Various cleanups to the management of multiple TCP stacks.

[jhb]

Summary

I could split this up into separate commits, or do them all as one merged
commit. I've only done very simple testing of these (boot and pass
traffic with the default stack). Feel free to add more reviewers.

Use strlcpy() with sizeof() instead of strncpy().

Simplify initialization of TCP functions structures.

init_tcp_functions() was already called before the first call to register
a stack. Just inline the work in the SYSINIT and remove the racy helper
variable. Instead, KASSERT that the rw lock is initialized when registering
a stack.

Protect the default stack via a direct pointer comparison.

The default stack uses the name "freebsd" instead of "default" so this
protection wasn't working for the default stack anyway.

head/sys/netinet/tcp_subr.c

 SYSCTL_NODE(_net_link, OID_AUTO, vxlan, CTLFLAG_RW | CTLFLAG_MPSAFE, 0,
     "Virtual eXtensible Local Area Network");
 
 static int vxlan_legacy_port = 0;
 TUNABLE_INT("net.link.vxlan.legacy_port", &vxlan_legacy_port);
 static int vxlan_reuse_port = 0;
 TUNABLE_INT("net.link.vxlan.reuse_port", &vxlan_reuse_port);
 
+/*
+ * This macro controls the default upper limitation on nesting of vxlan
+ * tunnels. By default it is 3, as the overhead of IPv6 vxlan tunnel is 70
+ * bytes, this will create at most 210 bytes overhead and the most inner
+ * tunnel's MTU will be 1290 which will meet IPv6 minimum MTU size 1280.
+ * Be careful to configure the tunnels when raising the limit. A large
+ * number of nested tunnels can introduce system crash.
+ */
+#ifndef MAX_VXLAN_NEST
+#define MAX_VXLAN_NEST  3
+#endif
+static int max_vxlan_nesting = MAX_VXLAN_NEST;
+SYSCTL_INT(_net_link_vxlan, OID_AUTO, max_nesting, CTLFLAG_RW,
+    &max_vxlan_nesting, 0, "Max nested tunnels");
+
 /* Default maximum number of addresses in the forwarding table. */
 #ifndef VXLAN_FTABLE_MAX
 #define VXLAN_FTABLE_MAX        2000
 #endif
 
 /* Timeout (in seconds) of addresses learned in the forwarding table. */
 #ifndef VXLAN_FTABLE_TIMEOUT
 #define VXLAN_FTABLE_TIMEOUT    (20 * 60)
#ifdef INET6
 
         return (error);
 #else
         m_freem(m);
         return (ENOTSUP);
 #endif
 }
 
+#define MTAG_VXLAN_LOOP 0x7876706c /* vxlp */
 static int
 vxlan_transmit(struct ifnet *ifp, struct mbuf *m)
 {
         struct rm_priotracker tracker;
         union vxlan_sockaddr vxlsa;
         struct vxlan_softc *sc;
         struct vxlan_ftable_entry *fe;
         struct ifnet *mcifp;
         struct ether_header *eh;
         int ipv4, error;
 
         sc = ifp->if_softc;
         eh = mtod(m, struct ether_header *);
         fe = NULL;
         mcifp = NULL;
 
         ETHER_BPF_MTAP(ifp, m);
 
         VXLAN_RLOCK(sc, &tracker);
         M_SETFIB(m, sc->vxl_fibnum);
         if ((ifp->if_drv_flags & IFF_DRV_RUNNING) == 0) {
                 VXLAN_RUNLOCK(sc, &tracker);
                 m_freem(m);
                 return (ENETDOWN);
+        }
+        if (__predict_false(if_tunnel_check_nesting(ifp, m, MTAG_VXLAN_LOOP,
+            max_vxlan_nesting) != 0)) {
+                VXLAN_RUNLOCK(sc, &tracker);
+                m_freem(m);
+                if_inc_counter(ifp, IFCOUNTER_OERRORS, 1);
+                return (ELOOP);
         }
 
         if ((m->m_flags & (M_BCAST | M_MCAST)) == 0)
                 fe = vxlan_ftable_entry_lookup(sc, eh->ether_dhost);
         if (fe == NULL)
                 fe = &sc->vxl_default_fe;
         vxlan_sockaddr_copy(&vxlsa, &fe->vxlfe_raddr.sa);