Re: [PATCH v2] qcow2-bitmap: Fix uint64_t left-shift overflow

[Vladimir Sementsov-Ogievskiy]

29.10.2019 14:14, Max Reitz wrote:
> On 28.10.19 07:33, Tuguoyi wrote:
>> In check_constraints_on_bitmap(), the sanity check on the
>> granularity will cause uint64_t integer left-shift overflow
>> when cluster_size is 2M and the granularity is BIGGER than
>> 32K. As a result, for a qcow2 disk with cluster_size set to
>> 2M, we could not even create a dirty bitmap with default
>> granularity. This patch fix the issue by dividing @len by
>> granularity instead.
>>
>> Signed-off-by: Guoyi Tu <address@hidden>
>> ---
>>   block/qcow2-bitmap.c | 4 ++--
>>   1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/block/qcow2-bitmap.c b/block/qcow2-bitmap.c
>> index 98294a7..71ac822 100644
>> --- a/block/qcow2-bitmap.c
>> +++ b/block/qcow2-bitmap.c
>> @@ -172,8 +172,8 @@ static int check_constraints_on_bitmap(BlockDriverState 
>> *bs,
>>       }
>>   
>>       if ((len > (uint64_t)BME_MAX_PHYS_SIZE << granularity_bits) ||
>> -        (len > (uint64_t)BME_MAX_TABLE_SIZE * s->cluster_size <<
>> -               granularity_bits))
>> +        (DIV_ROUND_UP(len, granularity) > (uint64_t)BME_MAX_TABLE_SIZE *
>> +                s->cluster_size))
> 
> This didn’t change because of this patch, but doesn’t this comparison
> need a conversion of bits to bytes somewhere?
> 
> len / granularity gives us the number of bits needed for the bitmap.
> 
> BME_MAX_TABLE_SIZE is, as far as I can see, a number of bitmap clusters,
> so multiplying it by the cluster size gives the number of bytes in the
> bitmap.  But the number of bits is eight times higher.

I think you are right. It would be better to fix it in the same patch..

> 
> Another topic: Isn’t BME_MAX_TABLE_SIZE too big?

Maybe) Still, I don't sure that we need to change it..

>  As it is, bitmap
> tables can have a size of 1 GB, and that’s the table alone.  Depending
> on the cluster size, the bitmap would take up at least 64 GB and cover
> at least 32 TB (at a granularity of 512 bytes).
> 
> Max
> 
>>       {
>>           error_setg(errp, "Too much space will be occupied by the bitmap. "
>>                      "Use larger granularity");
>>
> 
> 


-- 
Best regards,
Vladimir