lkdtm/heap: Avoid __alloc_size hint warning for VMALLOC_LINEAR_OVERFLOW

Once __alloc_size hints have been added, the compiler will (correctly!)
see this as an overflow. We are, however, trying to test for this
condition at run-time (not compile-time), so work around it with a
volatile int offset.

Cc: Arnd Bergmann <[email protected]>
Signed-off-by: Kees Cook <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Greg Kroah-Hartman <[email protected]>

Author: kees

drivers/misc/lkdtm/heap.c

@@ -12,6 +12,13 @@ static struct kmem_cache *double_free_cache;
 static struct kmem_cache *a_cache;
 static struct kmem_cache *b_cache;
 
+/*
+ * Using volatile here means the compiler cannot ever make assumptions
+ * about this value. This means compile-time length checks involving
+ * this variable cannot be performed; only run-time checks.
+ */
+static volatile int __offset = 1;
+
 /*
  * If there aren't guard pages, it's likely that a consecutive allocation will
  * let us overflow into the second allocation without overwriting something real.
@@ -24,7 +31,7 @@ void lkdtm_VMALLOC_LINEAR_OVERFLOW(void)
 	two = vzalloc(PAGE_SIZE);
 
 	pr_info("Attempting vmalloc linear overflow ...\n");
-	memset(one, 0xAA, PAGE_SIZE + 1);
+	memset(one, 0xAA, PAGE_SIZE + __offset);
 
 	vfree(two);
 	vfree(one);