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=-5.0 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE, SPF_PASS,URIBL_BLOCKED,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 6ECE0C43331 for ; Fri, 27 Mar 2020 19:20:15 +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 3D77B20663 for ; Fri, 27 Mar 2020 19:20:15 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="d8024E9p" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 3D77B20663 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]:45966 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1jHuWc-0003vq-FB for qemu-devel@archiver.kernel.org; Fri, 27 Mar 2020 15:20:14 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:50644) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1jHuW2-0003Ig-QX for qemu-devel@nongnu.org; Fri, 27 Mar 2020 15:19:39 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1jHuW1-0004GB-Bj for qemu-devel@nongnu.org; Fri, 27 Mar 2020 15:19:38 -0400 Received: from us-smtp-delivery-74.mimecast.com ([216.205.24.74]:27686) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1jHuW1-0004Ep-7e for qemu-devel@nongnu.org; Fri, 27 Mar 2020 15:19:37 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1585336776; 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=a5hYChqO8eGC5sfVCJjjy436q22PU8/2zpkmxNP5mhE=; b=d8024E9p0EccAln+CTIEI1IQrITUoeb8jQRv1Z0fvqRJTFH71TEgFBQ5DVDdDqicS0ETlT aOi3G44Gj7pI06HqnAf9l3IXTuCRL4Fv72d3Dv90MWMwUKltX6NQHII91nP1LujgGJAA/C BhTJKGj39APJ2yq6kPpHhiBxvXRMZJM= 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-115-HyPp0aNpMrmnN4TlVaFMSQ-1; Fri, 27 Mar 2020 15:19:32 -0400 X-MC-Unique: HyPp0aNpMrmnN4TlVaFMSQ-1 Received: from smtp.corp.redhat.com (int-mx02.intmail.prod.int.phx2.redhat.com [10.5.11.12]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 40BE98017CC; Fri, 27 Mar 2020 19:19:31 +0000 (UTC) Received: from [10.3.113.103] (ovpn-113-103.phx2.redhat.com [10.3.113.103]) by smtp.corp.redhat.com (Postfix) with ESMTPS id A524960BF3; Fri, 27 Mar 2020 19:19:30 +0000 (UTC) Subject: Re: [PATCH v4] qcow2: Forbid discard in qcow2 v2 images with backing files To: Alberto Garcia , qemu-devel@nongnu.org References: <20200327185930.19493-1-berto@igalia.com> From: Eric Blake Organization: Red Hat, Inc. Message-ID: Date: Fri, 27 Mar 2020 14:19:29 -0500 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.6.0 MIME-Version: 1.0 In-Reply-To: <20200327185930.19493-1-berto@igalia.com> Content-Language: en-US X-Scanned-By: MIMEDefang 2.79 on 10.5.11.12 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] [fuzzy] X-Received-From: 216.205.24.74 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: Kevin Wolf , =?UTF-8?Q?Daniel_P_=2e_Berrang=c3=a9?= , qemu-block@nongnu.org, Max Reitz Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 3/27/20 1:59 PM, Alberto Garcia wrote: > A discard request deallocates the selected clusters so they read back > as zeroes. This is done by clearing the cluster offset field and > setting QCOW_OFLAG_ZERO in the L2 entry. > > This flag is however only supported when qcow_version >= 3. In older > images the cluster is simply deallocated, exposing any possible stale > data from the backing file. > > Since discard is an advisory operation it's safer to simply forbid it > in this scenario. > > Note that we are adding this check to qcow2_co_pdiscard() and not to > qcow2_cluster_discard() or discard_in_l2_slice() because the last > two are also used by qcow2_snapshot_create() to discard the clusters > used by the VM state. In this case there's no risk of exposing stale > data to the guest and we really want that the clusters are always > discarded. > > Signed-off-by: Alberto Garcia > --- > v4: > - Show output of qemu-img map when there's no backing file [Eric] Reviewed-by: Eric Blake -- Eric Blake, Principal Software Engineer Red Hat, Inc. +1-919-301-3226 Virtualization: qemu.org | libvirt.org