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=-13.5 required=3.0 tests=BAYES_00,DKIM_INVALID, DKIM_SIGNED,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED 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 36E2FC433ED for ; Mon, 3 May 2021 09:43:12 +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 BFF44611CD for ; Mon, 3 May 2021 09:43:11 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org BFF44611CD 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]:38004 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1ldV6c-0003g9-NV for qemu-devel@archiver.kernel.org; Mon, 03 May 2021 05:43:10 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:37910) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1ldV3v-0000hJ-On for qemu-devel@nongnu.org; Mon, 03 May 2021 05:40:23 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]:27782) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1ldV3s-0004Ib-Nn for qemu-devel@nongnu.org; Mon, 03 May 2021 05:40:23 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1620034816; 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: in-reply-to:in-reply-to:references:references; bh=0fTNUFTP/KLyu5TgKRL890DYaVX0ljsgI303EQDWpR0=; b=Kr7SKMLxoXP9ZKRWJpOFUzYgkT8GDQvnxznepz6zWvKazhqRBJigm7Hyas5Hw+uZSgREtS VOdCtnACrMcOQEm3oOGg+C18fBZPuvyH6pfyS6/HeA4l1ma2wYh6dVcVmOt7l0PSG/B/0t r5c6gQFCcvWyNCpCTED/t//bRFhn+CQ= 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-170-E_F-W65MOiOt6zgWz-q0tw-1; Mon, 03 May 2021 05:40:15 -0400 X-MC-Unique: E_F-W65MOiOt6zgWz-q0tw-1 Received: from smtp.corp.redhat.com (int-mx04.intmail.prod.int.phx2.redhat.com [10.5.11.14]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 472DD801106; Mon, 3 May 2021 09:40:14 +0000 (UTC) Received: from merkur.fritz.box (ovpn-113-251.ams2.redhat.com [10.36.113.251]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 520AA5E26A; Mon, 3 May 2021 09:40:13 +0000 (UTC) Date: Mon, 3 May 2021 11:40:11 +0200 From: Kevin Wolf To: Peter Maydell Subject: Re: [PULL 37/64] block/snapshot: Fix fallback Message-ID: References: <20200907110936.261684-1-kwolf@redhat.com> <20200907110936.261684-38-kwolf@redhat.com> MIME-Version: 1.0 In-Reply-To: X-Scanned-By: MIMEDefang 2.79 on 10.5.11.14 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=kwolf@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Received-SPF: pass client-ip=170.10.133.124; envelope-from=kwolf@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -34 X-Spam_score: -3.5 X-Spam_bar: --- X-Spam_report: (-3.5 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.697, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H4=-0.01, RCVD_IN_MSPIKE_WL=-0.01, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action 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: QEMU Developers , Qemu-block , mreitz@redhat.com Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" Am 01.05.2021 um 00:30 hat Peter Maydell geschrieben: > On Mon, 7 Sept 2020 at 12:11, Kevin Wolf wrote: > > > > From: Max Reitz > > > > If the top node's driver does not provide snapshot functionality and we > > want to fall back to a node down the chain, we need to snapshot all > > non-COW children. For simplicity's sake, just do not fall back if there > > is more than one such child. Furthermore, we really only can fall back > > to bs->file and bs->backing, because bdrv_snapshot_goto() has to modify > > the child link (notably, set it to NULL). > > > > Suggested-by: Vladimir Sementsov-Ogievskiy > > Signed-off-by: Max Reitz > > Reviewed-by: Andrey Shinkevich > > Reviewed-by: Kevin Wolf > > Hi; Coverity thinks it's found a problem with this code > (CID 1452774): Cc: Max as the patch author > > @@ -205,39 +258,46 @@ int bdrv_snapshot_goto(BlockDriverState *bs, > > return ret; > > } > > > > - if (bs->file) { > > - BlockDriverState *file; > > - QDict *options = qdict_clone_shallow(bs->options); > > + fallback_ptr = bdrv_snapshot_fallback_ptr(bs); > > + if (fallback_ptr) { > > + QDict *options; > > QDict *file_options; > > Error *local_err = NULL; > > + BlockDriverState *fallback_bs = (*fallback_ptr)->bs; > > + char *subqdict_prefix = g_strdup_printf("%s.", (*fallback_ptr)->name); > > + > > + options = qdict_clone_shallow(bs->options); > > > > - file = bs->file->bs; > > /* Prevent it from getting deleted when detached from bs */ > > - bdrv_ref(file); > > + bdrv_ref(fallback_bs); > > > > - qdict_extract_subqdict(options, &file_options, "file."); > > + qdict_extract_subqdict(options, &file_options, subqdict_prefix); > > qobject_unref(file_options); > > - qdict_put_str(options, "file", bdrv_get_node_name(file)); > > + g_free(subqdict_prefix); > > + > > + qdict_put_str(options, (*fallback_ptr)->name, > > + bdrv_get_node_name(fallback_bs)); > > > > if (drv->bdrv_close) { > > drv->bdrv_close(bs); > > } > > - bdrv_unref_child(bs, bs->file); > > - bs->file = NULL; > > > > - ret = bdrv_snapshot_goto(file, snapshot_id, errp); > > + bdrv_unref_child(bs, *fallback_ptr); > > + *fallback_ptr = NULL; > > Here we set *fallback_ptr to NULL... > > > + > > + ret = bdrv_snapshot_goto(fallback_bs, snapshot_id, errp); > > open_ret = drv->bdrv_open(bs, options, bs->open_flags, &local_err); > > qobject_unref(options); > > if (open_ret < 0) { > > - bdrv_unref(file); > > + bdrv_unref(fallback_bs); > > bs->drv = NULL; > > /* A bdrv_snapshot_goto() error takes precedence */ > > error_propagate(errp, local_err); > > return ret < 0 ? ret : open_ret; > > } > > > > - assert(bs->file->bs == file); > > - bdrv_unref(file); > > + assert(fallback_bs == (*fallback_ptr)->bs); > > ...but here we dereference *fallback_ptr, and Coverity doesn't see > anything that it recognizes as being able to change it. > > > + bdrv_unref(fallback_bs); > > return ret; > > } > > False positive, or real issue? (If a false positive, a comment > explaining what's going on wouldn't go amiss -- as a human reader > I'm kind of confused about whether there's some kind of hidden > magic going on here.) I think it's a false positive because drv->bdrv_open() is supposed to give it a non-NULL value again. Not sure if we can make the assumption in every case without checking it, but it feels reasonable to require that drv->bdrv_open() would return failure otherwise. Max? Kevin