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=-7.2 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 67D45C433F5 for ; Tue, 21 Sep 2021 18:23:04 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 4A3F661211 for ; Tue, 21 Sep 2021 18:23:04 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232752AbhIUSYc (ORCPT ); Tue, 21 Sep 2021 14:24:32 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]:52509 "EHLO us-smtp-delivery-124.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232729AbhIUSYa (ORCPT ); Tue, 21 Sep 2021 14:24:30 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1632248581; 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=fCioJD4GXQ8rL7BZ9hZH89l38r2D+NX3fPR72Eu5Xs4=; b=iNghnsdLyIUMOYVls5OwNsyn7eWPJuG9nbeNxvY6eVh39O3+rKXaQyyjqMkvm+HtO6wLi6 0jj4dNu/WpdM4Mx8y3sYTOovBzjqRSHT9KWxa6rTtI+JV56ijyKNDfoj9sYlTHzGUlhydM 74S8STpH/eumctgQYqh6+S0I+yXBFJM= Received: from mail-ed1-f69.google.com (mail-ed1-f69.google.com [209.85.208.69]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-236-acAhHqi4NYqxXJwBgrFvmA-1; Tue, 21 Sep 2021 14:22:59 -0400 X-MC-Unique: acAhHqi4NYqxXJwBgrFvmA-1 Received: by mail-ed1-f69.google.com with SMTP id h24-20020a50cdd8000000b003d8005fe2f8so15845051edj.6 for ; Tue, 21 Sep 2021 11:22:59 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:from:to:cc:subject:in-reply-to:references:date :message-id:mime-version:content-transfer-encoding; bh=fCioJD4GXQ8rL7BZ9hZH89l38r2D+NX3fPR72Eu5Xs4=; b=i8SlYHWzXnMoJccjMLqTGlXFbnw3L2On4Z1ZV2o0KFz+PAV40DkVzM6FYb+Y9iX5qu 3URxvQKRpYtlVNnU8gQawMEDXMVY2kW6nDAvYhG+Wca0mwb4TQAZV2tOhgSeY7Oaj7q7 sy8JTj9eCvMMxjDtRx2P6zdVkvuhnm1C6vdpnf4rSTwSxUdNhyJZhnIphg7x0HCXCXXa 16zU9wjh5S7VYR0LVFVqKEqLbZl5Pc3QkgO+KkLSQ91Tk/vQxVob44H9L51pd+C5/ZN+ JUEHvQl4YMmP4/nNPSStgKI3cD9albIDL8YnY3qA+7O12czDhw6JdOP6YsNLRnwrjdck U2/Q== X-Gm-Message-State: AOAM5321jW2D2+F19G/3AFpb1eWzgK0diOS2sK2NdexPhfesC9gpEjQS MQHarHvy75MWYMLOpJgmyA5EE/RlqCDoXlhDwecVwPBKNDYfv+plwLDH/yeAzHk1hE+lyr3AHK6 E5+3GC572hxqjuhin X-Received: by 2002:a05:6402:c8b:: with SMTP id cm11mr36945301edb.368.1632248577187; Tue, 21 Sep 2021 11:22:57 -0700 (PDT) X-Google-Smtp-Source: ABdhPJxWZnPEEIEkqbkx5GE9LEeskOMg0msXbBPJpqTSqI4kp6TMb1xb7FdR2kHCNqXGcCixoh2+nw== X-Received: by 2002:a05:6402:c8b:: with SMTP id cm11mr36945152edb.368.1632248575740; Tue, 21 Sep 2021 11:22:55 -0700 (PDT) Received: from alrua-x1.borgediget.toke.dk ([2a0c:4d80:42:443::2]) by smtp.gmail.com with ESMTPSA id c17sm8760153edu.11.2021.09.21.11.22.55 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 21 Sep 2021 11:22:55 -0700 (PDT) Received: by alrua-x1.borgediget.toke.dk (Postfix, from userid 1000) id 87F3118034A; Tue, 21 Sep 2021 20:22:53 +0200 (CEST) From: Toke =?utf-8?Q?H=C3=B8iland-J=C3=B8rgensen?= To: Zvi Effron Cc: Lorenz Bauer , Lorenzo Bianconi , Daniel Borkmann , John Fastabend , netdev@vger.kernel.org, bpf Subject: Re: Redux: Backwards compatibility for XDP multi-buff In-Reply-To: References: <87o88l3oc4.fsf@toke.dk> X-Clacks-Overhead: GNU Terry Pratchett Date: Tue, 21 Sep 2021 20:22:53 +0200 Message-ID: <87ilyt3i0y.fsf@toke.dk> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org Zvi Effron writes: > On Tue, Sep 21, 2021 at 9:06 AM Toke H=C3=B8iland-J=C3=B8rgensen wrote: >> >> Hi Lorenz (Cc. the other people who participated in today's discussion) >> >> Following our discussion at the LPC session today, I dug up my previous >> summary of the issue and some possible solutions[0]. Seems no on >> actually replied last time, which is why we went with the "do nothing" >> approach, I suppose. I'm including the full text of the original email >> below; please take a look, and let's see if we can converge on a >> consensus here. >> >> First off, a problem description: If an existing XDP program is exposed >> to an xdp_buff that is really a multi-buffer, while it will continue to >> run, it may end up with subtle and hard-to-debug bugs: If it's parsing >> the packet it'll only see part of the payload and not be aware of that >> fact, and if it's calculating the packet length, that will also only be >> wrong (only counting the first fragment). >> >> So what to do about