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 E18D4C433B4 for ; Mon, 3 May 2021 10:19:07 +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 0CA3861153 for ; Mon, 3 May 2021 10:19:07 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 0CA3861153 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]:57792 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1ldVfN-0008Ao-OF for qemu-devel@archiver.kernel.org; Mon, 03 May 2021 06:19:05 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:46112) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1ldVdg-0007DS-3O for qemu-devel@nongnu.org; Mon, 03 May 2021 06:17:24 -0400 Received: from us-smtp-delivery-124.mimecast.com ([216.205.24.124]:27778) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1ldVdc-0001ts-6T for qemu-devel@nongnu.org; Mon, 03 May 2021 06:17:19 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1620037033; 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=snsteE7M99/GqqEv4tXpJQ/j1Br4zhsvlNlSXBpS+7o=; b=WF+U10lEnh2BVufb/85N9zo6jhrjHCVLw2YGBcz+qNKTDpEUaMhsUzuGG/hyCYk0GXcOW5 C0b2WdUzDmjY1o2lKutcvO1W3idDuDnS/FA623/v2bV2Gt34luIykcQJSGO8bV2rCC4i1R Es7mzoYAfJRTrUcaLyJTrjpZMDn8Azw= 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-223-rIdJF9bTPJeyHX-5-zrNzA-1; Mon, 03 May 2021 06:17:11 -0400 X-MC-Unique: rIdJF9bTPJeyHX-5-zrNzA-1 Received: from smtp.corp.redhat.com (int-mx03.intmail.prod.int.phx2.redhat.com [10.5.11.13]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id D4324100A605; Mon, 3 May 2021 10:17:10 +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 A334460937; Mon, 3 May 2021 10:17:09 +0000 (UTC) Date: Mon, 3 May 2021 12:17:08 +0200 From: Kevin Wolf To: Max Reitz Subject: Re: [PULL 37/64] block/snapshot: Fix fallback Message-ID: References: <20200907110936.261684-1-kwolf@redhat.com> <20200907110936.261684-38-kwolf@redhat.com> <1b1f6fe6-1a0e-61f8-1197-d26667a3d6fc@redhat.com> MIME-Version: 1.0 In-Reply-To: <1b1f6fe6-1a0e-61f8-1197-d26667a3d6fc@redhat.com> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 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=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit Received-SPF: pass client-ip=216.205.24.124; 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: Peter Maydell , QEMU Developers , Qemu-block Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" Am 03.05.2021 um 11:45 hat Max Reitz geschrieben: > On 03.05.21 11:40, Kevin Wolf wrote: > > 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 > > Yes, I’m writing a patch to add a comment. > > > > > @@ -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? > > Yes. I think it’s sensible to add an *fallback_ptr non-NULL check to the > assert condition (i.e., > > assert(*fallback_ptr && fallback_bs == (*fallback_ptr)->bs); > > ), because the intention of the condition is already to verify that > .bdrv_open() has opened the right node. So we might say what’s missing is > to also assert that it has opened any node at all, but if we’re fine with > asserting that it has opened the right node (which we did since > 7a9e51198c24), we should definitely be fine with asserting that it has > opened any node at all. True, that's a good point. Kevin