static ssize_t ppp_read(struct file *file, char __user *buf,
			size_t count, loff_t *ppos)
{
	struct ppp_file *pf = file->private_data;
	DECLARE_WAITQUEUE(wait, current);
	ssize_t ret;
	struct sk_buff *skb = NULL;
	struct iovec iov;
	struct iov_iter to;

	ret = count;

	if (!pf)
		return -ENXIO;
	add_wait_queue(&pf->rwait, &wait);
	for (;;) {
		set_current_state(TASK_INTERRUPTIBLE);
		skb = skb_dequeue(&pf->rq);
		if (skb)
			break;
		ret = 0;
		if (pf->dead)
			break;
		if (pf->kind == INTERFACE) {
			/*
			 * Return 0 (EOF) on an interface that has no
			 * channels connected, unless it is looping
			 * network traffic (demand mode).
			 */
			struct ppp *ppp = PF_TO_PPP(pf);
			if (ppp->n_channels == 0 &&
			    (ppp->flags & SC_LOOP_TRAFFIC) == 0)
				break;
		}
		ret = -EAGAIN;
		if (file->f_flags & O_NONBLOCK)
			break;
		ret = -ERESTARTSYS;
		if (signal_pending(current))
			break;
		schedule();
	}
	set_current_state(TASK_RUNNING);
	remove_wait_queue(&pf->rwait, &wait);

	if (!skb)
		goto out;

	ret = -EOVERFLOW;
	if (skb->len > count)
		goto outf;
	ret = -EFAULT;
	iov.iov_base = buf;
	iov.iov_len = count;
	iov_iter_init(&to, READ, &iov, 1, count);
	if (skb_copy_datagram_iter(skb, 0, &to, skb->len))
		goto outf;
	ret = skb->len;

 outf:
	kfree_skb(skb);
 out:
	return ret;
}
Unknown