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Thu, 23 Apr 2020 11:15:00 +0000 (UTC) Received: from dresden.str.redhat.com (ovpn-113-212.ams2.redhat.com [10.36.113.212]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 294DF60F8D; Thu, 23 Apr 2020 11:14:57 +0000 (UTC) Subject: Re: [PATCH v5 7/9] block: truncate: Don't make backing file data visible To: Kevin Wolf , qemu-block@nongnu.org References: <20200422152129.167074-1-kwolf@redhat.com> <20200422152129.167074-8-kwolf@redhat.com> From: Max Reitz Autocrypt: addr=mreitz@redhat.com; prefer-encrypt=mutual; keydata= mQENBFXOJlcBCADEyyhOTsoa/2ujoTRAJj4MKA21dkxxELVj3cuILpLTmtachWj7QW+TVG8U /PsMCFbpwsQR7oEy8eHHZwuGQsNpEtNC2G/L8Yka0BIBzv7dEgrPzIu+W3anZXQW4702+uES U29G8TP/NGfXRRHGlbBIH9KNUnOSUD2vRtpOLXkWsV5CN6vQFYgQfFvmp5ZpPeUe6xNplu8V mcTw8OSEDW/ZnxJc8TekCKZSpdzYoxfzjm7xGmZqB18VFwgJZlIibt1HE0EB4w5GsD7x5ekh awIe3RwoZgZDLQMdOitJ1tUc8aqaxvgA4tz6J6st8D8pS//m1gAoYJWGwwIVj1DjTYLtABEB AAG0HU1heCBSZWl0eiA8bXJlaXR6QHJlZGhhdC5jb20+iQFTBBMBCAA9AhsDBQkSzAMABQsJ CAcCBhUICQoLAgQWAgMBAh4BAheABQJVzie5FRhoa3A6Ly9rZXlzLmdudXBnLm5ldAAKCRD0 B9sAYdXPQDcIB/9uNkbYEex1rHKz3mr12uxYMwLOOFY9fstP5aoVJQ1nWQVB6m2cfKGdcRe1 2/nFaHSNAzT0NnKz2MjhZVmcrpyd2Gp2QyISCfb1FbT82GMtXFj1wiHmPb3CixYmWGQUUh+I AvUqsevLA+WihgBUyaJq/vuDVM1/K9Un+w+Tz5vpeMidlIsTYhcsMhn0L9wlCjoucljvbDy/ 8C9L2DUdgi3XTa0ORKeflUhdL4gucWoAMrKX2nmPjBMKLgU7WLBc8AtV+84b9OWFML6NEyo4 4cP7cM/07VlJK53pqNg5cHtnWwjHcbpGkQvx6RUx6F1My3y52vM24rNUA3+ligVEgPYBuQEN BFXOJlcBCADAmcVUNTWT6yLWQHvxZ0o47KCP8OcLqD+67T0RCe6d0LP8GsWtrJdeDIQk+T+F xO7DolQPS6iQ6Ak2/lJaPX8L0BkEAiMuLCKFU6Bn3lFOkrQeKp3u05wCSV1iKnhg0UPji9V2 W5eNfy8F4ZQHpeGUGy+liGXlxqkeRVhLyevUqfU0WgNqAJpfhHSGpBgihUupmyUg7lfUPeRM DzAN1pIqoFuxnN+BRHdAecpsLcbR8sQddXmDg9BpSKozO/JyBmaS1RlquI8HERQoe6EynJhd 64aICHDfj61rp+/0jTIcevxIIAzW70IadoS/y3DVIkuhncgDBvGbF3aBtjrJVP+5ABEBAAGJ ASUEGAEIAA8FAlXOJlcCGwwFCRLMAwAACgkQ9AfbAGHVz0CbFwf9F/PXxQR9i4N0iipISYjU sxVdjJOM2TMut+ZZcQ6NSMvhZ0ogQxJ+iEQ5OjnIputKvPVd5U7WRh+4lF1lB/NQGrGZQ1ic alkj6ocscQyFwfib+xIe9w8TG1CVGkII7+TbS5pXHRxZH1niaRpoi/hYtgzkuOPp35jJyqT/ /ELbqQTDAWcqtJhzxKLE/ugcOMK520dJDeb6x2xVES+S5LXby0D4juZlvUj+1fwZu+7Io5+B bkhSVPb/QdOVTpnz7zWNyNw+OONo1aBUKkhq2UIByYXgORPFnbfMY7QWHcjpBVw9MgC4tGeF R4bv+1nAMMxKmb5VvQCExr0eFhJUAHAhVg== Message-ID: <1d0c34af-7efa-9be7-669e-fe15cf89e3cb@redhat.com> Date: Thu, 23 Apr 2020 13:14:56 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.6.0 MIME-Version: 1.0 In-Reply-To: <20200422152129.167074-8-kwolf@redhat.com> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.12 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="okEdHx1NWzlm7lcRVObVsvTnSd9HFVv2A" Received-SPF: pass client-ip=207.211.31.120; envelope-from=mreitz@redhat.com; helo=us-smtp-1.