Re: [PATCH 10/20] midx: bounds-check large offset chunk

When we see a large offset bit in the regular midx offset table, we
use the entry as an index into a separate large offset table (just like
a pack idx does). But we don't bounds-check the access to that large
offset table (nor even record its size when we parse the chunk!).

The equivalent code for a regular pack idx is in check_pack_index_ptr().
But things are a bit simpler here because of the chunked format: we can
just check our array index directly.

As a bonus, we can get rid of the st_mult() here. If our array
bounds-check is successful, then we know that the result will fit in a
size_t (and the bounds check uses a division to avoid overflow
entirely).

Signed-off-by: Jeff King <redacted>
---
 midx.c                      |  8 +++++---
 midx.h                      |  1 +
 t/t5319-multi-pack-index.sh | 20 ++++++++++++++++++++
 3 files changed, 26 insertions(+), 3 deletions(-)

diff --git a/midx.c b/midx.c
index 7b1b45f381..3e768d0df0 100644
--- a/midx.c
+++ b/midx.c

@@ -180,7 +180,8 @@ struct multi_pack_index *load_multi_pack_index(const char *object_dir, int local
 	if (read_chunk(cf, MIDX_CHUNKID_OBJECTOFFSETS, midx_read_object_offsets, m))
 		die(_("multi-pack-index required object offsets chunk missing or corrupted"));

-	pair_chunk_unsafe(cf, MIDX_CHUNKID_LARGEOFFSETS, &m->chunk_large_offsets);
+	pair_chunk(cf, MIDX_CHUNKID_LARGEOFFSETS, &m->chunk_large_offsets,
+		   &m->chunk_large_offsets_len);

 	if (git_env_bool("GIT_TEST_MIDX_READ_RIDX", 1))
 		pair_chunk_unsafe(cf, MIDX_CHUNKID_REVINDEX, &m->chunk_revindex);

@@ -303,8 +304,9 @@ off_t nth_midxed_offset(struct multi_pack_index *m, uint32_t pos)
 			die(_("multi-pack-index stores a 64-bit offset, but off_t is too small"));

 		offset32 ^= MIDX_LARGE_OFFSET_NEEDED;
-		return get_be64(m->chunk_large_offsets +
-				st_mult(sizeof(uint64_t), offset32));
+		if (offset32 >= m->chunk_large_offsets_len / sizeof(uint64_t))
+			die(_("multi-pack-index large offset out of bounds"));
+		return get_be64(m->chunk_large_offsets + sizeof(uint64_t) * offset32);