From patchwork Mon May 3 09:54:18 2021 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Max Reitz X-Patchwork-Id: 12235833 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=-13.6 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 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 CE5A7C433ED for ; Mon, 3 May 2021 09:55:41 +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 3040D61157 for ; Mon, 3 May 2021 09:55:41 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 3040D61157 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]:55064 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1ldVIi-0002en-1d for qemu-devel@archiver.kernel.org; Mon, 03 May 2021 05:55:40 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:40504) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1ldVHa-00029I-4k for qemu-devel@nongnu.org; Mon, 03 May 2021 05:54:30 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]:36972) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1ldVHV-0004ga-Fi for qemu-devel@nongnu.org; Mon, 03 May 2021 05:54:29 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1620035664; 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; bh=0uu9yOp3YpP9hNFDJtCvZxKBX4OinAcVSrafAr0AGJs=; b=GIOFW34rUNF6UA3Ge9qV7MbjtM+AbbIvfA9sNOmnlkwAE2kt0WGv7UgdMRSh0innvXufN6 /zo/xo/nHOEFYAohQmwR2FDHW6qQ9Ui/DyoIyrpfJkJj7nSwEJaODLMjf5K8Qf5ZHNK3I1 57H5Uzw/nYKzX2WlRPvp4MYmo/M56Rw= 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-98--Cd0-FCjNg-pcFJ3z-m79Q-1; Mon, 03 May 2021 05:54:21 -0400 X-MC-Unique: -Cd0-FCjNg-pcFJ3z-m79Q-1 Received: from smtp.corp.redhat.com (int-mx07.intmail.prod.int.phx2.redhat.com [10.5.11.22]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id C25F68049CB; Mon, 3 May 2021 09:54:20 +0000 (UTC) Received: from localhost (ovpn-114-107.ams2.redhat.com [10.36.114.107]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 4C47D1037E88; Mon, 3 May 2021 09:54:20 +0000 (UTC) From: Max Reitz To: qemu-block@nongnu.org Subject: [PATCH] block/snapshot: Clarify goto fallback behavior Date: Mon, 3 May 2021 11:54:18 +0200 Message-Id: <20210503095418.31521-1-mreitz@redhat.com> MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.84 on 10.5.11.22 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=mreitz@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Received-SPF: pass client-ip=170.10.133.124; envelope-from=mreitz@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=unavailable 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: Kevin Wolf , qemu-devel@nongnu.org, Max Reitz Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" In the bdrv_snapshot_goto() fallback code, we work with a pointer to either bs->file or bs->backing. We close that child, close the node (with .bdrv_close()), apply the snapshot on the child node, and then re-open the node (with .bdrv_open()). In order for .bdrv_open() to attach the same child node that we had before, we pass "file={child-node}" or "backing={child-node}" to it. Therefore, when .bdrv_open() has returned success, we can assume that bs->file or bs->backing (respectively) points to our original child again. This is verified by an assertion. All of this is not immediately clear from a quick glance at the code, so add a comment to the assertion what it is for, and why it is valid. It certainly confused Coverity. Reported-by: Coverity (CID 1452774) Signed-off-by: Max Reitz Reviewed-by: Vladimir Sementsov-Ogievskiy --- block/snapshot.c | 14 +++++++++++++- 1 file changed, 13 insertions(+), 1 deletion(-) diff --git a/block/snapshot.c b/block/snapshot.c index e8ae9a28c1..cce5e35b35 100644 --- a/block/snapshot.c +++ b/block/snapshot.c @@ -275,13 +275,16 @@ int bdrv_snapshot_goto(BlockDriverState *bs, qobject_unref(file_options); g_free(subqdict_prefix); + /* Force .bdrv_open() below to re-attach fallback_bs on *fallback_ptr */ qdict_put_str(options, (*fallback_ptr)->name, bdrv_get_node_name(fallback_bs)); + /* Now close bs, apply the snapshot on fallback_bs, and re-open bs */ if (drv->bdrv_close) { drv->bdrv_close(bs); } + /* .bdrv_open() will re-attach it */ bdrv_unref_child(bs, *fallback_ptr); *fallback_ptr = NULL; @@ -296,7 +299,16 @@ int bdrv_snapshot_goto(BlockDriverState *bs, return ret < 0 ? ret : open_ret; } - assert(fallback_bs == (*fallback_ptr)->bs); + /* + * fallback_ptr is &bs->file or &bs->backing. *fallback_ptr + * was closed above and set to NULL, but the .bdrv_open() call + * has opened it again, because we set the respective option + * (with the qdict_put_str() call above). + * Assert that .bdrv_open() has attached some child on + * *fallback_ptr, and that it has attached the one we wanted + * it to (i.e., fallback_bs). + */ + assert(*fallback_ptr && fallback_bs == (*fallback_ptr)->bs); bdrv_unref(fallback_bs); return ret; }