Re: UDP recvmsg blocks after select(), 2.6 bug?

[David S. Miller]

On Thu, 07 Oct 2004 16:24:13 -0600
Chris Friesen <cfriesen@xxxxxxxxxxxxxxxxxx> wrote:

> I believe you misread what I said.  Just before the above quote, I said "We are 
> discussing the case where the socket is nonblocking and the udp checksum is 
> corrupt, right? "

And in that case we return -EAGAIN and always have.

> What I had in mind was that the non-blocking file descriptor have select() 
> return without verifying the checksum, and if it was discovered to be bad at 
> recvmsg() time, we return EAGAIN.

That's what we do.  In net/ipv4/udp.c:udp_recvmsg()

	if (skb->ip_summed==CHECKSUM_UNNECESSARY) {
		err = skb_copy_datagram_iovec(skb, sizeof(struct udphdr), msg->msg_iov,
					      copied);
	} else if (msg->msg_flags&MSG_TRUNC) {
		if (__udp_checksum_complete(skb))
			goto csum_copy_err;
		err = skb_copy_datagram_iovec(skb, sizeof(struct udphdr), msg->msg_iov,
					      copied);
	} else {
		err = skb_copy_and_csum_datagram_iovec(skb, sizeof(struct udphdr), msg->msg_iov);

		if (err == -EINVAL)
			goto csum_copy_err;
	}
 ...
csum_copy_err:
	UDP_INC_STATS_BH(UDP_MIB_INERRORS);

	/* Clear queue. */
	if (flags&MSG_PEEK) {
		int clear = 0;
		spin_lock_irq(&sk->sk_receive_queue.lock);
		if (skb == skb_peek(&sk->sk_receive_queue)) {
			__skb_unlink(skb, &sk->sk_receive_queue);
			clear = 1;
		}
		spin_unlock_irq(&sk->sk_receive_queue.lock);
		if (clear)
			kfree_skb(skb);
	}

	skb_free_datagram(sk, skb);

	if (noblock)
		return -EAGAIN;	
	goto try_again;

If socket is non-blocking, return -EAGAIN, else go back to "try_again"
where we block on packet arrival or error.
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