net-2.6 : V2 - fix dev_get_valid_name

the commit:

commit d9031024
Author: Octavian Purdila <opurdila@ixiacom.com>
Date:   Wed Nov 18 02:36:59 2009 +0000

    net: device name allocation cleanups

introduced a bug when there is a hash collision making impossible
to rename a device with eth%d. This bug is very hard to reproduce
and appears rarely.

The problem is coming from we don't pass a temporary buffer to
__dev_alloc_name but 'dev->name' which is modified by the function.

A detailed explanation is here:

http://marc.info/?l=linux-netdev&m=127417784011987&w=2

Changelog:
 V2 : replaced strings comparison by pointers comparison
Signed-off-by: NDaniel Lezcano <daniel.lezcano@free.fr>
Reviewed-by: NOctavian Purdila <opurdila@ixiacom.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/core/dev.c

@@ -954,18 +954,22 @@ int dev_alloc_name(struct net_device *dev, const char *name)
 }
 EXPORT_SYMBOL(dev_alloc_name);
 
-static int dev_get_valid_name(struct net *net, const char *name, char *buf,
-			      bool fmt)
+static int dev_get_valid_name(struct net_device *dev, const char *name, bool fmt)
 {
+	struct net *net;
+
+	BUG_ON(!dev_net(dev));
+	net = dev_net(dev);
+
 	if (!dev_valid_name(name))
 		return -EINVAL;
 
 	if (fmt && strchr(name, '%'))
-		return __dev_alloc_name(net, name, buf);
+		return dev_alloc_name(dev, name);
 	else if (__dev_get_by_name(net, name))
 		return -EEXIST;
-	else if (buf != name)
-		strlcpy(buf, name, IFNAMSIZ);
+	else if (dev->name != name)
+		strlcpy(dev->name, name, IFNAMSIZ);
 
 	return 0;
 }
@@ -997,7 +1001,7 @@ int dev_change_name(struct net_device *dev, const char *newname)
 
 	memcpy(oldname, dev->name, IFNAMSIZ);
 
-	err = dev_get_valid_name(net, newname, dev->name, 1);
+	err = dev_get_valid_name(dev, newname, 1);
 	if (err < 0)
 		return err;
 
@@ -4965,7 +4969,7 @@ int register_netdevice(struct net_device *dev)
 		}
 	}
 
-	ret = dev_get_valid_name(net, dev->name, dev->name, 0);
+	ret = dev_get_valid_name(dev, dev->name, 0);
 	if (ret)
 		goto err_uninit;
 
@@ -5574,7 +5578,7 @@ int dev_change_net_namespace(struct net_device *dev, struct net *net, const char
 		/* We get here if we can't use the current device name */
 		if (!pat)
 			goto out;
-		if (dev_get_valid_name(net, pat, dev->name, 1))
+		if (dev_get_valid_name(dev, pat, 1))
 			goto out;
 	}