Re: [PATCH net] net: try harder to not reuse ifindex when moving interfaces

[Thomas Graf <tgraf@suug.ch>]

On 10/22/15 at 05:00pm, Jiri Benc wrote:
> On Thu, 22 Oct 2015 16:52:13 +0200, Nicolas Dichtel wrote:
> > With the proposed scenario:
> > 1. create netns 'new_netns'
> > 2. in root netns, move the interface with ifindex 2 to new_netns
> > 3. in new_netns, delete the interface with ifindex 2
> > 4. in new_netns, create an interface - it will get ifindex 2
> > 
> > Operation 2 and 4 are done by dev_change_net_namespace() under rtnl_lock().
> > RTM_DELLINK(root netns) and RTM_NEWLINK(new_netns) are sent by this function.
> > It means that operation 3 has been done before and that RTM_DELLINK(new_netns)
> > has been sent before.
> 
> Imagine the application trying to configure the interface with ifindex 2
> after your step 2. It constructs a netlink message and sends it to the
> kernel; but while doing so, steps 3 and 4 happen. Now the application
> ends up configuring a different interface than it intended to. After
> that, it polls the netlink socket and receives the notifications about
> interface disappearing and a new one appearing.
> 
> I don't see any way the user space application can prevent this. There
> will always be a race between receiving netlink notifications and
> sending config requests.
> 
> I guess Thomas Haller can elaborate more as he ran into this.

I understand the race but when does it occur? Whoever creates
the original interface owns it and is responsible for its
lifecycle. *Iff* for some reason multiple entities manipulate
the interface, then it's probably a lot safer to just use flock
or something similar to serialize access entirely in user space.