PM / hibernate: Avoid overflow in hibernate_preallocate_memory

When system has a lot of highmem(e.g. 16GiB using a 32 bits kernel), the code to calculate how much memory we need to preallocate in normal zone may cause overflow. As Leon has analysed:
"
It looks that during computing 'alloc' variable there is overflow:
alloc = (3943404 - 1970542) - 1978280 = -5418 (signed)
And this function goes to err_out.
"
Fix this by avoiding that overflow.

Reference: https://bugzilla.kernel.org/show_bug.cgi?id=60817

Reported-and-tested-by Leon Drugi <[email protected]>

Cc stable <[email protected]>

Signed-off-by Aaron Lu <[email protected]>

---
 kernel/power/snapshot.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

kernel/power/snapshot.c +0 -0

index 98c3b34a4cff..91aa616a1f4e 100644

kernel/power/snapshot.c +0 -0

 	pages = preallocate_image_memory(alloc, avail_normal);
 	if (pages < alloc) {
 		/* We have exhausted non-highmem pages, try highmem. */

Review

Quoted 4 lines · Aaron Lu

(quoted diff) +2 -1

 	pages_highmem = preallocate_image_highmem(highmem / 2);
-	alloc = (count - max_size) - pages_highmem;
+	alloc = (count - max_size) > pages_highmem ?
+		(count - max_size - pages_highmem) : 0;

Well, what about

	alloc = count - max_size;
	if (alloc > pages_highmem)
		alloc -= pages_highmem;
	else
		alloc = 0;