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

From
Taylor Blau <me@ttaylorr.com>
Date
Jul 12, 2023, 23:37 UTC
Message-ID
<750d38cbeba42416d98895d6e0004d1ca6329ce5.1689205042.git.me@ttaylorr.com>
In-Reply-To
<cover.1689205042.git.me@ttaylorr.com>

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 <me@ttaylorr.com>
---
 packfile.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)
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);
 	}

-- 
2.41.0.347.g7b976b8871f
Previous: Junio C HamanoNext: Taylor Blau
Message 11 of 28 in “guard object lookups against 32-bit overflow”
  1. 00/20 guard object lookups against 32-bit overflowTaylor Blau, Jul 12, 2023
  2. 01/20 packfile.c: prevent overflow in `nth_packed_object_id()`Taylor Blau, Jul 12, 2023
  3. 02/20 packfile.c: prevent overflow in `load_idx()`Taylor Blau, Jul 12, 2023
  4. Phillip WoodJul 13, 2023
  5. Taylor BlauJul 13, 2023
  6. Taylor BlauJul 14, 2023
  7. Phillip WoodJul 14, 2023
  8. Junio C HamanoJul 14, 2023
  9. Phillip WoodJul 14, 2023
  10. Junio C HamanoJul 13, 2023
  11. 03/20 packfile.c: use checked arithmetic in `nth_packed_object_offset()`Taylor Blau, Jul 12, 2023
  12. 04/20 midx.c: use `size_t`'s for fanout nr and allocTaylor Blau, Jul 12, 2023
  13. 05/20 midx.c: prevent overflow in `nth_midxed_object_oid()`Taylor Blau, Jul 12, 2023
  14. 06/20 midx.c: prevent overflow in `nth_midxed_offset()`Taylor Blau, Jul 12, 2023
  15. 08/20 midx.c: prevent overflow in `write_midx_internal()`Taylor Blau, Jul 12, 2023
  16. 09/20 midx.c: prevent overflow in `fill_included_packs_batch()`Taylor Blau, Jul 12, 2023
  17. 10/20 pack-bitmap.c: ensure that eindex lookups don't overflowTaylor Blau, Jul 12, 2023
  18. 11/20 commit-graph.c: prevent overflow in `write_commit_graph_file()`Taylor Blau, Jul 12, 2023
  19. 12/20 commit-graph.c: prevent overflow in add_graph_to_chain()Taylor Blau, Jul 12, 2023
  20. 13/20 commit-graph.c: prevent overflow in `load_oid_from_graph()`Taylor Blau, Jul 12, 2023
  21. 14/20 commit-graph.c: prevent overflow in `fill_commit_graph_info()`Taylor Blau, Jul 12, 2023
  22. 15/20 commit-graph.c: prevent overflow in `fill_commit_in_graph()`Taylor Blau, Jul 12, 2023
  23. 16/20 commit-graph.c: prevent overflow in `load_tree_for_commit()`Taylor Blau, Jul 12, 2023
  24. 17/20 commit-graph.c: prevent overflow in `split_graph_merge_strategy()`Taylor Blau, Jul 12, 2023
  25. 18/20 commit-graph.c: prevent overflow in `merge_commit_graph()`Taylor Blau, Jul 12, 2023
  26. 19/20 commit-graph.c: prevent overflow in `write_commit_graph()`Taylor Blau, Jul 12, 2023
  27. 20/20 commit-graph.c: prevent overflow in `verify_commit_graph()`Taylor Blau, Jul 12, 2023
  28. 07/20 midx.c: store `nr`, `alloc` variables as `size_t`'sTaylor Blau, Jul 12, 2023

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