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,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 6101BC43603 for ; Fri, 6 Dec 2019 15:50:45 +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 23D5C20659 for ; Fri, 6 Dec 2019 15:50:45 +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="QHP3D0aa" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 23D5C20659 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]:40106 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1idFsR-0005bC-Hz for qemu-devel@archiver.kernel.org; Fri, 06 Dec 2019 10:50:43 -0500 Received: from eggs.gnu.org ([2001:470:142:3::10]:38390) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1idEin-0006gz-8v for qemu-devel@nongnu.org; Fri, 06 Dec 2019 09:36:42 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1idEik-0006ji-W0 for qemu-devel@nongnu.org; Fri, 06 Dec 2019 09:36:40 -0500 Received: from us-smtp-delivery-1.mimecast.com ([205.139.110.120]:55606 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 1idEik-0006iK-Ql for qemu-devel@nongnu.org; Fri, 06 Dec 2019 09:36:38 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1575642998; 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=OlKgljMsmEapL+eFMQYTxahgK2MUl8C8CXSYTjo/bp0=; b=QHP3D0aa0GduXvAXNeLwK2xN4Mau7uQo8JhUPz1MN1OjfFBRDVMxgJ/epn/a/Ohmur83j4 0G4lQN2le1Trbk1UCcM8Ex/7YZ7nDnyTIDFD+26zy1yh3EW0enDZfdQHoGTuHfPC+qgukh z7WeEKg+MCKVw7LrL6NiXa9sd4JMTAc= 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-166-l6rSiumjOlCIwKbcLCdphA-1; Fri, 06 Dec 2019 09:36:37 -0500 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 BC5731856A9A; Fri, 6 Dec 2019 14:36:35 +0000 (UTC) Received: from [10.3.116.171] (ovpn-116-171.phx2.redhat.com [10.3.116.171]) by smtp.corp.redhat.com (Postfix) with ESMTPS id CCA1260BF4; Fri, 6 Dec 2019 14:36:29 +0000 (UTC) Subject: Re: [PATCH for-4.2?] block/qcow2-bitmap: fix crash bug in qcow2_co_remove_persistent_dirty_bitmap To: Vladimir Sementsov-Ogievskiy , John Snow , "qemu-block@nongnu.org" References: <20191205193049.30666-1-vsementsov@virtuozzo.com> <22af83d1-91e2-ab71-2dc4-349b5b24d1d8@redhat.com> <990da2e0-8223-b257-254d-a27659ef1d24@redhat.com> <6a74ae0a-e377-0f70-c347-0307580d7981@virtuozzo.com> From: Eric Blake Organization: Red Hat, Inc. Message-ID: <36655f7a-7ea5-e80a-ebfd-5b19c90622c0@redhat.com> Date: Fri, 6 Dec 2019 08:36:29 -0600 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.2.2 MIME-Version: 1.0 In-Reply-To: <6a74ae0a-e377-0f70-c347-0307580d7981@virtuozzo.com> Content-Language: en-US X-Scanned-By: MIMEDefang 2.79 on 10.5.11.12 X-MC-Unique: l6rSiumjOlCIwKbcLCdphA-1 X-Mimecast-Spam-Score: 0 Content-Type: text/plain; charset=WINDOWS-1252; 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: 205.139.110.120 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: "kwolf@redhat.com" , Peter Maydell , Peter Krempa , "libvirt-list@redhat.com" , "qemu-devel@nongnu.org" , "mreitz@redhat.com" Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" [adding in Peter Maydell, as there is now potential talk of other=20 4.2-worthy patches] On 12/6/19 4:18 AM, Vladimir Sementsov-Ogievskiy wrote: > 05.12.2019 23:16, John Snow wrote: >> >> >> On 12/5/19 3:09 PM, Eric Blake wrote: >>> On 12/5/19 1:30 PM, Vladimir Sementsov-Ogievskiy wrote: >>>> Here is double bug: >>>> >>>> First, return error but not set errp. This may lead to: >>>> qmp block-dirty-bitmap-remove may report success when actually failed >>>> >>>> block-dirty-bitmap-remove used in a transaction will crash, as >>>> qmp_transaction will think that it returned success and will cal >>> >>> call >>> >>>> block_dirty_bitmap_remove_commit