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=-3.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS 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 C7B4DC47082 for ; Wed, 26 May 2021 15:02:26 +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 4C9CD613BC for ; Wed, 26 May 2021 15:02:26 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 4C9CD613BC Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=bugs.launchpad.net Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Received: from localhost ([::1]:53768 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1llv3B-0005sE-BT for qemu-devel@archiver.kernel.org; Wed, 26 May 2021 11:02:25 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:47774) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1llv0T-0003mT-Mb for qemu-devel@nongnu.org; Wed, 26 May 2021 10:59:37 -0400 Received: from indium.canonical.com ([91.189.90.7]:49298) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_128_GCM_SHA256:128) (Exim 4.90_1) (envelope-from ) id 1llv0R-0001bR-IU for qemu-devel@nongnu.org; Wed, 26 May 2021 10:59:37 -0400 Received: from loganberry.canonical.com ([91.189.90.37]) by indium.canonical.com with esmtp (Exim 4.93 #5 (Debian)) id 1llv0P-0002n4-5j for ; Wed, 26 May 2021 14:59:33 +0000 Received: from loganberry.canonical.com (localhost [127.0.0.1]) by loganberry.canonical.com (Postfix) with ESMTP id 210712E8186 for ; Wed, 26 May 2021 14:59:33 +0000 (UTC) MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: quoted-printable Date: Wed, 26 May 2021 14:46:57 -0000 From: Thomas Huth <721825@bugs.launchpad.net> To: qemu-devel@nongnu.org X-Launchpad-Notification-Type: bug X-Launchpad-Bug: product=qemu; status=Incomplete; importance=Low; assignee=None; X-Launchpad-Bug-Tags: block vdi X-Launchpad-Bug-Information-Type: Public X-Launchpad-Bug-Private: no X-Launchpad-Bug-Security-Vulnerability: no X-Launchpad-Bug-Commenters: stefanha th-huth X-Launchpad-Bug-Reporter: Stefan Hajnoczi (stefanha) X-Launchpad-Bug-Modifier: Thomas Huth (th-huth) References: <20110219161957.9055.80104.malonedeb@gandwana.canonical.com> Message-Id: <162204041872.21753.4247976072730479104.launchpad@chaenomeles.canonical.com> Subject: [Bug 721825] Re: VDI block driver bugs X-Launchpad-Message-Rationale: Subscriber (QEMU) @qemu-devel-ml X-Launchpad-Message-For: qemu-devel-ml Precedence: bulk X-Generated-By: Launchpad (canonical.com); Revision="802ed26817d1cdd050553dbe99cc8a3cad1a3bc7"; Instance="production" X-Launchpad-Hash: 857aa5bff2d42030ddb935c9c3bfcbb5061fb2bd Received-SPF: none client-ip=91.189.90.7; envelope-from=bounces@canonical.com; helo=indium.canonical.com X-Spam_score_int: -65 X-Spam_score: -6.6 X-Spam_bar: ------ X-Spam_report: (-6.6 / 5.0 requ) BAYES_00=-1.9, HEADER_FROM_DIFFERENT_DOMAINS=0.248, RCVD_IN_DNSWL_HI=-5, RCVD_IN_MSPIKE_H3=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_NONE=0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Reply-To: Bug 721825 <721825@bugs.launchpad.net> Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" ** Changed in: qemu Assignee: Stefan Weil (ubuntu-weilnetz) =3D> (unassigned) -- = You received this bug notification because you are a member of qemu- devel-ml, which is subscribed to QEMU. https://bugs.launchpad.net/bugs/721825 Title: VDI block driver bugs Status in QEMU: Incomplete Bug description: Chunqiang Tang reports the following issues with the VDI block driver, these are present in QEMU 0.14: "Bug 1. The most serious bug is caused by race condition in updating a ne= w = bmap entry in memory and on disk. Considering the following operation = sequence. = O1: VM issues a write to sector X O2: VDI allocates a new bmap entry and updates in-memory s->bmap O3: VDI writes data to disk O4: The disk I/O for writing sector X fails O5: VDI reports error to VM and returns. Note that the bmap entry is updated in memory, but not persisted on disk. = Now consider another write that immediately follows: P1: VM issues a write to sector X+1, which locates in the same block as = the previously used sector X. P2: s->bmap already has one entry for the block, and hence VDI writes = data directly without persisting the new s->bmap entry on disk. P3: The write disk I/O succeeds P4: VDI report success to VM, but the bitmap entry is still not = persisted on disk. Now suppose the VM powers off gracefully (i.e., the QEMU process quits) = and reboots. The second write to sector X+1, which is reported as finishe= d = successfully, is simply lost, because the corresponding in-memory s->bmap = entry is never persisted on disk. This is exactly what FVD's testing tool = discovers. After the block device is closed and then re-opened, disk = content verification fails. This is just one example of the problem. Race condition plus host crash = also causes problems. Consider another example below. Q1: VM issues a write to sector X Q2: VDI allocates a new bmap entry and updates in-memory s->bmap Q3: VDI writes sector X to disk and waits for the callback Q4: VM issues a write to another sector X+1, which is in the same block = as sector X. Q5: VDI sees the bitmap entry in s->bmap is already allocated, and = writes sector X+1 to disk. Q6: Write to sector X+1 finishes, and VDI's callback is invoked. Q7: VDI acknowledges to the VM the completion of writing sector X+1 Q8: After observing the completion of writing sector X+1, VM issues a = flush to ensure that sector X+1 is persisted on disk. Q9: VDI finishes the flush and acknowledge the completion of the = operation. Q10: ... (some other arbitrary operations, but the disk I/O for writing = sector X is still not finished....) Q11: The host crashes Now the new bitmap entry is not persisted on disk, while both writing to = sector X+1 and the flush has been acknowledged as finished. Sector X+1 is = lost, which is a corruption. This problem exists even if it uses O_DSYNC. = The root cause of the problem is that, if a request updates in-memory = s->bmap, another request that sees this update assumes that the update is = already persisted on disk, which is not. Bug 2: Similar to the bugs the FVD testing tool found for QCOW2, there ar= e = several cases of the code below on failure handling path without setting = error return code, which mistakenly reports failure as success. This = mistake is caught by FVD when doing image content validation. if (acb->hd_aiocb =3D=3D NULL) { /* missing ret =3D -EIO; */ goto done; = } = Bug 3: Similar to the bugs the FVD testing tool found for QCOW2, = vdi_aio_cancel does not perform a complete clean up and there are several = related bugs. First, memory buffer is not freed, acb->orig_buf and = acb->block_buffer. Second, acb->bh is not cancelled. Third, = vdi_aio_setup() does not initialize acb->bh to NULL so that when a reques= t = acb is cancelled and then later reused for another request, its acb->bh != =3D = NULL and the new request fails in vdi_schedule_bh(). This is caught by = FVD's testing tool, when it observes that no I/O failure is injected but = VDI reports a failed I/O request, which indicates a bug in the driver." http://permalink.gmane.org/gmane.comp.emulators.qemu/94340 To manage notifications about this bug go to: https://bugs.launchpad.net/qemu/+bug/721825/+subscriptions