l2tp: fix UDP checksum support

Index: net-next-2.6-l2tp/drivers/net/pppol2tp.c
===================================================================
--- net-next-2.6-l2tp.orig/drivers/net/pppol2tp.c
+++ net-next-2.6-l2tp/drivers/net/pppol2tp.c
@@ -489,6 +489,30 @@  out:
 	spin_unlock_bh(&session->reorder_q.lock);
 }
 
+static inline int pppol2tp_verify_udp_checksum(struct sock *sk,
+					       struct sk_buff *skb)
+{
+	struct udphdr *uh = udp_hdr(skb);
+	u16 ulen = ntohs(uh->len);
+	struct inet_sock *inet;
+	__wsum psum;
+
+	if (sk->sk_no_check || skb_csum_unnecessary(skb) || !uh->check)
+		return 0;
+
+	inet = inet_sk(sk);
+	psum = csum_tcpudp_nofold(inet->saddr, inet->daddr, ulen,
+				  IPPROTO_UDP, 0);
+
+	if ((skb->ip_summed == CHECKSUM_COMPLETE) &&
+	    !csum_fold(csum_add(psum, skb->csum)))
+		return 0;
+
+	skb->csum = psum;
+
+	return __skb_checksum_complete(skb);
+}
+
 /* Internal receive frame. Do the real work of receiving an L2TP data frame
  * here. The skb is not on a list when we get here.
  * Returns 0 if the packet was a data packet and was successfully passed on.
@@ -509,6 +533,9 @@  static int pppol2tp_recv_core(struct soc
 	if (tunnel == NULL)
 		goto no_tunnel;
 
+	if (tunnel->sock && pppol2tp_verify_udp_checksum(tunnel->sock, skb))
+		goto discard_bad_csum;
+
 	/* UDP always verifies the packet length. */
 	__skb_pull(skb, sizeof(struct udphdr));
 
@@ -725,6 +752,14 @@  discard:
 
 	return 0;
 
+discard_bad_csum:
+	LIMIT_NETDEBUG("%s: UDP: bad checksum\n", tunnel->name);
+	UDP_INC_STATS_USER(&init_net, UDP_MIB_INERRORS, 0);
+	tunnel->stats.rx_errors++;
+	kfree_skb(skb);
+
+	return 0;
+
 error:
 	/* Put UDP header back */
 	__skb_push(skb, sizeof(struct udphdr));
@@ -851,7 +886,7 @@  static int pppol2tp_sendmsg(struct kiocb
 	static const unsigned char ppph[2] = { 0xff, 0x03 };
 	struct sock *sk = sock->sk;
 	struct inet_sock *inet;
-	__wsum csum = 0;
+	__wsum csum;
 	struct sk_buff *skb;
 	int error;
 	int hdr_len;
@@ -859,6 +894,8 @@  static int pppol2tp_sendmsg(struct kiocb
 	struct pppol2tp_tunnel *tunnel;
 	struct udphdr *uh;
 	unsigned int len;
+	struct sock *sk_tun;
+	u16 udp_len;
 
 	error = -ENOTCONN;
 	if (sock_flag(sk, SOCK_DEAD) || !(sk->sk_state & PPPOX_CONNECTED))
@@ -870,7 +907,8 @@  static int pppol2tp_sendmsg(struct kiocb
 	if (session == NULL)
 		goto error;
 
-	tunnel = pppol2tp_sock_to_tunnel(session->tunnel_sock);
+	sk_tun = session->tunnel_sock;
+	tunnel = pppol2tp_sock_to_tunnel(sk_tun);
 	if (tunnel == NULL)
 		goto error_put_sess;
 
@@ -893,11 +931,12 @@  static int pppol2tp_sendmsg(struct kiocb
 	skb_reset_transport_header(skb);
 
 	/* Build UDP header */
-	inet = inet_sk(session->tunnel_sock);
+	inet = inet_sk(sk_tun);
+	udp_len = hdr_len + sizeof(ppph) + total_len;
 	uh = (struct udphdr *) skb->data;
 	uh->source = inet->sport;
 	uh->dest = inet->dport;
-	uh->len = htons(hdr_len + sizeof(ppph) + total_len);
+	uh->len = htons(udp_len);
 	uh->check = 0;
 	skb_put(skb, sizeof(struct udphdr));
 
@@ -919,8 +958,22 @@  static int pppol2tp_sendmsg(struct kiocb
 	skb_put(skb, total_len);
 
