Allow 'out-of-line' routing table updates for Router and Prefix discovery events

This change removes the requirement that a new routing table be provided when a
router or prefix discovery event happens so that an updated routing table may
be provided to the stack at a later time from the event.

This change is to address the use case where the netstack integrator may need to
obtain a lock before providing updated routes in response to the events above.

As an example, say we have an integrator that performs the below two operations
operations as described:
A. Normal route update:
  1. Obtain integrator lock
  2. Update routes in the integrator
  3. Call Stack.SetRouteTable with the updated routes
    3.1. Obtain Stack lock
    3.2. Update routes in Stack
    3.3. Release Stack lock
  4. Release integrator lock
B. NDP event triggered route update:
  1. Obtain Stack lock
  2. Call event handler
    2.1. Obtain integrator lock
    2.2. Update routes in the integrator
    2.3. Release integrator lock
    2.4. Return updated routes to update Stack
  3. Update routes in Stack
  4. Release Stack lock

A deadlock may occur if a Normal route update was attemped at the same time an
NDP event triggered route update was attempted. With threads T1 and T2:
1) T1 -> A.1, A.2
2) T2 -> B.1
3) T1 -> A.3 (hangs at A.3.1 since Stack lock is taken in step 2)
4) T2 -> B.2 (hangs at B.2.1 since integrator lock is taken in step 1)

Test: Existing tests were modified to not provide or expect routing table
changes in response to Router and Prefix discovery events.
PiperOrigin-RevId: 286274712

1 files changed, 30 insertions, 39 deletions

diff --git a/pkg/tcpip/stack/ndp.go b/pkg/tcpip/stack/ndp.go
index 27bd02e76..90664ba8a 100644
--- a/pkg/tcpip/stack/ndp.go
+++ b/pkg/tcpip/stack/ndp.go
@@ -131,40 +131,34 @@ type NDPDispatcher interface {
 	OnDuplicateAddressDetectionStatus(nicID tcpip.NICID, addr tcpip.Address, resolved bool, err *tcpip.Error)
 
 	// OnDefaultRouterDiscovered will be called when a new default router is
-	// discovered. Implementations must return true along with a new valid
-	// route table if the newly discovered router should be remembered. If
-	// an implementation returns false, the second return value will be
-	// ignored.
+	// discovered. Implementations must return true if the newly discovered
+	// router should be remembered.
 	//
 	// This function is not permitted to block indefinitely. This function
 	// is also not permitted to call into the stack.
-	OnDefaultRouterDiscovered(nicID tcpip.NICID, addr tcpip.Address) (bool, []tcpip.Route)
+	OnDefaultRouterDiscovered(nicID tcpip.NICID, addr tcpip.Address) bool
 
 	// OnDefaultRouterInvalidated will be called when a discovered default
-	// router is invalidated. Implementers must return a new valid route
-	// table.
+	// router that was remembered is invalidated.
 	//
 	// This function is not permitted to block indefinitely. This function
 	// is also not permitted to call into the stack.
-	OnDefaultRouterInvalidated(nicID tcpip.NICID, addr tcpip.Address) []tcpip.Route
+	OnDefaultRouterInvalidated(nicID tcpip.NICID, addr tcpip.Address)
 
 	// OnOnLinkPrefixDiscovered will be called when a new on-link prefix is
-	// discovered. Implementations must return true along with a new valid
-	// route table if the newly discovered on-link prefix should be
-	// remembered. If an implementation returns false, the second return
-	// value will be ignored.
+	// discovered. Implementations must return true if the newly discovered
+	// on-link prefix should be remembered.
 	//
 	// This function is not permitted to block indefinitely. This function
 	// is also not permitted to call into the stack.
-	OnOnLinkPrefixDiscovered(nicID tcpip.NICID, prefix tcpip.Subnet) (bool, []tcpip.Route)
+	OnOnLinkPrefixDiscovered(nicID tcpip.NICID, prefix tcpip.Subnet) bool
 
