Restore listening connections when netstack s/r is enabled.

This CL restores the listening connections when netstack s/r is enabled.
The changes include:
- New method as a workaround to replace the new routes and nics to the loaded
stack after restore.
- New Restore() for transport layer protocols to restore the protocol level
background workers.
- Adds afterLoad() method for fdbased processors.
- Adds a test to verify listening connection is restored after checkpointing
with netstack s/r enabled.
- Few other changes to save restore fields to enable netstack s/r.

PiperOrigin-RevId: 698453124

pkg/sentry/socket/netstack/netstack_state.go

@@ -34,7 +34,7 @@ func (s *sock) loadTimestamp(_ context.Context, nsec int64) {
 }
 
 func (s *Stack) saveStack() *stack.Stack {
-	if s.shouldSaveRestoreStack {
+	if s.IsSaveRestoreEnabled() {
 		return s.Stack
 	}
 

pkg/sentry/socket/netstack/save_restore.go

@@ -22,7 +22,7 @@ import (
 
 // afterLoad is invoked by stateify.
 func (s *Stack) afterLoad(ctx context.Context) {
-	if s.shouldSaveRestoreStack {
+	if s.IsSaveRestoreEnabled() {
 		// This indicates that netstack s/r is enabled and the stack
 		// should not be replaced with the new stack from context.
 		return

pkg/sentry/socket/netstack/stack.go

@@ -40,16 +40,23 @@ import (
 //
 // +stateify savable
 type Stack struct {
-	Stack                  *stack.Stack `state:".(*stack.Stack)"`
-	shouldSaveRestoreStack bool
+	Stack *stack.Stack `state:".(*stack.Stack)"`
 }
 
 // EnableSaveRestore enables netstack s/r.
 func (s *Stack) EnableSaveRestore() error {
-	s.shouldSaveRestoreStack = true
+	s.Stack.EnableSaveRestore()
 	return nil
 }
 
+// IsSaveRestoreEnabled implements inet.Stack.IsSaveRestoreEnabled.
+func (s *Stack) IsSaveRestoreEnabled() bool {
+	if s.Stack == nil {
+		return false
+	}
+	return s.Stack.IsSaveRestoreEnabled()
+}
+
 // Destroy implements inet.Stack.Destroy.
 func (s *Stack) Destroy() {
 	s.Stack.Close()
@@ -912,6 +919,14 @@ func (s *Stack) Restore() {
 	s.Stack.Restore()
 }
 
+// ReplaceConfig implements inet.Stack.ReplaceConfig.
+func (s *Stack) ReplaceConfig(st inet.Stack) {
+	if _, ok := st.(*Stack); !ok {
+		panic("netstack.Stack cannot be nil when netstack s/r is enabled")
+	}
+	s.Stack.ReplaceConfig(st.(*Stack).Stack)
+}
+
 // Resume implements inet.Stack.Resume.
 func (s *Stack) Resume() {
 	s.Stack.Resume()