[PATCH net v4] failover: allow name change on IFF_UP slave interfaces

[Si-Wei Liu <si-wei.liu@oracle.com>]

When a netdev appears through hot plug then gets enslaved by a failover
master that is already up and running, the slave will be opened
right away after getting enslaved. Today there's a race that userspace
(udev) may fail to rename the slave if the kernel (net_failover)
opens the slave earlier than when the userspace rename happens.
Unlike bond or team, the primary slave of failover can't be renamed by
userspace ahead of time, since the kernel initiated auto-enslavement is
unable to, or rather, is never meant to be synchronized with the rename
request from userspace.

As the failover slave interfaces are not designed to be operated
directly by userspace apps: IP configuration, filter rules with
regard to network traffic passing and etc., should all be done on master
interface. In general, userspace apps only care about the
name of master interface, while slave names are less important as long
as admin users can see reliable names that may carry
other information describing the netdev. For e.g., they can infer that
"ens3nsby" is a standby slave of "ens3", while for a
name like "eth0" they can't tell which master it belongs to.

Historically the name of IFF_UP interface can't be changed because
there might be admin script or management software that is already
relying on such behavior and assumes that the slave name can't be
changed once UP. But failover is special: with the in-kernel
auto-enslavement mechanism, the userspace expectation for device
enumeration and bring-up order is already broken. Previously initramfs
and various userspace config tools were modified to bypass failover
slaves because of auto-enslavement and duplicate MAC address. Similarly,
in case that users care about seeing reliable slave name, the new type
of failover slaves needs to be taken care of specifically in userspace
anyway.

It's less risky to lift up the rename restriction on failover slave
which is already UP. Although it's possible this change may potentially
break userspace component (most likely configuration scripts or
management software) that assumes slave name can't be changed while
UP, it's relatively a limited and controllable set among all userspace
components, which can be fixed specifically to listen for the rename
and/or link down/up events on failover slaves. Userspace component
interacting with slaves is expected to be changed to operate on failover
master interface instead, as the failover slave is dynamic in nature
which may come and go at any point.  The goal is to make the role of
failover slaves less relevant, and userspace components should only
deal with failover master in the long run.

Fixes: 30c8bd5aa8b2 ("net: Introduce generic failover module")
Signed-off-by: Si-Wei Liu <si-wei.liu@oracle.com>
Reviewed-by: Liran Alon <liran.alon@oracle.com>

--
v1 -> v2:
- Drop configurable module parameter (Sridhar)

v2 -> v3:
- Drop additional IFF_SLAVE_RENAME_OK flag (Sridhar)
- Send down and up events around rename (Michael S. Tsirkin)

v3 -> v4:
- Simplify notification to be sent (Stephen Hemminger)
---
 net/core/dev.c | 24 +++++++++++++++++++++++-
 1 file changed, 23 insertions(+), 1 deletion(-)

diff --git a/net/core/dev.c b/net/core/dev.c
index 722d50d..6ae5874 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -1180,7 +1180,21 @@ int dev_change_name(struct net_device *dev, const char *newname)
 	BUG_ON(!dev_net(dev));
 
 	net = dev_net(dev);
-	if (dev->flags & IFF_UP)
+
+	/* Allow failover slave to rename even when
+	 * it is up and running.
+	 *
+	 * Failover slaves are special, since userspace
+	 * might rename the slave after the interface
+	 * has been brought up and running due to
+	 * auto-enslavement.
+	 *
+	 * Failover users don't actually care about slave
+	 * name change, as they are only expected to operate
+	 * on master interface directly.
+	 */
+	if (dev->flags & IFF_UP &&
+	    likely(!(dev->priv_flags & IFF_FAILOVER_SLAVE)))
 		return -EBUSY;
 
 	write_seqcount_begin(&devnet_rename_seq);
@@ -1227,6 +1241,14 @@ int dev_change_name(struct net_device *dev, const char *newname)
 	hlist_add_head_rcu(&dev->name_hlist, dev_name_hash(net, dev->name));
 	write_unlock_bh(&dev_base_lock);
 
+	if (unlikely(dev->flags & IFF_UP)) {
+		struct netdev_notifier_change_info change_info;
+
+		change_info.flags_changed = 0;
+		call_netdevice_notifiers_info(NETDEV_CHANGE, dev,
+					      &change_info.info);
+	}
+
 	ret = call_netdevice_notifiers(NETDEV_CHANGENAME, dev);
 	ret = notifier_to_errno(ret);
 
-- 
1.8.3.1