From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-3.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id C1766C43457 for ; Wed, 14 Oct 2020 09:46:44 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 6E20C20757 for ; Wed, 14 Oct 2020 09:46:44 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728264AbgJNJqn (ORCPT ); Wed, 14 Oct 2020 05:46:43 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:44208 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728087AbgJNJqn (ORCPT ); Wed, 14 Oct 2020 05:46:43 -0400 Received: from orbyte.nwl.cc (orbyte.nwl.cc [IPv6:2001:41d0:e:133a::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 220CFC0613D2 for ; Wed, 14 Oct 2020 02:46:43 -0700 (PDT) Received: from n0-1 by orbyte.nwl.cc with local (Exim 4.94) (envelope-from ) id 1kSdMm-0006r9-Am; Wed, 14 Oct 2020 11:46:40 +0200 Date: Wed, 14 Oct 2020 11:46:40 +0200 From: Phil Sutter To: Pablo Neira Ayuso Cc: netfilter-devel@vger.kernel.org, Florian Westphal Subject: Re: [iptables PATCH 3/3] nft: Fix for concurrent noflush restore calls Message-ID: <20201014094640.GA13016@orbyte.nwl.cc> Mail-Followup-To: Phil Sutter , Pablo Neira Ayuso , netfilter-devel@vger.kernel.org, Florian Westphal References: <20201005144858.11578-1-phil@nwl.cc> <20201005144858.11578-4-phil@nwl.cc> <20201012125450.GA26934@salvia> <20201013100803.GW13016@orbyte.nwl.cc> <20201013101502.GA29142@salvia> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20201013101502.GA29142@salvia> Sender: Precedence: bulk List-ID: X-Mailing-List: netfilter-devel@vger.kernel.org On Tue, Oct 13, 2020 at 12:15:02PM +0200, Pablo Neira Ayuso wrote: > On Tue, Oct 13, 2020 at 12:08:03PM +0200, Phil Sutter wrote: > [...] > > On Mon, Oct 12, 2020 at 02:54:50PM +0200, Pablo Neira Ayuso wrote: > > [...] > > > Patch LGTM, thanks Phil. > > > > > > What I don't clearly see yet is what scenario is triggering the bug in > > > the existing code, if you don't mind to explain. > > > > See the test case attached to the patch: An other iptables-restore > > process may add references (i.e., jumps) to a chain the own > > iptables-restore process wants to delete. This should not be a problem > > because these references are added to a chain that is being flushed by > > the own process as well. But if that chain doesn't exist while the own > > process fetches kernel's ruleset, this flush job is not created. > > Let me rephrase this: > > 1) process A fetches the ruleset, finds no chain C (no flush job then) > 2) process B adds new chain C, flush job is present > 3) process B adds the ruleset > 4) process A appends rules to the existing chain C (because there is > no flush job) > > Is this the scenario? If so, I wonder why the generation ID is not > helping to refresh and retry. Not quite, let me try to put this more clearly: * Dump A: | *filter | :FOO - [0:0] # flush chain FOO | -X BAR # remove chain BAR | COMMIT * Dump B: | *filter | -A FOO -j BAR # reference BAR from a rule in FOO | COMMIT * Kernel ruleset: | *filter | :BAR - [0:0] | COMMIT * Process A: * read dump A * fetch cache * Process B: * read dump B * fetch ruleset * commit to kernel * Process A: * skip flush chain FOO job: not present * add delete chain BAR job: chain exists * commit fails (genid outdated) * refresh transaction: * delete chain BAR job remains active * genid updated * commit fails: can't remove chain BAR: EBUSY I realize the test case is not quite effective, ruleset should be emptied upon each iteration of concurrent restore job startup. Cheers, Phil