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=-5.5 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE, SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_1 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 BCAE1C433E0 for ; Mon, 8 Feb 2021 15:44:02 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 7C62164E6D for ; Mon, 8 Feb 2021 15:44:02 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232174AbhBHPni (ORCPT ); Mon, 8 Feb 2021 10:43:38 -0500 Received: from www62.your-server.de ([213.133.104.62]:55680 "EHLO www62.your-server.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233804AbhBHPmO (ORCPT ); Mon, 8 Feb 2021 10:42:14 -0500 Received: from sslproxy01.your-server.de ([78.46.139.224]) by www62.your-server.de with esmtpsa (TLSv1.3:TLS_AES_256_GCM_SHA384:256) (Exim 4.92.3) (envelope-from ) id 1l98fF-000E8Z-Uo; Mon, 08 Feb 2021 16:41:26 +0100 Received: from [85.7.101.30] (helo=pc-9.home) by sslproxy01.your-server.de with esmtpsa (TLSv1.3:TLS_AES_256_GCM_SHA384:256) (Exim 4.92) (envelope-from ) id 1l98fF-000P5I-Kx; Mon, 08 Feb 2021 16:41:25 +0100 Subject: Re: [PATCH bpf-next V15 2/7] bpf: fix bpf_fib_lookup helper MTU check for SKB ctx To: Jesper Dangaard Brouer Cc: bpf@vger.kernel.org, netdev@vger.kernel.org, Daniel Borkmann , Alexei Starovoitov , maze@google.com, lmb@cloudflare.com, shaun@tigera.io, Lorenzo Bianconi , marek@cloudflare.com, John Fastabend , Jakub Kicinski , eyal.birger@gmail.com, colrack@gmail.com, David Ahern References: <161228314075.576669.15427172810948915572.stgit@firesoul> <161228321177.576669.11521750082473556168.stgit@firesoul> <20210208145713.4ee3e9ba@carbon> <20210208162056.44b0236e@carbon> From: Daniel Borkmann Message-ID: <547131a3-5125-d419-8e61-0fc675d663a8@iogearbox.net> Date: Mon, 8 Feb 2021 16:41:24 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.7.2 MIME-Version: 1.0 In-Reply-To: <20210208162056.44b0236e@carbon> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit X-Authenticated-Sender: daniel@iogearbox.net X-Virus-Scanned: Clear (ClamAV 0.102.4/26074/Mon Feb 8 13:20:40 2021) Precedence: bulk List-ID: X-Mailing-List: bpf@vger.kernel.org On 2/8/21 4:20 PM, Jesper Dangaard Brouer wrote: > On Mon, 8 Feb 2021 14:57:13 +0100 > Jesper Dangaard Brouer wrote: >> On Fri, 5 Feb 2021 01:06:35 +0100 >> Daniel Borkmann wrote: >>> On 2/2/21 5:26 PM, Jesper Dangaard Brouer wrote: >>>> BPF end-user on Cilium slack-channel (Carlo Carraro) wants to use >>>> bpf_fib_lookup for doing MTU-check, but *prior* to extending packet size, >>>> by adjusting fib_params 'tot_len' with the packet length plus the expected >>>> encap size. (Just like the bpf_check_mtu helper supports). He discovered >>>> that for SKB ctx the param->tot_len was not used, instead skb->len was used >>>> (via MTU check in is_skb_forwardable() that checks against netdev MTU). >>>> >>>> Fix this by using fib_params 'tot_len' for MTU check. If not provided (e.g. >>>> zero) then keep existing TC behaviour intact. Notice that 'tot_len' for MTU >>>> check is done like XDP code-path, which checks against FIB-dst MTU. [...] >>>> - if (!rc) { >>>> - struct net_device *dev; >>>> - >>>> - dev = dev_get_by_index_rcu(net, params->ifindex); >>>> + if (rc == BPF_FIB_LKUP_RET_SUCCESS && !check_mtu) { >>>> + /* When tot_len isn't provided by user, >>>> + * check skb against net_device MTU >>>> + */ >>>> if (!is_skb_forwardable(dev, skb)) >>>> rc = BPF_FIB_LKUP_RET_FRAG_NEEDED; >>> >>> ... so using old cached dev from above will result in wrong MTU check & >>> subsequent passing of wrong params->mtu_result = dev->mtu this way. >> >> Yes, you are right, params->ifindex have a chance to change in the calls. >> So, our attempt to save an ifindex lookup (dev_get_by_index_rcu) is not >> correct. >> >>> So one >>> way to fix is that we would need to pass &dev to bpf_ipv{4,6}_fib_lookup(). >> >> Ok, I will try to code it up, and see how ugly it looks, but I'm no >> longer sure that it is worth saving this ifindex lookup, as it will >> only happen if BPF-prog didn't specify params->tot_len. > > I guess we can still do this as an "optimization", but the dev/ifindex > will very likely be another at this point. I would say for sake of progress, lets ship your series w/o this optimization so it can land, and we revisit this later on independent from here. Actually DavidA back then acked the old poc patch I posted, so maybe that's worth a revisit as well but needs more testing first. Thanks, Daniel