@matheusbsbs / matheus-bagliano / commits / f874c0ed90

midx repack: avoid potential integer overflow on 64 bit systems

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. Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk> Signed-off-by: Junio C Hamano <gitster@pobox.com>

Phillip Wood committed May 22, 2025 at 16:55 UTC f874c0ed90c63276e0ebc445ad6fee5dcbfacb86

1 file changed +8 -2

midx-write.c

+8 -2

@@ -1704,9 +1704,15 @@ static void fill_included_packs_batch(struct repository *r,

1704 if (!want_included_pack(r, m, pack_kept_objects, pack_int_id))

1705 continue;

1706

1707 - expected_size = uint64_mult(p->pack_size,

1708 - pack_info[i].referenced_objects);

1707 + /*

1708 + * Use shifted integer arithmetic to calculate the

1709 + * expected pack size to ~4 significant digits without

1710 + * overflow for packsizes less that 1PB.

1711 + */

1712 + expected_size = (uint64_t)pack_info[i].referenced_objects << 14;

1713 expected_size /= p->num_objects;

1714 + expected_size = u64_mult(expected_size, p->pack_size);

1715 + expected_size = u64_add(expected_size, 1u << 13) >> 14;

1716

1717 if (expected_size >= batch_size)

1718 continue;