Fix sendto for dual stack UDP sockets

Previously, dual stack UDP sockets bound to an IPv4 address could not use
sendto to communicate with IPv4 addresses. Further, dual stack UDP sockets
bound to an IPv6 address could use sendto to communicate with IPv4 addresses.
Neither of these behaviors are consistent with Linux.

PiperOrigin-RevId: 197036024
Change-Id: Ic3713efc569f26196e35bb41e6ad63f23675fc90

3 files changed, 54 insertions, 23 deletions

diff --git a/pkg/tcpip/transport/udp/endpoint.go b/pkg/tcpip/transport/udp/endpoint.go
index f86fc6d5a..6fcddd028 100644
--- a/pkg/tcpip/transport/udp/endpoint.go
+++ b/pkg/tcpip/transport/udp/endpoint.go
@@ -62,7 +62,6 @@ type endpoint struct {
 	id         stack.TransportEndpointID
 	state      endpointState
 	bindNICID  tcpip.NICID
-	bindAddr   tcpip.Address
 	regNICID   tcpip.NICID
 	route      stack.Route `state:"manual"`
 	dstPort    uint16
@@ -267,13 +266,13 @@ func (e *endpoint) Write(p tcpip.Payload, opts tcpip.WriteOptions) (uintptr, *tc
 
 		toCopy := *to
 		to = &toCopy
-		netProto, err := e.checkV4Mapped(to, true)
+		netProto, err := e.checkV4Mapped(to, false)
 		if err != nil {
 			return 0, err
 		}
 
 		// Find the enpoint.
-		r, err := e.stack.FindRoute(nicid, e.bindAddr, to.Addr, netProto)
+		r, err := e.stack.FindRoute(nicid, e.id.LocalAddress, to.Addr, netProto)
 		if err != nil {
 			return 0, err
 		}
@@ -439,11 +438,16 @@ func (e *endpoint) checkV4Mapped(addr *tcpip.FullAddress, allowMismatch bool) (t
 		if addr.Addr == "\x00\x00\x00\x00" {
 			addr.Addr = ""
 		}
+
+		// Fail if we are bound to an IPv6 address.
+		if !allowMismatch && len(e.id.LocalAddress) == 16 {
+			return 0, tcpip.ErrNetworkUnreachable
+		}
 	}
 
 	// Fail if we're bound to an address length different from the one we're
 	// checking.
-	if l := len(e.id.LocalAddress); !allowMismatch && l != 0 && l != len(addr.Addr) {
+	if l := len(e.id.LocalAddress); l != 0 && l != len(addr.Addr) {
 		return 0, tcpip.ErrInvalidEndpointState
 	}
 
@@ -485,7 +489,7 @@ func (e *endpoint) Connect(addr tcpip.FullAddress) *tcpip.Error {
 	}
 
 	// Find a route to the desired destination.
-	r, err := e.stack.FindRoute(nicid, e.bindAddr, addr.Addr, netProto)
+	r, err := e.stack.FindRoute(nicid, e.id.LocalAddress, addr.Addr, netProto)
 	if err != nil {
 		return err
 	}
@@ -605,7 +609,7 @@ func (e *endpoint) bindLocked(addr tcpip.FullAddress, commit func() *tcpip.Error
 		return tcpip.ErrInvalidEndpointState
 	}
 
-	netProto, err := e.checkV4Mapped(&addr, false)
+	netProto, err := e.checkV4Mapped(&addr, true)
 	if err != nil {
 		return err
 	}
@@ -670,7 +674,6 @@ func (e *endpoint) Bind(addr tcpip.FullAddress, commit func() *tcpip.Error) *tcp
 	}
 
 	e.bindNICID = addr.NIC
-	e.bindAddr = addr.Addr
 
 	return nil
 }
diff --git a/pkg/tcpip/transport/udp/endpoint_state.go b/pkg/tcpip/transport/udp/endpoint_state.go
index e20d59ca3..93784fb05 100644
--- a/pkg/tcpip/transport/udp/endpoint_state.go
+++ b/pkg/tcpip/transport/udp/endpoint_state.go
@@ -72,7 +72,7 @@ func (e *endpoint) afterLoad() {
 
 	var err *tcpip.Error
 	if e.state == stateConnected {
-		e.route, err = e.stack.FindRoute(e.regNICID, e.bindAddr, e.id.RemoteAddress, netProto)
+		e.route, err = e.stack.FindRoute(e.regNICID, e.id.LocalAddress, e.id.RemoteAddress, netProto)
 		if err != nil {
 			panic(*err)
 		}
diff --git a/pkg/tcpip/transport/udp/udp_test.go b/pkg/tcpip/transport/udp/udp_test.go
index 1eb9ecb80..cc342c69b 100644
--- a/pkg/tcpip/transport/udp/udp_test.go
+++ b/pkg/tcpip/transport/udp/udp_test.go
@@ -401,7 +401,7 @@ func TestV4ReadOnV4(t *testing.T) {
 	testV4Read(c)
 }
 
