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=-5.3 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED 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 CA6A8C43466 for ; Mon, 21 Sep 2020 16:27:03 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 83E89239A1 for ; Mon, 21 Sep 2020 16:27:03 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="F4ZGPedA" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728469AbgIUQ1B (ORCPT ); Mon, 21 Sep 2020 12:27:01 -0400 Received: from us-smtp-2.mimecast.com ([205.139.110.61]:23805 "EHLO us-smtp-delivery-1.mimecast.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1728468AbgIUQ1A (ORCPT ); Mon, 21 Sep 2020 12:27:00 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1600705618; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=Z2+uwvEiNHa4aouNC9wNaWxR76qMrVPlLtc8ZKmzEB4=; b=F4ZGPedA+jEUMlcQ/geC9P6A/AdRJaNWVW54Zd2eNjDwTzYDHiWWSrGNjBtZIPHlERIoMi I+fooT3ExPrDs4CWs3FIcRs/cdWnBVSC1P4DnMGRGSVUHnlP2TY7BOoVhz3dvfIuqK2X74 r/3JF2sOUvT2DyAInbh7trc47m734hg= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-588-xH5kqItFNx2NG482Spac9A-1; Mon, 21 Sep 2020 12:26:51 -0400 X-MC-Unique: xH5kqItFNx2NG482Spac9A-1 Received: from smtp.corp.redhat.com (int-mx04.intmail.prod.int.phx2.redhat.com [10.5.11.14]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id A674C393B5; Mon, 21 Sep 2020 16:26:49 +0000 (UTC) Received: from carbon (unknown [10.36.110.30]) by smtp.corp.redhat.com (Postfix) with ESMTP id 6A8655D9CD; Mon, 21 Sep 2020 16:26:40 +0000 (UTC) Date: Mon, 21 Sep 2020 18:26:38 +0200 From: Jesper Dangaard Brouer To: Daniel Borkmann Cc: Lorenz Bauer , Maciej =?UTF-8?B?xbtlbmN6eWtvd3Nr?= =?UTF-8?B?aQ==?= , Saeed Mahameed , Daniel Borkmann , Alexei Starovoitov , BPF-dev-list , "netdev@vger.kernel.org" , Lorenzo Bianconi , John Fastabend , Jakub Kicinski , Shaun Crampton , David Miller , Marek Majkowski , brouer@redhat.com Subject: Re: BPF redirect API design issue for BPF-prog MTU feedback? Message-ID: <20200921182638.5d8343fd@carbon> In-Reply-To: <340f209d-58d4-52a6-0804-7102d80c1468@iogearbox.net> References: <20200917143846.37ce43a0@carbon> <56ccfc21195b19d5b25559aca4cef5c450d0c402.camel@kernel.org> <20200918120016.7007f437@carbon> <20200921144953.6456d47d@carbon> <340f209d-58d4-52a6-0804-7102d80c1468@iogearbox.net> MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable X-Scanned-By: MIMEDefang 2.79 on 10.5.11.14 Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org On Mon, 21 Sep 2020 17:08:17 +0200 Daniel Borkmann wrote: > On 9/21/20 2:49 PM, Jesper Dangaard Brouer wrote: > > On Mon, 21 Sep 2020 11:37:18 +0100 > > Lorenz Bauer wrote: =20 > >> On Sat, 19 Sep 2020 at 00:06, Maciej =C5=BBenczykowski wrote: =20 > >>> =20 > >>>> This is a good point. As bpf_skb_adjust_room() can just be run after > >>>> bpf_redirect() call, then a MTU check in bpf_redirect() actually > >>>> doesn't make much sense. As clever/bad BPF program can then avoid t= he > >>>> MTU check anyhow. This basically means that we have to do the MTU > >>>> check (again) on kernel side anyhow to catch such clever/bad BPF > >>>> programs. (And I don't like wasting cycles on doing the same check = two > >>>> times). =20 > >>> > >>> If you get rid of the check in bpf_redirect() you might as well get > >>> rid of *all* the checks for excessive mtu in all the helpers that > >>> adjust packet size one way or another way. They *all* then become > >>> useless overhead. > >>> > >>> I don't like that. There may be something the bpf program could do to > >>> react to the error condition (for example in my case, not modify > >>> things and just let the core stack deal with things - which will > >>> probably