which will crash, as state->bitmap is >>>> NULL >>>> >>>> Second (like in anecdote), this case is not an error at all. As it is >>>> documented in the comment above bdrv_co_remove_persistent_dirty_bitmap >>>> definition, absence of bitmap is not an error, and similar case handle= d >>>> at start of qcow2_co_remove_persistent_dirty_bitmap, it returns 0 when >>>> there is no bitmaps at all.. >>> >>> double . >>> >>>> >>>> But when there are some bitmaps, but not the requested one, it return >>>> error with errp unset. >>>> >>>> Fix that. >>>> >>>> Fixes: b56a1e31759b750 >>>> Signed-off-by: Vladimir Sementsov-Ogievskiy >>>> --- >>>> >>>> Hi all! >>>> >>>> Ohm, suddenly we faced this bug. It's a regression in 4.2. I'm very >>>> sorry for introducing it, and it sad that it's found so late.. >>>> >>>> Personally, I think that this one worth rc5, as it makes new bitmap >>>> interfaces unusable. But the decision is yours. >>>> >>>> Last minute edit: hmm, actually, transaction action introduced in >>>> 4.2, so crash is not a regression, only broken block-dirty-bitmap-remo= ve >>>> command is a regression... Maybe it's OK for stable. >>> >>> Libvirt REALLY wants to use transaction bitmap management (and require >>> qemu 4.2) for its incremental backup patches that Peter is almost ready >>> to merge in.=A0 I'm trying to ascertain: >>> >>> When exactly can this bug hit?=A0 Can you give a sequence of QMP comman= ds >>> that will trigger it for easy reproduction?=A0 Are there workarounds (s= uch >>> as checking that a bitmap exists prior to attempting to remove it)?=A0 = If >>> it does NOT get fixed in rc5, how will libvirt be able to probe whether >>> the fix is in place? >>> >> >> It looks like: >> >> - You need to have at least one bitmap >> - You need to use transactional remove >> - you need to misspell the bitmap name >> - The remove will fail (return -EINVAL) but doesn't set errp >> - The caller chokes on incomplete information, state->bitmap is NULL >=20 >=20 > No, that would be too simple, the thing is worse. Absolutele correct remo= ving is broken, without any misspelling >=20 > Bug triggers when we are removing persistent bitmap that is not stored ye= t in the image AND at least one (another) bitmap already stored in the imag= e. So, something like: >=20 > 1. create persistent bitmap A > 2. shutdown vm (bitmap A is synced) > 3. start vm > 4. create persistent bitmap B > 5. remove bitmap B - it fails (and crashes if in transaction) >=20 > =3D=3D=3D=3D >=20 > Hmm, workaround.. >=20 > I'm afraid that the only thing is not remove persistent bitmaps, which we= re never synced to the image. So, instead the sequence above, we need >=20 >=20 > 1. create persistent bitmap A > 2. shutdown vm > 3. start vm > 4. create persistent bitmap B > 5. remember, that we want to remove bitmap B after vm shutdown > ... > some other operations > ... > 6. vm shutdown > 7. start vm in stopped mode, and remove all bitmaps marked for removing > 8. stop vm >=20 > But, I think that in real circumstances, vm shutdown is rare thing... This is sounding a bit more serious. As I said earlier, it shouldn't=20 delay 4.2 on its own, but if the fix is obvious (and other than=20 comments, it is a single change from 'ret =3D -EINVAL' to 'ret =3D 0' which= =20 fixes a definite reproducible crash), I think it rises to the level of=20 acceptable. I've been so worried about the question of which release, that I don't=20 know if I've previously offered: Reviewed-by: Eric Blake --=20 Eric Blake, Principal Software Engineer Red Hat, Inc. +1-919-301-3226 Virtualization: qemu.org | libvirt.org