Decrement e.synRcvdCount once handshake is complete.

Earlier the count was dropped only after calling e.deliverAccepted. This lead to
an issue where there were no connections in SYN-RCVD state for the listening
endpoint but e.synRcvdCount would not be zero because it was being reduced only
when handleSynSegment returned after deliverAccepted returned.

This issue is seen when the Nth SYN for a listen backlog of size N which would
cause the listen backlog to be full gets dropped occasionally. This happens when
the new SYN comes at when the previous completed endpoint has been delivered to
the accept queue but the synRcvdCount hasn't yet been decremented because the
goroutine running handleSynSegment has not yet completed.

PiperOrigin-RevId: 338690646

2 files changed, 12 insertions, 10 deletions

diff --git a/pkg/tcpip/transport/tcp/accept.go b/pkg/tcpip/transport/tcp/accept.go
index b706438bd..6b3238d6b 100644
--- a/pkg/tcpip/transport/tcp/accept.go
+++ b/pkg/tcpip/transport/tcp/accept.go
@@ -425,20 +425,17 @@ func (e *endpoint) notifyAborted() {
 // cookies to accept connections.
 func (e *endpoint) handleSynSegment(ctx *listenContext, s *segment, opts *header.TCPSynOptions) {
 	defer ctx.synRcvdCount.dec()
-	defer func() {
-		e.mu.Lock()
-		e.decSynRcvdCount()
-		e.mu.Unlock()
-	}()
 	defer s.decRef()
 
 	n, err := ctx.createEndpointAndPerformHandshake(s, opts, &waiter.Queue{}, e.owner)
 	if err != nil {
 		e.stack.Stats().TCP.FailedConnectionAttempts.Increment()
 		e.stats.FailedConnectionAttempts.Increment()
+		e.decSynRcvdCount()
 		return
 	}
 	ctx.removePendingEndpoint(n)
+	e.decSynRcvdCount()
 	n.startAcceptedLoop()
 	e.stack.Stats().TCP.PassiveConnectionOpenings.Increment()
 
@@ -456,7 +453,9 @@ func (e *endpoint) incSynRcvdCount() bool {
 }
 
 func (e *endpoint) decSynRcvdCount() {
+	e.mu.Lock()
 	e.synRcvdCount--
+	e.mu.Unlock()
 }
 
 func (e *endpoint) acceptQueueIsFull() bool {
diff --git a/pkg/tcpip/transport/tcp/tcp_test.go b/pkg/tcpip/transport/tcp/tcp_test.go
index a7149efd0..93a580bad 100644
--- a/pkg/tcpip/transport/tcp/tcp_test.go
+++ b/pkg/tcpip/transport/tcp/tcp_test.go
@@ -5131,6 +5131,7 @@ func TestKeepalive(t *testing.T) {
 }
 
 func executeHandshake(t *testing.T, c *context.Context, srcPort uint16, synCookieInUse bool) (irs, iss seqnum.Value) {
+	t.Helper()
 	// Send a SYN request.
 	irs = seqnum.Value(789)
 	c.SendPacket(nil, &context.Headers{
@@ -5175,6 +5176,7 @@ func executeHandshake(t *testing.T, c *context.Context, srcPort uint16, synCooki
 }
 
 func executeV6Handshake(t *testing.T, c *context.Context, srcPort uint16, synCookieInUse bool) (irs, iss seqnum.Value) {
+	t.Helper()
 	// Send a SYN request.
 	irs = seqnum.Value(789)
 	c.SendV6Packet(nil, &context.Headers{
@@ -5238,13 +5240,14 @@ func TestListenBacklogFull(t *testing.T) {
 
 	// Test acceptance.
 	// Start listening.
-	listenBacklog := 2
+	listenBacklog := 10
 	if err := c.EP.Listen(listenBacklog); err != nil {
 		t.Fatalf("Listen failed: %s", err)
 	}
 
-	for i := 0; i < listenBacklog; i++ {
-		executeHandshake(t, c, context.TestPort+uint16(i), false /*synCookieInUse */)
+	lastPortOffset := uint16(0)
+	for ; int(lastPortOffset) < listenBacklog; lastPortOffset++ {
+		executeHandshake(t, c, context.TestPort+lastPortOffset, false /*synCookieInUse */)
 	}
 
 	time.Sleep(50 * time.Millisecond)
@@ -5252,7 +5255,7 @@ func TestListenBacklogFull(t *testing.T) {
 	// Now execute send one more SYN. The stack should not respond as the backlog
 	// is full at this point.
 	c.SendPacket(nil, &context.Headers{
-		SrcPort: context.TestPort + 2,
+		SrcPort: context.TestPort + uint16(lastPortOffset),
 		DstPort: context.StackPort,
 		Flags:   header.TCPFlagSyn,
 		SeqNum:  seqnum.Value(789),
@@ -5293,7 +5296,7 @@ func TestListenBacklogFull(t *testing.T) {
 	}
 
 	// Now a new handshake must succeed.
-	executeHandshake(t, c, context.TestPort+2, false /*synCookieInUse */)
+	executeHandshake(t, c, context.TestPort+lastPortOffset, false /*synCookieInUse */)
 
 	newEP, _, err := c.EP.Accept(nil)
 	if err == tcpip.ErrWouldBlock {