wgengine/monitor: add monitor for link change events

Change-Id: I45100c7a5b785ad6824080fe0a38751e3d246eaa
Signed-off-by: Andrew Dunham <andrew@tailscale.com>

wgengine/monitor/monitor.go

@@ -10,13 +10,16 @@ package monitor
 import (
 	"encoding/json"
 	"errors"
+	"net"
 	"net/netip"
 	"runtime"
 	"sync"
 	"time"
 
+	"golang.org/x/exp/slices"
 	"tailscale.com/net/interfaces"
 	"tailscale.com/types/logger"
+	"tailscale.com/util/mak"
 )
 
 // pollWallTimeInterval is how often we check the time to check
@@ -64,19 +67,21 @@ type Mon struct {
 	change chan struct{}
 	stop   chan struct{} // closed on Stop
 
-	mu         sync.Mutex // guards all following fields
-	cbs        map[*callbackHandle]ChangeFunc
-	ruleDelCB  map[*callbackHandle]RuleDeleteCallback
-	ifState    *interfaces.State
-	gwValid    bool       // whether gw and gwSelfIP are valid
-	gw         netip.Addr // our gateway's IP
-	gwSelfIP   netip.Addr // our own IP address (that corresponds to gw)
-	started    bool
-	closed     bool
-	goroutines sync.WaitGroup
-	wallTimer  *time.Timer // nil until Started; re-armed AfterFunc per tick
-	lastWall   time.Time
-	timeJumped bool // whether we need to send a changed=true after a big time jump
+	mu            sync.Mutex // guards all following fields
+	cbs           map[*callbackHandle]ChangeFunc
+	ruleDelCB     map[*callbackHandle]RuleDeleteCallback
+	linkChangedCB map[*callbackHandle]LinkChangedCallback
+	ifState       *interfaces.State
+	ifState2      []*net.Interface
+	gwValid       bool       // whether gw and gwSelfIP are valid
+	gw            netip.Addr // our gateway's IP
+	gwSelfIP      netip.Addr // our own IP address (that corresponds to gw)
+	started       bool
+	closed        bool
+	goroutines    sync.WaitGroup
+	wallTimer     *time.Timer // nil until Started; re-armed AfterFunc per tick
+	lastWall      time.Time
+	timeJumped    bool // whether we need to send a changed=true after a big time jump
 }
 
 // New instantiates and starts a monitoring instance.
@@ -179,6 +184,26 @@ func (m *Mon) RegisterRuleDeleteCallback(callback RuleDeleteCallback) (unregiste
 	}
 }
 
+// LinkChangedCallback is a callback when a network link changes.
+type LinkChangedCallback func(iif *net.Interface, deleted bool)
+
+// RegisterLinkChangedCallback adds a callback to the set of parties to be
+// notified (in their own goroutine) whenever a link (a.k.a. "interface") is
+// changed.
+//
+// To remove this callback, call unregister or close the monitor.
+func (m *Mon) RegisterLinkChangedCallback(callback LinkChangedCallback) (unregister func()) {
+	handle := new(callbackHandle)
+	m.mu.Lock()
+	defer m.mu.Unlock()
+	mak.Set(&m.linkChangedCB, handle, callback)
+	return func() {
+		m.mu.Lock()
+		defer m.mu.Unlock()
+		delete(m.linkChangedCB, handle)
+	}
+}
+
 // Start starts the monitor.
 // A monitor can only be started & closed once.
 func (m *Mon) Start() {
@@ -256,6 +281,7 @@ func (m *Mon) stopped() bool {
 // the change channel of changes, and stopping when a stop is issued.
 func (m *Mon) pump() {
 	defer m.goroutines.Done()
+pumpLoop:
 	for !m.stopped() {
 		msg, err := m.om.Receive()
 		if err != nil {
@@ -265,14 +291,18 @@ func (m *Mon) pump() {
 			// Keep retrying while we're not closed.
 			m.logf("error from link monitor: %v", err)
 			time.Sleep(time.Second)
-			continue
+			continue pumpLoop
 		}
-		if rdm, ok := msg.(ipRuleDeletedMessage); ok {
-			m.notifyRuleDeleted(rdm)
-			continue
+		switch v := msg.(type) {
+		case ipRuleDeletedMessage:
+			m.notifyRuleDeleted(v)
+			continue pumpLoop
+		case newLinkMessage:
+			m.notifyLinkChanged(v)
+			continue pumpLoop
 		}
 		if msg.ignore() {
-			continue
+			continue pumpLoop
 		}
 		m.InjectEvent()
 	}
@@ -286,6 +316,38 @@ func (m *Mon) notifyRuleDeleted(rdm ipRuleDeletedMessage) {
 	}
 }
 
