Disallow setting a coordinator for a coordinator device.

The error code is based on the runc's behavior:

```shell
# ip link set dev br0 master br0
Error: Can not enslave a bridge to a bridge.
# echo $?
2
```

Reported-by: syzbot+9dcefbdcc9ecdeb4df96@syzkaller.appspotmail.com
PiperOrigin-RevId: 679761515

pkg/tcpip/stack/bridge_test.go

@@ -48,7 +48,7 @@ func TestWritePacketFromBridge(t *testing.T) {
 	bridgeEndpoint := stack.NewBridgeEndpoint(1500)
 	bridgeEndpoint.SetLinkAddress(bridgeLinkAddr)
 	if err := s.CreateNIC(bridgeID, bridgeEndpoint); err != nil {
-		t.Fatalf("s.CreateNIC(%d, _): %s", nicID, err)
+		t.Fatalf("s.CreateNIC(%d, _): %s", bridgeID, err)
 	}
 	if err := s.SetNICCoordinator(nicID, bridgeID); err != nil {
 		t.Fatalf("s.SetNICCoordinator")
@@ -106,7 +106,7 @@ func TestWritePacketBetweenDevices(t *testing.T) {
 	bridgeEndpoint := stack.NewBridgeEndpoint(1500)
 	bridgeEndpoint.SetLinkAddress(bridgeLinkAddr)
 	if err := s.CreateNIC(bridgeID, bridgeEndpoint); err != nil {
-		t.Fatalf("s.CreateNIC(%d, _): %s", nicID, err)
+		t.Fatalf("s.CreateNIC(%d, _): %s", bridgeID, err)
 	}
 
 	c := channel.New(1, header.EthernetMinimumSize, localLinkAddr)
@@ -150,6 +150,23 @@ func TestWritePacketBetweenDevices(t *testing.T) {
 	}
 }
 
+func TestSetCoordinator(t *testing.T) {
+	const (
+		bridgeLinkAddr = tcpip.LinkAddress("\x02\x02\x03\x04\x05\x08")
+		bridgeID       = 6
+	)
+
+	s := stack.New(stack.Options{})
+	bridgeEndpoint := stack.NewBridgeEndpoint(1500)
+	bridgeEndpoint.SetLinkAddress(bridgeLinkAddr)
+	if err := s.CreateNIC(bridgeID, bridgeEndpoint); err != nil {
+		t.Fatalf("s.CreateNIC(%d, _): %s", bridgeID, err)
+	}
+	if err := s.SetNICCoordinator(bridgeID, bridgeID); err == nil {
+		t.Fatalf("s.SetNICCoordinator(%d, %d) = %s, want = %s", bridgeID, bridgeID, err, tcpip.ErrNoSuchFile{})
+	}
+}
+
 func TestMTU(t *testing.T) {
 	e := stack.NewBridgeEndpoint(1500)
 	mtus := []uint32{1000, 2000}

pkg/tcpip/stack/stack.go

@@ -1053,7 +1053,10 @@ func (s *Stack) SetNICCoordinator(id tcpip.NICID, mid tcpip.NICID) tcpip.Error {
 	if !ok {
 		return &tcpip.ErrUnknownNICID{}
 	}
-
+	// Setting a coordinator for a coordinator NIC is not allowed.
+	if _, ok := nic.NetworkLinkEndpoint.(CoordinatorNIC); ok {
+		return &tcpip.ErrNoSuchFile{}
+	}
 	m, ok := s.nics[mid]
 	if !ok {
 		return &tcpip.ErrUnknownNICID{}

test/rtnetlink/linux/bridge_test.sh

@@ -31,6 +31,10 @@ ip link add hveth1 type veth peer name veth1 netns test1
 ip link set up dev hveth0
 ip link set up dev hveth1
 
+if ip link set br0 master br0; then
+  exit 1
+fi
+
 ip link set hveth0 master br0
 ip link set hveth1 master br0