RDS: have sockets get transport module references

Right now there's nothing to stop the various paths that use
rs->rs_transport from racing with rmmod and executing freed transport
code. The simple fix is to have binding to a transport also hold a
reference to the transport's module, removing this class of races.

We already had an unused t_owner field which was set for the modular
transports and which wasn't set for the built-in loop transport.

Signed-off-by: Zach Brown <[e-mail]>

net/rds/af_rds.c

···
 	rds_sock_count--;
 	spin_unlock_irqrestore(&rds_sock_lock, flags);
 
+	rds_trans_put(rs->rs_transport);
+
 	sock->sk = NULL;
 	sock_put(sk);
 out:

net/rds/connection.c

···
 {
 	struct rds_connection *conn, *parent = NULL;
 	struct hlist_head *head = rds_conn_bucket(laddr, faddr);
+	struct rds_transport *loop_trans;
 	unsigned long flags;
 	int ret;
 
···
 	 * can bind to the destination address then we'd rather the messages
 	 * flow through loopback rather than either transport.
 	 */
-	if (rds_trans_get_preferred(faddr)) {
+	loop_trans = rds_trans_get_preferred(faddr);
+	if (loop_trans) {
+		rds_trans_put(loop_trans);
 		conn->c_loopback = 1;
 		if (is_outgoing && trans->t_prefer_loopback) {
 			/* "outgoing" connection - and the transport

net/rds/rds.h

···
 int rds_trans_register(struct rds_transport *trans);
 void rds_trans_unregister(struct rds_transport *trans);
 struct rds_transport *rds_trans_get_preferred(__be32 addr);
+void rds_trans_put(struct rds_transport *trans);
 unsigned int rds_trans_stats_info_copy(struct rds_info_iterator *iter,
 				       unsigned int avail);
 int rds_trans_init(void);

net/rds/transport.c

···
 }
 EXPORT_SYMBOL_GPL(rds_trans_unregister);
 
+void rds_trans_put(struct rds_transport *trans)
+{
+	if (trans && trans->t_owner)
+		module_put(trans->t_owner);
+}
+
 struct rds_transport *rds_trans_get_preferred(__be32 addr)
 {
 	struct rds_transport *ret = NULL;
-	int i;
+	struct rds_transport *trans;
+	unsigned int i;
 
 	if (IN_LOOPBACK(ntohl(addr)))
 		return &rds_loop_transport;
 
 	down_read(&rds_trans_sem);
-	for (i = 0; i < RDS_TRANS_COUNT; i++)
-	{
-		if (transports[i] && (transports[i]->laddr_check(addr) == 0)) {
-			ret = transports[i];
+	for (i = 0; i < RDS_TRANS_COUNT; i++) {
+		trans = transports[i];
+
+		if (trans && (trans->laddr_check(addr) == 0) &&
+		    (!trans->t_owner || try_module_get(trans->t_owner))) {
+			ret = trans;
 			break;
 		}
 	}