datapath: geneve: Refine MTU limit.

Upstream commit:
    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>
Upstream: aeee0e66c6b4 ("geneve: Refine MTU limit")
Signed-off-by: Joe Stringer <joe@ovn.org>
Acked-by: Jesse Gross <jesse@kernel.org>

diff --git a/datapath/linux/compat/geneve.c b/datapath/linux/compat/geneve.c
index 50ed393..92feeef 100644 (file)
--- a/datapath/linux/compat/geneve.c
+++ b/datapath/linux/compat/geneve.c
@@ -742,17 +742,34 @@ static netdev_tx_t geneve_dev_xmit(struct sk_buff *skb, struct net_device *dev)
        return NETDEV_TX_OK;
 }
 
-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 const struct net_device_ops geneve_netdev_ops = {
 #ifdef HAVE_DEV_TSTATS
        .ndo_init               = geneve_init,
@@ -1045,7 +1062,7 @@ struct net_device *rpl_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;