 	// OnOnLinkPrefixInvalidated will be called when a discovered on-link
-	// prefix is invalidated. Implementers must return a new valid route
-	// table.
+	// prefix that was remembered is invalidated.
 	//
 	// This function is not permitted to block indefinitely. This function
 	// is also not permitted to call into the stack.
-	OnOnLinkPrefixInvalidated(nicID tcpip.NICID, prefix tcpip.Subnet) []tcpip.Route
+	OnOnLinkPrefixInvalidated(nicID tcpip.NICID, prefix tcpip.Subnet)
 
 	// OnAutoGenAddress will be called when a new prefix with its
 	// autonomous address-configuration flag set has been received and SLAAC
@@ -668,7 +662,7 @@ func (ndp *ndpState) handleRA(ip tcpip.Address, ra header.NDPRouterAdvert) {
 
 // invalidateDefaultRouter invalidates a discovered default router.
 //
-// The NIC that ndp belongs to and its associated stack MUST be locked.
+// The NIC that ndp belongs to MUST be locked.
 func (ndp *ndpState) invalidateDefaultRouter(ip tcpip.Address) {
 	rtr, ok := ndp.defaultRouters[ip]
 
@@ -686,8 +680,8 @@ func (ndp *ndpState) invalidateDefaultRouter(ip tcpip.Address) {
 	delete(ndp.defaultRouters, ip)
 
 	// Let the integrator know a discovered default router is invalidated.
-	if ndp.nic.stack.ndpDisp != nil {
-		ndp.nic.stack.routeTable = ndp.nic.stack.ndpDisp.OnDefaultRouterInvalidated(ndp.nic.ID(), ip)
+	if ndpDisp := ndp.nic.stack.ndpDisp; ndpDisp != nil {
+		ndpDisp.OnDefaultRouterInvalidated(ndp.nic.ID(), ip)
 	}
 }
 
@@ -696,15 +690,15 @@ func (ndp *ndpState) invalidateDefaultRouter(ip tcpip.Address) {
 //
 // The router identified by ip MUST NOT already be known by the NIC.
 //
-// The NIC that ndp belongs to and its associated stack MUST be locked.
+// The NIC that ndp belongs to MUST be locked.
 func (ndp *ndpState) rememberDefaultRouter(ip tcpip.Address, rl time.Duration) {
-	if ndp.nic.stack.ndpDisp == nil {
+	ndpDisp := ndp.nic.stack.ndpDisp
+	if ndpDisp == nil {
 		return
 	}
 
 	// Inform the integrator when we discovered a default router.
-	remember, routeTable := ndp.nic.stack.ndpDisp.OnDefaultRouterDiscovered(ndp.nic.ID(), ip)
-	if !remember {
+	if !ndpDisp.OnDefaultRouterDiscovered(ndp.nic.ID(), ip) {
 		// Informed by the integrator to not remember the router, do
 		// nothing further.
 		return
@@ -731,8 +725,6 @@ func (ndp *ndpState) rememberDefaultRouter(ip tcpip.Address, rl time.Duration) {
 		}),
 		doNotInvalidate: &doNotInvalidate,
 	}
-
-	ndp.nic.stack.routeTable = routeTable
 }
 
 // rememberOnLinkPrefix remembers a newly discovered on-link prefix with IPv6
@@ -740,15 +732,15 @@ func (ndp *ndpState) rememberDefaultRouter(ip tcpip.Address, rl time.Duration) {
 //
 // The prefix identified by prefix MUST NOT already be known.
 //
-// The NIC that ndp belongs to and its associated stack MUST be locked.
+// The NIC that ndp belongs to MUST be locked.
 func (ndp *ndpState) rememberOnLinkPrefix(prefix tcpip.Subnet, l time.Duration) {
-	if ndp.nic.stack.ndpDisp == nil {
+	ndpDisp := ndp.nic.stack.ndpDisp
+	if ndpDisp == nil {
 		return
 	}
 
