Merge branch 'l2tp-mtu'

Guillaume Nault says:

====================
l2tp: sanitise MTU handling on sessions

Most of the code handling sessions' MTU has no effect. The ->mtu field
in struct l2tp_session might be used at session creation time, but
neither PPP nor Ethernet pseudo-wires take updates into account.

L2TP sessions don't have a concept of MTU, which is the reason why
->mtu is mostly ignored. MTU should remain a network device thing.
Therefore this patch set does not try to propagate/update ->mtu to/from
the device. That would complicate the code unnecessarily. Instead this
field and the associated ioctl commands and netlink attributes are
removed.

Patch #1 defines l2tp_tunnel_dst_mtu() in order to simplify the
following patches. Then patches #2 and #3 remove MTU handling from PPP
and Ethernet pseudo-wires respectively.
====================
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

@@ -12,6 +12,9 @@
 #ifndef _L2TP_CORE_H_
 #define _L2TP_CORE_H_
 
+#include <net/dst.h>
+#include <net/sock.h>
+
 /* Just some random numbers */
 #define L2TP_TUNNEL_MAGIC	0x42114DDA
 #define L2TP_SESSION_MAGIC	0x0C04EB7D
@@ -61,7 +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;
 };
 
@@ -105,7 +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 */
@@ -268,6 +269,21 @@ static inline int l2tp_get_l2specific_len(struct l2tp_session *session)
 	}
 }
 
+static inline u32 l2tp_tunnel_dst_mtu(const struct l2tp_tunnel *tunnel)
+{
+	struct dst_entry *dst;
+	u32 mtu;
+
+	dst = sk_dst_get(tunnel->sock);
+	if (!dst)
+		return 0;
+
+	mtu = dst_mtu(dst);
+	dst_release(dst);
+
+	return mtu;
+}
+
 #define l2tp_printk(ptr, type, func, fmt, ...)				\
 do {									\
 	if (((ptr)->debug) & (type))					\

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

@@ -226,22 +226,19 @@ static void l2tp_eth_adjust_mtu(struct l2tp_tunnel *tunnel,
 				struct net_device *dev)
 {
 	unsigned int overhead = 0;
-	struct dst_entry *dst;
 	u32 l3_overhead = 0;
+	u32 mtu;
 
 	/* if the encap is UDP, account for UDP header size */
 	if (tunnel->encap == L2TP_ENCAPTYPE_UDP) {
 		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
@@ -255,18 +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 */
-	dst = sk_dst_get(tunnel->sock);
-	if (dst) {
-		/* dst_mtu will use PMTU if found, else fallback to intf MTU */
-		u32 pmtu = dst_mtu(dst);
+	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;
 
-		if (pmtu != 0)
-			dev->mtu = pmtu;
-		dst_release(dst);
-	}
-	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] &&

net/l2tp/l2tp_ppp.c

@@ -93,7 +93,6 @@
 #include <linux/nsproxy.h>
 #include <net/net_namespace.h>
 #include <net/netns/generic.h>
-#include <net/dst.h>
 #include <net/ip.h>
 #include <net/udp.h>
 #include <net/xfrm.h>
@@ -554,7 +553,6 @@ static void pppol2tp_show(struct seq_file *m, void *arg)
 static void pppol2tp_session_init(struct l2tp_session *session)
 {
 	struct pppol2tp_session *ps;
-	struct dst_entry *dst;
 
 	session->recv_skb = pppol2tp_recv;
 #if IS_ENABLED(CONFIG_L2TP_DEBUGFS)
@@ -564,17 +562,6 @@ static void pppol2tp_session_init(struct l2tp_session *session)
 	ps = l2tp_session_priv(session);
 	mutex_init(&ps->sk_lock);
 	ps->owner = current->pid;
-
-	/* If PMTU discovery was enabled, use the MTU that was discovered */
-	dst = sk_dst_get(session->tunnel->sock);
-	if (dst) {
-		u32 pmtu = dst_mtu(dst);
-
-		if (pmtu)
-			session->mtu = pmtu - PPPOL2TP_HEADER_OVERHEAD;
-
-		dst_release(dst);
-	}
 }
 
 struct l2tp_connect_info {
@@ -661,6 +648,22 @@ static int pppol2tp_sockaddr_get_info(const void *sa, int sa_len,
 	return 0;
 }
 
