net: vxlan: use custom ndo_change_mtu handler

When adding a new vxlan device to an "underlying carrier" (here:
dst->remote_ifindex), the MTU size assigned to the vxlan device
is the MTU at setup time of the carrier - needed headroom, when
adding a vxlan device w/o explicit carrier, then it defaults
to 1500.

In case of an explicit carrier that supports jumbo frames, we
currently cannot change vxlan MTU via ip(8) to > 1500 in
post-setup time, as vxlan driver uses eth_change_mtu() as default
method for manually setting MTU.

Hence, use a custom implementation that only falls back to
eth_change_mtu() in case we didn't use a dev parameter on device
setup time, and otherwise allow a max MTU setting of the carrier
incl. adjustment for headroom.
Reported-by: NShahed Shaikh <shahed.shaikh@qlogic.com>
Signed-off-by: NDaniel Borkmann <dborkman@redhat.com>
Acked-by: NStephen Hemminger <stephen@networkplumber.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/vxlan.c

@@ -2014,6 +2014,29 @@ static void vxlan_set_multicast_list(struct net_device *dev)
 {
 }
 
+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(dev_net(dev), 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;
+}
+
 static const struct net_device_ops vxlan_netdev_ops = {
 	.ndo_init		= vxlan_init,
 	.ndo_uninit		= vxlan_uninit,
@@ -2022,7 +2045,7 @@ static const struct net_device_ops vxlan_netdev_ops = {
 	.ndo_start_xmit		= vxlan_xmit,
 	.ndo_get_stats64	= ip_tunnel_get_stats64,
 	.ndo_set_rx_mode	= vxlan_set_multicast_list,
-	.ndo_change_mtu		= eth_change_mtu,
+	.ndo_change_mtu		= vxlan_change_mtu,
 	.ndo_validate_addr	= eth_validate_addr,
 	.ndo_set_mac_address	= eth_mac_addr,
 	.ndo_fdb_add		= vxlan_fdb_add,