3 files changed, 77 insertions, 42 deletions

diff --git a/pkg/tcpip/transport/tcp/accept.go b/pkg/tcpip/transport/tcp/accept.go
index b4c4c8ab1..375ca21f6 100644
--- a/pkg/tcpip/transport/tcp/accept.go
+++ b/pkg/tcpip/transport/tcp/accept.go
@@ -365,21 +365,29 @@ func (l *listenContext) closeAllPendingEndpoints() {
 }
 
 // deliverAccepted delivers the newly-accepted endpoint to the listener. If the
-// endpoint has transitioned out of the listen state, the new endpoint is closed
-// instead.
+// endpoint has transitioned out of the listen state (acceptedChan is nil),
+// the new endpoint is closed instead.
 func (e *endpoint) deliverAccepted(n *endpoint) {
 	e.mu.Lock()
-	state := e.EndpointState()
 	e.pendingAccepted.Add(1)
-	defer e.pendingAccepted.Done()
-	acceptedChan := e.acceptedChan
 	e.mu.Unlock()
+	defer e.pendingAccepted.Done()
 
-	if state == StateListen {
-		acceptedChan <- n
-		e.waiterQueue.Notify(waiter.EventIn)
-	} else {
-		n.Close()
+	e.acceptMu.Lock()
+	for {
+		if e.acceptedChan == nil {
+			e.acceptMu.Unlock()
+			n.Close()
+			return
+		}
+		select {
+		case e.acceptedChan <- n:
+			e.acceptMu.Unlock()
+			e.waiterQueue.Notify(waiter.EventIn)
+			return
+		default:
+			e.acceptCond.Wait()
+		}
 	}
 }
 
@@ -420,11 +428,13 @@ func (e *endpoint) handleSynSegment(ctx *listenContext, s *segment, opts *header
 }
 
 func (e *endpoint) incSynRcvdCount() bool {
-	if e.synRcvdCount >= cap(e.acceptedChan) {
-		return false
+	e.acceptMu.Lock()
+	canInc := e.synRcvdCount < cap(e.acceptedChan)
+	e.acceptMu.Unlock()
+	if canInc {
+		e.synRcvdCount++
 	}
-	e.synRcvdCount++
-	return true
+	return canInc
 }
 
 func (e *endpoint) decSynRcvdCount() {
@@ -432,10 +442,10 @@ func (e *endpoint) decSynRcvdCount() {
 }
 
 func (e *endpoint) acceptQueueIsFull() bool {
-	if l, c := len(e.acceptedChan)+e.synRcvdCount, cap(e.acceptedChan); l >= c {
-		return true
-	}
-	return false
+	e.acceptMu.Lock()
+	full := len(e.acceptedChan)+e.synRcvdCount >= cap(e.acceptedChan)
+	e.acceptMu.Unlock()
+	return full
 }
 
 // handleListenSegment is called when a listening endpoint receives a segment
diff --git a/pkg/tcpip/transport/tcp/endpoint.go b/pkg/tcpip/transport/tcp/endpoint.go
index b6e571361..1ebee0cfe 100644
--- a/pkg/tcpip/transport/tcp/endpoint.go
+++ b/pkg/tcpip/transport/tcp/endpoint.go
@@ -291,6 +291,7 @@ func (*EndpointInfo) IsEndpointInfo() {}
 // The following three mutexes can be acquired independent of e.mu but if
 // acquired with e.mu then e.mu must be acquired first.
 //
+// e.acceptMu -> protects acceptedChan.
 // e.rcvListMu -> Protects the rcvList and associated fields.
 // e.sndBufMu -> Protects the sndQueue and associated fields.
 // e.lastErrorMu -> Protects the lastError field.
@@ -533,6 +534,23 @@ type endpoint struct {
 	// to the acceptedChan below terminate before we close acceptedChan.
 	pendingAccepted sync.WaitGroup `state:"nosave"`
 
+	// acceptMu protects acceptedChan.
+	acceptMu sync.Mutex `state:"nosave"`
+
+	// acceptCond is a condition variable that can be used to block on when
+	// acceptedChan is full and an endpoint is ready to be delivered.
+	//
+	// This condition variable is required because just blocking on sending
+	// to acceptedChan does not work in cases where endpoint.Listen is
+	// called twice with different backlog values. In such cases the channel
+	// is closed and a new one created. Any pending goroutines blocking on
+	// the write to the channel will panic.
+	//
+	// We use this condition variable to block/unblock goroutines which
+	// tried to deliver an endpoint but couldn't because accept backlog was
+	// full ( See: endpoint.deliverAccepted ).
+	acceptCond *sync.Cond `state:"nosave"`
+
 	// acceptedChan is used by a listening endpoint protocol goroutine to
 	// send newly accepted connections to the endpoint so that they can be
 	// read by Accept() calls.
@@ -814,6 +832,7 @@ func newEndpoint(s *stack.Stack, netProto tcpip.NetworkProtocolNumber, waiterQue
 
 	e.segmentQueue.setLimit(MaxUnprocessedSegments)
 	e.tsOffset = timeStampOffset()
+	e.acceptCond = sync.NewCond(&e.acceptMu)
 
