Do not allow unbinding network protocol

Once a packet socket is bound to a network protocol, it cannot be
unbound from that protocol; the network protocol binding may only be
updated to a different network protocol.

To comply with Linux.

PiperOrigin-RevId: 397810878

pkg/tcpip/transport/packet/endpoint.go

@@ -82,11 +82,9 @@ type endpoint struct {
 
 	mu sync.RWMutex `state:"nosave"`
 	// +checklocks:mu
-	netProto tcpip.NetworkProtocolNumber
-	// +checklocks:mu
 	closed bool
 	// +checklocks:mu
-	bound bool
+	boundNetProto tcpip.NetworkProtocolNumber
 	// +checklocks:mu
 	boundNIC tcpip.NICID
 
@@ -98,10 +96,10 @@ type endpoint struct {
 // NewEndpoint returns a new packet endpoint.
 func NewEndpoint(s *stack.Stack, cooked bool, netProto tcpip.NetworkProtocolNumber, waiterQueue *waiter.Queue) (tcpip.Endpoint, tcpip.Error) {
 	ep := &endpoint{
-		stack:       s,
-		cooked:      cooked,
-		netProto:    netProto,
-		waiterQueue: waiterQueue,
+		stack:         s,
+		cooked:        cooked,
+		boundNetProto: netProto,
+		waiterQueue:   waiterQueue,
 	}
 	ep.ops.InitHandler(ep, ep.stack, tcpip.GetStackSendBufferLimits, tcpip.GetStackReceiveBufferLimits)
 	ep.ops.SetReceiveBufferSize(32*1024, false /* notify */)
@@ -137,7 +135,7 @@ func (ep *endpoint) Close() {
 		return
 	}
 
-	ep.stack.UnregisterPacketEndpoint(0, ep.netProto, ep)
+	ep.stack.UnregisterPacketEndpoint(ep.boundNIC, ep.boundNetProto, ep)
 
 	ep.rcvMu.Lock()
 	defer ep.rcvMu.Unlock()
@@ -150,7 +148,6 @@ func (ep *endpoint) Close() {
 	}
 
 	ep.closed = true
-	ep.bound = false
 	ep.waiterQueue.Notify(waiter.EventHUp | waiter.EventErr | waiter.ReadableEvents | waiter.WritableEvents)
 }
 
@@ -211,7 +208,7 @@ func (ep *endpoint) Write(p tcpip.Payloader, opts tcpip.WriteOptions) (int64, tc
 	ep.mu.Lock()
 	closed := ep.closed
 	nicID := ep.boundNIC
-	proto := ep.netProto
+	proto := ep.boundNetProto
 	ep.mu.Unlock()
 	if closed {
 		return 0, &tcpip.ErrClosedForSend{}
@@ -294,24 +291,29 @@ func (ep *endpoint) Bind(addr tcpip.FullAddress) tcpip.Error {
 	defer ep.mu.Unlock()
 
 	netProto := tcpip.NetworkProtocolNumber(addr.Port)
-	if ep.bound && ep.boundNIC == addr.NIC && ep.netProto == netProto {
-		// If the NIC being bound is the same then just return success.
+	if netProto == 0 {
+		// Do not allow unbinding the network protocol.
+		netProto = ep.boundNetProto
+	}
+
+	if ep.boundNIC == addr.NIC && ep.boundNetProto == netProto {
+		// Already bound to the requested NIC and network protocol.
 		return nil
 	}
 
-	// Unregister endpoint with all the nics.
-	ep.stack.UnregisterPacketEndpoint(0, ep.netProto, ep)
-	ep.bound = false
+	// TODO(https://gvisor.dev/issue/6618): Unregister after registering the new
+	// binding.
+	ep.stack.UnregisterPacketEndpoint(ep.boundNIC, ep.boundNetProto, ep)
+	ep.boundNIC = 0
+	ep.boundNetProto = 0
 
 	// Bind endpoint to receive packets from specific interface.
 	if err := ep.stack.RegisterPacketEndpoint(addr.NIC, netProto, ep); err != nil {
 		return err
 	}
 
-	ep.bound = true
 	ep.boundNIC = addr.NIC
-	ep.netProto = netProto
-
+	ep.boundNetProto = netProto
 	return nil
 }
 
@@ -473,7 +475,7 @@ func (*endpoint) State() uint32 {
 func (ep *endpoint) Info() tcpip.EndpointInfo {
 	ep.mu.RLock()
 	defer ep.mu.RUnlock()
-	return &stack.TransportEndpointInfo{NetProto: ep.netProto}
+	return &stack.TransportEndpointInfo{NetProto: ep.boundNetProto}
 }
 
 // Stats returns a pointer to the endpoint stats.

pkg/tcpip/transport/packet/endpoint_state.go

@@ -15,6 +15,7 @@
 package packet
 
 import (
+	"fmt"
 	"time"
 
 	"gvisor.dev/gvisor/pkg/tcpip"
@@ -57,9 +58,8 @@ func (ep *endpoint) afterLoad() {
 	ep.stack = stack.StackFromEnv
 	ep.ops.InitHandler(ep, ep.stack, tcpip.GetStackSendBufferLimits, tcpip.GetStackReceiveBufferLimits)
 
-	// TODO(gvisor.dev/173): Once bind is supported, choose the right NIC.
-	if err := ep.stack.RegisterPacketEndpoint(0, ep.netProto, ep); err != nil {
-		panic(err)
+	if err := ep.stack.RegisterPacketEndpoint(ep.boundNIC, ep.boundNetProto, ep); err != nil {
+		panic(fmt.Sprintf("RegisterPacketEndpoint(%d, %d, _): %s", ep.boundNIC, ep.boundNetProto, err))
 	}
 
 	ep.rcvMu.Lock()

test/syscalls/linux/packet_socket.cc

@@ -214,6 +214,11 @@ TEST_P(PacketSocketTest, RebindProtocol) {
   ASSERT_NO_FATAL_FAILURE(bind_to_network_protocol(ETH_P_IP));
   ASSERT_NO_FATAL_FAILURE(send_udp_message(++counter));
   ASSERT_NO_FATAL_FAILURE(test_recv(counter));
+
+  // A zero valued protocol number should not change the bound network protocol.
+  ASSERT_NO_FATAL_FAILURE(bind_to_network_protocol(0));
+  ASSERT_NO_FATAL_FAILURE(send_udp_message(++counter));
+  ASSERT_NO_FATAL_FAILURE(test_recv(counter));
 }
 
 INSTANTIATE_TEST_SUITE_P(AllPacketSocketTests, PacketSocketTest,