udp: Also bind() connected ports for "splice" forwarding

pasta handles "spliced" port forwarding by resending datagrams received on
a bound socket in the init namespace to a connected socket in the guest
namespace.  This means there are actually three ports associated with each
"connection".  First there's the source and destination ports of the
originating datagram.  That's also the destination port of the forwarded
datagram, but the source port of the forwarded datagram is the kernel
allocated bound address of the connected socket.

However, by bind()ing as well as connect()ing the forwarding socket we can
choose the source port of the forwarded datagrams.  By choosing it to match
the original source port we remove that surprising third port number and
no longer need to store port numbers in struct udp_splice_port.

As a bonus this means that the recipient of the packets will see the
original source port if they call getpeername().  This rarely matters, but
it can't hurt.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

1 files changed, 32 insertions, 52 deletions

diff --git a/udp.c b/udp.c
index 0aa6308..a025a48 100644
--- a/udp.c
+++ b/udp.c
@@ -51,20 +51,19 @@
  *       - send packet to udp4_splice_map[5000].ns_conn_sock
  *     - otherwise:
  *       - create new socket udp_splice_map[V4][5000].ns_conn_sock
+ *       - bind in namespace to 127.0.0.1:5000
  *       - connect in namespace to 127.0.0.1:80 (note: this destination port
  *         might be remapped to another port instead)
- *       - get source port of new connected socket (10000) with getsockname()
- *       - add to epoll with reference: index = 10000, splice: UDP_BACK_TO_INIT
- *       - set udp_splice_map[V4][10000].init_bound_sock to s
- *       - set udp_splice_map[V4][10000].init_dst_port to 5000
+ *       - add to epoll with reference: index = 5000, splice: UDP_BACK_TO_INIT
+ *       - set udp_splice_map[V4][5000].init_bound_sock to s
  *   - update udp_splice_map[V4][5000].ns_conn_ts with current time
  *
- *   - reverse direction: 127.0.0.1:80 -> 127.0.0.1:10000 in namespace from
- *     connected socket s, having epoll reference: index = 10000,
+ *   - reverse direction: 127.0.0.1:80 -> 127.0.0.1:5000 in namespace from
+ *     connected socket s, having epoll reference: index = 5000,
  *     splice = UDP_BACK_TO_INIT
- *     - if udp_splice_map[V4][10000].init_bound_sock:
- *       - send to udp_splice_map[V4][10000].init_bound_sock, with destination
- *         port udp_splice_map[V4][10000].init_dst_port (5000)
+ *     - if udp_splice_map[V4][5000].init_bound_sock:
+ *       - send to udp_splice_map[V4][5000].init_bound_sock, with destination
+ *         port 5000
  *     - otherwise, discard
  *
  * - from namespace to init:
@@ -75,19 +74,18 @@
  *       - send packet to udp4_splice_map[2000].init_conn_sock
  *     - otherwise:
  *       - create new socket udp_splice_map[V4][2000].init_conn_sock
+ *       - bind in init to 127.0.0.1:2000
  *       - connect in init to 127.0.0.1:22 (note: this destination port
  *         might be remapped to another port instead)
- *       - get source port of new connected socket (4000) with getsockname()
- *       - add to epoll with reference: index = 4000, splice = UDP_BACK_TO_NS
- *       - set udp_splice_map[V4][4000].ns_bound_sock to s
- *       - set udp_splice_map[V4][4000].ns_dst_port to 2000
- *     - update udp_splice_map[V4][4000].init_conn_ts with current time
+ *       - add to epoll with reference: index = 2000, splice = UDP_BACK_TO_NS
+ *       - set udp_splice_map[V4][2000].ns_bound_sock to s
+ *     - update udp_splice_map[V4][2000].init_conn_ts with current time
  *
- *   - reverse direction: 127.0.0.1:22 -> 127.0.0.1:4000 in init from connected
- *     socket s, having epoll reference: index = 4000, splice = UDP_BACK_TO_NS
- *   - if udp_splice_map[V4][4000].ns_bound_sock:
- *     - send to udp_splice_map[V4][4000].ns_bound_sock, with destination port
- *       udp_splice_map[4000].ns_dst_port (2000)
+ *   - reverse direction: 127.0.0.1:22 -> 127.0.0.1:2000 in init from connected
+ *     socket s, having epoll reference: index = 2000, splice = UDP_BACK_TO_NS
+ *   - if udp_splice_map[V4][2000].ns_bound_sock:
+ *     - send to udp_splice_map[V4][2000].ns_bound_sock, with destination port
+ *       2000
  *   - otherwise, discard
  */
 
