Re: [Qemu-devel] [PATCH 0/2] qcow2: Add two more unalignment checks

[Max Reitz <mreitz@redhat.com>]

On 2015-01-19 at 16:04, Eric Blake wrote:
> On 01/19/2015 01:49 PM, Max Reitz wrote:
>> With the series adding unalignment checks and the series reworking the
>> zero cluster expansion code overlapping, the unalignment checks have not
>> been implemented in the latter code.
>>
>> This series fixes it.
>>
>> There are other places which would require unalignment checks, like the
>> offsets of L1 tables, especially for snapshots; but because it would be
>> best to add these checks in the function which reads the snapshot table,
>> this would make images with broken snapshots completely unusable, which
>> is something I opted to avoid for now.
>>
>> Ideally, we need to make the qcow2 repair function repair such cases,
>> but until that is done there is not much we can do about them.
> What's the best repair?

That's what I was asking myself...

> Read the data from the unaligned location, and
> write a fresh copy into a new aligned allocation?

Maybe. Maybe there is no way of repairing them and the only way is 
asking the user whether it's fine to delete the snapshot (qemu-img check 
-r make_consistent_whatever_it_takes).

Also, the question remains for every unaligned data structure: L2 
tables, data clusters… The refcount structures are the only ones that 
can be easily recovered. For data clusters, reading from the unaligned 
location would probably be best; for L2 tables… maybe the same, then run 
a consistency check on the data and if it seems off™, throw the whole L2 
table away.

> At any rate, I agree
> that repairing unaligned locations is harder than detecting them, so
> your series is fine as is.
>
> Reviewed-by: Eric Blake <eblake@redhat.com>

Thanks!

Max