l2tp: define l2tp_tunnel_dst_mtu()

Consolidate retrieval of tunnel's socket mtu in order to simplify
l2tp_eth and l2tp_ppp a bit.
Signed-off-by: Guillaume Nault <g.nault@alphalink.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>

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
@@ -268,6 +271,21 @@ 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_eth.c

@@ -226,8 +226,8 @@ 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) {
@@ -256,15 +256,9 @@ 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);
-
-		if (pmtu != 0)
-			dev->mtu = pmtu;
-		dst_release(dst);
-	}
+	mtu = l2tp_tunnel_dst_mtu(tunnel);
+	if (mtu)
+		dev->mtu = mtu;
 	session->mtu = dev->mtu - overhead;
 	dev->mtu = session->mtu;
 	dev->needed_headroom += session->hdr_len;

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 +553,7 @@ 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;
+	u32 mtu;
 
 	session->recv_skb = pppol2tp_recv;
 #if IS_ENABLED(CONFIG_L2TP_DEBUGFS)
@@ -566,15 +565,9 @@ static void pppol2tp_session_init(struct l2tp_session *session)
 	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);
-	}
+	mtu = l2tp_tunnel_dst_mtu(session->tunnel);
+	if (mtu)
+		session->mtu = mtu - PPPOL2TP_HEADER_OVERHEAD;
 }
 
 struct l2tp_connect_info {