This is what DEBUG_NOTIFIERS performs (see kernel/notifier.c):

```
#ifdef CONFIG_DEBUG_NOTIFIERS
		if (unlikely(!func_ptr_is_kernel_text(nb->notifier_call))) {
			WARN(1, "Invalid notifier called!");
			nb = next_nb;
			continue;
		}
#endif
```

CFI can do the same better.

Thanks to @thestinger for the idea.

Refers to #99.