-func testDualWrite(c *testContext) uint16 {
+func testV4Write(c *testContext) uint16 {
 	// Write to V4 mapped address.
 	payload := buffer.View(newPayload())
 	n, err := c.ep.Write(tcpip.SlicePayload(payload), tcpip.WriteOptions{
@@ -423,16 +423,18 @@ func testDualWrite(c *testContext) uint16 {
 		),
 	)
 
-	port := udp.SourcePort()
-
 	// Check the payload.
 	if !bytes.Equal(payload, udp.Payload()) {
 		c.t.Fatalf("Bad payload: got %x, want %x", udp.Payload(), payload)
 	}
 
+	return udp.SourcePort()
+}
+
+func testV6Write(c *testContext) uint16 {
 	// Write to v6 address.
-	payload = buffer.View(newPayload())
-	n, err = c.ep.Write(tcpip.SlicePayload(payload), tcpip.WriteOptions{
+	payload := buffer.View(newPayload())
+	n, err := c.ep.Write(tcpip.SlicePayload(payload), tcpip.WriteOptions{
 		To: &tcpip.FullAddress{Addr: testV6Addr, Port: testPort},
 	})
 	if err != nil {
@@ -442,14 +444,12 @@ func testDualWrite(c *testContext) uint16 {
 		c.t.Fatalf("Bad number of bytes written: got %v, want %v", n, len(payload))
 	}
 
-	// Check that we received the packet, and that the source port is the
-	// same as the one used in ipv4.
-	b = c.getV6Packet()
-	udp = header.UDP(header.IPv6(b).Payload())
+	// Check that we received the packet.
+	b := c.getV6Packet()
+	udp := header.UDP(header.IPv6(b).Payload())
 	checker.IPv6(c.t, b,
 		checker.UDP(
 			checker.DstPort(testPort),
-			checker.SrcPort(port),
 		),
 	)
 
@@ -458,7 +458,17 @@ func testDualWrite(c *testContext) uint16 {
 		c.t.Fatalf("Bad payload: got %x, want %x", udp.Payload(), payload)
 	}
 
-	return port
+	return udp.SourcePort()
+}
+
+func testDualWrite(c *testContext) uint16 {
+	v4Port := testV4Write(c)
+	v6Port := testV6Write(c)
+	if v4Port != v6Port {
+		c.t.Fatalf("expected v4 and v6 ports to be equal: got v4Port = %d, v6Port = %d", v4Port, v6Port)
+	}
+
+	return v4Port
 }
 
 func TestDualWriteUnbound(t *testing.T) {
@@ -498,7 +508,16 @@ func TestDualWriteConnectedToV6(t *testing.T) {
 		c.t.Fatalf("Bind failed: %v", err)
 	}
 
-	testDualWrite(c)
+	testV6Write(c)
+
+	// Write to V4 mapped address.
+	payload := buffer.View(newPayload())
+	_, err := c.ep.Write(tcpip.SlicePayload(payload), tcpip.WriteOptions{
+		To: &tcpip.FullAddress{Addr: testV4MappedAddr, Port: testPort},
+	})
+	if err != tcpip.ErrNetworkUnreachable {
+		c.t.Fatalf("Write returned unexpected error: got %v, want %v", err, tcpip.ErrNetworkUnreachable)
+	}
 }
 
 func TestDualWriteConnectedToV4Mapped(t *testing.T) {
@@ -512,7 +531,16 @@ func TestDualWriteConnectedToV4Mapped(t *testing.T) {
 		c.t.Fatalf("Bind failed: %v", err)
 	}
 
-	testDualWrite(c)
+	testV4Write(c)
+
+	// Write to v6 address.
+	payload := buffer.View(newPayload())
+	_, err := c.ep.Write(tcpip.SlicePayload(payload), tcpip.WriteOptions{
+		To: &tcpip.FullAddress{Addr: testV6Addr, Port: testPort},
+	})
+	if err != tcpip.ErrInvalidEndpointState {
+		c.t.Fatalf("Write returned unexpected error: got %v, want %v", err, tcpip.ErrInvalidEndpointState)
+	}
 }
 
 func TestV4WriteOnV6Only(t *testing.T) {
@@ -547,8 +575,8 @@ func TestV6WriteOnBoundToV4Mapped(t *testing.T) {
 	_, err := c.ep.Write(tcpip.SlicePayload(payload), tcpip.WriteOptions{
 		To: &tcpip.FullAddress{Addr: testV6Addr, Port: testPort},
 	})
-	if err != tcpip.ErrNoRoute {
-		c.t.Fatalf("Write returned unexpected error: got %v, want %v", err, tcpip.ErrNoRoute)
+	if err != tcpip.ErrInvalidEndpointState {
+		c.t.Fatalf("Write returned unexpected error: got %v, want %v", err, tcpip.ErrInvalidEndpointState)
 	}
 }