sctp: enable PLPMTUD when the transport is ready

sctp_transport_pl_reset() is called whenever any of these 3 members in
transport is changed:

  - probe_interval
  - param_flags & SPP_PMTUD_ENABLE
  - state == ACTIVE

If all are true, start the PLPMTUD when it's not yet started. If any of
these is false, stop the PLPMTUD when it's already running.

sctp_transport_pl_update() is called when the transport dst has changed.
It will restart the PLPMTUD probe. Again, the pathmtu won't change but
use the dst's mtu until the Search phase is done.

Note that after using PLPMTUD, the pathmtu is only initialized with the
dst mtu when the transport dst changes. At other time it is updated by
pl.pmtu. So sctp_transport_pmtu_check() will be called only when PLPMTUD
is disabled in sctp_packet_config().

After this patch, the PLPMTUD feature from RFC8899 will be activated
and can be used by users.
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Acked-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sctp/associola.c

@@ -716,6 +716,8 @@ struct sctp_transport *sctp_assoc_add_peer(struct sctp_association *asoc,
 		return NULL;
 	}
 
+	sctp_transport_pl_reset(peer);
+
 	/* Attach the remote transport to our asoc.  */
 	list_add_tail_rcu(&peer->transports, &asoc->peer.transport_addr_list);
 	asoc->peer.transport_count++;
@@ -814,6 +816,7 @@ void sctp_assoc_control_transport(struct sctp_association *asoc,
 			spc_state = SCTP_ADDR_CONFIRMED;
 
 		transport->state = SCTP_ACTIVE;
+		sctp_transport_pl_reset(transport);
 		break;
 
 	case SCTP_TRANSPORT_DOWN:
@@ -823,6 +826,7 @@ void sctp_assoc_control_transport(struct sctp_association *asoc,
 		 */
 		if (transport->state != SCTP_UNCONFIRMED) {
 			transport->state = SCTP_INACTIVE;
+			sctp_transport_pl_reset(transport);
 			spc_state = SCTP_ADDR_UNREACHABLE;
 		} else {
 			sctp_transport_dst_release(transport);

net/sctp/output.c

@@ -103,7 +103,8 @@ void sctp_packet_config(struct sctp_packet *packet, __u32 vtag,
 		sctp_transport_route(tp, NULL, sp);
 		if (asoc->param_flags & SPP_PMTUD_ENABLE)
 			sctp_assoc_sync_pmtu(asoc);
-	} else if (!sctp_transport_pmtu_check(tp)) {
+	} else if (!sctp_transport_pl_enabled(tp) &&
+		   !sctp_transport_pmtu_check(tp)) {
 		if (asoc->param_flags & SPP_PMTUD_ENABLE)
 			sctp_assoc_sync_pmtu(asoc);
 	}

net/sctp/socket.c

@@ -2496,6 +2496,7 @@ static int sctp_apply_peer_addr_params(struct sctp_paddrparams *params,
 				sctp_transport_pmtu(trans, sctp_opt2sk(sp));
 				sctp_assoc_sync_pmtu(asoc);
 			}
+			sctp_transport_pl_reset(trans);
 		} else if (asoc) {
 			asoc->param_flags =
 				(asoc->param_flags & ~SPP_PMTUD) | pmtud_change;
@@ -4506,6 +4507,7 @@ static int sctp_setsockopt_probe_interval(struct sock *sk,
 			return -EINVAL;
 
 		t->probe_interval = msecs_to_jiffies(probe_interval);
+		sctp_transport_pl_reset(t);
 		return 0;
 	}
 
@@ -4522,8 +4524,10 @@ static int sctp_setsockopt_probe_interval(struct sock *sk,
 	 * each transport.
 	 */
 	if (asoc) {
-		list_for_each_entry(t, &asoc->peer.transport_addr_list, transports)
+		list_for_each_entry(t, &asoc->peer.transport_addr_list, transports) {
 			t->probe_interval = msecs_to_jiffies(probe_interval);
+			sctp_transport_pl_reset(t);
+		}
 
 		asoc->probe_interval = msecs_to_jiffies(probe_interval);
 		return 0;

net/sctp/transport.c

@@ -259,6 +259,8 @@ void sctp_transport_pmtu(struct sctp_transport *transport, struct sock *sk)
 		transport->pathmtu = sctp_dst_mtu(transport->dst);
 	else
 		transport->pathmtu = SCTP_DEFAULT_MAXSEGMENT;
+
+	sctp_transport_pl_update(transport);
 }
 
 void sctp_transport_pl_send(struct sctp_transport *t)