udp_tunnel: add config option to bind to a device

UDP tunnel sockets are always opened unbound to a specific device. This
patch allow the socket to be bound on a custom device, which
incidentally makes UDP tunnels VRF-aware if binding to an l3mdev.
Signed-off-by: Alexis Bauvin <abauvin@scaleway.com>
Reviewed-by: Amine Kherbouche <akherbouche@scaleway.com>
Tested-by: Amine Kherbouche <akherbouche@scaleway.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/udp_tunnel.h

@@ -30,6 +30,7 @@ struct udp_port_cfg {
 
 	__be16			local_udp_port;
 	__be16			peer_udp_port;
+	int			bind_ifindex;
 	unsigned int		use_udp_checksums:1,
 				use_udp6_tx_checksums:1,
 				use_udp6_rx_checksums:1,

net/ipv4/udp_tunnel.c

@@ -20,6 +20,23 @@ int udp_sock_create4(struct net *net, struct udp_port_cfg *cfg,
 	if (err < 0)
 		goto error;
 
+	if (cfg->bind_ifindex) {
+		struct net_device *dev;
+
+		dev = dev_get_by_index(net, cfg->bind_ifindex);
+		if (!dev) {
+			err = -ENODEV;
+			goto error;
+		}
+
+		err = kernel_setsockopt(sock, SOL_SOCKET, SO_BINDTODEVICE,
+					dev->name, strlen(dev->name) + 1);
+		dev_put(dev);
+
+		if (err < 0)
+			goto error;
+	}
+
 	udp_addr.sin_family = AF_INET;
 	udp_addr.sin_addr = cfg->local_ip;
 	udp_addr.sin_port = cfg->local_udp_port;

net/ipv6/ip6_udp_tunnel.c

@@ -31,6 +31,22 @@ int udp_sock_create6(struct net *net, struct udp_port_cfg *cfg,
 		if (err < 0)
 			goto error;
 	}
+	if (cfg->bind_ifindex) {
+		struct net_device *dev;
+
+		dev = dev_get_by_index(net, cfg->bind_ifindex);
+		if (!dev) {
+			err = -ENODEV;
+			goto error;
+		}
+
+		err = kernel_setsockopt(sock, SOL_SOCKET, SO_BINDTODEVICE,
+					dev->name, strlen(dev->name) + 1);
+		dev_put(dev);
+
+		if (err < 0)
+			goto error;
+	}
 
 	udp6_addr.sin6_family = AF_INET6;
 	memcpy(&udp6_addr.sin6_addr, &cfg->local_ip6,