net: plumb extack in __dev_change_net_namespace()

It could be hard to understand why the netlink command fails. For example,
if dev->netns_immutable is set, the error is "Invalid argument".

Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

include/linux/netdevice.h

@@ -4191,12 +4191,13 @@ int dev_change_flags(struct net_device *dev, unsigned int flags,
 int dev_set_alias(struct net_device *, const char *, size_t);
 int dev_get_alias(const struct net_device *, char *, size_t);
 int __dev_change_net_namespace(struct net_device *dev, struct net *net,
-			       const char *pat, int new_ifindex);
+			       const char *pat, int new_ifindex,
+			       struct netlink_ext_ack *extack);
 static inline
 int dev_change_net_namespace(struct net_device *dev, struct net *net,
 			     const char *pat)
 {
-	return __dev_change_net_namespace(dev, net, pat, 0);
+	return __dev_change_net_namespace(dev, net, pat, 0, NULL);
 }
 int __dev_set_mtu(struct net_device *, int);
 int dev_set_mtu(struct net_device *, int);

net/core/dev.c

@@ -12131,6 +12131,7 @@ EXPORT_SYMBOL(unregister_netdev);
  *	      is already taken in the destination network namespace.
  *	@new_ifindex: If not zero, specifies device index in the target
  *	              namespace.
+ *	@extack: netlink extended ack
  *
  *	This function shuts down a device interface and moves it
  *	to a new network namespace. On success 0 is returned, on
@@ -12140,7 +12141,8 @@ EXPORT_SYMBOL(unregister_netdev);
  */
 
 int __dev_change_net_namespace(struct net_device *dev, struct net *net,
-			       const char *pat, int new_ifindex)
+			       const char *pat, int new_ifindex,
+			       struct netlink_ext_ack *extack)
 {
 	struct netdev_name_node *name_node;
 	struct net *net_old = dev_net(dev);
@@ -12151,12 +12153,16 @@ int __dev_change_net_namespace(struct net_device *dev, struct net *net,
 
 	/* Don't allow namespace local devices to be moved. */
 	err = -EINVAL;
-	if (dev->netns_immutable)
+	if (dev->netns_immutable) {
+		NL_SET_ERR_MSG(extack, "The interface netns is immutable");
 		goto out;
+	}
 
 	/* Ensure the device has been registered */
-	if (dev->reg_state != NETREG_REGISTERED)
+	if (dev->reg_state != NETREG_REGISTERED) {
+		NL_SET_ERR_MSG(extack, "The interface isn't registered");
 		goto out;
+	}
 
 	/* Get out if there is nothing todo */
 	err = 0;
@@ -12169,30 +12175,49 @@ int __dev_change_net_namespace(struct net_device *dev, struct net *net,
 	err = -EEXIST;
 	if (netdev_name_in_use(net, dev->name)) {
 		/* We get here if we can't use the current device name */
-		if (!pat)
+		if (!pat) {
+			NL_SET_ERR_MSG(extack,
+				       "An interface with the same name exists in the target netns");
 			goto out;
+		}
 		err = dev_prep_valid_name(net, dev, pat, new_name, EEXIST);
-		if (err < 0)
+		if (err < 0) {
+			NL_SET_ERR_MSG_FMT(extack,
+					   "Unable to use '%s' for the new interface name in the target netns",
+					   pat);
 			goto out;
+		}
 	}
 	/* Check that none of the altnames conflicts. */
 	err = -EEXIST;
-	netdev_for_each_altname(dev, name_node)
-		if (netdev_name_in_use(net, name_node->name))
+	netdev_for_each_altname(dev, name_node) {
+		if (netdev_name_in_use(net, name_node->name)) {
+			NL_SET_ERR_MSG_FMT(extack,
+					   "An interface with the altname %s exists in the target netns",
+					   name_node->name);
 			goto out;
+		}
+	}
 
 	/* Check that new_ifindex isn't used yet. */
 	if (new_ifindex) {
 		err = dev_index_reserve(net, new_ifindex);
-		if (err < 0)
+		if (err < 0) {
+			NL_SET_ERR_MSG_FMT(extack,
+					   "The ifindex %d is not available in the target netns",
+					   new_ifindex);
 			goto out;
+		}
 	} else {
 		/* If there is an ifindex conflict assign a new one */
 		err = dev_index_reserve(net, dev->ifindex);
 		if (err == -EBUSY)
 			err = dev_index_reserve(net, 0);
-		if (err < 0)
+		if (err < 0) {
+			NL_SET_ERR_MSG(extack,
+				       "Unable to allocate a new ifindex in the target netns");
 			goto out;
+		}
 		new_ifindex = err;
 	}
 

net/core/rtnetlink.c

@@ -3028,7 +3028,7 @@ static int do_setlink(const struct sk_buff *skb, struct net_device *dev,
 
 		new_ifindex = nla_get_s32_default(tb[IFLA_NEW_IFINDEX], 0);
 
-		err = __dev_change_net_namespace(dev, tgt_net, pat, new_ifindex);
+		err = __dev_change_net_namespace(dev, tgt_net, pat, new_ifindex, extack);
 		if (err)
 			goto errout;