4 files changed, 37 insertions, 8 deletions

diff --git a/include/net/tcp.h b/include/net/tcp.h
index c0258100d70c..e07878d246aa 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -409,9 +409,11 @@ extern bool tcp_peer_is_proven(struct request_sock *req, struct dst_entry *dst,
 extern bool tcp_remember_stamp(struct sock *sk);
 extern bool tcp_tw_remember_stamp(struct inet_timewait_sock *tw);
 extern void tcp_fastopen_cache_get(struct sock *sk, u16 *mss,
-                                   struct tcp_fastopen_cookie *cookie);
+                                   struct tcp_fastopen_cookie *cookie,
+                                   int *syn_loss, unsigned long *last_syn_loss);
 extern void tcp_fastopen_cache_set(struct sock *sk, u16 mss,
-                                   struct tcp_fastopen_cookie *cookie);
+                                   struct tcp_fastopen_cookie *cookie,
+                                   bool syn_lost);
 extern void tcp_fetch_timewait_stamp(struct sock *sk, struct dst_entry *dst);
 extern void tcp_disable_fack(struct tcp_sock *tp);
 extern void tcp_close(struct sock *sk, long timeout);
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 38b6a811edfc..c49a4fc175bd 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -5652,6 +5652,7 @@ static bool tcp_rcv_fastopen_synack(struct sock *sk, struct sk_buff *synack,
         struct tcp_sock *tp = tcp_sk(sk);
         struct sk_buff *data = tcp_write_queue_head(sk);
         u16 mss = tp->rx_opt.mss_clamp;
+        bool syn_drop;
 
         if (mss == tp->rx_opt.user_mss) {
                 struct tcp_options_received opt;
@@ -5664,7 +5665,14 @@ static bool tcp_rcv_fastopen_synack(struct sock *sk, struct sk_buff *synack,
                 mss = opt.mss_clamp;
         }
 
-        tcp_fastopen_cache_set(sk, mss, cookie);
+        /* The SYN-ACK neither has cookie nor acknowledges the data. Presumably
+         * the remote receives only the retransmitted (regular) SYNs: either
+         * the original SYN-data or the corresponding SYN-ACK is lost.
+         */
+        syn_drop = (cookie->len <= 0 && data &&
+                    inet_csk(sk)->icsk_retransmits);
+
+        tcp_fastopen_cache_set(sk, mss, cookie, syn_drop);
 
         if (data) { /* Retransmit unacked data in SYN */
                 tcp_retransmit_skb(sk, data);
diff --git a/net/ipv4/tcp_metrics.c b/net/ipv4/tcp_metrics.c
index d02ff3777785..99779ae44f64 100644
--- a/net/ipv4/tcp_metrics.c
+++ b/net/ipv4/tcp_metrics.c
@@ -32,6 +32,8 @@ enum tcp_metric_index {
 
 struct tcp_fastopen_metrics {
         u16     mss;
+        u16     syn_loss:10;            /* Recurring Fast Open SYN losses */
+        unsigned long   last_syn_loss;  /* Last Fast Open SYN loss */
         struct  tcp_fastopen_cookie     cookie;
 };
 
@@ -125,6 +127,7 @@ static void tcpm_suck_dst(struct tcp_metrics_block *tm, struct dst_entry *dst)
         tm->tcpm_ts = 0;
         tm->tcpm_ts_stamp = 0;
         tm->tcpm_fastopen.mss = 0;
+        tm->tcpm_fastopen.syn_loss = 0;
         tm->tcpm_fastopen.cookie.len = 0;
 }
 
@@ -644,7 +647,8 @@ bool tcp_tw_remember_stamp(struct inet_timewait_sock *tw)
 static DEFINE_SEQLOCK(fastopen_seqlock);
 
 void tcp_fastopen_cache_get(struct sock *sk, u16 *mss,
-                            struct tcp_fastopen_cookie *cookie)
+                            struct tcp_fastopen_cookie *cookie,
+                            int *syn_loss, unsigned long *last_syn_loss)
 {
         struct tcp_metrics_block *tm;
 
@@ -659,14 +663,15 @@ void tcp_fastopen_cache_get(struct sock *sk, u16 *mss,
                         if (tfom->mss)
                                 *mss = tfom->mss;
                         *cookie = tfom->cookie;
+                        *syn_loss = tfom->syn_loss;
+                        *last_syn_loss = *syn_loss ? tfom->last_syn_loss : 0;
                 } while (read_seqretry(&fastopen_seqlock, seq));
         }
         rcu_read_unlock();
 }
 
-
 void tcp_fastopen_cache_set(struct sock *sk, u16 mss,
-                            struct tcp_fastopen_cookie *cookie)
+                            struct tcp_fastopen_cookie *cookie, bool syn_lost)
 {
         struct tcp_metrics_block *tm;
 
@@ -679,6 +684,11 @@ void tcp_fastopen_cache_set(struct sock *sk, u16 mss,
                 tfom->mss = mss;
                 if (cookie->len > 0)
                         tfom->cookie = *cookie;
+                if (syn_lost) {
+                        ++tfom->syn_loss;
+                        tfom->last_syn_loss = jiffies;
+                } else
+                        tfom->syn_loss = 0;
                 write_sequnlock_bh(&fastopen_seqlock);
         }
         rcu_read_unlock();
diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 88693281da4c..c5cfd5ec3184 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -2860,10 +2860,19 @@ static int tcp_send_syn_data(struct sock *sk, struct sk_buff *syn)
 {
         struct tcp_sock *tp = tcp_sk(sk);
         struct tcp_fastopen_request *fo = tp->fastopen_req;
-        int space, i, err = 0, iovlen = fo->data->msg_iovlen;
+        int syn_loss = 0, space, i, err = 0, iovlen = fo->data->msg_iovlen;
         struct sk_buff *syn_data = NULL, *data;
+        unsigned long last_syn_loss = 0;
+
+        tcp_fastopen_cache_get(sk, &tp->rx_opt.mss_clamp, &fo->cookie,
+                               &syn_loss, &last_syn_loss);
+        /* Recurring FO SYN losses: revert to regular handshake temporarily */
+        if (syn_loss > 1 &&
+            time_before(jiffies, last_syn_loss + (60*HZ << syn_loss))) {
+                fo->cookie.len = -1;
+                goto fallback;
+        }
 
-        tcp_fastopen_cache_get(sk, &tp->rx_opt.mss_clamp, &fo->cookie);
         if (fo->cookie.len <= 0)
                 goto fallback;