midx: use 64-bit multiplication for chunk sizes

When calculating the sizes of certain chunks, we should use 64-bit
multiplication always. This allows us to properly predict the chunk
sizes without risk of overflow.

Other possible overflows were discovered by evaluating each
multiplication in midx.c and ensuring that at least one side of the
operator was of type size_t or off_t.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

midx.c

@@ -244,7 +244,7 @@ static off_t nth_midxed_offset(struct multi_pack_index *m, uint32_t pos)
 	const unsigned char *offset_data;
 	uint32_t offset32;
 
-	offset_data = m->chunk_object_offsets + pos * MIDX_CHUNK_OFFSET_WIDTH;
+	offset_data = m->chunk_object_offsets + (off_t)pos * MIDX_CHUNK_OFFSET_WIDTH;
 	offset32 = get_be32(offset_data + sizeof(uint32_t));
 
 	if (m->chunk_large_offsets && offset32 & MIDX_LARGE_OFFSET_NEEDED) {
@@ -260,7 +260,8 @@ static off_t nth_midxed_offset(struct multi_pack_index *m, uint32_t pos)
 
 static uint32_t nth_midxed_pack_int_id(struct multi_pack_index *m, uint32_t pos)
 {
-	return get_be32(m->chunk_object_offsets + pos * MIDX_CHUNK_OFFSET_WIDTH);
+	return get_be32(m->chunk_object_offsets +
+			(off_t)pos * MIDX_CHUNK_OFFSET_WIDTH);
 }
 
 static int nth_midxed_pack_entry(struct repository *r,
@@ -912,15 +913,15 @@ static int write_midx_internal(const char *object_dir, struct multi_pack_index *
 	add_chunk(cf, MIDX_CHUNKID_OIDFANOUT, MIDX_CHUNK_FANOUT_SIZE,
 		  write_midx_oid_fanout);
 	add_chunk(cf, MIDX_CHUNKID_OIDLOOKUP,
-		  ctx.entries_nr * the_hash_algo->rawsz,
+		  (size_t)ctx.entries_nr * the_hash_algo->rawsz,
 		  write_midx_oid_lookup);
 	add_chunk(cf, MIDX_CHUNKID_OBJECTOFFSETS,
-		  ctx.entries_nr * MIDX_CHUNK_OFFSET_WIDTH,
+		  (size_t)ctx.entries_nr * MIDX_CHUNK_OFFSET_WIDTH,
 		  write_midx_object_offsets);
 
 	if (ctx.large_offsets_needed)
 		add_chunk(cf, MIDX_CHUNKID_LARGEOFFSETS,
-			ctx.num_large_offsets * MIDX_CHUNK_LARGE_OFFSET_WIDTH,
+			(size_t)ctx.num_large_offsets * MIDX_CHUNK_LARGE_OFFSET_WIDTH,
 			write_midx_large_offsets);
 
 	write_midx_header(f, get_num_chunks(cf), ctx.nr - dropped_packs);