *skb,
         struct inet_connection_sock *icsk = inet_csk(sk);
         struct tcp_sock *tp = tcp_sk(sk);
         struct tcp_cookie_values *cvp = tp->cookie_values;
+        struct tcp_fastopen_cookie foc = { .len = -1 };
         int saved_clamp = tp->rx_opt.mss_clamp;
 
-        tcp_parse_options(skb, &tp->rx_opt, &hash_location, 0, NULL);
+        tcp_parse_options(skb, &tp->rx_opt, &hash_location, 0, &foc);
 
         if (th->ack) {
                 /* rfc793:
@@ -5665,11 +5694,9 @@ static int tcp_rcv_synsent_state_process(struct sock *sk, struct sk_buff *skb,
                  *        If SEG.ACK =< ISS, or SEG.ACK > SND.NXT, send
                  *        a reset (unless the RST bit is set, if so drop
                  *        the segment and return)"
-                 *
-                 *  We do not send data with SYN, so that RFC-correct
-                 *  test reduces to:
                  */
-                if (TCP_SKB_CB(skb)->ack_seq != tp->snd_nxt)
+                if (!after(TCP_SKB_CB(skb)->ack_seq, tp->snd_una) ||
+                    after(TCP_SKB_CB(skb)->ack_seq, tp->snd_nxt))
                         goto reset_and_undo;
 
                 if (tp->rx_opt.saw_tstamp && tp->rx_opt.rcv_tsecr &&
@@ -5781,6 +5808,9 @@ static int tcp_rcv_synsent_state_process(struct sock *sk, struct sk_buff *skb,
 
                 tcp_finish_connect(sk, skb);
 
+                if (tp->syn_fastopen && tcp_rcv_fastopen_synack(sk, skb, &foc))
+                        return -1;
+
                 if (sk->sk_write_pending ||
                     icsk->icsk_accept_queue.rskq_defer_accept ||
                     icsk->icsk_ack.pingpong) {