[NET]: Factor out __dev_alloc_name from dev_alloc_name

When forcibly changing the network namespace of a device
I need something that can generate a name for the device
in the new namespace without overwriting the old name.

__dev_alloc_name provides me that functionality.
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/dev.c

@@ -739,9 +739,10 @@ int dev_valid_name(const char *name)
 }
 
 /**
- *	dev_alloc_name - allocate a name for a device
- *	@dev: device
+ *	__dev_alloc_name - allocate a name for a device
+ *	@net: network namespace to allocate the device name in
  *	@name: name format string
+ *	@buf:  scratch buffer and result name string
  *
  *	Passed a format string - eg "lt%d" it will try and find a suitable
  *	id. It scans list of devices to build up a free map, then chooses
@@ -752,18 +753,13 @@ int dev_valid_name(const char *name)
  *	Returns the number of the unit assigned or a negative errno code.
  */
 
-int dev_alloc_name(struct net_device *dev, const char *name)
+static int __dev_alloc_name(struct net *net, const char *name, char *buf)
 {
 	int i = 0;
-	char buf[IFNAMSIZ];
 	const char *p;
 	const int max_netdevices = 8*PAGE_SIZE;
 	long *inuse;
 	struct net_device *d;
-	struct net *net;
-
-	BUG_ON(!dev->nd_net);
-	net = dev->nd_net;
 
 	p = strnchr(name, IFNAMSIZ-1, '%');
 	if (p) {
@@ -787,7 +783,7 @@ int dev_alloc_name(struct net_device *dev, const char *name)
 				continue;
 
 			/*  avoid cases where sscanf is not exact inverse of printf */
-			snprintf(buf, sizeof(buf), name, i);
+			snprintf(buf, IFNAMSIZ, name, i);
 			if (!strncmp(buf, d->name, IFNAMSIZ))
 				set_bit(i, inuse);
 		}
@@ -796,11 +792,9 @@ int dev_alloc_name(struct net_device *dev, const char *name)
 		free_page((unsigned long) inuse);
 	}
 
-	snprintf(buf, sizeof(buf), name, i);
-	if (!__dev_get_by_name(net, buf)) {
-		strlcpy(dev->name, buf, IFNAMSIZ);
+	snprintf(buf, IFNAMSIZ, name, i);
+	if (!__dev_get_by_name(net, buf))
 		return i;
-	}
 
 	/* It is possible to run out of possible slots
 	 * when the name is long and there isn't enough space left
@@ -809,6 +803,34 @@ int dev_alloc_name(struct net_device *dev, const char *name)
 	return -ENFILE;
 }
 
+/**
+ *	dev_alloc_name - allocate a name for a device
+ *	@dev: device
+ *	@name: name format string
+ *
+ *	Passed a format string - eg "lt%d" it will try and find a suitable
+ *	id. It scans list of devices to build up a free map, then chooses
+ *	the first empty slot. The caller must hold the dev_base or rtnl lock
+ *	while allocating the name and adding the device in order to avoid
+ *	duplicates.
+ *	Limited to bits_per_byte * page size devices (ie 32K on most platforms).
+ *	Returns the number of the unit assigned or a negative errno code.
+ */
+
+int dev_alloc_name(struct net_device *dev, const char *name)
+{
+	char buf[IFNAMSIZ];
+	struct net *net;
+	int ret;
+
+	BUG_ON(!dev->nd_net);
+	net = dev->nd_net;
+	ret = __dev_alloc_name(net, name, buf);
+	if (ret >= 0)
+		strlcpy(dev->name, buf, IFNAMSIZ);
+	return ret;
+}
+
 
 /**
  *	dev_change_name - change name of a device