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

From: Taylor Blau <me@ttaylorr.com>
Date: May 20, 2025, 17:59 UTC
Message-ID: <aCzDCEK7OhCYyAZa@nand.local>
In-Reply-To: <9f07da4fe71d21b14226d8f0132cd3c8600fba13.1747753388.git.phillip.wood@dunelm.org.uk>

On Tue, May 20, 2025 at 04:04:25PM +0100, Phillip Wood wrote:

Show 15 quoted lines

> 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.

I like this approach quite a bit, thanks!

Thanks, Taylor