net: fix ifname in netlink ntf during netns move

dev_get_valid_name() overwrites the netdev's name on success.
This makes it hard to use in prepare-commit-like fashion,
where we do validation first, and "commit" to the change
later.

Factor out a helper which lets us save the new name to a buffer.
Use it to fix the problem of notification on netns move having
incorrect name:

 5: eth0: <BROADCAST,NOARP> mtu 1500 qdisc noop state DOWN group default
     link/ether be:4d:58:f9:d5:40 brd ff:ff:ff:ff:ff:ff
 6: eth1: <BROADCAST,NOARP> mtu 1500 qdisc noop state DOWN group default
     link/ether 1e:4a:34:36:e3:cd brd ff:ff:ff:ff:ff:ff

 [ ~]# ip link set dev eth0 netns 1 name eth1

ip monitor inside netns:
 Deleted inet eth0
 Deleted inet6 eth0
 Deleted 5: eth1: <BROADCAST,NOARP> mtu 1500 qdisc noop state DOWN group default
     link/ether be:4d:58:f9:d5:40 brd ff:ff:ff:ff:ff:ff new-netnsid 0 new-ifindex 7

Name is reported as eth1 in old netns for ifindex 5, already renamed.

Fixes: d90310243f ("net: device name allocation cleanups")
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

net/core/dev.c

@@ -1123,6 +1123,26 @@ static int __dev_alloc_name(struct net *net, const char *name, char *buf)
 	return -ENFILE;
 }
 
+static int dev_prep_valid_name(struct net *net, struct net_device *dev,
+			       const char *want_name, char *out_name)
+{
+	int ret;
+
+	if (!dev_valid_name(want_name))
+		return -EINVAL;
+
+	if (strchr(want_name, '%')) {
+		ret = __dev_alloc_name(net, want_name, out_name);
+		return ret < 0 ? ret : 0;
+	} else if (netdev_name_in_use(net, want_name)) {
+		return -EEXIST;
+	} else if (out_name != want_name) {
+		strscpy(out_name, want_name, IFNAMSIZ);
+	}
+
+	return 0;
+}
+
 static int dev_alloc_name_ns(struct net *net,
 			     struct net_device *dev,
 			     const char *name)
@@ -1160,19 +1180,13 @@ EXPORT_SYMBOL(dev_alloc_name);
 static int dev_get_valid_name(struct net *net, struct net_device *dev,
 			      const char *name)
 {
-	BUG_ON(!net);
+	char buf[IFNAMSIZ];
+	int ret;
 
-	if (!dev_valid_name(name))
-		return -EINVAL;
-
-	if (strchr(name, '%'))
-		return dev_alloc_name_ns(net, dev, name);
-	else if (netdev_name_in_use(net, name))
-		return -EEXIST;
-	else if (dev->name != name)
-		strscpy(dev->name, name, IFNAMSIZ);
-
-	return 0;
+	ret = dev_prep_valid_name(net, dev, name, buf);
+	if (ret >= 0)
+		strscpy(dev->name, buf, IFNAMSIZ);
+	return ret;
 }
 
 /**
@@ -11038,6 +11052,7 @@ int __dev_change_net_namespace(struct net_device *dev, struct net *net,
 			       const char *pat, int new_ifindex)
 {
 	struct net *net_old = dev_net(dev);
+	char new_name[IFNAMSIZ] = {};
 	int err, new_nsid;
 
 	ASSERT_RTNL();
@@ -11064,7 +11079,7 @@ int __dev_change_net_namespace(struct net_device *dev, struct net *net,
 		/* We get here if we can't use the current device name */
 		if (!pat)
 			goto out;
-		err = dev_get_valid_name(net, dev, pat);
+		err = dev_prep_valid_name(net, dev, pat, new_name);
 		if (err < 0)
 			goto out;
 	}
@@ -11135,6 +11150,9 @@ int __dev_change_net_namespace(struct net_device *dev, struct net *net,
 	kobject_uevent(&dev->dev.kobj, KOBJ_ADD);
 	netdev_adjacent_add_links(dev);
 
+	if (new_name[0]) /* Rename the netdev to prepared name */
+		strscpy(dev->name, new_name, IFNAMSIZ);
+
 	/* Fixup kobjects */
 	err = device_rename(&dev->dev, dev->name);
 	WARN_ON(err);