mptcp: Use full MPTCP-level disconnect state machine

RFC 8684 appendix D describes the connection state machine for
MPTCP. This patch implements the DATA_FIN / DATA_ACK exchanges and
MPTCP-level socket state changes described in that appendix, rather than
simply sending DATA_FIN along with TCP FIN when disconnecting subflows.

DATA_FIN is now sent and acknowledged before shutting down the
subflows. Received DATA_FIN information (if not part of a data packet)
is written to the MPTCP socket when the incoming DSS option is parsed by
the subflow, and the MPTCP worker is scheduled to process the
flag. DATA_FIN received as part of a full DSS mapping will be handled
when the mapping is processed.

The DATA_FIN is acknowledged by the worker if the reader is caught
up. If there is still data to be moved to the MPTCP-level queue, ack_seq
will be incremented to account for the DATA_FIN when it reaches the end
of the stream and a DATA_ACK will be sent to the peer.

Signed-off-by: Mat Martineau <[email protected]>
Signed-off-by: David S. Miller <[email protected]>

1 files changed, 11 insertions, 0 deletions

diff --git a/net/mptcp/options.c b/net/mptcp/options.c
index 38583d1b9b5f..b4458ecd01f8 100644
--- a/net/mptcp/options.c
+++ b/net/mptcp/options.c
@@ -868,6 +868,17 @@ void mptcp_incoming_options(struct sock *sk, struct sk_buff *skb,
 	if (mp_opt.use_ack)
 		update_una(msk, &mp_opt);
 
+	/* Zero-length packets, like bare ACKs carrying a DATA_FIN, are
+	 * dropped by the caller and not propagated to the MPTCP layer.
+	 * Copy the DATA_FIN information now.
+	 */
+	if (TCP_SKB_CB(skb)->seq == TCP_SKB_CB(skb)->end_seq) {
+		if (mp_opt.data_fin && mp_opt.data_len == 1 &&
+		    mptcp_update_rcv_data_fin(msk, mp_opt.data_seq) &&
+		    schedule_work(&msk->work))
+			sock_hold(subflow->conn);
+	}
+
 	mpext = skb_ext_add(skb, SKB_EXT_MPTCP);
 	if (!mpext)
 		return;