udp: fix behavior of wrong checksums

We have two problems in UDP stack related to bogus checksums :

1) We return -EAGAIN to application even if receive queue is not empty.
   This breaks applications using edge trigger epoll()

2) Under UDP flood, we can loop forever without yielding to other
   processes, potentially hanging the host, especially on non SMP.

This patch is an attempt to make things better.

We might in the future add extra support for rt applications
wanting to better control time spent doing a recv() in a hostile
environment. For example we could validate checksums before queuing
packets in socket receive queue.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
int udp_recvmsg(struct sock *sk, struct msghdr *msg, size_t len, int noblock,
		int flags, int *addr_len)
{
	struct inet_sock *inet = inet_sk(sk);
	DECLARE_SOCKADDR(struct sockaddr_in *, sin, msg->msg_name);
	struct sk_buff *skb;
	unsigned int ulen, copied;
	int peeked, off = 0;
	int err;
	int is_udplite = IS_UDPLITE(sk);
	bool slow;

	if (flags & MSG_ERRQUEUE)
		return ip_recv_error(sk, msg, len, addr_len);

try_again:
	skb = __skb_recv_datagram(sk, flags | (noblock ? MSG_DONTWAIT : 0),
				  &peeked, &off, &err);
	if (!skb)
		goto out;

	ulen = skb->len - sizeof(struct udphdr);
	copied = len;
	if (copied > ulen)
		copied = ulen;
	else if (copied < ulen)
		msg->msg_flags |= MSG_TRUNC;

	/*
	 * If checksum is needed at all, try to do it while copying the
	 * data.  If the data is truncated, or if we only want a partial
	 * coverage checksum (UDP-Lite), do it before the copy.
	 */

	if (copied < ulen || UDP_SKB_CB(skb)->partial_cov) {
		if (udp_lib_checksum_complete(skb))
			goto csum_copy_err;
	}

	if (skb_csum_unnecessary(skb))
		err = skb_copy_datagram_msg(skb, sizeof(struct udphdr),
					    msg, copied);
	else {
		err = skb_copy_and_csum_datagram_msg(skb, sizeof(struct udphdr),
						     msg);

		if (err == -EINVAL)
			goto csum_copy_err;
	}

	if (unlikely(err)) {
		trace_kfree_skb(skb, udp_recvmsg);
		if (!peeked) {
			atomic_inc(&sk->sk_drops);
			UDP_INC_STATS_USER(sock_net(sk),
					   UDP_MIB_INERRORS, is_udplite);
		}
		goto out_free;
	}

	if (!peeked)
		UDP_INC_STATS_USER(sock_net(sk),
				UDP_MIB_INDATAGRAMS, is_udplite);

	sock_recv_ts_and_drops(msg, sk, skb);

	/* Copy the address. */
	if (sin) {
		sin->sin_family = AF_INET;
		sin->sin_port = udp_hdr(skb)->source;
		sin->sin_addr.s_addr = ip_hdr(skb)->saddr;
		memset(sin->sin_zero, 0, sizeof(sin->sin_zero));
		*addr_len = sizeof(*sin);
	}
	if (inet->cmsg_flags)
		ip_cmsg_recv_offset(msg, skb, sizeof(struct udphdr));

	err = copied;
	if (flags & MSG_TRUNC)
		err = ulen;

out_free:
	skb_free_datagram_locked(sk, skb);
out:
	return err;

csum_copy_err:
	slow = lock_sock_fast(sk);
	if (!skb_kill_datagram(sk, skb, flags)) {
		UDP_INC_STATS_USER(sock_net(sk), UDP_MIB_CSUMERRORS, is_udplite);
		UDP_INC_STATS_USER(sock_net(sk), UDP_MIB_INERRORS, is_udplite);
 	}
 	unlock_sock_fast(sk, slow);
 
	/* starting over for a new packet, but check if we need to yield */
	cond_resched();
 	msg->msg_flags &= ~MSG_TRUNC;
 	goto try_again;
 }

int udp_recvmsg(struct sock *sk, struct msghdr *msg, size_t len, int noblock,
		int flags, int *addr_len)
{
	struct inet_sock *inet = inet_sk(sk);
	DECLARE_SOCKADDR(struct sockaddr_in *, sin, msg->msg_name);
	struct sk_buff *skb;
	unsigned int ulen, copied;
	int peeked, off = 0;
	int err;
	int is_udplite = IS_UDPLITE(sk);
	bool slow;

	if (flags & MSG_ERRQUEUE)
		return ip_recv_error(sk, msg, len, addr_len);

try_again:
	skb = __skb_recv_datagram(sk, flags | (noblock ? MSG_DONTWAIT : 0),
				  &peeked, &off, &err);
	if (!skb)
		goto out;

	ulen = skb->len - sizeof(struct udphdr);
	copied = len;
	if (copied > ulen)
		copied = ulen;
	else if (copied < ulen)
		msg->msg_flags |= MSG_TRUNC;

	/*
	 * If checksum is needed at all, try to do it while copying the
	 * data.  If the data is truncated, or if we only want a partial
	 * coverage checksum (UDP-Lite), do it before the copy.
	 */

	if (copied < ulen || UDP_SKB_CB(skb)->partial_cov) {
		if (udp_lib_checksum_complete(skb))
			goto csum_copy_err;
	}

	if (skb_csum_unnecessary(skb))
		err = skb_copy_datagram_msg(skb, sizeof(struct udphdr),
					    msg, copied);
	else {
		err = skb_copy_and_csum_datagram_msg(skb, sizeof(struct udphdr),
						     msg);

		if (err == -EINVAL)
			goto csum_copy_err;
	}

	if (unlikely(err)) {
		trace_kfree_skb(skb, udp_recvmsg);
		if (!peeked) {
			atomic_inc(&sk->sk_drops);
			UDP_INC_STATS_USER(sock_net(sk),
					   UDP_MIB_INERRORS, is_udplite);
		}
		goto out_free;
	}

	if (!peeked)
		UDP_INC_STATS_USER(sock_net(sk),
				UDP_MIB_INDATAGRAMS, is_udplite);

	sock_recv_ts_and_drops(msg, sk, skb);

	/* Copy the address. */
	if (sin) {
		sin->sin_family = AF_INET;
		sin->sin_port = udp_hdr(skb)->source;
		sin->sin_addr.s_addr = ip_hdr(skb)->saddr;
		memset(sin->sin_zero, 0, sizeof(sin->sin_zero));
		*addr_len = sizeof(*sin);
	}
	if (inet->cmsg_flags)
		ip_cmsg_recv_offset(msg, skb, sizeof(struct udphdr));

	err = copied;
	if (flags & MSG_TRUNC)
		err = ulen;

out_free:
	skb_free_datagram_locked(sk, skb);
out:
	return err;

csum_copy_err:
	slow = lock_sock_fast(sk);
	if (!skb_kill_datagram(sk, skb, flags)) {
		UDP_INC_STATS_USER(sock_net(sk), UDP_MIB_CSUMERRORS, is_udplite);
		UDP_INC_STATS_USER(sock_net(sk), UDP_MIB_INERRORS, is_udplite);
 	}
 	unlock_sock_fast(sk, slow);
 
	if (noblock)
		return -EAGAIN;
	/* starting over for a new packet */
 	msg->msg_flags &= ~MSG_TRUNC;
 	goto try_again;
 }
