3 files changed, 37 insertions, 4 deletions

diff --git a/include/linux/dccp.h b/include/linux/dccp.h
index 749f01ccd26e..eed52bcd35d0 100644
--- a/include/linux/dccp.h
+++ b/include/linux/dccp.h
@@ -462,6 +462,7 @@ struct dccp_ackvec;
  * @dccps_hc_rx_insert_options - receiver wants to add options when acking
  * @dccps_hc_tx_insert_options - sender wants to add options when sending
  * @dccps_server_timewait - server holds timewait state on close (RFC 4340, 8.3)
+ * @dccps_sync_scheduled - flag which signals "send out-of-band message soon"
  * @dccps_xmitlet - tasklet scheduled by the TX CCID to dequeue data packets
  * @dccps_xmit_timer - used by the TX CCID to delay sending (rate-based pacing)
  * @dccps_syn_rtt - RTT sample from Request/Response exchange (in usecs)
@@ -503,6 +504,7 @@ struct dccp_sock {
         __u8                            dccps_hc_rx_insert_options:1;
         __u8                            dccps_hc_tx_insert_options:1;
         __u8                            dccps_server_timewait:1;
+        __u8                            dccps_sync_scheduled:1;
         struct tasklet_struct           dccps_xmitlet;
         struct timer_list               dccps_xmit_timer;
 };
diff --git a/net/dccp/options.c b/net/dccp/options.c
index 7743df00f5b1..dabd6ee34d45 100644
--- a/net/dccp/options.c
+++ b/net/dccp/options.c
@@ -427,6 +427,7 @@ static int dccp_insert_option_ackvec(struct sock *sk, struct sk_buff *skb)
 {
         struct dccp_sock *dp = dccp_sk(sk);
         struct dccp_ackvec *av = dp->dccps_hc_rx_ackvec;
+        struct dccp_skb_cb *dcb = DCCP_SKB_CB(skb);
         const u16 buflen = dccp_ackvec_buflen(av);
         /* Figure out how many options do we need to represent the ackvec */
         const u8 nr_opts = DIV_ROUND_UP(buflen, DCCP_SINGLE_OPT_MAXLEN);
@@ -435,10 +436,25 @@ static int dccp_insert_option_ackvec(struct sock *sk, struct sk_buff *skb)
         const unsigned char *tail, *from;
         unsigned char *to;
 
-        if (DCCP_SKB_CB(skb)->dccpd_opt_len + len > DCCP_MAX_OPT_LEN)
+        if (dcb->dccpd_opt_len + len > DCCP_MAX_OPT_LEN) {
+                DCCP_WARN("Lacking space for %u bytes on %s packet\n", len,
+                          dccp_packet_name(dcb->dccpd_type));
                 return -1;
-
-        DCCP_SKB_CB(skb)->dccpd_opt_len += len;
+        }
+        /*
+         * Since Ack Vectors are variable-length, we can not always predict
+         * their size. To catch exception cases where the space is running out
+         * on the skb, a separate Sync is scheduled to carry the Ack Vector.
+         */
+        if (len > DCCPAV_MIN_OPTLEN &&
+            len + dcb->dccpd_opt_len + skb->len > dp->dccps_mss_cache) {
+                DCCP_WARN("No space left for Ack Vector (%u) on skb (%u+%u), "
+                          "MPS=%u ==> reduce payload size?\n", len, skb->len,
+                          dcb->dccpd_opt_len, dp->dccps_mss_cache);
+                dp->dccps_sync_scheduled = 1;
+                return 0;
+        }
+        dcb->dccpd_opt_len += len;
 
         to   = skb_push(skb, len);
         len  = buflen;
@@ -479,7 +495,7 @@ static int dccp_insert_option_ackvec(struct sock *sk, struct sk_buff *skb)
         /*
          * Each sent Ack Vector is recorded in the list, as per A.2 of RFC 4340.
          */
-        if (dccp_ackvec_update_records(av, DCCP_SKB_CB(skb)->dccpd_seq, nonce))
+        if (dccp_ackvec_update_records(av, dcb->dccpd_seq, nonce))
                 return -ENOBUFS;
         return 0;
 }
diff --git a/net/dccp/output.c b/net/dccp/output.c
index 45b91853f5ae..d96dd9d362ae 100644
--- a/net/dccp/output.c
+++ b/net/dccp/output.c
@@ -283,6 +283,15 @@ static void dccp_xmit_packet(struct sock *sk)
          * any local drop will eventually be reported via receiver feedback.
          */
         ccid_hc_tx_packet_sent(dp->dccps_hc_tx_ccid, sk, len);
+
+        /*
+         * If the CCID needs to transfer additional header options out-of-band
+         * (e.g. Ack Vectors or feature-negotiation options), it activates this
+         * flag to schedule a Sync. The Sync will automatically incorporate all
+         * currently pending header options, thus clearing the backlog.
+         */
+        if (dp->dccps_sync_scheduled)
+                dccp_send_sync(sk, dp->dccps_gsr, DCCP_PKT_SYNC);
 }
 
 /**
@@ -636,6 +645,12 @@ void dccp_send_sync(struct sock *sk, const u64 ackno,
         DCCP_SKB_CB(skb)->dccpd_type = pkt_type;
         DCCP_SKB_CB(skb)->dccpd_ack_seq = ackno;
 
+        /*
+         * Clear the flag in case the Sync was scheduled for out-of-band data,
+         * such as carrying a long Ack Vector.
+         */
+        dccp_sk(sk)->dccps_sync_scheduled = 0;
+
         dccp_transmit_skb(sk, skb);
 }