+/* Rough estimation of the maximum payload size a tunnel can transmit without
+ * fragmenting at the lower IP layer. Assumes L2TPv2 with sequence
+ * numbers and no IP option. Not quite accurate, but the result is mostly
+ * unused anyway.
+ */
+static int pppol2tp_tunnel_mtu(const struct l2tp_tunnel *tunnel)
+{
+	int mtu;
+
+	mtu = l2tp_tunnel_dst_mtu(tunnel);
+	if (mtu <= PPPOL2TP_HEADER_OVERHEAD)
+		return 1500 - PPPOL2TP_HEADER_OVERHEAD;
+
+	return mtu - PPPOL2TP_HEADER_OVERHEAD;
+}
+
 /* connect() handler. Attach a PPPoX socket to a tunnel UDP socket
  */
 static int pppol2tp_connect(struct socket *sock, struct sockaddr *uservaddr,
@@ -778,8 +781,6 @@ static int pppol2tp_connect(struct socket *sock, struct sockaddr *uservaddr,
 			goto end;
 		}
 	} else {
-		/* Default MTU must allow space for UDP/L2TP/PPP headers */
-		cfg.mtu = 1500 - PPPOL2TP_HEADER_OVERHEAD;
 		cfg.pw_type = L2TP_PWTYPE_PPP;
 
 		session = l2tp_session_create(sizeof(struct pppol2tp_session),
@@ -824,7 +825,7 @@ static int pppol2tp_connect(struct socket *sock, struct sockaddr *uservaddr,
 
 	po->chan.private = sk;
 	po->chan.ops	 = &pppol2tp_chan_ops;
-	po->chan.mtu	 = session->mtu;
+	po->chan.mtu	 = pppol2tp_tunnel_mtu(tunnel);
 
 	error = ppp_register_net_channel(sock_net(sk), &po->chan);
 	if (error) {
@@ -880,10 +881,6 @@ static int pppol2tp_session_create(struct net *net, struct l2tp_tunnel *tunnel,
 		goto err;
 	}
 
-	/* Default MTU values. */
-	if (cfg->mtu == 0)
-		cfg->mtu = 1500 - PPPOL2TP_HEADER_OVERHEAD;
-
 	/* Allocate and initialize a new session context. */
 	session = l2tp_session_create(sizeof(struct pppol2tp_session),
 				      tunnel, session_id,
@@ -1047,7 +1044,6 @@ static void pppol2tp_copy_stats(struct pppol2tp_ioc_stats *dest,
 static int pppol2tp_session_ioctl(struct l2tp_session *session,
 				  unsigned int cmd, unsigned long arg)
 {
-	struct ifreq ifr;
 	int err = 0;
 	struct sock *sk;
 	int val = (int) arg;
@@ -1063,39 +1059,6 @@ static int pppol2tp_session_ioctl(struct l2tp_session *session,
 		return -EBADR;
 
 	switch (cmd) {
-	case SIOCGIFMTU:
-		err = -ENXIO;
-		if (!(sk->sk_state & PPPOX_CONNECTED))
-			break;
-
-		err = -EFAULT;
-		if (copy_from_user(&ifr, (void __user *) arg, sizeof(struct ifreq)))
-			break;
-		ifr.ifr_mtu = session->mtu;
-		if (copy_to_user((void __user *) arg, &ifr, sizeof(struct ifreq)))
-			break;
-
-		l2tp_info(session, L2TP_MSG_CONTROL, "%s: get mtu=%d\n",
-			  session->name, session->mtu);
-		err = 0;
-		break;
-
-	case SIOCSIFMTU:
-		err = -ENXIO;
-		if (!(sk->sk_state & PPPOX_CONNECTED))
-			break;
-
-		err = -EFAULT;
-		if (copy_from_user(&ifr, (void __user *) arg, sizeof(struct ifreq)))
-			break;
-
-		session->mtu = ifr.ifr_mtu;
-
-		l2tp_info(session, L2TP_MSG_CONTROL, "%s: set mtu=%d\n",
-			  session->name, session->mtu);
-		err = 0;
-		break;
-
 	case PPPIOCGMRU:
 	case PPPIOCGFLAGS:
 		err = -EFAULT;
@@ -1692,8 +1655,7 @@ static void pppol2tp_seq_session_show(struct seq_file *m, void *v)
 		   tunnel->peer_tunnel_id,
 		   session->peer_session_id,
 		   state, user_data_ok);
-	seq_printf(m, "   %d/0/%c/%c/%s %08x %u\n",
-		   session->mtu,
+	seq_printf(m, "   0/0/%c/%c/%s %08x %u\n",
 		   session->recv_seq ? 'R' : '-',
 		   session->send_seq ? 'S' : '-',
 		   session->lns_mode ? "LNS" : "LAC",