Re: [Qemu-devel] [PATCH 2/2] docs/qcow2: Correct refcount_block_entries

[Max Reitz <mreitz@redhat.com>]

On 30.08.2014 00:37, Eric Blake wrote:
> On 08/29/2014 03:45 PM, Max Reitz wrote:
>> A refblock entry may have a different size than 16 bits, it may even be
>> smaller than a byte. Correct the refcount_block_entries calculation
>> accordingly.
>>
>> Signed-off-by: Max Reitz <mreitz@redhat.com>
>> ---
>>   docs/specs/qcow2.txt | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/docs/specs/qcow2.txt b/docs/specs/qcow2.txt
>> index cfbc8b0..531c478 100644
>> --- a/docs/specs/qcow2.txt
>> +++ b/docs/specs/qcow2.txt
>> @@ -183,7 +183,7 @@ blocks and are exactly one cluster in size.
>>   Given a offset into the image file, the refcount of its cluster can be obtained
>>   as follows:
>>   
>> -    refcount_block_entries = (cluster_size / sizeof(uint16_t))
>> +    refcount_block_entries = (cluster_size / (refcount_bits / 8))
>>   
>
> Consider refcount_order == 0 (that is, no shared blocks, ALL blocks have
> at most refcount 1).  Then, refcount_bits is (1 << 0) == 1.  But 1/8 in
> integer math truncates to 0 (oops, division by zero is undefined); when
> in reality, the expression you want here is (cluster_size * 8 /
> refcount_bits).

If it is integer division, that is. ;-)

I'm counting on you accepting "cluster_size * 8 / refcount_bits" and not 
rejecting it because of a possible integer overflow. *g*

Max