geneve: Refine MTU limit

Calculate the maximum MTU taking into account the size of headers
involved in GENEVE encapsulation, as for other tunnel types.

Changes in v3:
- Correct comment style
Changes in v2:
- Conform more closely to ip_tunnel_change_mtu
- Exclude GENEVE options from max MTU calculation
Signed-off-by: David Wragg <david@weave.works>
Acked-by: Jesse Gross <jesse@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/geneve.c

@@ -1039,17 +1039,34 @@ static netdev_tx_t geneve_xmit(struct sk_buff *skb, struct net_device *dev)
 	return geneve_xmit_skb(skb, dev, info);
 }
 
-static int geneve_change_mtu(struct net_device *dev, int new_mtu)
+static int __geneve_change_mtu(struct net_device *dev, int new_mtu, bool strict)
 {
-	/* GENEVE overhead is not fixed, so we can't enforce a more
-	 * precise max MTU.
+	/* The max_mtu calculation does not take account of GENEVE
+	 * options, to avoid excluding potentially valid
+	 * configurations.
 	 */
-	if (new_mtu < 68 || new_mtu > IP_MAX_MTU)
+	int max_mtu = IP_MAX_MTU - GENEVE_BASE_HLEN - sizeof(struct iphdr)
+		- dev->hard_header_len;
+
+	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 geneve_change_mtu(struct net_device *dev, int new_mtu)
+{
+	return __geneve_change_mtu(dev, new_mtu, true);
+}
+
 static int geneve_fill_metadata_dst(struct net_device *dev, struct sk_buff *skb)
 {
 	struct ip_tunnel_info *info = skb_tunnel_info(skb);
@@ -1459,7 +1476,7 @@ struct net_device *geneve_dev_create_fb(struct net *net, const char *name,
 	/* openvswitch users expect packet sizes to be unrestricted,
 	 * so set the largest MTU we can.
 	 */
-	err = geneve_change_mtu(dev, IP_MAX_MTU);
+	err = __geneve_change_mtu(dev, IP_MAX_MTU, false);
 	if (err)
 		goto err;