 	/* Calculate UDP checksum if configured to do so */
-	if (session->tunnel_sock->sk_no_check != UDP_CSUM_NOXMIT)
-		csum = udp_csum_outgoing(sk, skb);
+	if (sk_tun->sk_no_check == UDP_CSUM_NOXMIT)
+		skb->ip_summed = CHECKSUM_NONE;
+	else if (!(skb->dst->dev->features & NETIF_F_V4_CSUM)) {
+		skb->ip_summed = CHECKSUM_COMPLETE;
+		csum = skb_checksum(skb, 0, udp_len, 0);
+		uh->check = csum_tcpudp_magic(inet->saddr, inet->daddr,
+					      udp_len, IPPROTO_UDP, csum);
+		if (uh->check == 0)
+			uh->check = CSUM_MANGLED_0;
+	} else {
+		skb->ip_summed = CHECKSUM_PARTIAL;
+		skb->csum_start = skb_transport_header(skb) - skb->head;
+		skb->csum_offset = offsetof(struct udphdr, check);
+		uh->check = ~csum_tcpudp_magic(inet->saddr, inet->daddr,
+					       udp_len, IPPROTO_UDP, 0);
+	}
 
 	/* Debug */
 	if (session->send_seq)
@@ -1008,13 +1061,14 @@  static int pppol2tp_xmit(struct ppp_chan
 	struct sock *sk = (struct sock *) chan->private;
 	struct sock *sk_tun;
 	int hdr_len;
+	u16 udp_len;
 	struct pppol2tp_session *session;
 	struct pppol2tp_tunnel *tunnel;
 	int rc;
 	int headroom;
 	int data_len = skb->len;
 	struct inet_sock *inet;
-	__wsum csum = 0;
+	__wsum csum;
 	struct udphdr *uh;
 	unsigned int len;
 	int old_headroom;
@@ -1060,6 +1114,8 @@  static int pppol2tp_xmit(struct ppp_chan
 	/* Setup L2TP header */
 	pppol2tp_build_l2tp_header(session, __skb_push(skb, hdr_len));
 
+	udp_len = sizeof(struct udphdr) + hdr_len + sizeof(ppph) + data_len;
+
 	/* Setup UDP header */
 	inet = inet_sk(sk_tun);
 	__skb_push(skb, sizeof(*uh));
@@ -1067,13 +1123,9 @@  static int pppol2tp_xmit(struct ppp_chan
 	uh = udp_hdr(skb);
 	uh->source = inet->sport;
 	uh->dest = inet->dport;
-	uh->len = htons(sizeof(struct udphdr) + hdr_len + sizeof(ppph) + data_len);
+	uh->len = htons(udp_len);
 	uh->check = 0;
 
-	/* *BROKEN* Calculate UDP checksum if configured to do so */
-	if (sk_tun->sk_no_check != UDP_CSUM_NOXMIT)
-		csum = udp_csum_outgoing(sk_tun, skb);
-
 	/* Debug */
 	if (session->send_seq)
 		PRINTK(session->debug, PPPOL2TP_MSG_DATA, KERN_DEBUG,
@@ -1108,6 +1160,24 @@  static int pppol2tp_xmit(struct ppp_chan
 	skb->dst = dst_clone(__sk_dst_get(sk_tun));
 	pppol2tp_skb_set_owner_w(skb, sk_tun);
 
+	/* Calculate UDP checksum if configured to do so */
+	if (sk_tun->sk_no_check == UDP_CSUM_NOXMIT)
+		skb->ip_summed = CHECKSUM_NONE;
+	else if (!(skb->dst->dev->features & NETIF_F_V4_CSUM)) {
+		skb->ip_summed = CHECKSUM_COMPLETE;
+		csum = skb_checksum(skb, 0, udp_len, 0);
+		uh->check = csum_tcpudp_magic(inet->saddr, inet->daddr,
+					      udp_len, IPPROTO_UDP, csum);
+		if (uh->check == 0)
+			uh->check = CSUM_MANGLED_0;
+	} else {
+		skb->ip_summed = CHECKSUM_PARTIAL;
+		skb->csum_start = skb_transport_header(skb) - skb->head;
+		skb->csum_offset = offsetof(struct udphdr, check);
+		uh->check = ~csum_tcpudp_magic(inet->saddr, inet->daddr,
+					       udp_len, IPPROTO_UDP, 0);
+	}
+
 	/* Queue the packet to IP for output */
 	len = skb->len;
 	rc = ip_queue_xmit(skb, 1);