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=-5.4 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=ham 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 D2B23C10F27 for ; Mon, 9 Mar 2020 16:05:14 +0000 (UTC) Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 9BA2F208C3 for ; Mon, 9 Mar 2020 16:05:14 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="hcmhozFO" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 9BA2F208C3 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Received: from localhost ([::1]:46070 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1jBKu1-0007w7-OQ for qemu-devel@archiver.kernel.org; Mon, 09 Mar 2020 12:05:13 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:41403) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1jBKfW-0001Ul-6Q for qemu-devel@nongnu.org; Mon, 09 Mar 2020 11:50:15 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1jBKfU-0004mw-Ou for qemu-devel@nongnu.org; Mon, 09 Mar 2020 11:50:14 -0400 Received: from us-smtp-delivery-1.mimecast.com ([207.211.31.120]:59034 helo=us-smtp-1.mimecast.com) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1jBKfU-0004mh-KI for qemu-devel@nongnu.org; Mon, 09 Mar 2020 11:50:12 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1583769012; 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=vvvQ/1gvSSy3w9iEGsDplJQWxDpJfkvOXWkqxfR7ks8=; b=hcmhozFOShYCRXirkmm4npIpaW9enyaF66RYDA3vADfxNeGOndHePxWL641aUZujfni9nI 7RBm3j1zgUY0ECaKb5r3kKzoNQvROOU640G+KIAbSgRgvUpRyP0vY5cOmsqKIz2BeLNZmv UIRZZTTdIpWWviioYR07xdUwVAoiqu4= 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-253--u-dNmUzPjGYkZ28cFuQjg-1; Mon, 09 Mar 2020 11:50:08 -0400 X-MC-Unique: -u-dNmUzPjGYkZ28cFuQjg-1 Received: from smtp.corp.redhat.com (int-mx08.intmail.prod.int.phx2.redhat.com [10.5.11.23]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id CA37780256C; Mon, 9 Mar 2020 15:50:06 +0000 (UTC) Received: from [10.3.116.177] (ovpn-116-177.phx2.redhat.com [10.3.116.177]) by smtp.corp.redhat.com (Postfix) with ESMTPS id DDBF5272D1; Mon, 9 Mar 2020 15:50:02 +0000 (UTC) Subject: Re: [PATCH v3 1/4] block: Add trivial backing_fmt support to qcow, sheepdog, vmdk To: =?UTF-8?Q?Daniel_P=2e_Berrang=c3=a9?= , Kevin Wolf References: <20200306225121.3199279-1-eblake@redhat.com> <20200306225121.3199279-2-eblake@redhat.com> <20200309152112.GC6478@linux.fritz.box> <20200309153624.GK3033513@redhat.com> From: Eric Blake Organization: Red Hat, Inc. Message-ID: <04b776d3-8c35-d00a-5ff0-d55b76cf4969@redhat.com> Date: Mon, 9 Mar 2020 10:50:02 -0500 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.5.0 MIME-Version: 1.0 In-Reply-To: <20200309153624.GK3033513@redhat.com> Content-Language: en-US X-Scanned-By: MIMEDefang 2.84 on 10.5.11.23 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: quoted-printable X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] [fuzzy] X-Received-From: 207.211.31.120 X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Fam Zheng , pkrempa@redhat.com, "open list:Sheepdog" , qemu-block@nongnu.org, libvir-list@redhat.com, Michael Tokarev , qemu-devel@nongnu.org, mreitz@redhat.com, "open list:Trivial patches" , Liu Yuan , Laurent Vivier Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 3/9/20 10:36 AM, Daniel P. Berrang=C3=A9 wrote: > On Mon, Mar 09, 2020 at 04:21:12PM +0100, Kevin Wolf wrote: >> Am 06.03.2020 um 23:51 hat Eric Blake geschrieben: >>> For qcow2 and qed, we want to encourage the use of -F always, as these >>> formats can suffer from data corruption or security holes if backing >>> format is probed. But for other formats, the backing format cannot be >>> recorded. Making the user decide on a per-format basis whether to >>> supply a backing format string is awkward, better is to just blindly >>> accept a backing format argument even if it is ignored by the >>> contraints of the format at hand. >>> >>> Signed-off-by: Eric Blake >> >> I'm not sure if I agree with this reasoning. Accepting and silently >> ignoring -F could give users a false sense of security. If I specify a >> -F raw and QEMU later probes qcow2, that would be very surprising. >=20 > And if the user specifies "-F raw" and we probe qcow2, and the user > does not realize this, they can become silently reliant on always > probing qcow2. If we then honour the "-F raw" option in a later > QEMU release, we'll break the behaviour they've relied on. >=20 > IMHO, we must not accept "-F fmt" unless we're in a position to > honour it. So I'm thinking: qemu-img create -f qcow -b backing.qcow -F qcow img.qcow =3D> okay qemu-img create -f qcow -b backing.raw -F raw img.qcow =3D> okay,=20 slightly risky (if backing.raw is ever changed to be non-raw), but then=20 again, backing files tend to be read-only (do we even support commit on=20 qcow images, or do we limit that to qcow2?) qemu-img create -f qcow -b backing.qcow -F raw img.qcow =3D> fails, due= =20 to mismatch qemu-img create -u -f qcow -b anything -F anything img.qcow $size =3D>=20 fails: we can't write -F into the image, nor can we open anything to=20 probe its type to check that -F was correct qemu-img create -f qcow -b backing.qcow img.qcow =3D> warns, but okay=20 (we did not get -F, but the probe works out) qemu-img create -f qcow -b backing.raw img.qcow =3D> likewise warns qemu-img create -f qcow -b backing.qcow2 img.qcow =3D> error; new qcow=20 images (which you should avoid where possible anyways) must be backed by=20 only raw or qcow, going forward Other scenarios? Do the above ideas look reasonable? --=20 Eric Blake, Principal Software Engineer Red Hat, Inc. +1-919-301-3226 Virtualization: qemu.org | libvirt.org