vxlan: Relax MTU constraints

Allow the MTU of vxlan devices without an underlying device to be set
to larger values (up to a maximum based on IP packet limits and vxlan
overhead).

Previously, their MTUs could not be set to higher than the
conventional ethernet value of 1500.  This is a very arbitrary value
in the context of vxlan, and prevented vxlan devices from being able
to take advantage of jumbo frames etc.

The default MTU remains 1500, for compatibility.

Signed-off-by: David Wragg <david@weave.works>
Acked-by: Roopa Prabhu <roopa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/drivers/net/vxlan.c b/drivers/net/vxlan.c
index 6543918..e992c6a 100644
--- a/drivers/net/vxlan.c
+++ b/drivers/net/vxlan.c
@@ -2367,27 +2367,41 @@
 {
 }
 
+static int __vxlan_change_mtu(struct net_device *dev,
+			      struct net_device *lowerdev,
+			      struct vxlan_rdst *dst, int new_mtu, bool strict)
+{
+	int max_mtu = IP_MAX_MTU;
+
+	if (lowerdev)
+		max_mtu = lowerdev->mtu;
+
+	if (dst->remote_ip.sa.sa_family == AF_INET6)
+		max_mtu -= VXLAN6_HEADROOM;
+	else
+		max_mtu -= VXLAN_HEADROOM;
+
+	if (new_mtu < 68)
+		return -EINVAL;
+
+	if (new_mtu > max_mtu) {
+		if (strict)
+			return -EINVAL;
+
+		new_mtu = max_mtu;
+	}
+
+	dev->mtu = new_mtu;
+	return 0;
+}
+
 static int vxlan_change_mtu(struct net_device *dev, int new_mtu)
 {
 	struct vxlan_dev *vxlan = netdev_priv(dev);
 	struct vxlan_rdst *dst = &vxlan->default_dst;
-	struct net_device *lowerdev;
-	int max_mtu;
-
-	lowerdev = __dev_get_by_index(vxlan->net, dst->remote_ifindex);
-	if (lowerdev == NULL)
-		return eth_change_mtu(dev, new_mtu);
-
-	if (dst->remote_ip.sa.sa_family == AF_INET6)
-		max_mtu = lowerdev->mtu - VXLAN6_HEADROOM;
-	else
-		max_mtu = lowerdev->mtu - VXLAN_HEADROOM;
-
-	if (new_mtu < 68 || new_mtu > max_mtu)
-		return -EINVAL;
-
-	dev->mtu = new_mtu;
-	return 0;
+	struct net_device *lowerdev = __dev_get_by_index(vxlan->net,
+							 dst->remote_ifindex);
+	return __vxlan_change_mtu(dev, lowerdev, dst, new_mtu, true);
 }
 
 static int egress_ipv4_tun_info(struct net_device *dev, struct sk_buff *skb,