Re: [Qemu-devel] [RFC] Check backing_file chain's loop

[Eric Blake]

On 05/30/2013 04:14 AM, Dongxu Wang wrote:
> Hi,
> 
> Now block layer does not check whether backing_file chain can be a
> circle, do you think it is necessary to do a check?
> 
> For example,  1.qcow2's backing_file is 2.qcow2 and 2.qcow2's
> backing_file is 1.qcow2, then any IO operation will be a fault.
> 
> If necessary, I will post a patch to fix.

Yes, it's worth patching.

> 
> [$] qemu-img create -f qcow2 1.qcow2
> qemu-img: Image creation needs a size parameter
> [$] qemu-img create -f qcow2 1.qcow2 8M
> Formatting '1.qcow2', fmt=qcow2 size=8388608 encryption=off
> cluster_size=65536 lazy_refcounts=off
> [$] qemu-img create -f qcow2 2.qcow2 -o backing_file=1.qcow2
> Formatting '2.qcow2', fmt=qcow2 size=8388608 backing_file='1.qcow2'
> encryption=off cluster_size=65536 lazy_refcounts=off
> [$] qemu-img create -f qcow2 1.qcow2 -o backing_file=2.qcow2
> Formatting '1.qcow2', fmt=qcow2 size=8388608 backing_file='2.qcow2'
> encryption=off cluster_size=65536 lazy_refcounts=off
> [$] qemu-io -c "read -v 0 512" 1.qcow2
> ^CSegmentation fault (core dumped)

Also make sure that 'qemu-img info 1.qcow2' and 'qemu-img info
--backing-chain 1.qcow2' don't crash.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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