Libguestfs - Nov 2018 - nbdkit: Could not read L1 table when reading exported qcow2

Hello, I face a problem on the nbdkit server:

Version-Release number of selected component (if applicable):
nbdkit-v1.9.1
qemu-utils-2.12+dfsg-3+b1

How reproducible:
100%

Steps:
1. Create a qcow2 file
# qemu-img create disk 1G -f qcow2

2. Export the qcow2 file then try to read it via nbd url
# nbdkit file file=disk -e raw -v
nbdkit: debug: TLS disabled: could not load TLS certificates
nbdkit: debug: registering plugin
/usr/local/lib/nbdkit/plugins/nbdkit-file-plugin.so
nbdkit: debug: registered plugin
/usr/local/lib/nbdkit/plugins/nbdkit-file-plugin.so (name file)
nbdkit: debug: file: load
nbdkit: debug: file: config key=file, value=/tmp/disk
nbdkit: debug: file: config_complete
nbdkit: debug: bound to IP address <any>:10809 (2 socket(s))
nbdkit: debug: forked into background (new pid = 32524)

3. Try to read the file via nbd:
# qemu-img info
nbd://localhost/raw

qemu-img: Could not open 'nbd://localhost/raw': Could not read L1 table:
Invalid argument

Logs from nbdkit:
nbdkit: debug: file: open readonly=0
nbdkit: file[1]: debug: newstyle negotiation: flags: global 0x3
nbdkit: file[1]: debug: newstyle negotiation: client flags: 0x3
nbdkit: file[1]: debug: newstyle negotiation: NBD_OPT_GO: client requested
export 'raw' (ignored)
nbdkit: file[1]: debug: get_size
nbdkit: file[1]: debug: can_write
nbdkit: file[1]: debug: can_zero
nbdkit: file[1]: debug: can_write
nbdkit: file[1]: debug: can_trim
nbdkit: file[1]: debug: can_fua
nbdkit: file[1]: debug: can_flush
nbdkit: file[1]: debug: is_rotational
nbdkit: file[1]: debug: newstyle negotiation: flags: export 0x6d
nbdkit: file[1]: debug: newstyle negotiation: NBD_OPT_GO: ignoring
NBD_INFO_* request 3
nbdkit: file[1]: debug: handshake complete, processing requests with 16
threads
nbdkit: debug: starting worker thread file.0
nbdkit: debug: starting worker thread file.1
nbdkit: debug: starting worker thread file.3
nbdkit: debug: starting worker thread file.2
nbdkit: debug: starting worker thread file.4
nbdkit: debug: starting worker thread file.5
nbdkit: debug: starting worker thread file.7
nbdkit: debug: starting worker thread file.6
nbdkit: debug: starting worker thread file.8
nbdkit: debug: starting worker thread file.9
nbdkit: debug: starting worker thread file.10
nbdkit: debug: starting worker thread file.11
nbdkit: debug: starting worker thread file.12
nbdkit: debug: starting worker thread file.13
nbdkit: debug: starting worker thread file.14
nbdkit: debug: starting worker thread file.15
nbdkit: file.1: debug: pread count=512 offset=0
nbdkit: file.0: debug: pread count=512 offset=0
nbdkit: file.3: debug: sending error reply: Invalid argument
nbdkit: file.4: debug: client sent disconnect command, closing connection
nbdkit: file.4: debug: exiting worker thread file.4
nbdkit: file.2: debug: exiting worker thread file.2
nbdkit: file.7: debug: exiting worker thread file.7
nbdkit: file.5: debug: exiting worker thread file.5
nbdkit: file.6: debug: exiting worker thread file.6
nbdkit: file.8: debug: exiting worker thread file.8
nbdkit: file.9: debug: exiting worker thread file.9
nbdkit: file.10: debug: exiting worker thread file.10
nbdkit: file.11: debug: exiting worker thread file.11
nbdkit: file.12: debug: exiting worker thread file.12
nbdkit: file.0: debug: exiting worker thread file.0
nbdkit: file.14: debug: exiting worker thread file.14
nbdkit: file.15: debug: exiting worker thread file.15
nbdkit: file.1: debug: exiting worker thread file.1
nbdkit: file.13: debug: exiting worker thread file.13
nbdkit: file.3: debug: exiting worker thread file.3
nbdkit: file[1]: debug: close


Could you please check if it is a bug from nbdkit? Thanks
-- 
Best regards,
-----------------------------------
Han Han
Quality Engineer
Redhat.

Email: hhan@redhat.com
Phone: +861065339333

On Wed, Nov 21, 2018 at 11:33:33AM +0800, Han Han wrote:
> Hello, I face a problem on the nbdkit server:
> 
> Version-Release number of selected component (if applicable):
> nbdkit-v1.9.1
> qemu-utils-2.12+dfsg-3+b1
> 
> How reproducible:
> 100%
> 
> Steps:
> 1. Create a qcow2 file
> # qemu-img create disk 1G -f qcow2
This creates a 1GB virtual size qcow2 (real size is about 196K):

$ ll disk
-rw-rw-r--. 1 rjones rjones 196624 Nov 21 09:20 disk
$ file disk
disk: QEMU QCOW Image (v3), 1073741824 bytes
> 2. Export the qcow2 file then try to read it via nbd url
> # nbdkit file file=disk -e raw -v
This exports the disk using the export name "raw".  However nbdkit
doesn't know anything about qcow2, so this exports bytes of qcow2 data
to the client.  The virtual size of the disk will be about 196K, and
it won't be resizable (so you cannot write to it).

Note that the -e (export name) option is not used by nbdkit so you
don't need it.
> 3. Try to read the file via nbd:
> # qemu-img info
> nbd://localhost/raw
What precisely happens here is that qemu-img sees a qcow2 file and
tries to read beyond the end of the file:

nbdkit: file.2: error: invalid request: offset and count are out of range:
offset=196608 count=512

If you want to export a qcow2 file, then it's better to use qemu-nbd
since it knows about the qcow2 format:

$ qemu-nbd -t disk &
$ qemu-img info nbd://localhost
image: nbd://localhost:10809
file format: raw
virtual size: 1.0G (1073741824 bytes)
disk size: unavailable

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
virt-builder quickly builds VMs from scratch
http://libguestfs.org/virt-builder.1.html

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.

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:

$ qemu-img info nbd://localhost
image: nbd://localhost:10809
file format: qcow2
virtual size: 1.0G (1073741824 bytes)
disk size: unavailable
cluster_size: 65536
Format specific information:
    compat: 1.1
    lazy refcounts: false
    refcount bits: 16
    corrupt: false

However the fundamental problem remains that you're exporting qcow2
bytes over the NBD connection which is not really what you wanted to
do.

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.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
libguestfs lets you edit virtual machines.  Supports shell scripting,
bindings from many languages.  http://libguestfs.org