Re: [PATCH v5 3/7] qcow: Tolerate backing_fmt=, but warn on backing_fmt=raw

On 5/5/20 10:30 AM, Eric Blake wrote: > On 5/5/20 2:35 AM, Kevin Wolf wrote: > > Am 03.04.2020 um 19:58 hat Eric Blake geschrieben: > > > qcow has no space in the metadata to store a backing format, and there > > > are existing qcow images backed both by raw or by other formats > > > (usually qcow) images, reliant on probing to tell the difference. > > > While we don't recommend the creation of new qcow images (as qcow2 is > > > hands-down better), we can at least insist that if the user does > > > request a specific format without using -u, then it must be non-raw > > > (as a raw backing file that gets inadvertently edited into some other > > > format can form a security hole); if the user does not request a > > > specific format or lies when using -u, then the status quo of probing > > > for the backing format remains intact (although an upcoming patch will > > > warn when omitting a format request).  Thus, when this series is > > > complete, the only way to use a backing file for qcow without > > > triggering a warning is when using -F if the backing file is non-raw > > > to begin with.  Note that this is only for QemuOpts usage; there is no > > > change to the QAPI to allow a format through -blockdev. > > > > > > Add a new iotest 290 just for qcow, to demonstrate the new warning. > > > > > > Signed-off-by: Eric Blake <eblake(a)redhat.com&gt; > > > > Somehow this feels backwards. Not specifying the backing file format at > > all isn't any safer than explicitly specifying raw. > > > > If there is a difference at all, I would say that explicitly specifying > > raw means that the user is aware what they are doing. So we would have > > more reason to warn against raw images if the backing format isn't > > specified at all because then the user might not be aware that they are > > using a backing file that probes as raw. > > Prior to this patch, -F does not work with qcow.  And even with this > patch, we still cannot store the explicit value of -F in the qcow file. > Anything that does not use -F must continue to work for now (although it > may now warn, and in fact must warn if we deprecate it), while anything > explicit is free to fail (since it failed already), but could also be > made to work (if letting it work is nicer than making it fail, and where > "work" may still include a warning, although it's pointless to have > something brand new that works but is deprecated out of the box).  So > the following is my summary of the two options we can choose between: > > Option 1, qcow backed by raw is more common than qcow backed by other, > so we want: > raw <- qcow, no -F: work without warning (but if backing file is edited, > a future probe seeing non-raw would break image) > raw <- qcow, with -F: work without warning (but if backing file is > edited, a future probe seeing non-raw would break image) > other <- qcow, no -F: works but issues a warning (but backing file will > always probe correctly) > other <- qcow, with -F: fails (we cannot honor the user's explicit > request, because we would still have to probe) > > Option 2, qcow backed by other is more common than qcow backed by raw, > so we want: > raw <- qcow, no -F: works but issues a warning (using a raw backing file > without explicit buy-in is risky) > raw <- qcow, with -F: works but issues a warning (explicit buy-in will > still require subsequent probing, and a backing file could change which > would break image) > other <- qcow, no -F: works without warning > other <- qcow, with -F: works without warning (later probing will still > see non-raw) > > It looks like you are leaning more towards option 1, while my patch > leaned more towards option 2.  Anyone else want to chime in with an > opinion on which is safer vs. easier? > Option 3: > completely deprecate qcow images with backing files, as there is no safe > way to do things favoring either raw (option 1) or non-raw (option 2), > and therefore accept -F solely for convenience with the rest of the > series, but always issue a warning regardless of whether -F was present. Hearing no other opinion in the meantime, I've come up with option 4: raw <- qcow, no -F: works but issues a warning to use -F (the user should be explicit that they know they are using raw) raw <- qcow, with -F raw: a probe is attempted, if it returns anything other than raw, then fail (since we can't store the backing type, and the user's explicit type didn't match reality); otherwise works without warning (users tend to treat backing files as read-only, so even though editing a backing file could make it appear non-raw, that's less likely to happen)

other <- qcow, no -F: works without warning (we'll probe in future opens, but the probe will see the same file type and not corrupt user data) other <- qcow, with -F: a probe is attempted and must match, but otherwise works without warning (we'll still have to probe in future opens, but it's no worse than before)