[PATCH v3] lkdtm: fix memory copy size for WRITE_KERN

[Candle Sun]

From: Candle Sun <candle.sun@xxxxxxxxxx>

Though do_overwritten() follows do_nothing() in source code, the final
memory address order is determined by the compiler. We can't always
assume address of do_overwritten() is bigger than do_nothing(). At least
the Clang we are using places do_overwritten() before do_nothing() in the
object. This causes the copy size in lkdtm_WRITE_KERN() is *really* big
and WRITE_KERN test on ARM32 arch will fail.

Get absolute value of the address subtraction for memcpy() size.

Signed-off-by: Candle Sun <candle.sun@xxxxxxxxxx>
---
Changes in v3:
- Clean some typo.
- Remove one comment line.
Changes in v2:
- Use abs() in place of address comparison.
---
 drivers/misc/lkdtm/perms.c | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/drivers/misc/lkdtm/perms.c b/drivers/misc/lkdtm/perms.c
index 2dede2ef658f..c9227e08f97f 100644
--- a/drivers/misc/lkdtm/perms.c
+++ b/drivers/misc/lkdtm/perms.c
@@ -31,13 +31,12 @@ static unsigned long ro_after_init __ro_after_init = 0x55AA5500;
  * This just returns to the caller. It is designed to be copied into
  * non-executable memory regions.
  */
-static void do_nothing(void)
+static noinline void do_nothing(void)
 {
 	return;
 }
 
-/* Must immediately follow do_nothing for size calculuations to work out. */
-static void do_overwritten(void)
+static noinline void do_overwritten(void)
 {
 	pr_info("do_overwritten wasn't overwritten!\n");
 	return;
@@ -113,7 +112,7 @@ void lkdtm_WRITE_KERN(void)
 	size_t size;
 	volatile unsigned char *ptr;
 
-	size = (unsigned long)do_overwritten - (unsigned long)do_nothing;
+	size = (size_t)abs((uintptr_t)do_overwritten - (uintptr_t)do_nothing);
 	ptr = (unsigned char *)do_overwritten;
 
 	pr_info("attempting bad %zu byte write at %px\n", size, ptr);
-- 
2.17.0