[03/20] packfile.c: use checked arithmetic in `nth_packed_object_offset()`

Commit Message

Taylor Blau July 12, 2023, 11:37 p.m. UTC

In a similar spirit as the previous commits, ensure that we use
`st_add()` or `st_mult()` when computing values that may overflow the
32-bit unsigned limit.

Note that in each of these instances, we prevent 32-bit overflow
already since we have explicit casts to `size_t`.

So this code is OK as-is, but let's clarify it by using the `st_xyz()`
helpers to make it obvious that we are performing the relevant
computations using 64 bits.

Signed-off-by: Taylor Blau <[e-mail]>
---
 packfile.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

Patch

diff --git a/packfile.c b/packfile.c
index 70acf1694b..e8e01e348e 100644
--- a/packfile.c
+++ b/packfile.c
@@ -1948,14 +1948,15 @@  off_t nth_packed_object_offset(const struct packed_git *p, uint32_t n)
 	const unsigned int hashsz = the_hash_algo->rawsz;
 	index += 4 * 256;
 	if (p->index_version == 1) {
-		return ntohl(*((uint32_t *)(index + (hashsz + 4) * (size_t)n)));
+		return ntohl(*((uint32_t *)(index + st_mult(hashsz + 4, n))));
 	} else {
 		uint32_t off;
-		index += 8 + (size_t)p->num_objects * (hashsz + 4);
-		off = ntohl(*((uint32_t *)(index + 4 * n)));
+		index += st_add(8, st_mult(p->num_objects, hashsz + 4));
+		off = ntohl(*((uint32_t *)(index + st_mult(4, n))));
 		if (!(off & 0x80000000))
 			return off;
-		index += (size_t)p->num_objects * 4 + (off & 0x7fffffff) * 8;
+		index += st_add(st_mult(p->num_objects, 4),
+				st_mult(off & 0x7fffffff, 8));
 		check_pack_index_ptr(p, index);
 		return get_be64(index);
 	}