 	// Inform the integrator when we discovered an on-link prefix.
-	remember, routeTable := ndp.nic.stack.ndpDisp.OnOnLinkPrefixDiscovered(ndp.nic.ID(), prefix)
-	if !remember {
+	if !ndpDisp.OnOnLinkPrefixDiscovered(ndp.nic.ID(), prefix) {
 		// Informed by the integrator to not remember the prefix, do
 		// nothing further.
 		return
@@ -769,13 +761,11 @@ func (ndp *ndpState) rememberOnLinkPrefix(prefix tcpip.Subnet, l time.Duration)
 		invalidationTimer: timer,
 		doNotInvalidate:   &doNotInvalidate,
 	}
-
-	ndp.nic.stack.routeTable = routeTable
 }
 
 // invalidateOnLinkPrefix invalidates a discovered on-link prefix.
 //
-// The NIC that ndp belongs to and its associated stack MUST be locked.
+// The NIC that ndp belongs to MUST be locked.
 func (ndp *ndpState) invalidateOnLinkPrefix(prefix tcpip.Subnet) {
 	s, ok := ndp.onLinkPrefixes[prefix]
 
@@ -796,8 +786,8 @@ func (ndp *ndpState) invalidateOnLinkPrefix(prefix tcpip.Subnet) {
 	delete(ndp.onLinkPrefixes, prefix)
 
 	// Let the integrator know a discovered on-link prefix is invalidated.
-	if ndp.nic.stack.ndpDisp != nil {
-		ndp.nic.stack.routeTable = ndp.nic.stack.ndpDisp.OnOnLinkPrefixInvalidated(ndp.nic.ID(), prefix)
+	if ndpDisp := ndp.nic.stack.ndpDisp; ndpDisp != nil {
+		ndpDisp.OnOnLinkPrefixInvalidated(ndp.nic.ID(), prefix)
 	}
 }
 
@@ -829,7 +819,7 @@ func (ndp *ndpState) prefixInvalidationCallback(prefix tcpip.Subnet, vl time.Dur
 // handleOnLinkPrefixInformation assumes that the prefix this pi is for is
 // not the link-local prefix and the on-link flag is set.
 //
-// The NIC that ndp belongs to and its associated stack MUST be locked.
+// The NIC that ndp belongs to MUST be locked.
 func (ndp *ndpState) handleOnLinkPrefixInformation(pi header.NDPPrefixInformation) {
 	prefix := pi.Subnet()
 	prefixState, ok := ndp.onLinkPrefixes[prefix]
@@ -1066,10 +1056,11 @@ func (ndp *ndpState) handleAutonomousPrefixInformation(pi header.NDPPrefixInform
 	}
 
 	// Inform the integrator that we have a new SLAAC address.
-	if ndp.nic.stack.ndpDisp == nil {
+	ndpDisp := ndp.nic.stack.ndpDisp
+	if ndpDisp == nil {
 		return
 	}
-	if !ndp.nic.stack.ndpDisp.OnAutoGenAddress(ndp.nic.ID(), addrWithPrefix) {
+	if !ndpDisp.OnAutoGenAddress(ndp.nic.ID(), addrWithPrefix) {
 		// Informed by the integrator not to add the address.
 		return
 	}
@@ -1135,8 +1126,8 @@ func (ndp *ndpState) cleanupAutoGenAddrResourcesAndNotify(addr tcpip.Address) bo
 
 	delete(ndp.autoGenAddresses, addr)
 
-	if ndp.nic.stack.ndpDisp != nil {
-		ndp.nic.stack.ndpDisp.OnAutoGenAddressInvalidated(ndp.nic.ID(), tcpip.AddressWithPrefix{
+	if ndpDisp := ndp.nic.stack.ndpDisp; ndpDisp != nil {
+		ndpDisp.OnAutoGenAddressInvalidated(ndp.nic.ID(), tcpip.AddressWithPrefix{
 			Address:   addr,
 			PrefixLen: validPrefixLenForAutoGen,
 		})