[PATCH net-next v3 2/2] net: plumb extack in __dev_change_net_namespace()

[Nicolas Dichtel <nicolas.dichtel@6wind.com>]

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

Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
---
 include/linux/netdevice.h |  5 +++--
 net/core/dev.c            | 41 +++++++++++++++++++++++++++++++--------
 net/core/rtnetlink.c      |  2 +-
 3 files changed, 37 insertions(+), 11 deletions(-)

diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index fccc03cd2164..58d9f052f154 100644
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h
@@ -4155,12 +4155,13 @@ 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);
diff --git a/net/core/dev.c b/net/core/dev.c
index ebc000b56828..9605fa2e7415 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -11917,6 +11917,7 @@ 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
@@ -11926,7 +11927,8 @@ 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);
@@ -11937,12 +11939,17 @@ 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_local)
+	if (dev->netns_local) {
+		NL_SET_ERR_MSG(extack,
+			       "The interface has the 'netns local' property");
 		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;
@@ -11955,30 +11962,48 @@ 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",
+					   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))
+		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;
 	}
 
diff --git a/net/core/rtnetlink.c b/net/core/rtnetlink.c
index acf787e4d22d..717f2e3e333e 100644
--- a/net/core/rtnetlink.c
+++ b/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;
 
-- 
2.47.1