Re: [PATCH v8 1/2] seccomp: add a return code to trap to userspace

[Oleg Nesterov <oleg@redhat.com>]

On 10/29, Tycho Andersen wrote:
>
> +static struct file *init_listener(struct seccomp_filter *filter)
> +{
> +	struct file *ret = ERR_PTR(-EBUSY);
> +	struct seccomp_filter *cur, *last_locked = NULL;
> +	int filter_nesting = 0;
> +
> +	for (cur = current->seccomp.filter; cur; cur = cur->prev) {
> +		mutex_lock_nested(&cur->notify_lock, filter_nesting);
> +		filter_nesting++;
> +		last_locked = cur;
> +		if (cur->notif)
> +			goto out;
> +	}

Somehow I no longer understand why do you need to take all locks. Isn't
the first filter's notify_lock enough? IOW,

		for (cur = current->seccomp.filter; cur; cur = cur->prev) {
			if (cur->notif)
				return ERR_PTR(-EBUSY);
			first = cur;
		}

		if (first)
			mutex_lock(&first->notify_lock);

		... initialize filter->notif ...

	out:
		if (first)
			mutex_unlock(&first->notify_lock);

		return ret;

Oleg.