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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 88ABDC61DA4 for ; Thu, 2 Feb 2023 21:32:05 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229972AbjBBVcE (ORCPT ); Thu, 2 Feb 2023 16:32:04 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:40002 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229667AbjBBVcD (ORCPT ); Thu, 2 Feb 2023 16:32:03 -0500 Received: from mail.netfilter.org (mail.netfilter.org [217.70.188.207]) by lindbergh.monkeyblade.net (Postfix) with ESMTP id 9E86C2712 for ; Thu, 2 Feb 2023 13:32:02 -0800 (PST) Date: Thu, 2 Feb 2023 22:31:58 +0100 From: Pablo Neira Ayuso To: Phil Sutter , Florian Westphal , netfilter-devel@vger.kernel.org Subject: Re: [nf-next PATCH v2] netfilter: nf_tables: Introduce NFTA_RULE_ACTUAL_EXPR Message-ID: References: <20221221142221.27211-1-phil@nwl.cc> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: netfilter-devel@vger.kernel.org Hi Phil, On Wed, Jan 18, 2023 at 02:48:20PM +0100, Phil Sutter wrote: [...] > The crucial aspect of this implementation is to provide a compatible > rule representation for old software which is not aware of it. This is > only possible by dumping the compat representation in the well-known > NFTA_RULE_EXPRESSIONS attribute. OK, so NFTA_RULE_EXPRESSIONS contains the xt expressions. Then, _ACTUAL_EXPR is taken if kernel supports it and these are expressions that run from datapath, if present. > This means what is contained in NFTA_RULE_EXPRESSIONS may not be what > the kernel actually executes. To make this less scary, the kernel should > dump the actual rule in a second attribute for the sake of verification > in user space. > > While rule dumps are pretty much fixed given the above, there is > flexibility when it comes to loading the rule: > > A) Submit the compat representation as additional attribute > > This was my initial approach, but Florian objected because the changing > content of NFTA_RULE_EXPRESSIONS attribute may be confusing: It is indeed. > On input, NFTA_RULE_EXPRESSIONS contains the new rule representation, on > output it contains the compat one. The extra attribute I introduced > behaves identical, i.e. on input it holds the compat representation > while on output it holds the new one. > > B) Submit the new representation as additional attribute > > This is the current approach: If the additional attribute is present, > the kernel will use it to build the rule and leave NFTA_RULE_EXPRESSIONS > alone (actually: store it for dumps). Otherwise it will "fall back" to > using NFTA_RULE_EXPRESSIONS just as usual. > > When dumping, if a stored NFTA_RULE_EXPRESSIONS content is present, it > will dump that as-is and serialize the active rule into an additional > attribute. Otherwise the active rule will go into NFTA_RULE_EXPRESSIONS > just as usual. So this is not swapping things, right? Probably I am still getting confused but the initial approach described in A. When, dumping back to userspace, NFTA_RULE_EXPRESSIONS still stores the xt compat representation and NFTA_RULE_ACTUAL_EXPRS the one that runs from kernel datapath (if the kernel supports this attribute). [...] > I am swapping things around in libnftnl - it uses NFTA_RULE_ACTUAL_EXPRS > if present and puts NFTA_RULE_EXPRESSIONS into a second list for > verification only. In iptables, I parse both lists separately into > iptables_command_state objects and compare them. If not identical, > there's a bug. Old kernels would simply discard the ACTUAL_ attribute. Maybe _ALT_ standing by alternative is a better name? Sorry, this is a bit confusing but I understand something like this is required as you explained during the NFWS.