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Mon, 17 Feb 2020 16:56:13 +0000 (UTC) To: Qemu-block , Kevin Wolf From: Max Reitz Subject: qcow2: Zero-initialization of external data files 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: Date: Mon, 17 Feb 2020 17:56:11 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.4.1 MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.79 on 10.5.11.15 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="ncPtxHryCGBDiFaTTn88jPF7UYRvp1h1n" X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] [fuzzy] X-Received-From: 207.211.31.81 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: "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) --ncPtxHryCGBDiFaTTn88jPF7UYRvp1h1n Content-Type: multipart/mixed; boundary="CYUhujAji5i8HcYkJOFmFKAFX9x5lDO3y" --CYUhujAji5i8HcYkJOFmFKAFX9x5lDO3y Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: quoted-printable Hi, AFAIU, external data files with data_file_raw=3Don are supposed to return the same data as the qcow2 file when read. But we still use the qcow2 metadata structures (which are by default initialized to =E2=80=9Ceverythin= g unallocated=E2=80=9D), even though we never ensure that the external data f= ile is zero, too, so this can happen: $ dd if=3D/dev/urandom of=3Dfoo.raw 64M [...] $ sudo losetup -f --show foo.raw /dev/loop0 $ sudo ./qemu-img create -f qcow2 -o \ data_file=3D/dev/loop0,data_file_raw=3Don foo.qcow2 64M [...] $ sudo ./qemu-io -c 'read -P 0 0 64M' foo.qcow2 read 67108864/67108864 bytes at offset 0 64 MiB, 1 ops; 00.00 sec (25.036 GiB/sec and 400.5751 ops/sec) $ sudo ./qemu-io -c 'read -P 0 0 64M' -f raw foo.raw Pattern verification failed at offset 0, 67108864 bytes read 67108864/67108864 bytes at offset 0 64 MiB, 1 ops; 00.01 sec (5.547 GiB/sec and 88.7484 ops/sec) I suppose this behavior is fine for blockdev-create because I guess it=E2= =80=99s the user=E2=80=99s responsibility to ensure that the external data file is = zero. But maybe it isn=E2=80=99t, so that=E2=80=99s my first question: Is it rea= lly the user=E2=80=99s responsibility or should we always ensure it=E2=80=99s zero? My second question is: If we decide that this is fine for blockdev-create, should at least qcow2_co_create_opts() ensure the data file it just created is zero? Max --CYUhujAji5i8HcYkJOFmFKAFX9x5lDO3y-- --ncPtxHryCGBDiFaTTn88jPF7UYRvp1h1n Content-Type: application/pgp-signature; name="signature.asc" Content-Description: OpenPGP digital signature Content-Disposition: attachment; filename="signature.asc" -----BEGIN PGP SIGNATURE----- iQEzBAEBCAAdFiEEkb62CjDbPohX0Rgp9AfbAGHVz0AFAl5KxasACgkQ9AfbAGHV z0BgmQf6A/cjD5z51M6/bmWn82QMEBH2kpeH6Sy+7gE/AxcwVmuHbUdoG0NQy6hy wYSgU4BdXAHv3puYLmzqg9rkDL5DQl9ETd1Z9pE1Xg41GjWZYRS+WaU4E3tqz2JW CxSgYbhmpuMLchyLrANSwbO3i+AP55YPPL+rj0HChUIe6LxtnP/iM3Mc0pKMDHoU 3RNA4u3605ghNA531+y5nSgkWgF/80MhcBQfLQ4L4XawDPicXLrn1RTRAgMLXz8w NLEVbBnfJDuv4TajHIXIWPfxdwIPHHP2ZKigefqEdkycgWb+8v2JKISELRy/KYc3 dnb+a572YAzBvONybokXGvVMktaACg== =sGgq -----END PGP SIGNATURE----- --ncPtxHryCGBDiFaTTn88jPF7UYRvp1h1n--