mptcp: infinite mapping receiving

This patch adds the infinite mapping receiving logic. When the infinite
mapping is received, set the map_data_len of the subflow to 0.

In subflow_check_data_avail(), only reset the subflow when the map_data_len
of the subflow is non-zero.

Suggested-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Geliang Tang <geliang.tang@suse.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/mptcp/subflow.c

@@ -1006,7 +1006,9 @@ static enum mapping_status get_mapping_status(struct sock *ssk,
 
 	data_len = mpext->data_len;
 	if (data_len == 0) {
+		pr_debug("infinite mapping received");
 		MPTCP_INC_STATS(sock_net(ssk), MPTCP_MIB_INFINITEMAPRX);
+		subflow->map_data_len = 0;
 		return MAPPING_INVALID;
 	}
 
@@ -1220,7 +1222,7 @@ fallback:
 			return true;
 		}
 
-		if (subflow->mp_join || subflow->fully_established) {
+		if ((subflow->mp_join || subflow->fully_established) && subflow->map_data_len) {
 			/* fatal protocol error, close the socket.
 			 * subflow_error_report() will introduce the appropriate barriers
 			 */