sctp: add encap_port for netns sock asoc and transport

encap_port is added as per netns/sock/assoc/transport, and the
latter one's encap_port inherits the former one's by default.
The transport's encap_port value would mostly decide if one
packet should go out with udp encapsulated or not.

This patch also allows users to set netns' encap_port by sysctl.

v1->v2:
  - Change to define encap_port as __be16 for sctp_sock, asoc and
    transport.
v2->v3:
  - No change.
v3->v4:
  - Add 'encap_port' entry in ip-sysctl.rst.
v4->v5:
  - Improve the description of encap_port in ip-sysctl.rst.
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Acked-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Documentation/networking/ip-sysctl.rst

@@ -2642,6 +2642,22 @@ addr_scope_policy - INTEGER
 
 	Default: 1
 
+encap_port - INTEGER
+	The default remote UDP encapsulation port.
+
+	This value is used to set the dest port of the UDP header for the
+	outgoing UDP-encapsulated SCTP packets by default. Users can also
+	change the value for each sock/asoc/transport by using setsockopt.
+	For further information, please refer to RFC6951.
+
+	Note that when connecting to a remote server, the client should set
+	this to the port that the UDP tunneling sock on the peer server is
+	listening to and the local UDP tunneling sock on the client also
+	must be started. On the server, it would get the encap_port from
+	the incoming packet's source port.
+
+	Default: 0
+
 
 ``/proc/sys/net/core/*``
 ========================

include/net/netns/sctp.h

@@ -27,6 +27,8 @@ struct netns_sctp {
 	struct sock *udp6_sock;
 	/* UDP tunneling listening port. */
 	int udp_port;
+	/* UDP tunneling remote encap port. */
+	int encap_port;
 
 	/* This is the global local address list.
 	 * We actively maintain this complete list of addresses on

include/net/sctp/structs.h

@@ -178,6 +178,8 @@ struct sctp_sock {
 	 */
 	__u32 hbinterval;
 
+	__be16 encap_port;
+
 	/* This is the max_retrans value for new associations. */
 	__u16 pathmaxrxt;
 
@@ -877,6 +879,8 @@ struct sctp_transport {
 	 */
 	unsigned long last_time_ecne_reduced;
 
+	__be16 encap_port;
+
 	/* This is the max_retrans value for the transport and will
 	 * be initialized from the assocs value.  This can be changed
 	 * using the SCTP_SET_PEER_ADDR_PARAMS socket option.
@@ -1790,6 +1794,8 @@ struct sctp_association {
 	 */
 	unsigned long hbinterval;
 
+	__be16 encap_port;
+
 	/* This is the max_retrans value for new transports in the
 	 * association.
 	 */

net/sctp/associola.c

@@ -99,6 +99,8 @@ static struct sctp_association *sctp_association_init(
 	 */
 	asoc->hbinterval = msecs_to_jiffies(sp->hbinterval);
 
+	asoc->encap_port = sp->encap_port;
+
 	/* Initialize path max retrans value. */
 	asoc->pathmaxrxt = sp->pathmaxrxt;
 
@@ -624,6 +626,8 @@ struct sctp_transport *sctp_assoc_add_peer(struct sctp_association *asoc,
 	 */
 	peer->hbinterval = asoc->hbinterval;
 
+	peer->encap_port = asoc->encap_port;
+
 	/* Set the path max_retrans.  */
 	peer->pathmaxrxt = asoc->pathmaxrxt;
 

net/sctp/protocol.c

@@ -1359,6 +1359,9 @@ static int __net_init sctp_defaults_init(struct net *net)
 	/* Set UDP tunneling listening port to 0 by default */
 	net->sctp.udp_port = 0;
 
+	/* Set remote encap port to 0 by default */
+	net->sctp.encap_port = 0;
+
 	/* Set SCOPE policy to enabled */
 	net->sctp.scope_policy = SCTP_SCOPE_POLICY_ENABLE;
 

net/sctp/socket.c

@@ -4876,6 +4876,7 @@ static int sctp_init_sock(struct sock *sk)
 	 * be modified via SCTP_PEER_ADDR_PARAMS
 	 */
 	sp->hbinterval  = net->sctp.hb_interval;
+	sp->encap_port  = htons(net->sctp.encap_port);
 	sp->pathmaxrxt  = net->sctp.max_retrans_path;
 	sp->pf_retrans  = net->sctp.pf_retrans;
 	sp->ps_retrans  = net->sctp.ps_retrans;

net/sctp/sysctl.c

@@ -36,6 +36,7 @@ static int rto_alpha_max = 1000;
 static int rto_beta_max = 1000;
 static int pf_expose_max = SCTP_PF_EXPOSE_MAX;
 static int ps_retrans_max = SCTP_PS_RETRANS_MAX;
+static int udp_port_max = 65535;
 
 static unsigned long max_autoclose_min = 0;
 static unsigned long max_autoclose_max =
@@ -290,6 +291,15 @@ static struct ctl_table sctp_net_table[] = {
 		.mode		= 0644,
 		.proc_handler	= proc_dointvec,
 	},
+	{
+		.procname	= "encap_port",
+		.data		= &init_net.sctp.encap_port,
+		.maxlen		= sizeof(int),
+		.mode		= 0644,
+		.proc_handler	= proc_dointvec,
+		.extra1		= SYSCTL_ZERO,
+		.extra2		= &udp_port_max,
+	},
 	{
 		.procname	= "addr_scope_policy",
 		.data		= &init_net.sctp.scope_policy,