just generate packet too big icmp error). > >>> > >>> btw. right now our forwarding programs first adjust the packet size > >>> then call bpf_redirect() and almost immediately return what it > >>> returned. > >>> > >>> but this could I think easily be changed to reverse the ordering, so > >>> we wouldn't increase packet size before the core stack was informed we > >>> would be forwarding via a different interface. =20 > >> > >> We do the same, except that we also use XDP_TX when appropriate. This > >> complicates the matter, because there is no helper call we could > >> return an error from. =20 > >=20 > > Do notice that my MTU work is focused on TC-BPF. For XDP-redirect the > > MTU check is done in xdp_ok_fwd_dev() via __xdp_enqueue(), which also > > happens too late to give BPF-prog knowledge/feedback. For XDP_TX I > > audited the drivers when I implemented xdp_buff.frame_sz, and they > > handled (or I added) handling against max HW MTU. E.g. mlx5 [1]. > >=20 > > [1] https://elixir.bootlin.com/linux/v5.9-rc6/source/drivers/net/ethern= et/mellanox/mlx5/core/en/xdp.c#L267 > > =20 > >> My preference would be to have three helpers: get MTU for a device, > >> redirect ctx to a device (with MTU check), resize ctx (without MTU > >> check) but that doesn't work with XDP_TX. Your idea of doing checks > >> in redirect and adjust_room is pragmatic and seems easier to > >> implement. =20 > > =20 > > I do like this plan/proposal (with 3 helpers), but it is not possible > > with current API. The main problem is the current bpf_redirect API > > doesn't provide the ctx, so we cannot do the check in the BPF-helper. > >=20 > > Are you saying we should create a new bpf_redirect API (that incl packe= t ctx)? =20 >=20 > Sorry for jumping in late here... one thing that is not clear to me > is that if we are fully sure that skb is dropped by stack anyway due > to invalid MTU (redirect to ingress does this via dev_forward_skb(), Yes, TC-redirecting to *INGRESS* have a slightly relaxed MTU check via is_skb_forwardable() called via ____dev_forward_skb(). This MTU check seems redundant as netstack will do MTU checks anyhow. > it's not fully clear to me whether it's also the case for the > dev_queue_xmit()), This seems the problematic case; TC-ingress redirect to netdev-egress, that basically calls dev_queue_xmit(). I tried to follow the code all the way into ixgbe driver, and I didn't see any MTU checks. We might have to add a MTU check here, as it could be considered a bug/problematic that we allow this. (e.g. netdev with large MTU can redirect frames larger than MTU of egress netdev). > then why not dropping all the MTU checks aside > from SKB_MAX_ALLOC sanity check for BPF helpers=20 I agree, and think that the MTU checks in the BPF-helpers, make little sense, as we have found ways to circumvent these checks (as discussed in thread). > and have something like a device object (similar to e.g. TCP sockets) > exposed to BPF prog where we can retrieve the object and read > dev->mtu from the prog, so the BPF program could then do the > "exception" handling internally w/o extra prog needed (we also > already expose whether skb is GSO or not). I do think we need some BPF-helper that allows BPF-prog to lookup MTU of a netdev, so it can do proper ICMP exception handling. I looked at doing ICMP exception handling on kernel-side, but realized that this is not possible at the TC-redirect layer, as we have not decoded the L3 protocol at this point (e.g. cannot know if I need to call icmp_send or icmp6_send). --=20 Best regards, Jesper Dangaard Brouer MSc.CS, Principal Kernel Engineer at Red Hat LinkedIn: http://www.linkedin.com/in/brouer