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=-5.1 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, 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 C38CFC2D0F1 for ; Tue, 31 Mar 2020 15:14:48 +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 92EAE20BED for ; Tue, 31 Mar 2020 15:14:48 +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="YUFyrQio" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 92EAE20BED 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]:39970 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1jJIbG-0002Kn-OD for qemu-devel@archiver.kernel.org; Tue, 31 Mar 2020 11:14:47 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:59119) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1jJIZe-0008S8-3k for qemu-devel@nongnu.org; Tue, 31 Mar 2020 11:13:07 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1jJIZc-0002Qq-P4 for qemu-devel@nongnu.org; Tue, 31 Mar 2020 11:13:05 -0400 Received: from us-smtp-1.mimecast.com ([207.211.31.81]:47688 helo=us-smtp-delivery-1.mimecast.com) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1jJIZc-0002QS-Km for qemu-devel@nongnu.org; Tue, 31 Mar 2020 11:13:04 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1585667583; 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=5GKiJBzl4x58r7tlFgM7ELEcO62DVc1ZKolrrOgJdgg=; b=YUFyrQio874qpMG2syzcV63TSEq9CxeEjnU8OVk16GFNNQR+3pvsY9wYhHMBYo1rmiip6n 84k9AAm0rlAn+fBpBTWcQHaCq2fdSvZKS8EpnkEFHDJ8850acbE6C5RjHaQauekZee8mxK fpANzq0md6k8k5TSXw8whK47ezy2Rtw= 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-259-fB_05eJBNe2mxV3X5uAuJA-1; Tue, 31 Mar 2020 11:13:01 -0400 X-MC-Unique: fB_05eJBNe2mxV3X5uAuJA-1 Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.phx2.redhat.com [10.5.11.11]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id C761C1902EAD; Tue, 31 Mar 2020 15:12:59 +0000 (UTC) Received: from [10.3.113.246] (ovpn-113-246.phx2.redhat.com [10.3.113.246]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 1B3678F34F; Tue, 31 Mar 2020 15:12:58 +0000 (UTC) Subject: Re: [PATCH v5] qcow2: Forbid discard in qcow2 v2 images with backing files To: Alberto Garcia , qemu-devel@nongnu.org References: <20200331114345.29993-1-berto@igalia.com> From: Eric Blake Organization: Red Hat, Inc. Message-ID: <3fb921af-06f8-2157-bc1e-cc25fee96319@redhat.com> Date: Tue, 31 Mar 2020 10:12:57 -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: <20200331114345.29993-1-berto@igalia.com> Content-Language: en-US X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 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: 207.211.31.81 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 3/31/20 6:43 AM, 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 > --- > v5: > - Fix iotests 046 and 177 with compat=0.10 [Max] Reviewed-by: Eric Blake -- Eric Blake, Principal Software Engineer Red Hat, Inc. +1-919-301-3226 Virtualization: qemu.org | libvirt.org