From mboxrd@z Thu Jan 1 00:00:00 1970 From: Evgeniy Polyakov Subject: Re: [NETLINK] Don't attach callback to a going-away netlink socket Date: Wed, 18 Apr 2007 12:42:07 +0400 Message-ID: <20070418084206.GA4241@2ka.mipt.ru> References: <4625D3D2.9030507@sw.ru> <20070418081707.GA29267@2ka.mipt.ru> <4625D637.2040308@trash.net> Mime-Version: 1.0 Content-Type: text/plain; charset=koi8-r Cc: Pavel Emelianov , David Miller , Linux Netdev List , Andrew Morton , Linux Kernel Mailing List , devel@openvz.org, Kirill Korotaev To: Patrick McHardy Return-path: Received: from relay.2ka.mipt.ru ([194.85.82.65]:58738 "EHLO 2ka.mipt.ru" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1030477AbXDRInA (ORCPT ); Wed, 18 Apr 2007 04:43:00 -0400 Content-Disposition: inline In-Reply-To: <4625D637.2040308@trash.net> Sender: netdev-owner@vger.kernel.org List-Id: netdev.vger.kernel.org On Wed, Apr 18, 2007 at 10:26:31AM +0200, Patrick McHardy (kaber@trash.net) wrote: > Evgeniy Polyakov wrote: > > On Wed, Apr 18, 2007 at 12:16:18PM +0400, Pavel Emelianov (xemul@sw.ru) wrote: > > > >>Sorry, I forgot to put netdev and David in Cc when I first sent it. > >> > >>There is a race between netlink_dump_start() and netlink_release() > >>that can lead to the situation when a netlink socket with non-zero > >>callback is freed. > > > > > > Out of curiosity, why not to fix a netlink_dump_start() to remove > > callback in error path, since in 'no-error' path it removes it in > > netlink_dump(). > > > It already does (netlink_destroy_callback), but that doesn't help > with this race though since without this patch we don't enter the > error path. I thought that with releasing a socket, which will have a callback attached only results in a leak of the callback? In that case we can just free it in dump() just like it is done in no-error path already. Or do I miss something additional? > > And, btw, can release method be called while socket is being used, I > > thought about proper reference counters should prevent this, but not > > 100% sure with RCU dereferencing of the descriptor. > > > The problem is asynchronous processing of the dump request in the > context of a different process. Process requests a dump, message > is queued and process returns from sendmsg since some other process > is already processing the queue. Then the process closes the socket, > resulting in netlink_release being called. When the dump request > is finally processed the race Pavel described might happen. This > can only happen for netlink families that use mutex_try_lock for > queue processing of course. Doesn't it called from ->sk_data_ready() which is synchronous with respect to sendmsg, not sure about conntrack though, but it looks so? -- Evgeniy Polyakov