+func (m *Mon) notifyLinkChanged(nlm newLinkMessage) {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+	for _, cb := range m.linkChangedCB {
+		go cb(nlm.Link, nlm.Delete)
+	}
+
+	// Update our cached state
+	updated := false
+	for i, iif := range m.ifState2 {
+		if iif.Index == nlm.Link.Index {
+			if nlm.Delete {
+				m.ifState2 = slices.Delete(m.ifState2, i, i)
+			} else {
+				m.ifState2[i] = nlm.Link
+			}
+			updated = true
+			break
+		}
+	}
+	if updated {
+		return
+	}
+
+	// Need to append
+	// TODO(andrew): insert sorted instead of insert then sort?
+	m.ifState2 = append(m.ifState2, nlm.Link)
+	slices.SortFunc(m.ifState2, func(x, y *net.Interface) bool {
+		return x.Index < y.Index
+	})
+}
+
 // isInterestingInterface reports whether the provided interface should be
 // considered when checking for network state changes.
 // The ips parameter should be the IPs of the provided interface.

wgengine/monitor/monitor_linux.go

@@ -54,14 +54,18 @@ func newOSMon(logf logger.Logf, m *Mon) (osMon, error) {
 		// but all reachability would.
 		Groups: unix.RTMGRP_IPV4_IFADDR | unix.RTMGRP_IPV6_IFADDR |
 			unix.RTMGRP_IPV4_ROUTE | unix.RTMGRP_IPV6_ROUTE |
-			unix.RTMGRP_IPV4_RULE, // no IPV6_RULE in x/sys/unix
+			unix.RTMGRP_IPV4_RULE | unix.RTMGRP_LINK, // no IPV6_RULE in x/sys/unix
 	})
 	if err != nil {
 		// Google Cloud Run does not implement NETLINK_ROUTE RTMGRP support
 		logf("monitor_linux: AF_NETLINK RTMGRP failed, falling back to polling")
 		return newPollingMon(logf, m)
 	}
-	return &nlConn{logf: logf, conn: conn, addrCache: make(map[uint32]map[netip.Addr]bool)}, nil
+	return &nlConn{
+		logf:      logf,
+		conn:      conn,
+		addrCache: make(map[uint32]map[netip.Addr]bool),
+	}, nil
 }
 
 func (c *nlConn) IsInterestingInterface(iface string) bool { return true }
@@ -229,6 +233,53 @@ func (c *nlConn) Receive() (message, error) {
 			c.logf("%+v", rdm)
 		}
 		return rdm, nil
+	case unix.RTM_NEWLINK, unix.RTM_DELLINK:
+		typeStr := "RTM_NEWLINK"
+		if msg.Header.Type == unix.RTM_DELLINK {
+			typeStr = "RTM_DELLINK"
+		}
+
+		var lmsg rtnetlink.LinkMessage
+		if err := lmsg.UnmarshalBinary(msg.Data); err != nil {
+			c.logf("%s: failed to parse: %v", typeStr, err)
+			return unspecifiedMessage{}, nil
+		}
+
+		// Make a *net.Interface
+		netif := &net.Interface{
+			Index: int(lmsg.Index),
+		}
+		if attrs := lmsg.Attributes; attrs != nil {
+			netif.HardwareAddr = attrs.Address
+			netif.MTU = int(attrs.MTU)
+			netif.Name = attrs.Name
+		}
+
+		// Handle flags
+		if lmsg.Flags&unix.IFF_UP != 0 {
+			netif.Flags |= net.FlagUp
+		}
+		if lmsg.Flags&unix.IFF_BROADCAST != 0 {
+			netif.Flags |= net.FlagBroadcast
+		}
+		if lmsg.Flags&unix.IFF_LOOPBACK != 0 {
+			netif.Flags |= net.FlagLoopback
+		}
+		if lmsg.Flags&unix.IFF_POINTOPOINT != 0 {
+			netif.Flags |= net.FlagPointToPoint
+		}
+		if lmsg.Flags&unix.IFF_MULTICAST != 0 {
+			netif.Flags |= net.FlagMulticast
+		}
+
+		nlm := &newLinkMessage{
+			Link:   netif,
+			Delete: msg.Header.Type == unix.RTM_DELLINK,
+		}
+		if debugNetlinkMessages() {
+			c.logf("newLinkMessage{Link: %+v, Delete: %v}", nlm.Link, nlm.Delete)
+		}
+		return nlm, nil
 	default:
 		c.logf("unhandled netlink msg type %+v, %q", msg.Header, msg.Data)
 		return unspecifiedMessage{}, nil
@@ -286,3 +337,11 @@ func (m *newAddrMessage) ignore() bool {
 type ignoreMessage struct{}
 
 func (ignoreMessage) ignore() bool { return true }
+
+// newLinkMessage is a message for a link being added.
+type newLinkMessage struct {
+	Link   *net.Interface
+	Delete bool
+}
+
+func (newLinkMessage) ignore() bool { return true }