lkdtm: Prevent the compiler from optimising lkdtm_CORRUPT_STACK()

At least on powerpc with GCC 6, the compiler is smart enough to optimise
lkdtm_CORRUPT_STACK() into an empty function that just returns.

If we print the buffer after we've written to it that prevents the
compiler from optimising away data and the memset().
Signed-off-by: NMichael Ellerman <mpe@ellerman.id.au>
Acked-by: NKees Cook <keescook@chromium.org>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/misc/lkdtm_bugs.c

@@ -80,7 +80,8 @@ noinline void lkdtm_CORRUPT_STACK(void)
 	/* Use default char array length that triggers stack protection. */
 	char data[8];
 
-	memset((void *)data, 0, 64);
+	memset((void *)data, 'a', 64);
+	pr_info("Corrupted stack with '%16s'...\n", data);
 }
 
 void lkdtm_UNALIGNED_LOAD_STORE_WRITE(void)