net: qualcomm: rmnet: Allow configuration updates to existing devices

This allows the changelink operation to succeed if the mux_id was
specified as an argument. Note that the mux_id must match the
existing mux_id of the rmnet device or should be an unused mux_id.

Fixes: 1dc49e9d ("net: rmnet: do not allow to change mux id if mux id is duplicated")
Reported-and-tested-by: Alex Elder <elder@linaro.org>
Signed-off-by: Sean Tranchetti <stranche@codeaurora.org>
Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/qualcomm/rmnet/rmnet_config.c

@@ -282,7 +282,6 @@ static int rmnet_changelink(struct net_device *dev, struct nlattr *tb[],
 {
 	struct rmnet_priv *priv = netdev_priv(dev);
 	struct net_device *real_dev;
-	struct rmnet_endpoint *ep;
 	struct rmnet_port *port;
 	u16 mux_id;
 
@@ -297,20 +296,28 @@ static int rmnet_changelink(struct net_device *dev, struct nlattr *tb[],
 
 	if (data[IFLA_RMNET_MUX_ID]) {
 		mux_id = nla_get_u16(data[IFLA_RMNET_MUX_ID]);
-		if (rmnet_get_endpoint(port, mux_id)) {
-			NL_SET_ERR_MSG_MOD(extack, "MUX ID already exists");
-			return -EINVAL;
-		}
+
+		if (mux_id != priv->mux_id) {
+			struct rmnet_endpoint *ep;
+
 			ep = rmnet_get_endpoint(port, priv->mux_id);
 			if (!ep)
 				return -ENODEV;
 
+			if (rmnet_get_endpoint(port, mux_id)) {
+				NL_SET_ERR_MSG_MOD(extack,
+						   "MUX ID already exists");
+				return -EINVAL;
+			}
+
 			hlist_del_init_rcu(&ep->hlnode);
-		hlist_add_head_rcu(&ep->hlnode, &port->muxed_ep[mux_id]);
+			hlist_add_head_rcu(&ep->hlnode,
+					   &port->muxed_ep[mux_id]);
 
 			ep->mux_id = mux_id;
 			priv->mux_id = mux_id;
 		}
+	}
 
 	if (data[IFLA_RMNET_FLAGS]) {
 		struct ifla_rmnet_flags *flags;