l2tp: fix manual sequencing (de)activation in L2TPv2

Commit e0d4435f "l2tp: Update PPP-over-L2TP driver to work over L2TPv3"
broke the PPPOL2TP_SO_SENDSEQ setsockopt. The L2TP header length was
previously computed by pppol2tp_l2t_header_len() before each call to
l2tp_xmit_skb(). Now that header length is retrieved from the hdr_len
session field, this field must be updated every time the L2TP header
format is modified, or l2tp_xmit_skb() won't push the right amount of
data for the L2TP header.

This patch uses l2tp_session_set_header_len() to adjust hdr_len every
time sequencing is (de)activated from userspace (either by the
PPPOL2TP_SO_SENDSEQ setsockopt or the L2TP_ATTR_SEND_SEQ netlink
attribute).
Signed-off-by: Guillaume Nault <g.nault@alphalink.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/l2tp/l2tp_core.c

@@ -112,7 +112,6 @@ struct l2tp_net {
 	spinlock_t l2tp_session_hlist_lock;
 };
 
-static void l2tp_session_set_header_len(struct l2tp_session *session, int version);
 static void l2tp_tunnel_free(struct l2tp_tunnel *tunnel);
 
 static inline struct l2tp_tunnel *l2tp_tunnel(struct sock *sk)
@@ -1863,7 +1862,7 @@ EXPORT_SYMBOL_GPL(l2tp_session_delete);
 /* We come here whenever a session's send_seq, cookie_len or
  * l2specific_len parameters are set.
  */
-static void l2tp_session_set_header_len(struct l2tp_session *session, int version)
+void l2tp_session_set_header_len(struct l2tp_session *session, int version)
 {
 	if (version == L2TP_HDR_VER_2) {
 		session->hdr_len = 6;
@@ -1876,6 +1875,7 @@ static void l2tp_session_set_header_len(struct l2tp_session *session, int versio
 	}
 
 }
+EXPORT_SYMBOL_GPL(l2tp_session_set_header_len);
 
 struct l2tp_session *l2tp_session_create(int priv_size, struct l2tp_tunnel *tunnel, u32 session_id, u32 peer_session_id, struct l2tp_session_cfg *cfg)
 {

net/l2tp/l2tp_core.h

@@ -263,6 +263,7 @@ void l2tp_recv_common(struct l2tp_session *session, struct sk_buff *skb,
 		      int length, int (*payload_hook)(struct sk_buff *skb));
 int l2tp_session_queue_purge(struct l2tp_session *session);
 int l2tp_udp_encap_recv(struct sock *sk, struct sk_buff *skb);
+void l2tp_session_set_header_len(struct l2tp_session *session, int version);
 
 int l2tp_xmit_skb(struct l2tp_session *session, struct sk_buff *skb,
 		  int hdr_len);

net/l2tp/l2tp_netlink.c

@@ -578,8 +578,10 @@ static int l2tp_nl_cmd_session_modify(struct sk_buff *skb, struct genl_info *inf
 	if (info->attrs[L2TP_ATTR_RECV_SEQ])
 		session->recv_seq = nla_get_u8(info->attrs[L2TP_ATTR_RECV_SEQ]);
 
-	if (info->attrs[L2TP_ATTR_SEND_SEQ])
+	if (info->attrs[L2TP_ATTR_SEND_SEQ]) {
 		session->send_seq = nla_get_u8(info->attrs[L2TP_ATTR_SEND_SEQ]);
+		l2tp_session_set_header_len(session, session->tunnel->version);
+	}
 
 	if (info->attrs[L2TP_ATTR_LNS_MODE])
 		session->lns_mode = nla_get_u8(info->attrs[L2TP_ATTR_LNS_MODE]);

net/l2tp/l2tp_ppp.c

@@ -1312,6 +1312,7 @@ static int pppol2tp_session_setsockopt(struct sock *sk,
 			po->chan.hdrlen = val ? PPPOL2TP_L2TP_HDR_SIZE_SEQ :
 				PPPOL2TP_L2TP_HDR_SIZE_NOSEQ;
 		}
+		l2tp_session_set_header_len(session, session->tunnel->version);
 		l2tp_info(session, PPPOL2TP_MSG_CONTROL,
 			  "%s: set send_seq=%d\n",
 			  session->name, session->send_seq);