lkdtm/fortify: Swap memcpy() for strncpy()

The memcpy() runtime defenses are still not landed, so test with
strncpy() for now.

Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reported-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Reviewed-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20220216202548.2093883-1-keescook@chromium.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/misc/lkdtm/fortify.c

@@ -44,14 +44,14 @@ void lkdtm_FORTIFIED_SUBOBJECT(void)
 	strscpy(src, "over ten bytes", size);
 	size = strlen(src) + 1;
 
-	pr_info("trying to strcpy past the end of a member of a struct\n");
+	pr_info("trying to strncpy past the end of a member of a struct\n");
 
 	/*
-	 * memcpy(target.a, src, 20); will hit a compile error because the
+	 * strncpy(target.a, src, 20); will hit a compile error because the
 	 * compiler knows at build time that target.a < 20 bytes. Use a
 	 * volatile to force a runtime error.
 	 */
-	memcpy(target.a, src, size);
+	strncpy(target.a, src, size);
 
 	/* Store result to global to prevent the code from being eliminated */
 	fortify_scratch_space = target.a[3];