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

1 files changed, 10 insertions, 7 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
 }