Re: [Qemu-devel] [PATCH 07/21] qcow2: Helper for refcount array size calculation

[Max Reitz <mreitz@redhat.com>]

On 2014-11-10 at 23:49, Eric Blake wrote:
> On 11/10/2014 06:45 AM, Max Reitz wrote:
>> Add a helper function which correctly calculates the byte size of a
>> refcount array for any refcount order, and use that function.
>>
>> Signed-off-by: Max Reitz <mreitz@redhat.com>
>> ---
>>   block/qcow2-refcount.c | 39 ++++++++++++++++++++++++++++-----------
>>   1 file changed, 28 insertions(+), 11 deletions(-)
>>
>> diff --git a/block/qcow2-refcount.c b/block/qcow2-refcount.c
>> index 16652da..cfb4807 100644
>> --- a/block/qcow2-refcount.c
>> +++ b/block/qcow2-refcount.c
>> @@ -1132,6 +1132,20 @@ fail:
>>   /* refcount checking functions */
>>   
>>   
>> +static size_t refcount_array_byte_size(BDRVQcowState *s, uint64_t entries)
>> +{
>> +    if (s->refcount_order < 3) {
>> +        /* sub-byte width */
>> +        int shift = 3 - s->refcount_order;
>> +        return (entries + (1 << shift) - 1) >> shift;
>> +    } else if (s->refcount_order == 3) {
>> +        /* byte width */
>> +        return entries;
>> +    } else {
>> +        /* multiple bytes wide */
>> +        return entries << (s->refcount_order - 3);
>> +    }
> A comment proving why this can't overflow might be nice (if I analyzed
> correctly, entries will be computed by file size / clusters, and in the
> worst case, the smallest cluster and largest refcount_order results in
> '(size >> 9) << (6 - 3)' which is still safe).

Yes, will do.

>> @@ -1161,12 +1175,13 @@ static int inc_refcounts(BlockDriverState *bs,
>>           k = cluster_offset >> s->cluster_bits;
>>           if (k >= *refcount_table_size) {
>>               int64_t old_refcount_table_size = *refcount_table_size;
>> +            size_t new_byte_size;
>>               void *new_refcount_table;
>>   
>>               *refcount_table_size = k + 1;
>> -            new_refcount_table = g_try_realloc(*refcount_table,
>> -                                               *refcount_table_size *
>> -                                               s->refcount_bits / 8);
>> +            new_byte_size = refcount_array_byte_size(s, *refcount_table_size);
>> +
>> +            new_refcount_table = g_try_realloc(*refcount_table, new_byte_size);
> Yay - this addresses one of my possible overflow comments on 6/21.
>
> I wonder if the series would have less churn if you rearranged this
> patch to come before 6/21.

Why not, I'll add an __attribute__((used)) to it (which should be fine 
for the duration of a single patch).

Max