 	return e
 }
@@ -834,9 +853,11 @@ func (e *endpoint) Readiness(mask waiter.EventMask) waiter.EventMask {
 	case StateListen:
 		// Check if there's anything in the accepted channel.
 		if (mask & waiter.EventIn) != 0 {
+			e.acceptMu.Lock()
 			if len(e.acceptedChan) > 0 {
 				result |= waiter.EventIn
 			}
+			e.acceptMu.Unlock()
 		}
 	}
 	if e.EndpointState().connected() {
@@ -981,29 +1002,19 @@ func (e *endpoint) closeNoShutdownLocked() {
 // closePendingAcceptableConnections closes all connections that have completed
 // handshake but not yet been delivered to the application.
 func (e *endpoint) closePendingAcceptableConnectionsLocked() {
-	done := make(chan struct{})
-	// Spin a goroutine up as ranging on e.acceptedChan will just block when
-	// there are no more connections in the channel. Using a non-blocking
-	// select does not work as it can potentially select the default case
-	// even when there are pending writes but that are not yet written to
-	// the channel.
-	go func() {
-		defer close(done)
-		for n := range e.acceptedChan {
-			n.notifyProtocolGoroutine(notifyReset)
-			// close all connections that have completed but
-			// not accepted by the application.
-			n.Close()
-		}
-	}()
-	// pendingAccepted(see endpoint.deliverAccepted) tracks the number of
-	// endpoints which have completed handshake but are not yet written to
-	// the e.acceptedChan. We wait here till the goroutine above can drain
-	// all such connections from e.acceptedChan.
-	e.pendingAccepted.Wait()
+	e.acceptMu.Lock()
+	if e.acceptedChan == nil {
+		e.acceptMu.Unlock()
+		return
+	}
+
 	close(e.acceptedChan)
-	<-done
 	e.acceptedChan = nil
+	e.acceptCond.Broadcast()
+	e.acceptMu.Unlock()
+
+	// Wait for all pending endpoints to close.
+	e.pendingAccepted.Wait()
 }
 
 // cleanupLocked frees all resources associated with the endpoint. It is called
@@ -1012,9 +1023,7 @@ func (e *endpoint) closePendingAcceptableConnectionsLocked() {
 func (e *endpoint) cleanupLocked() {
 	// Close all endpoints that might have been accepted by TCP but not by
 	// the client.
-	if e.acceptedChan != nil {
-		e.closePendingAcceptableConnectionsLocked()
-	}
+	e.closePendingAcceptableConnectionsLocked()
 
 	e.workerCleanup = false
 
@@ -2204,6 +2213,8 @@ func (e *endpoint) listen(backlog int) *tcpip.Error {
 	if e.EndpointState() == StateListen && !e.workerCleanup {
 		// Adjust the size of the channel iff we can fix existing
 		// pending connections into the new one.
+		e.acceptMu.Lock()
+		defer e.acceptMu.Unlock()
 		if len(e.acceptedChan) > backlog {
 			return tcpip.ErrInvalidEndpointState
 		}
@@ -2216,6 +2227,11 @@ func (e *endpoint) listen(backlog int) *tcpip.Error {
 		for ep := range origChan {
 			e.acceptedChan <- ep
 		}
+
+		// Notify any blocked goroutines that they can attempt to
+		// deliver endpoints again.
+		e.acceptCond.Broadcast()
+
 		return nil
 	}
 
@@ -2245,9 +2261,12 @@ func (e *endpoint) listen(backlog int) *tcpip.Error {
 	// The channel may be non-nil when we're restoring the endpoint, and it
 	// may be pre-populated with some previously accepted (but not Accepted)
 	// endpoints.
+	e.acceptMu.Lock()
 	if e.acceptedChan == nil {
 		e.acceptedChan = make(chan *endpoint, backlog)
 	}
+	e.acceptMu.Unlock()
+
 	e.workerRunning = true
 	go e.protocolListenLoop( // S/R-SAFE: drained on save.
 		seqnum.Size(e.receiveBufferAvailable()))
@@ -2276,9 +2295,12 @@ func (e *endpoint) Accept() (tcpip.Endpoint, *waiter.Queue, *tcpip.Error) {
 	}
 
 	// Get the new accepted endpoint.
+	e.acceptMu.Lock()
+	defer e.acceptMu.Unlock()
 	var n *endpoint
 	select {
 	case n = <-e.acceptedChan:
+		e.acceptCond.Signal()
 	default:
 		return nil, nil, tcpip.ErrWouldBlock
 	}
diff --git a/pkg/tcpip/transport/tcp/endpoint_state.go b/pkg/tcpip/transport/tcp/endpoint_state.go
index 9175de441..c3c692555 100644
--- a/pkg/tcpip/transport/tcp/endpoint_state.go
+++ b/pkg/tcpip/transport/tcp/endpoint_state.go
@@ -173,6 +173,9 @@ func (e *endpoint) afterLoad() {
 	// Restore the endpoint to InitialState as it will be moved to
 	// its origEndpointState during Resume.
 	e.state = StateInitial
+	// Condition variables and mutexs are not S/R'ed so reinitialize
+	// acceptCond with e.acceptMu.
+	e.acceptCond = sync.NewCond(&e.acceptMu)
 	stack.StackFromEnv.RegisterRestoredEndpoint(e)
 }