net: mqtt_sn: udp: fix joining v6 multicast group

GCC gave the following warning:

mqtt_sn_transport_udp.c: In function ‘tp_udp_init’:
mqtt_sn_transport_udp.c:117:9: warning: ‘memcpy’ forming offset [12, 105]
is out of the bounds [0, 12] of object ‘mreqn’ with type ‘struct ip_mreqn’
[-Warray-bounds=]
  117 |         memcpy(&mreqn.imr_multiaddr, &udp->bcaddr.data[2], si...
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
mqtt_sn_transport_udp.c:48:25: note: ‘mreqn’ declared here
   48 |         struct ip_mreqn mreqn;
      |                         ^~~~~

And it turns out that it's right. The original code looks like it tries to
copy the IP address to imr_multiaddr in a way that works for both v4 and
v6. It ignores, that bcaddr.data may be way larger than even a v6 addr,
because it's a buffer that's big enough to hold many other structs as well.

Not only that, IP_ADD_MEMBERSHIP is for IPv4 only and imr_multiaddr can
only hold an IPv4 address anyway.

This modifies the code to have separate code paths for v4 and v6 and call
the correct APIs. This also gets rid of the memcpy, since it can be a
simple struct assignment now.

Signed-off-by: Michael Zimmermann <[email protected]>

Author: M1cha

subsys/net/lib/mqtt_sn/mqtt_sn_transport_udp.c

@@ -45,7 +45,6 @@ static int tp_udp_init(struct mqtt_sn_transport *transport)
 	struct mqtt_sn_transport_udp *udp = UDP_TRANSPORT(transport);
 	int err;
 	struct sockaddr addrm;
-	struct ip_mreqn mreqn;
 	int optval;
 	struct net_if *iface;
 
@@ -112,22 +111,43 @@ static int tp_udp_init(struct mqtt_sn_transport *transport)
 		return errno;
 	}
 
-	memcpy(&mreqn.imr_multiaddr, &udp->bcaddr.data[2], sizeof(udp->bcaddr.data) - 2);
 	if (udp->bcaddr.sa_family == AF_INET && IS_ENABLED(CONFIG_NET_IPV4)) {
+		struct sockaddr_in *bcaddr_in = (struct sockaddr_in *)&udp->bcaddr;
+		struct ip_mreqn mreqn;
+
 		iface = net_if_ipv4_select_src_iface(
 			&((struct sockaddr_in *)&udp->bcaddr)->sin_addr);
+
+		mreqn = (struct ip_mreqn) {
+			.imr_multiaddr = bcaddr_in->sin_addr,
+			.imr_ifindex = net_if_get_by_iface(iface),
+		};
+
+		err = zsock_setsockopt(udp->sock, IPPROTO_IP, IP_ADD_MEMBERSHIP, &mreqn,
+				       sizeof(mreqn));
+		if (err < 0) {
+			return errno;
+		}
 	} else if (udp->bcaddr.sa_family == AF_INET6 && IS_ENABLED(CONFIG_NET_IPV6)) {
+		struct sockaddr_in6 *bcaddr_in6 = (struct sockaddr_in6 *)&udp->bcaddr;
+		struct ipv6_mreq mreq;
+
 		iface = net_if_ipv6_select_src_iface(&((struct sockaddr_in6 *)&addrm)->sin6_addr);
+
+		mreq = (struct ipv6_mreq) {
+			.ipv6mr_multiaddr = bcaddr_in6->sin6_addr,
+			.ipv6mr_ifindex = net_if_get_by_iface(iface),
+		};
+
+		err = zsock_setsockopt(udp->sock, IPPROTO_IPV6, IPV6_ADD_MEMBERSHIP, &mreq,
+				       sizeof(mreq));
+		if (err < 0) {
+			return errno;
+		}
 	} else {
 		LOG_ERR("Unknown AF");
 		return -EINVAL;
 	}
-	mreqn.imr_ifindex = net_if_get_by_iface(iface);
-
-	err = zsock_setsockopt(udp->sock, IPPROTO_IP, IP_ADD_MEMBERSHIP, &mreqn, sizeof(mreqn));
-	if (err < 0) {
-		return errno;
-	}
 
 	optval = CONFIG_MQTT_SN_LIB_BROADCAST_RADIUS;
 	err = zsock_setsockopt(udp->sock, IPPROTO_IP, IP_MULTICAST_TTL, &optval, sizeof(optval));