From mboxrd@z Thu Jan 1 00:00:00 1970 From: Thomas Graf Subject: Re: [PATCH net] net: try harder to not reuse ifindex when moving interfaces Date: Thu, 22 Oct 2015 18:45:58 +0200 Message-ID: <20151022164558.GH23554@pox.localdomain> References: <20151021164613.24650836@griffin> <20151021.083214.534622235927401863.davem@davemloft.net> <20151021172502.63220dbb@griffin> <20151021.085635.1582760365341524949.davem@davemloft.net> <1445447578.1265325.416533273.3D5599FD@webmail.messagingengine.com> <5628F81D.1070009@6wind.com> <20151022170100.1e45a8b8@griffin> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Nicolas Dichtel , Hannes Frederic Sowa , David Miller , netdev@vger.kernel.org, thaller@redhat.com To: Jiri Benc Return-path: Received: from mail-wi0-f169.google.com ([209.85.212.169]:37782 "EHLO mail-wi0-f169.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752025AbbJVQqB (ORCPT ); Thu, 22 Oct 2015 12:46:01 -0400 Received: by wicfv8 with SMTP id fv8so128970123wic.0 for ; Thu, 22 Oct 2015 09:45:59 -0700 (PDT) Content-Disposition: inline In-Reply-To: <20151022170100.1e45a8b8@griffin> Sender: netdev-owner@vger.kernel.org List-ID: 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.