l2tp: ignore L2TP_ATTR_MTU

This attribute's handling is broken. It can only be used when creating
Ethernet pseudo-wires, in which case its value can be used as the
initial MTU for the l2tpeth device.
However, when handling update requests, L2TP_ATTR_MTU only modifies
session->mtu. This value is never propagated to the l2tpeth device.
Dump requests also return the value of session->mtu, which is not
synchronised anymore with the device MTU.

The same problem occurs if the device MTU is properly updated using the
generic IFLA_MTU attribute. In this case, session->mtu is not updated,
and L2TP_ATTR_MTU will report an invalid value again when dumping the
session.

It does not seem worthwhile to complexify l2tp_eth.c to synchronise
session->mtu with the device MTU. Even the ip-l2tp manpage advises to
use 'ip link' to initialise the MTU of l2tpeth devices (iproute2 does
not handle L2TP_ATTR_MTU at all anyway). So let's just ignore it
entirely.
Signed-off-by: Guillaume Nault <g.nault@alphalink.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/uapi/linux/l2tp.h

@@ -119,7 +119,7 @@ enum {
 	L2TP_ATTR_IP_DADDR,		/* u32 */
 	L2TP_ATTR_UDP_SPORT,		/* u16 */
 	L2TP_ATTR_UDP_DPORT,		/* u16 */
-	L2TP_ATTR_MTU,			/* u16 */
+	L2TP_ATTR_MTU,			/* u16 (not used) */
 	L2TP_ATTR_MRU,			/* u16 (not used) */
 	L2TP_ATTR_STATS,		/* nested */
 	L2TP_ATTR_IP6_SADDR,		/* struct in6_addr */

net/l2tp/l2tp_core.c

@@ -1674,7 +1674,6 @@ struct l2tp_session *l2tp_session_create(int priv_size, struct l2tp_tunnel *tunn
 		if (cfg) {
 			session->pwtype = cfg->pw_type;
 			session->debug = cfg->debug;
-			session->mtu = cfg->mtu;
 			session->send_seq = cfg->send_seq;
 			session->recv_seq = cfg->recv_seq;
 			session->lns_mode = cfg->lns_mode;

net/l2tp/l2tp_core.h

@@ -64,7 +64,6 @@ struct l2tp_session_cfg {
 	int			peer_cookie_len; /* 0, 4 or 8 bytes */
 	int			reorder_timeout; /* configured reorder timeout
 						  * (in jiffies) */
-	int			mtu;
 	char			*ifname;
 };
 
@@ -108,7 +107,6 @@ struct l2tp_session {
 	int			reorder_timeout; /* configured reorder timeout
 						  * (in jiffies) */
 	int			reorder_skip;	/* set if skip to next nr */
-	int			mtu;
 	enum l2tp_pwtype	pwtype;
 	struct l2tp_stats	stats;
 	struct hlist_node	global_hlist;	/* Global hash list node */

net/l2tp/l2tp_debugfs.c

@@ -191,8 +191,7 @@ static void l2tp_dfs_seq_session_show(struct seq_file *m, void *v)
 	if (session->send_seq || session->recv_seq)
 		seq_printf(m, "   nr %hu, ns %hu\n", session->nr, session->ns);
 	seq_printf(m, "   refcnt %d\n", refcount_read(&session->ref_count));
-	seq_printf(m, "   config %d/0/%c/%c/-/%s %08x %u\n",
-		   session->mtu,
+	seq_printf(m, "   config 0/0/%c/%c/-/%s %08x %u\n",
 		   session->recv_seq ? 'R' : '-',
 		   session->send_seq ? 'S' : '-',
 		   session->lns_mode ? "LNS" : "LAC",

net/l2tp/l2tp_eth.c

@@ -234,14 +234,11 @@ static void l2tp_eth_adjust_mtu(struct l2tp_tunnel *tunnel,
 		overhead += sizeof(struct udphdr);
 		dev->needed_headroom += sizeof(struct udphdr);
 	}
-	if (session->mtu != 0) {
-		dev->mtu = session->mtu;
-		dev->needed_headroom += session->hdr_len;
-		return;
-	}
+
 	lock_sock(tunnel->sock);
 	l3_overhead = kernel_sock_ip_overhead(tunnel->sock);
 	release_sock(tunnel->sock);
+
 	if (l3_overhead == 0) {
 		/* L3 Overhead couldn't be identified, this could be
 		 * because tunnel->sock was NULL or the socket's
@@ -255,12 +252,12 @@ static void l2tp_eth_adjust_mtu(struct l2tp_tunnel *tunnel,
 	 */
 	overhead += session->hdr_len + ETH_HLEN + l3_overhead;
 
-	/* If PMTU discovery was enabled, use discovered MTU on L2TP device */
-	mtu = l2tp_tunnel_dst_mtu(tunnel);
-	if (mtu)
+	mtu = l2tp_tunnel_dst_mtu(tunnel) - overhead;
+	if (mtu < dev->min_mtu || mtu > dev->max_mtu)
+		dev->mtu = ETH_DATA_LEN - overhead;
+	else
 		dev->mtu = mtu;
-	session->mtu = dev->mtu - overhead;
-	dev->mtu = session->mtu;
+
 	dev->needed_headroom += session->hdr_len;
 }
 

net/l2tp/l2tp_netlink.c

@@ -608,9 +608,6 @@ static int l2tp_nl_cmd_session_create(struct sk_buff *skb, struct genl_info *inf
 	if (info->attrs[L2TP_ATTR_RECV_TIMEOUT])
 		cfg.reorder_timeout = nla_get_msecs(info->attrs[L2TP_ATTR_RECV_TIMEOUT]);
 
-	if (info->attrs[L2TP_ATTR_MTU])
-		cfg.mtu = nla_get_u16(info->attrs[L2TP_ATTR_MTU]);
-
 #ifdef CONFIG_MODULES
 	if (l2tp_nl_cmd_ops[cfg.pw_type] == NULL) {
 		genl_unlock();
@@ -698,9 +695,6 @@ static int l2tp_nl_cmd_session_modify(struct sk_buff *skb, struct genl_info *inf
 	if (info->attrs[L2TP_ATTR_RECV_TIMEOUT])
 		session->reorder_timeout = nla_get_msecs(info->attrs[L2TP_ATTR_RECV_TIMEOUT]);
 
-	if (info->attrs[L2TP_ATTR_MTU])
-		session->mtu = nla_get_u16(info->attrs[L2TP_ATTR_MTU]);
-
 	ret = l2tp_session_notify(&l2tp_nl_family, info,
 				  session, L2TP_CMD_SESSION_MODIFY);
 
@@ -730,8 +724,7 @@ static int l2tp_nl_session_send(struct sk_buff *skb, u32 portid, u32 seq, int fl
 	    nla_put_u32(skb, L2TP_ATTR_PEER_SESSION_ID,
 			session->peer_session_id) ||
 	    nla_put_u32(skb, L2TP_ATTR_DEBUG, session->debug) ||
-	    nla_put_u16(skb, L2TP_ATTR_PW_TYPE, session->pwtype) ||
-	    nla_put_u16(skb, L2TP_ATTR_MTU, session->mtu))
+	    nla_put_u16(skb, L2TP_ATTR_PW_TYPE, session->pwtype))
 		goto nla_put_failure;
 
 	if ((session->ifname[0] &&