[patch] perf: potential underflow in perf_sample_ustack_size()

[Dan Carpenter <dan.carpenter@oracle.com>]

The code here is trying to ensure that we don't have a
"header_size + stack_size" which is more than USHRT_MAX.  I changed
the overflow check a little to make it more clear.

My concern here is that if "header_size + sizeof(u64)" is very large
then we could end up with underflow doing the subtraction and end up
with a "stack_size" larger than intended.

I don't know perf well enough to say if this is possible.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>

diff --git a/kernel/events/core.c b/kernel/events/core.c
index dd236b6..eec9e683 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -4217,11 +4217,13 @@ perf_sample_ustack_size(u16 stack_size, u16 header_size,
 	header_size += 2 * sizeof(u64);
 
 	/* Do we fit in with the current stack dump size? */
-	if ((u16) (header_size + stack_size) < header_size) {
+	if (header_size > USHRT_MAX - stack_size) {
 		/*
 		 * If we overflow the maximum size for the sample,
 		 * we customize the stack dump size to fit in.
 		 */
+		if (header_size + sizeof(u64) > USHRT_MAX)
+			return 0;
 		stack_size = USHRT_MAX - header_size - sizeof(u64);
 		stack_size = round_up(stack_size, sizeof(u64));
 	}