Subject: [PATCH v2 4/6] shallow.c: avoid theoretical pointer wrap-around

From: Nguyễn Thái Ngọc Duy <[email-masked]>
Date: Tue, 6 Dec 2016 19:53:37 +0700

From: Rasmus Villemoes <[email-masked]>

The expression info->free+size is technically undefined behaviour in
exactly the case we want to test for. Moreover, the compiler is likely
to translate the expression to

  (unsigned long)info->free + size > (unsigned long)info->end

where there's at least a theoretical chance that the LHS could wrap
around 0, giving a false negative.

This might as well be written using pointer subtraction avoiding these
issues.

Signed-off-by: Rasmus Villemoes <[email-masked]>
Signed-off-by: Nguyễn Thái Ngọc Duy <[email-masked]>
---
 shallow.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/shallow.c b/shallow.c
index 75e1702..719f699 100644
--- a/shallow.c
+++ b/shallow.c
@@ -446,7 +446,7 @@ static uint32_t *paint_alloc(struct paint_info *info)
 	unsigned nr = (info->nr_bits + 31) / 32;
 	unsigned size = nr * sizeof(uint32_t);
 	void *p;
-	if (!info->pool_count || info->free + size > info->end) {
+	if (!info->pool_count || size > info->end - info->free) {
 		if (size > POOL_SIZE)
 			die("BUG: pool size too small for %d in paint_alloc()",
 			    size);
-- 
2.8.2.524.g6ff3d78