mptcp: send ADD_ADDR echo before create subflows

In some corner cases, the peer handing an incoming ADD_ADDR option, can
receive a retransmitted ADD_ADDR for the same address before the subflow
creation completes.

We can avoid the above issue by generating and sending the ADD_ADDR echo
before starting the MPJ subflow connection.

This slightly changes the behaviour of the packetdrill tests as the
ADD_ADDR echo packet is sent earlier.

Acked-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Yonglong Li <liyonglong@chinatelecom.cn>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Link: https://lore.kernel.org/r/20220317221444.426335-1-mathew.j.martineau@linux.intel.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/mptcp/pm_netlink.c

@@ -650,7 +650,6 @@ static void mptcp_pm_nl_add_addr_received(struct mptcp_sock *msk)
 	unsigned int add_addr_accept_max;
 	struct mptcp_addr_info remote;
 	unsigned int subflows_max;
-	bool reset_port = false;
 	int i, nr;
 
 	add_addr_accept_max = mptcp_pm_get_add_addr_accept_max(msk);
@@ -661,14 +660,15 @@ static void mptcp_pm_nl_add_addr_received(struct mptcp_sock *msk)
 		 msk->pm.remote.family);
 
 	remote = msk->pm.remote;
+	mptcp_pm_announce_addr(msk, &remote, true);
+	mptcp_pm_nl_addr_send_ack(msk);
+
 	if (lookup_subflow_by_daddr(&msk->conn_list, &remote))
-		goto add_addr_echo;
+		return;
 
 	/* pick id 0 port, if none is provided the remote address */
-	if (!remote.port) {
-		reset_port = true;
+	if (!remote.port)
 		remote.port = sk->sk_dport;
-	}
 
 	/* connect to the specified remote address, using whatever
 	 * local address the routing configuration will pick.
@@ -684,14 +684,6 @@ static void mptcp_pm_nl_add_addr_received(struct mptcp_sock *msk)
 	for (i = 0; i < nr; i++)
 		__mptcp_subflow_connect(sk, &addrs[i], &remote);
 	spin_lock_bh(&msk->pm.lock);
-
-	/* be sure to echo exactly the received address */
-	if (reset_port)
-		remote.port = 0;
-
-add_addr_echo:
-	mptcp_pm_announce_addr(msk, &remote, true);
-	mptcp_pm_nl_addr_send_ack(msk);
 }
 
 void mptcp_pm_nl_addr_send_ack(struct mptcp_sock *msk)