@@ -145,8 +143,6 @@ struct udp_tap_port {
  * @init_conn_sock:	Socket connected in init for namespace source port
  * @ns_conn_ts:		Timestamp of activity for socket connected in namespace
  * @init_conn_ts:	Timestamp of activity for socket connceted in init
- * @ns_dst_port:	Destination port in namespace for init source port
- * @init_dst_port:	Destination port in init for namespace source port
  * @ns_bound_sock:	Bound socket in namespace for this source port in init
  * @init_bound_sock:	Bound socket in init for this source port in namespace
  */
@@ -157,9 +153,6 @@ struct udp_splice_port {
 	time_t ns_conn_ts;
 	time_t init_conn_ts;
 
-	in_port_t ns_dst_port;
-	in_port_t init_dst_port;
-
 	int ns_bound_sock;
 	int init_bound_sock;
 };
@@ -425,11 +418,10 @@ int udp_splice_connect(const struct ctx *c, int v6, int bound_sock,
 {
 	struct epoll_event ev = { .events = EPOLLIN | EPOLLRDHUP | EPOLLHUP };
 	union epoll_ref ref = { .r.proto = IPPROTO_UDP,
-				.r.p.udp.udp = { .splice = splice, .v6 = v6 }
+				.r.p.udp.udp = { .splice = splice, .v6 = v6,
+						 .port = src }
 			      };
-	struct sockaddr_storage sa;
-	struct udp_splice_port *sp;
-	socklen_t sl = sizeof(sa);
+	struct udp_splice_port *sp = &udp_splice_map[v6 ? V6 : V4][src];
 	int s;
 
 	s = socket(v6 ? AF_INET6 : AF_INET, SOCK_DGRAM | SOCK_NONBLOCK,
@@ -448,46 +440,34 @@ int udp_splice_connect(const struct ctx *c, int v6, int bound_sock,
 	if (v6) {
 		struct sockaddr_in6 addr6 = {
 			.sin6_family = AF_INET6,
-			.sin6_port = htons(dst),
+			.sin6_port = htons(src),
 			.sin6_addr = IN6ADDR_LOOPBACK_INIT,
 		};
+		if (bind(s, (struct sockaddr *)&addr6, sizeof(addr6)))
+			goto fail;
+		addr6.sin6_port = htons(dst);
 		if (connect(s, (struct sockaddr *)&addr6, sizeof(addr6)))
 			goto fail;
 	} else {
 		struct sockaddr_in addr4 = {
 			.sin_family = AF_INET,
-			.sin_port = htons(dst),
+			.sin_port = htons(src),
 			.sin_addr = { .s_addr = htonl(INADDR_LOOPBACK) },
 		};
+		if (bind(s, (struct sockaddr *)&addr4, sizeof(addr4)))
+			goto fail;
+		addr4.sin_port = htons(dst);
 		if (connect(s, (struct sockaddr *)&addr4, sizeof(addr4)))
 			goto fail;
 	}
 
-	if (getsockname(s, (struct sockaddr *)&sa, &sl))
-		goto fail;
-
-	if (v6) {
-		struct sockaddr_in6 sa6;
-
-		memcpy(&sa6, &sa, sizeof(sa6));
-		ref.r.p.udp.udp.port = ntohs(sa6.sin6_port);
-	} else {
-		struct sockaddr_in sa4;
-
-		memcpy(&sa4, &sa, sizeof(sa4));
-		ref.r.p.udp.udp.port = ntohs(sa4.sin_port);
-	}
-
-	sp = &udp_splice_map[v6 ? V6 : V4][ref.r.p.udp.udp.port];
 	if (splice == UDP_BACK_TO_INIT) {
 		sp->init_bound_sock = bound_sock;
-		sp->init_dst_port = src;
-		udp_splice_map[v6 ? V6 : V4][src].ns_conn_sock = s;
+		sp->ns_conn_sock = s;
 		bitmap_set(udp_act[v6 ? V6 : V4][UDP_ACT_NS_CONN], src);
 	} else if (splice == UDP_BACK_TO_NS) {
 		sp->ns_bound_sock = bound_sock;
-		sp->ns_dst_port = src;
-		udp_splice_map[v6 ? V6 : V4][src].init_conn_sock = s;
+		sp->init_conn_sock = s;
 		bitmap_set(udp_act[v6 ? V6 : V4][UDP_ACT_INIT_CONN], src);
 	}
 
@@ -591,7 +571,7 @@ static void udp_sock_handler_splice(const struct ctx *c, union epoll_ref ref,
 		if (!(s = udp_splice_map[v6][dst].init_bound_sock))
 			return;
 
-		send_dst = udp_splice_map[v6][dst].init_dst_port;
+		send_dst = dst;
 		break;
 	case UDP_TO_INIT:
 		src += c->udp.fwd_in.rdelta[src];
@@ -608,7 +588,7 @@ static void udp_sock_handler_splice(const struct ctx *c, union epoll_ref ref,
 		if (!(s = udp_splice_map[v6][dst].ns_bound_sock))
 			return;
 
-		send_dst = udp_splice_map[v6][dst].ns_dst_port;
+		send_dst = dst;
 		break;
 	default:
 		return;