Subject: Re: [BUG] rtnl_newlink: Rogue MOVE event delivered on netns change

From: Jakub Kicinski <[email-masked]>
Date: Mon, 16 Oct 2023 07:32:51 -0700

On Sat, 14 Oct 2023 10:58:20 +0200 Greg Kroah-Hartman wrote:
> On Fri, Oct 13, 2023 at 03:43:02PM -0700, Jakub Kicinski wrote:
> > On Fri, 13 Oct 2023 15:36:05 -0700 Jakub Kicinski wrote:  
> > >    kobject_uevent(&dev->dev.kobj, KOBJ_REMOVE);
> > >    dev_net_set(dev, net);
> > >    kobject_uevent(&dev->dev.kobj, KOBJ_ADD);  
> > 
> > Greg, we seem to have a problem in networking with combined
> > netns move and name change.
> > 
> > We have this code in __dev_change_net_namespace():
> > 
> > 	kobject_uevent(&dev->dev.kobj, KOBJ_REMOVE);
> > 	dev_net_set(dev, net);
> > 	kobject_uevent(&dev->dev.kobj, KOBJ_ADD);
> > 
> > 	err = device_rename(&dev->dev, dev->name);
> > 
> > Is there any way we can only get the REMOVE (old name) and ADD
> > (new name) events, without the move? I.e. silence the rename? 
> > 
> > Daniel is reporting that with current code target netns sees an 
> > add of an interface with the old (duplicated) name. And then a rename.  
> 
> But that's how this has always been, right?  What problems is this
> causing?

Original report is up-thread:
https://lore.kernel.org/all/[email-masked]/
With a link to a GH issue for lxc:
https://github.com/lxc/incus/issues/146

> > Without a silent move best we can do is probably:
> > 
> > 	kobject_uevent(&dev->dev.kobj, KOBJ_REMOVE);
> > 	dev_net_set(dev, net);
> > 	err = device_rename(&dev->dev, dev->name);
> > 	kobject_uevent(&dev->dev.kobj, KOBJ_ADD);
> > 
> > which will give us:
> > 
> > 	MOVE new-name
> > 	ADD new-name
> > 
> > in target netns, which, hm.  
> 
> That wouldn't make much sense.
> 
> What is the real problem here?  What changed to cause a problem?

IIUC what happens is:

 - systemd controls "real" eth0
 - we move a "to be renamed" eth0 from a container into main ns
 - we rename "to be renamed" eth0 to something else
 - seeing the rename of eth0 system thinks it's the "real" one
   that is being renamed, ergo there's no eth0 any more,
   so it shuts down its "unit" for eth0

I don't think anything changed. Sounds more like someone finally tried
to use this in anger.