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Mon, 03 May 2021 05:54:29 -0400 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 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) 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= X-MC-Unique: -Cd0-FCjNg-pcFJ3z-m79Q-1 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 Content-Transfer-Encoding: quoted-printable Received-SPF: pass (zohomail.com: domain of gnu.org designates 209.51.188.17 as permitted sender) client-ip=209.51.188.17; envelope-from=qemu-devel-bounces+importer=patchew.org@nongnu.org; helo=lists.gnu.org; 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+importer=patchew.org@nongnu.org Sender: "Qemu-devel" X-ZohoMail-DKIM: fail (Header signature does not verify) Content-Type: text/plain; charset="utf-8" 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=3D{child-node}" or "backing=3D{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); =20 + /* 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)); =20 + /* Now close bs, apply the snapshot on fallback_bs, and re-open bs= */ if (drv->bdrv_close) { drv->bdrv_close(bs); } =20 + /* .bdrv_open() will re-attach it */ bdrv_unref_child(bs, *fallback_ptr); *fallback_ptr =3D NULL; =20 @@ -296,7 +299,16 @@ int bdrv_snapshot_goto(BlockDriverState *bs, return ret < 0 ? ret : open_ret; } =20 - assert(fallback_bs =3D=3D (*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 =3D=3D (*fallback_ptr)->bs); bdrv_unref(fallback_bs); return ret; } --=20 2.31.1