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

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

Cc: Arnd Bergmann <[e-mail]>
Cc: Greg Kroah-Hartman <[e-mail]>
Reported-by: Muhammad Usama Anjum <[e-mail]>
Reviewed-by: Muhammad Usama Anjum <[e-mail]>
Signed-off-by: Kees Cook <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/misc/lkdtm/fortify.c

···
 	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];