macvtap: Fix the minor device number allocation

On systems that create and delete lots of dynamic devices the
31bit linux ifindex fails to fit in the 16bit macvtap minor,
resulting in unusable macvtap devices. I have systems running
automated tests that that hit this condition in just a few days.

Use a linux idr allocator to track which mavtap minor numbers
are available and and to track the association between macvtap
minor numbers and macvtap network devices.

Remove the unnecessary unneccessary check to see if the network
device we have found is indeed a macvtap device. With macvtap
specific data structures it is impossible to find any other
kind of networking device.

Increase the macvtap minor range from 65536 to the full 20 bits
that is supported by linux device numbers. It doesn't solve the
original problem but there is no penalty for a larger minor
device range.

Signed-off-by: Eric W. Biederman <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

drivers/net/macvtap.c

···
 };
 
 /*
- * Minor number matches netdev->ifindex, so need a potentially
- * large value. This also makes it possible to split the
- * tap functionality out again in the future by offering it
- * from other drivers besides macvtap. As long as every device
- * only has one tap, the interface numbers assure that the
- * device nodes are unique.
+ * Variables for dealing with macvtaps device numbers.
  */
 static dev_t macvtap_major;
-#define MACVTAP_NUM_DEVS 65536
+#define MACVTAP_NUM_DEVS (1U << MINORBITS)
+static DEFINE_MUTEX(minor_lock);
+static DEFINE_IDR(minor_idr);
+
 #define GOODCOPY_LEN 128
 static struct class *macvtap_class;
 static struct cdev macvtap_cdev;
···
 	return macvtap_forward(skb->dev, skb);
 }
 
+static int macvtap_get_minor(struct macvlan_dev *vlan)
+{
+	int retval = -ENOMEM;
+	int id;
+
+	mutex_lock(&minor_lock);
+	if (idr_pre_get(&minor_idr, GFP_KERNEL) == 0)
+		goto exit;
+
+	retval = idr_get_new_above(&minor_idr, vlan, 1, &id);
+	if (retval < 0) {
+		if (retval == -EAGAIN)
+			retval = -ENOMEM;
+		goto exit;
+	}
+	if (id < MACVTAP_NUM_DEVS) {
+		vlan->minor = id;
+	} else {
+		printk(KERN_ERR "too many macvtap devices\n");
+		retval = -EINVAL;
+		idr_remove(&minor_idr, id);
+	}
+exit:
+	mutex_unlock(&minor_lock);
+	return retval;
+}
+
+static void macvtap_free_minor(struct macvlan_dev *vlan)
+{
+	mutex_lock(&minor_lock);
+	if (vlan->minor) {
+		idr_remove(&minor_idr, vlan->minor);
+		vlan->minor = 0;
+	}
+	mutex_unlock(&minor_lock);
+}
+
+static struct net_device *dev_get_by_macvtap_minor(int minor)
+{
+	struct net_device *dev = NULL;
+	struct macvlan_dev *vlan;
+
+	mutex_lock(&minor_lock);
+	vlan = idr_find(&minor_idr, minor);
+	if (vlan) {
+		dev = vlan->dev;
+		dev_hold(dev);
+	}
+	mutex_unlock(&minor_lock);
+	return dev;
+}
+
 static int macvtap_newlink(struct net *src_net,
 			   struct net_device *dev,
 			   struct nlattr *tb[],
···
 static int macvtap_open(struct inode *inode, struct file *file)
 {
 	struct net *net = current->nsproxy->net_ns;
-	struct net_device *dev = dev_get_by_index(net, iminor(inode));
+	struct net_device *dev = dev_get_by_macvtap_minor(iminor(inode));
 	struct macvtap_queue *q;
 	int err;
 
 	err = -ENODEV;
 	if (!dev)
-		goto out;
-
-	/* check if this is a macvtap device */
-	err = -EINVAL;
-	if (dev->rtnl_link_ops != &macvtap_link_ops)
 		goto out;
 
 	err = -ENOMEM;
···
 				unsigned long event, void *ptr)
 {
 	struct net_device *dev = ptr;
+	struct macvlan_dev *vlan;
 	struct device *classdev;
 	dev_t devt;
+	int err;
 
 	if (dev->rtnl_link_ops != &macvtap_link_ops)
 		return NOTIFY_DONE;
 
+	vlan = netdev_priv(dev);
 
 	switch (event) {
 	case NETDEV_REGISTER:
···
 		 * been registered but before register_netdevice has
 		 * finished running.
 		 */
-		devt = MKDEV(MAJOR(macvtap_major), dev->ifindex);
+		err = macvtap_get_minor(vlan);
+		if (err)
+			return notifier_from_errno(err);
+
+		devt = MKDEV(MAJOR(macvtap_major), vlan->minor);
 		classdev = device_create(macvtap_class, &dev->dev, devt,
 					 dev, "tap%d", dev->ifindex);
-		if (IS_ERR(classdev))
+		if (IS_ERR(classdev)) {
+			macvtap_free_minor(vlan);
 			return notifier_from_errno(PTR_ERR(classdev));
+		}
 		break;
 	case NETDEV_UNREGISTER:
-		devt = MKDEV(MAJOR(macvtap_major), dev->ifindex);
+		devt = MKDEV(MAJOR(macvtap_major), vlan->minor);
 		device_destroy(macvtap_class, devt);
+		macvtap_free_minor(vlan);
 		break;
 	}
 

include/linux/if_macvlan.h

···
 	int (*forward)(struct net_device *dev, struct sk_buff *skb);
 	struct macvtap_queue	*taps[MAX_MACVTAP_QUEUES];
 	int			numvtaps;
+	int			minor;
 };
 
 static inline void macvlan_count_rx(const struct macvlan_dev *vlan,