mptcp: more DATA FIN fixes

Currently data fin on data packet are not handled properly:
the 'rcv_data_fin_seq' field is interpreted as the last
sequence number carrying a valid data, but for data fin
packet with valid maps we currently store map_seq + map_len,
that is, the next value.

The 'write_seq' fields carries instead the value subseguent
to the last valid byte, so in mptcp_write_data_fin() we
never detect correctly the last DSS map.

Fixes: 7279da61 ("mptcp: Use MPTCP-level flag for sending DATA_FIN")
Fixes: 1a49b2c2 ("mptcp: Handle incoming 32-bit DATA_FIN values")
Reviewed-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/mptcp/options.c

@@ -451,7 +451,10 @@ static bool mptcp_established_options_mp(struct sock *sk, struct sk_buff *skb,
 static void mptcp_write_data_fin(struct mptcp_subflow_context *subflow,
 				 struct sk_buff *skb, struct mptcp_ext *ext)
 {
-	u64 data_fin_tx_seq = READ_ONCE(mptcp_sk(subflow->conn)->write_seq);
+	/* The write_seq value has already been incremented, so the actual
+	 * sequence number for the DATA_FIN is one less.
+	 */
+	u64 data_fin_tx_seq = READ_ONCE(mptcp_sk(subflow->conn)->write_seq) - 1;
 
 	if (!ext->use_map || !skb->len) {
 		/* RFC6824 requires a DSS mapping with specific values
@@ -460,10 +463,7 @@ static void mptcp_write_data_fin(struct mptcp_subflow_context *subflow,
 		ext->data_fin = 1;
 		ext->use_map = 1;
 		ext->dsn64 = 1;
-		/* The write_seq value has already been incremented, so
-		 * the actual sequence number for the DATA_FIN is one less.
-		 */
-		ext->data_seq = data_fin_tx_seq - 1;
+		ext->data_seq = data_fin_tx_seq;
 		ext->subflow_seq = 0;
 		ext->data_len = 1;
 	} else if (ext->data_seq + ext->data_len == data_fin_tx_seq) {

net/mptcp/subflow.c

@@ -749,7 +749,7 @@ static enum mapping_status get_mapping_status(struct sock *ssk,
 				return MAPPING_DATA_FIN;
 			}
 		} else {
-			u64 data_fin_seq = mpext->data_seq + data_len;
+			u64 data_fin_seq = mpext->data_seq + data_len - 1;
 
 			/* If mpext->data_seq is a 32-bit value, data_fin_seq
 			 * must also be limited to 32 bits.