Re: [PATCH] Ipvlan should return an error when an address is already in use.

[David Miller <davem@davemloft.net>]

From: Krister Johansen <kjlx@templeofstupid.com>
Date: Fri, 30 Dec 2016 20:10:58 -0800

> The ipvlan code already knows how to detect when a duplicate address is
> about to be assigned to an ipvlan device.  However, that failure is not
> propogated outward and leads to a silent failure.  This teaches the ip
> address addition functions how to report this error to the user
> applications so that a notifier chain failure during ip address addition
> will not appear to succeed when it actually has not.
> 
> This can be especially useful if it is necessary to provision many
> ipvlans in containers.  The provisioning software (or operator) can use
> this to detect situations where an ip address is unexpectedly in use.
> 
> Signed-off-by: Krister Johansen <kjlx@templeofstupid.com>

Your patch isn't handling the case of primary address promotions,
which also issue NETDEV_UP events on these notifier chains.

But on a more basic level, it's extremely important that once you
start using the notifier_{from,to}_errno() handling for a notifier,
you must start doing so for all such cases of that notifier.