this? First of all, to do anything about it, XDP >> programs need to be able to declare themselves "multi-buffer aware" (but >> see point 1 below). We could try to auto-detect it in the verifier by >> which helpers the program is using, but since existing programs could be >> perfectly happy to just keep running, it probably needs to be something >> the program communicates explicitly. One option is to use the >> expected_attach_type to encode this; programs can then declare it in the >> source by section name, or the userspace loader can set the type for >> existing programs if needed. >> >> With this, the kernel will know if a given XDP program is multi-buff >> aware and can decide what to do with that information. For this we came >> up with basically three options: >> >> 1. Do nothing. This would make it up to users / sysadmins to avoid >> anything breaking by manually making sure to not enable multi-buffer >> support while loading any XDP programs that will malfunction if >> presented with an mb frame. This will probably break in interesting >> ways, but it's nice and simple from an implementation PoV. With this >> we don't need the declaration discussed above either. >> >> 2. Add a check at runtime and drop the frames if they are mb-enabled and >> the program doesn't understand it. This is relatively simple to >> implement, but it also makes for difficult-to-understand issues (why >> are my packets suddenly being dropped?), and it will incur runtime >> overhead. >> >> 3. Reject loading of programs that are not MB-aware when running in an >> MB-enabled mode. This would make things break in more obvious ways, >> and still allow a userspace loader to declare a program "MB-aware" to >> force it to run if necessary. The problem then becomes at what level >> to block this? >> > > I think there's another potential problem with this as well: what happens= to > already loaded programs that are not MB-aware? Are they forcibly unloaded? I'd say probably the opposite: You can't toggle whatever switch we end up with if there are any non-MB-aware programs (you'd have to unload them first)... >> Doing this at the driver level is not enough: while a particular >> driver knows if it's running in multi-buff mode, we can't know for >> sure if a particular XDP program is multi-buff aware at attach time: >> it could be tail-calling other programs, or redirecting packets to >> another interface where it will be processed by a non-MB aware >> program. >> >> So another option is to make it a global toggle: e.g., create a new >> sysctl to enable multi-buffer. If this is set, reject loading any XDP >> program that doesn't support multi-buffer mode, and if it's unset, >> disable multi-buffer mode in all drivers. This will make it explicit >> when the multi-buffer mode is used, and prevent any accidental subtle >> malfunction of existing XDP programs. The drawback is that it's a >> mode switch, so more configuration complexity. >> > > Could we combine the last two bits here into a global toggle that doesn't > require a sysctl? If any driver is put into multi-buffer mode, then the s= ystem > switches to requiring all programs be multi-buffer? When the last multi-b= uffer > enabled driver switches out of multi-buffer, remove the system-wide > restriction? Well, the trouble here is that we don't necessarily have an explicit "multi-buf mode" for devices. For instance, you could raise the MTU of a device without it necessarily involving any XDP multi-buffer stuff (if you're not running XDP on that device). So if we did turn "raising the MTU" into such a mode switch, we would end up blocking any MTU changes if any XDP programs are loaded. Or having an MTU change cause a force-unload of all XDP programs. Neither of those are desirable outcomes, I think; and if we add a separate "XDP multi-buff" switch, we might as well make it system-wide? > Regarding my above question, if non-MB-aware XDP programs are not forcibly > unloaded, then a global toggle is also insufficient. An existing non-MB-a= ware > XDP program would still beed to be rejected at attach time by the > driver. See above. -Toke