mptcp: always include dack if possible.

Currently passive MPTCP socket can skip including the DACK
option - if the peer sends data before accept() completes.

The above happens because the msk 'can_ack' flag is set
only after the accept() call.

Such missing DACK option may cause - as per RFC spec -
unwanted fallback to TCP.

This change addresses the issue using the key material
available in the current subflow, if any, to create a suitable
dack option when msk ack seq is not yet available.

v1 -> v2:
 - adavance the generated ack after the initial MPC packet

Fixes: de25b6ecfdb8 ("mptcp: process MP_CAPABLE data option")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/net/mptcp/options.c b/net/mptcp/options.c
index 45acd877bef345259ca2a84c58cd0d375f88386f..fd2c3150e59193e189a5e85f901d21c2bfca0115 100644 (file)
--- a/net/mptcp/options.c
+++ b/net/mptcp/options.c
@@ -334,6 +334,8 @@ static bool mptcp_established_options_dss(struct sock *sk, struct sk_buff *skb,
        struct mptcp_sock *msk;
        unsigned int ack_size;
        bool ret = false;
+       bool can_ack;
+       u64 ack_seq;
        u8 tcp_fin;
 
        if (skb) {
@@ -360,9 +362,22 @@ static bool mptcp_established_options_dss(struct sock *sk, struct sk_buff *skb,
                ret = true;
        }
 
+       /* passive sockets msk will set the 'can_ack' after accept(), even
+        * if the first subflow may have the already the remote key handy
+        */
+       can_ack = true;
        opts->ext_copy.use_ack = 0;
        msk = mptcp_sk(subflow->conn);
-       if (!msk || !READ_ONCE(msk->can_ack)) {
+       if (likely(msk && READ_ONCE(msk->can_ack))) {
+               ack_seq = msk->ack_seq;
+       } else if (subflow->can_ack) {
+               mptcp_crypto_key_sha(subflow->remote_key, NULL, &ack_seq);
+               ack_seq++;
+       } else {
+               can_ack = false;
+       }
+
+       if (unlikely(!can_ack)) {
                *size = ALIGN(dss_size, 4);
                return ret;
        }
@@ -375,7 +390,7 @@ static bool mptcp_established_options_dss(struct sock *sk, struct sk_buff *skb,
 
        dss_size += ack_size;
 
-       opts->ext_copy.data_ack = msk->ack_seq;
+       opts->ext_copy.data_ack = ack_seq;
        opts->ext_copy.ack64 = 1;
        opts->ext_copy.use_ack = 1;