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

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

[Dongxu Wang]

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.

[$] qemu-img create -f qcow2 1.qcow2
qemu-img: Image creation needs a size parameter
[$] 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)



Thanks.

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

[Eric Blake]

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

[Stefan Hajnoczi]

On Thu, May 30, 2013 at 06:12:00AM -0600, Eric Blake wrote:
> 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.

qemu-img info --backing-chain detects cycles and returns an error, see
the hash table in collect_image_info_list().

We should protect bdrv_open() too.

Stefan

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

[Dongxu Wang]

On 2013/5/30 20:48, Stefan Hajnoczi wrote:
>> hain 1.qcow2' don't crash.
> qemu-img info --backing-chain detects cycles and returns an error, see
> the hash table in collect_image_info_list().
>
> We should protect bdrv_open() too.
>
Okay, will send a patch.
> Stefan
>
>