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Fri, 19 Jun 2020 07:57:30 +0000 (UTC) Subject: Re: [PATCH for-5.1] qcow2: Don't open images with a backing file and the data-file-raw bit To: Alberto Garcia , qemu-devel@nongnu.org References: <20200415190207.21118-1-berto@igalia.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: <80d5af9c-0b8b-472a-fba2-cace33be6053@redhat.com> Date: Fri, 19 Jun 2020 09:57:27 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.8.0 MIME-Version: 1.0 In-Reply-To: X-Scanned-By: MIMEDefang 2.84 on 10.5.11.22 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="5jxoDAsnco8fPQZCHiW0NglSH6p27nGzu" Received-SPF: pass client-ip=207.211.31.81; envelope-from=mreitz@redhat.com; helo=us-smtp-delivery-1.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/06/19 01:50:04 X-ACL-Warn: Detected OS = Linux 2.2.x-3.x [generic] [fuzzy] X-Spam_score_int: -30 X-Spam_score: -3.1 X-Spam_bar: --- X-Spam_report: (-3.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-1, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H4=-0.01, RCVD_IN_MSPIKE_WL=-0.01, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=_AUTOLEARN 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-block@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) --5jxoDAsnco8fPQZCHiW0NglSH6p27nGzu Content-Type: multipart/mixed; boundary="33qzK0gy6wKKoJSDpZRMCRnASyY6hPqmu" --33qzK0gy6wKKoJSDpZRMCRnASyY6hPqmu Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: quoted-printable On 18.06.20 14:03, Alberto Garcia wrote: > On Wed 03 Jun 2020 03:53:03 PM CEST, Max Reitz wrote: >> Sorry for the long delay. :/ >=20 > And sorry for my long delay as well. >=20 >> The patch itself looks good, but I=E2=80=99m not sure whether it is exte= nsive >> enough. Let me just jump straight to the problem: >> >> $ ./qemu-img create -f qcow2 \ >> -o data_file=3Dfoo.qcow2.raw,data_file_raw=3Don \ >> foo.qcow2 64M >> (Create some file empty foo.qcow2 with external data file that=E2=80=99s= raw) >> >> $ ./qemu-img create -f qcow2 backing.qcow2 64M >> $ ./qemu-io -c 'write -P 42 0 64M' backing.qcow2 >> (Create some file filled with 42s) >> >> $ ./qemu-img compare foo.qcow2 foo.qcow2.raw >> Images are identical. >> (As expected, foo.qcow2 is identical to its raw data file) >> >> $ ./qemu-img compare --image-opts \ >> file.filename=3Dfoo.qcow2,backing.file.filename=3Dbacking.qcow2 \ >> file.filename=3Dfoo.qcow2.raw >> Content mismatch at offset 0! >> (Oops.) >=20 > If two images have the same contents but then you compare them changing > the backing file of one of them you can also get a content mismatch. How > is this different? It=E2=80=99s different in that files with data-file-raw can=E2=80=99t have = backing files at all. So maybe users shouldn=E2=80=99t be allowed to give them backing f= iles at runtime either. Or at least, if we have data-file-raw, *all* data visible on such an image should be taken from the raw data file, never from any backing file. > Regardless of how we should ideally handle bs->backing and data-file-raw > (and yes I agree that it would be nice that QEMU would say "you cannot > have a backing file and an external raw file" in all cases) I think that > if the problem is that the user can override the backing file name and > get different contents, then that's not a problem that we should be > worried about. Well, not really be worried about, but I do think it=E2=80=99s indicative o= f some problem, though I=E2=80=99m not sure whether the problem is error reporting. I think it=E2=80=99s rather the fact that data-file-raw should = imply metadata preallocation. With preallocation, we=E2=80=99d ensure that we always take all data from t= he raw data file. So we=E2=80=99d always ignore any potential backing file. (It makes sense to guard users against giving images with data-file-raw a backing file, and to consider such images corrupt, as done by this patch. But if users can force a backing file at runtime, I think showing its contents is another bug.) Max --33qzK0gy6wKKoJSDpZRMCRnASyY6hPqmu-- --5jxoDAsnco8fPQZCHiW0NglSH6p27nGzu Content-Type: application/pgp-signature; name="signature.asc" Content-Description: OpenPGP digital signature Content-Disposition: attachment; filename="signature.asc" -----BEGIN PGP SIGNATURE----- iQEzBAEBCAAdFiEEkb62CjDbPohX0Rgp9AfbAGHVz0AFAl7sb+gACgkQ9AfbAGHV z0CSVwf9E6CIbYc7bH4zwTpIawNA8OlrWcTw0Lx8XIrpjVYqLFkxhPuHpW5fLcWB vOpUOqV74142+Otp3UiNDUusKTksWHEEiqsG8QXx/N7NPGx72dQiQqbOdPCwOCPz I9yD8xkQHINiaFv+r2tLtbCIdL9/3KwJ9i5P6al4CNxcAWuHI8C1hFwSx29vKgLK 1l1/T+/U2S2DV1Emv7B8EkmcLza6krSlB9mGSc7Qf6qYc8X2zuauGCrMYddtsl+C M4CwU4CJFsdj/LMJ639DFGP9AJgZzjg3hTxuCU/qu4lRj6T6wTISrSM6iYjNEUPu EbXw4tgaJcFrnh6amSJx8zfnAeN/nw== =I/1F -----END PGP SIGNATURE----- --5jxoDAsnco8fPQZCHiW0NglSH6p27nGzu--