[Libguestfs] nbdkit: Could not read L1 table when reading exported qcow2

Han Han hhan at redhat.com
Fri Nov 23 09:56:08 UTC 2018

On Wed, Nov 21, 2018 at 10:33 PM Eric Blake <eblake at redhat.com> wrote:

> On 11/21/18 4:39 AM, Richard W.M. Jones wrote:
> > On Wed, Nov 21, 2018 at 09:25:05AM +0000, Richard W.M. Jones wrote:
> >> nbdkit: file.2: error: invalid request: offset and count are out of
> range: offset=196608 count=512
> >
> > Actually what happens even more precisely is that the underlying file
> > is not a multiple of 512 bytes (196624 ≡ 16 mod 512).
> >
> > qemu-img (the client) issues a request for the final "sector" of the
> > file which goes beyond the end of the file.
> I'm still hoping to fix that bug in qemu before the 3.1 release, but
> that date is getting close :)
> >
> > You can "fix" this by using the truncate filter
> > (
> https://github.com/libguestfs/nbdkit/blob/master/filters/truncate/nbdkit-truncate-filter.pod
> )
> > telling it to round up the size to the next multiple of 512 bytes:
> >
> > $ nbdkit --filter=truncate file file=disk -f -v round-up=512
> >
> > and this allows qemu-img to work:
> Yes, this is the right workaround for now.
> >
> > $ qemu-img info nbd://localhost
> > image: nbd://localhost:10809
> > file format: qcow2
> Invoking qemu-img without '-f $format' means that qemu-img has to probe
> the image to guess its format. Here, the probe succeeded, but in general
> this is insecure (if the file was 'raw', but the probe returned anything
> else, then a malicious guest could have tried to write metadata into the
> initial sector of their disk to try and get qemu to perform something on
> the guest's behalf due to the incorrect probe).  A more robust solution
> pairs either:
> qemu-nbd -f raw file.qcow2   <=>   qemu-img info -f qcow2 nbd://localhost
> nbdkit file file.qcow2   <=>   qemu-img info -f qcow2 nbd://localhost
> where NBD is exposing the qcow2 metadata over the network (whether via
> qemu-nbd or nbdkit), and the client HAS to be aware of qcow2, or:
> qemu-nbd -f qcow2 file.qcow2   <=>   qemu-img info -f raw nbd://localhost
> qemu-nbd -f qcow2 file.qcow2   <=>   any other nbd client
> where qemu-nbd is interpreting the qcow2 file, and only exposing the
> guest-visible data over the network.
> Or, repeating the above in a different formulation - when you are
> interested in serving qcow2 data over the network, you HAVE to have
> exactly one of your two endpoints be aware that the qcow2 format is
> involved - either the server is agnostic and the client interprets qcow2
> (the former setup), or the server interprets qcow2 and the client is
> agnostic (the latter setup).
> Then, as Rich pointed out, when you expose qcow2 over NBD (the former
> setup), the current NBD protocol does NOT allow resizes - so while
> reading the image works okay, writing to the image is likely to hit
> ENOSPC unless you pre-allocated the .qcow2 file being served.  That is
> likely to change upstream in the next six months (it's on my todo list
> to get NBD resize implemented), but there's still a percolation delay
> from upstream to using it in default OS installs.  On the other hand,
> when you expose raw over NBD by using qemu-nbd to convert qcow2 to raw
> on the server end, then the server can resize things as needed and the
> client need not be aware that qcow2 is even involved.
> After nbd resize implemented, we can get actual disk size instread of
'unavailable', right?
Will it support disk extend, or just support disk shrinking?

> As for why nbdkit doesn't have a plugin for exposing a local qcow2 file
> as raw - we don't see the point in duplicating what qemu-nbd can already
> do. Yes, someone could write such a plugin, but it will probably have
> worse performance than what you can already get out of using qemu-nbd as
> the server.
> > However the fundamental problem remains that you're exporting qcow2
> > bytes over the NBD connection which is not really what you wanted to
> > do.
> Maybe it is, if the qcow2 file is fully preallocated, but yeah, until
> NBD resize exists, most people don't want that combination.
> >
> > You should use qemu-nbd instead since it understands qcow2 natively,
> > or use raw format disks if you want to use nbdkit for its other
> > features.
> >
> > Rich.
> >
> --
> Eric Blake, Principal Software Engineer
> Red Hat, Inc.           +1-919-301-3266
> Virtualization:  qemu.org | libvirt.org

Best regards,
Han Han
Quality Engineer

Email: hhan at redhat.com
Phone: +861065339333
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