use st_add and st_mult for allocation size computation

If our size computation overflows size_t, we may allocate a
much smaller buffer than we expected and overflow it. It's
probably impossible to trigger an overflow in most of these
sites in practice, but it is easy enough convert their
additions and multiplications into overflow-checking
variants. This may be fixing real bugs, and it makes
auditing the code easier.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 4 insertions, 2 deletions

diff --git a/diffcore-delta.c b/diffcore-delta.c
index 7cf431d26..4159748a7 100644
--- a/diffcore-delta.c
+++ b/diffcore-delta.c
@@ -53,7 +53,8 @@ static struct spanhash_top *spanhash_rehash(struct spanhash_top *orig)
 	int osz = 1 << orig->alloc_log2;
 	int sz = osz << 1;
 
-	new = xmalloc(sizeof(*orig) + sizeof(struct spanhash) * sz);
+	new = xmalloc(st_add(sizeof(*orig),
+			     st_mult(sizeof(struct spanhash), sz)));
 	new->alloc_log2 = orig->alloc_log2 + 1;
 	new->free = INITIAL_FREE(new->alloc_log2);
 	memset(new->data, 0, sizeof(struct spanhash) * sz);
@@ -130,7 +131,8 @@ static struct spanhash_top *hash_chars(struct diff_filespec *one)
 	int is_text = !diff_filespec_is_binary(one);
 
 	i = INITIAL_HASH_SIZE;
-	hash = xmalloc(sizeof(*hash) + sizeof(struct spanhash) * (1<<i));
+	hash = xmalloc(st_add(sizeof(*hash),
+			      st_mult(sizeof(struct spanhash), 1<<i)));
 	hash->alloc_log2 = i;
 	hash->free = INITIAL_FREE(i);
 	memset(hash->data, 0, sizeof(struct spanhash) * (1<<i));