ipv: Re-enable IP when MTU > 68

Re-enable IP when the MTU gets back to a valid size. 

This patch just checks if the in_dev is NULL on a NETDEV_CHANGEMTU event
and if MTU is valid (bigger than 68), then re-enable in_dev. 

Also a function that checks valid MTU size was created.
Signed-off-by: NBreno Leitao <leitao@linux.vnet.ibm.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/devinet.c

@@ -1029,6 +1029,11 @@ static void inetdev_changename(struct net_device *dev, struct in_device *in_dev)
 	}
 }
 
+static inline bool inetdev_valid_mtu(unsigned mtu)
+{
+	return mtu >= 68;
+}
+
 /* Called only under RTNL semaphore */
 
 static int inetdev_event(struct notifier_block *this, unsigned long event,
@@ -1048,6 +1053,10 @@ static int inetdev_event(struct notifier_block *this, unsigned long event,
 				IN_DEV_CONF_SET(in_dev, NOXFRM, 1);
 				IN_DEV_CONF_SET(in_dev, NOPOLICY, 1);
 			}
+		} else if (event == NETDEV_CHANGEMTU) {
+			/* Re-enabling IP */
+			if (inetdev_valid_mtu(dev->mtu))
+				in_dev = inetdev_init(dev);
 		}
 		goto out;
 	}
@@ -1058,7 +1067,7 @@ static int inetdev_event(struct notifier_block *this, unsigned long event,
 		dev->ip_ptr = NULL;
 		break;
 	case NETDEV_UP:
-		if (dev->mtu < 68)
+		if (!inetdev_valid_mtu(dev->mtu))
 			break;
 		if (dev->flags & IFF_LOOPBACK) {
 			struct in_ifaddr *ifa;
@@ -1080,9 +1089,9 @@ static int inetdev_event(struct notifier_block *this, unsigned long event,
 		ip_mc_down(in_dev);
 		break;
 	case NETDEV_CHANGEMTU:
-		if (dev->mtu >= 68)
+		if (inetdev_valid_mtu(dev->mtu))
 			break;
-		/* MTU falled under 68, disable IP */
+		/* disable IP when MTU is not enough */
 	case NETDEV_UNREGISTER:
 		inetdev_destroy(in_dev);
 		break;