Re: [PATCH] block: Improve backing file validation

[address@hidden]

On 2021/5/10 16:41, Daniel P. Berrangé wrote:
> On Mon, May 10, 2021 at 12:30:45PM +0800, Li Zhijian wrote:
>> Image below user cases:
>> case 1:
>> ```
>> $ qemu-img create -f raw source.raw 1G
>> $ qemu-img create -f qcow2 -F raw -b source.raw ./source.raw
>> qemu-img info source.raw
>> image: source.raw
>> file format: qcow2
>> virtual size: 193K (197120 bytes)
>> disk size: 196K
>> cluster_size: 65536
>> backing file: source.raw <<<<<<
>> backing file format: raw
>> Format specific information:
>>      compat: 1.1
>>      lazy refcounts: false
>>      refcount bits: 16
>>      corrupt: false
>> ```
>>
>> case 2:
>> ```
>> $ qemu-img create -f raw source.raw 1G
>> $ ln -sf source.raw destination.qcow2
>> $ qemu-img create -f qcow2 -F raw -b source.raw ./destination.qcow2
>> qemu-img info source.raw
>> image: source.raw
>> file format: qcow2 <<<<<<
>> virtual size: 2.0G (2147483648 bytes)
>> disk size: 196K
>> cluster_size: 65536
>> backing file: source.raw
>> backing file format: raw
>> Format specific information:
>>      compat: 1.1
>>      lazy refcounts: false
>>      refcount bits: 16
>>      corrupt: false
>> ```
>> Generally, we don't expect to corrupte the source.raw anyway, while
>> actually it does.
>>
>> Here we validate the realpath of file instead the input string.
> That still won't handle the case where you use hard links
>
>    $ ln source.raw destination.qcow2
>
> To properly validate the scenarios I think it is neccessary
> to ignore the filename sentirely.
>
> Instead attempt to open both files, and if successful, fstat()
> them both, and then compare the st_dev + st_ino  fields.


Sounds great, i will update it.

Thanks

Zhijian