Re: [PATCH 2/4] midx repack: avoid potential integer overflow on 64 bit systems

[Phillip Wood <phillip.wood123@gmail.com>]

On 20/05/2025 18:59, Taylor Blau wrote:
> On Tue, May 20, 2025 at 04:04:25PM +0100, Phillip Wood wrote:
>> From: Phillip Wood <phillip.wood@dunelm.org.uk>
>>
>> On a 64 bit system the calculation
>>
>>      p->pack_size * pack_info[i].referenced_objects
>>
>> could overflow. If a pack file contains 2^28 objects with an average
>> compressed size of 1KB then the pack size will be 2^38B. If all of the
>> objects are referenced by the multi-pack index the sum above will
>> overflow. Avoid this by using shifted integer arithmetic and changing
>> the order of the calculation so that the pack size is divided by the
>> total number of objects in the pack before multiplying by the number of
>> objects referenced by the multi-pack index. Using a shift of 14 bits
>> should give reasonable accuracy while avoiding overflow for pack sizes
>> less that 1PB.
> 
> Ahhh, this renders some of comments on the previous patch moot. I think
> that this is a not-unreasonable concern to be addressing even on modern
> 64-bit systems, since I have definitely encountered packs that have on
> the order of ~2^28 objects in them.

Thanks, that's good to know

Phillip

> I like this approach quite a bit, thanks!
> 
> Thanks,
> Taylor