Re: [Qemu-devel] [PATCH] qcow2: Unset zero_beyond_eof in save_vmstate

[Max Reitz <mreitz@redhat.com>]

On 2013-10-21 22:37, Eric Blake wrote:
> On 10/20/2013 08:52 PM, Max Reitz wrote:
>> Saving the VM state is done using bdrv_pwrite. This function may perform
>> a read-modify-write, which in this case results in data being read from
>> beyond the end of the virtual disk. Since we are actually trying to
>> access an area which is not a part of the virtual disk, zero_beyond_eof
>> has to be set to false before performing the partial write, otherwise
>> the VM state may become corrupted.
>>
>> Signed-off-by: Max Reitz <mreitz@redhat.com>
>> ---
>> Follow-up to (depends on):
>>   - qcow2: Restore total_sectors value in save_vmstate
> Reviewed-by: Eric Blake <eblake@redhat.com>
>
> Do you have test cases that demonstrate the corruption pre-patch?

I could write a test case for the other patch, but for this one it would 
probably be rather difficult. What I did to bisect the bug was just 
starting a VM over and over while saving a snapshot at some time during 
boot-up and trying to load that snapshot again. Sometimes, qemu itself 
would report a corrupted VM state, but most of the time, the guest 
simply hang or paniced. This is something I can detect interactively, 
but I don't know if I could write a test for this (at least not for 
hanging).

Max