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/04/23 06:43:51 X-ACL-Warn: Detected OS = Linux 2.2.x-3.x [generic] [fuzzy] X-Received-From: 207.211.31.120 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: berto@igalia.com, vsementsov@virtuozzo.com, qemu-devel@nongnu.org Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" This is an OpenPGP/MIME signed message (RFC 4880 and 3156) --okEdHx1NWzlm7lcRVObVsvTnSd9HFVv2A Content-Type: multipart/mixed; boundary="dUI0aC4PS4rmjekEmNUUvWEhIeeGJSH5V" --dUI0aC4PS4rmjekEmNUUvWEhIeeGJSH5V Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: quoted-printable On 22.04.20 17:21, Kevin Wolf wrote: > When extending the size of an image that has a backing file larger than > its old size, make sure that the backing file data doesn't become > visible in the guest, but the added area is properly zeroed out. >=20 > Consider the following scenario where the overlay is shorter than its > backing file: >=20 > base.qcow2: AAAAAAAA > overlay.qcow2: BBBB >=20 > When resizing (extending) overlay.qcow2, the new blocks should not stay > unallocated and make the additional As from base.qcow2 visible like > before this patch, but zeros should be read. >=20 > A similar case happens with the various variants of a commit job when an > intermediate file is short (- for unallocated): >=20 > base.qcow2: A-A-AAAA > mid.qcow2: BB-B > top.qcow2: C--C--C- >=20 > After commit top.qcow2 to mid.qcow2, the following happens: >=20 > mid.qcow2: CB-C00C0 (correct result) > mid.qcow2: CB-C--C- (before this fix) >=20 > Without the fix, blocks that previously read as zeros on top.qcow2 > suddenly turn into A. >=20 > Signed-off-by: Kevin Wolf > Reviewed-by: Alberto Garcia > --- > block/io.c | 14 ++++++++++++++ > 1 file changed, 14 insertions(+) >=20 > diff --git a/block/io.c b/block/io.c > index 795075954e..8fbb607515 100644 > --- a/block/io.c > +++ b/block/io.c > @@ -3394,6 +3394,20 @@ int coroutine_fn bdrv_co_truncate(BdrvChild *child= , int64_t offset, bool exact, > goto out; > } > =20 > + /* > + * If the image has a backing file that is large enough that it woul= d > + * provide data for the new area, we cannot leave it unallocated bec= ause > + * then the backing file content would become visible. Instead, zero= -fill > + * the new area. > + * > + * Note that if the image has a backing file, but was opened without= the > + * backing file, taking care of keeping things consistent with that = backing > + * file is the user's responsibility. > + */ > + if (new_bytes && bs->backing) { > + flags |=3D BDRV_REQ_ZERO_WRITE; > + } This breaks growing any non-qcow2 image with any backing file. Do we care about that? The comment says something about =E2=80=9Ca backing file that is large enou= gh that it would provide data for the new area=E2=80=9D, but that condition do= esn=E2=80=99t appear in the code. Should it? (If it did, I think the number of cases this change broke would be much smaller.) If it was deliberate to not have that condition here, and if we decide that we don=E2=80=99t care about non-qcow2 formats here, then I think at le= ast the error message deserves some improvement over =E2=80=9Cqemu-img: Block d= river does not support requested flags=E2=80=9D. 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