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midx repack: avoid integer overflow on 32 bit systems

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On a 32 bit system "git multi-pack-index --repack --batch-size=120M"
failed with

    fatal: size_t overflow: 6038786 * 1289

The calculation to estimated size of the objects in the pack referenced
by the multi-pack-index uses st_mult() to multiply the pack size by the
number of referenced objects before dividing by the total number of
objects in the pack. As size_t is 32 bits on 32 bit systems this
calculation easily overflows. Fix this by using 64bit arithmetic instead.

Also fix a potential overflow when caluculating the total size of the
objects referenced by the multipack index with a batch size larger
than SIZE_MAX / 2. In that case

    total_size += estimated_size

can overflow as both total_size and estimated_size can be greater that
SIZE_MAX / 2. This is addressed by using saturating arithmetic for the
addition. Although estimated_size is of type uint64_t by the time we
reach this sum it is bounded by the batch size which is of type size_t
and so casting estimated_size to size_t does not truncate the value.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
This commit is contained in:
Phillip Wood
2025-05-22 16:55:20 +01:00
committed by Junio C Hamano
parent 8613c2bb6c
commit b103881d4f
2 changed files with 24 additions and 4 deletions
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12
midx-write.c
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@@ -1699,19 +1699,23 @@ static void fill_included_packs_batch(struct repository *r,
for (i = 0; total_size < batch_size && i < m->num_packs; i++) {
int pack_int_id = pack_info[i].pack_int_id;
struct packed_git *p = m->packs[pack_int_id];
size_t expected_size;
uint64_t expected_size;
if (!want_included_pack(r, m, pack_kept_objects, pack_int_id))
continue;
expected_size = st_mult(p->pack_size,
pack_info[i].referenced_objects);
expected_size = uint64_mult(p->pack_size,
pack_info[i].referenced_objects);
expected_size /= p->num_objects;
if (expected_size >= batch_size)
continue;
total_size += expected_size;
if (unsigned_add_overflows(total_size, (size_t)expected_size))
total_size = SIZE_MAX;
else
total_size += expected_size;
include_pack[pack_int_id] = 1;
}