From patchwork Tue Jan 16 15:48:39 2024 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Fiona Ebner X-Patchwork-Id: 13520940 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org 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 smtp.lore.kernel.org (Postfix) with ESMTPS id EA903C47DA2 for ; Tue, 16 Jan 2024 15:49:32 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1rPlgO-0001aO-1F; Tue, 16 Jan 2024 10:48:56 -0500 Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1rPlgL-0001YK-5I; Tue, 16 Jan 2024 10:48:53 -0500 Received: from proxmox-new.maurer-it.com ([94.136.29.106]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1rPlgJ-0003sg-8Q; Tue, 16 Jan 2024 10:48:52 -0500 Received: from proxmox-new.maurer-it.com (localhost.localdomain [127.0.0.1]) by proxmox-new.maurer-it.com (Proxmox) with ESMTP id 638664916A; Tue, 16 Jan 2024 16:48:48 +0100 (CET) From: Fiona Ebner To: qemu-devel@nongnu.org Cc: qemu-block@nongnu.org, hreitz@redhat.com, kwolf@redhat.com, fam@euphon.net, stefanha@redhat.com Subject: [PATCH] block/io: clear BDRV_BLOCK_RECURSE flag after recursing in bdrv_co_block_status Date: Tue, 16 Jan 2024 16:48:39 +0100 Message-Id: <20240116154839.401030-1-f.ebner@proxmox.com> X-Mailer: git-send-email 2.39.2 MIME-Version: 1.0 Received-SPF: pass client-ip=94.136.29.106; envelope-from=f.ebner@proxmox.com; helo=proxmox-new.maurer-it.com X-Spam_score_int: -18 X-Spam_score: -1.9 X-Spam_bar: - X-Spam_report: (-1.9 / 5.0 requ) BAYES_00=-1.9, SPF_HELO_NONE=0.001, SPF_PASS=-0.001, T_SCC_BODY_TEXT_LINE=-0.01 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Using fleecing backup like in [0] on a qcow2 image (with metadata preallocation) can lead to the following assertion failure: > bdrv_co_do_block_status: Assertion `!(ret & BDRV_BLOCK_ZERO)' failed. In the reproducer [0], it happens because the BDRV_BLOCK_RECURSE flag will be set by the qcow2 driver, so the caller will recursively check the file child. Then the BDRV_BLOCK_ZERO set too. Later up the call chain, in bdrv_co_do_block_status() for the snapshot-access driver, the assertion failure will happen, because both flags are set. To fix it, clear the recurse flag after the recursive check was done. In detail: > #0 qcow2_co_block_status Returns 0x45 = BDRV_BLOCK_RECURSE | BDRV_BLOCK_DATA | BDRV_BLOCK_OFFSET_VALID. > #1 bdrv_co_do_block_status Because of the data flag, bdrv_co_do_block_status() will now also set BDRV_BLOCK_ALLOCATED. Because of the recurse flag, bdrv_co_do_block_status() for the bdrv_file child will be called, which returns 0x16 = BDRV_BLOCK_ALLOCATED | BDRV_BLOCK_OFFSET_VALID | BDRV_BLOCK_ZERO. Now the return value inherits the zero flag. Returns 0x57 = BDRV_BLOCK_RECURSE | BDRV_BLOCK_DATA | BDRV_BLOCK_OFFSET_VALID | BDRV_BLOCK_ALLOCATED | BDRV_BLOCK_ZERO. > #2 bdrv_co_common_block_status_above > #3 bdrv_co_block_status_above > #4 bdrv_co_block_status > #5 cbw_co_snapshot_block_status > #6 bdrv_co_snapshot_block_status > #7 snapshot_access_co_block_status > #8 bdrv_co_do_block_status Return value is propagated all the way up to here, where the assertion failure happens, because BDRV_BLOCK_RECURSE and BDRV_BLOCK_ZERO are both set. > #9 bdrv_co_common_block_status_above > #10 bdrv_co_block_status_above > #11 block_copy_block_status > #12 block_copy_dirty_clusters > #13 block_copy_common > #14 block_copy_async_co_entry > #15 coroutine_trampoline [0]: > #!/bin/bash > rm /tmp/disk.qcow2 > ./qemu-img create /tmp/disk.qcow2 -o preallocation=metadata -f qcow2 1G > ./qemu-img create /tmp/fleecing.qcow2 -f qcow2 1G > ./qemu-img create /tmp/backup.qcow2 -f qcow2 1G > ./qemu-system-x86_64 --qmp stdio \ > --blockdev qcow2,node-name=node0,file.driver=file,file.filename=/tmp/disk.qcow2 \ > --blockdev qcow2,node-name=node1,file.driver=file,file.filename=/tmp/fleecing.qcow2 \ > --blockdev qcow2,node-name=node2,file.driver=file,file.filename=/tmp/backup.qcow2 \ > < {"execute": "qmp_capabilities"} > {"execute": "blockdev-add", "arguments": { "driver": "copy-before-write", "file": "node0", "target": "node1", "node-name": "node3" } } > {"execute": "blockdev-add", "arguments": { "driver": "snapshot-access", "file": "node3", "node-name": "snap0" } } > {"execute": "blockdev-backup", "arguments": { "device": "snap0", "target": "node1", "sync": "full", "job-id": "backup0" } } > EOF Signed-off-by: Fiona Ebner Reviewed-by: Vladimir Sementsov-Ogievskiy --- I'm new to this part of the code, so I'm not sure if it is actually safe to clear the flag? Intuitively, I'd expect it to be only relevant until it was acted upon, but no clue. block/io.c | 10 ++++++++++ 1 file changed, 10 insertions(+) diff --git a/block/io.c b/block/io.c index 8fa7670571..33150c0359 100644 --- a/block/io.c +++ b/block/io.c @@ -2584,6 +2584,16 @@ bdrv_co_do_block_status(BlockDriverState *bs, bool want_zero, ret |= (ret2 & BDRV_BLOCK_ZERO); } } + + /* + * Now that the recursive search was done, clear the flag. Otherwise, + * with more complicated block graphs like snapshot-access -> + * copy-before-write -> qcow2, where the return value will be propagated + * further up to a parent bdrv_co_do_block_status() call, both the + * BDRV_BLOCK_RECURSE and BDRV_BLOCK_ZERO flags would be set, which is + * not allowed. + */ + ret &= ~BDRV_BLOCK_RECURSE; } out: