Implement Broadcast support

This change adds support for the SO_BROADCAST socket option in gVisor Netstack.
This support includes getsockopt()/setsockopt() functionality for both UDP and
TCP endpoints (the latter being a NOOP), dispatching broadcast messages up and
down the stack, and route finding/creation for broadcast packets. Finally, a
suite of tests have been implemented, exercising this functionality through the
Linux syscall API.

PiperOrigin-RevId: 234850781
Change-Id: If3e666666917d39f55083741c78314a06defb26c

1 files changed, 23 insertions, 0 deletions

diff --git a/pkg/tcpip/transport/udp/endpoint.go b/pkg/tcpip/transport/udp/endpoint.go
index 9c3881d63..05d35e526 100644
--- a/pkg/tcpip/transport/udp/endpoint.go
+++ b/pkg/tcpip/transport/udp/endpoint.go
@@ -82,6 +82,7 @@ type endpoint struct {
 	multicastAddr  tcpip.Address
 	multicastNICID tcpip.NICID
 	reusePort      bool
+	broadcast      bool
 
 	// shutdownFlags represent the current shutdown state of the endpoint.
 	shutdownFlags tcpip.ShutdownFlags
@@ -347,6 +348,10 @@ func (e *endpoint) Write(p tcpip.Payload, opts tcpip.WriteOptions) (uintptr, <-c
 			nicid = e.bindNICID
 		}
 
+		if to.Addr == header.IPv4Broadcast && !e.broadcast {
+			return 0, nil, tcpip.ErrBroadcastDisabled
+		}
+
 		r, _, _, err := e.connectRoute(nicid, *to)
 		if err != nil {
 			return 0, nil, err
@@ -502,6 +507,13 @@ func (e *endpoint) SetSockOpt(opt interface{}) *tcpip.Error {
 		e.mu.Lock()
 		e.reusePort = v != 0
 		e.mu.Unlock()
+
+	case tcpip.BroadcastOption:
+		e.mu.Lock()
+		e.broadcast = v != 0
+		e.mu.Unlock()
+
+		return nil
 	}
 	return nil
 }
@@ -581,6 +593,17 @@ func (e *endpoint) GetSockOpt(opt interface{}) *tcpip.Error {
 		*o = 0
 		return nil
 
+	case *tcpip.BroadcastOption:
+		e.mu.RLock()
+		v := e.broadcast
+		e.mu.RUnlock()
+
+		*o = 0
+		if v {
+			*o = 1
+		}
+		return nil
+
 	default:
 		return tcpip.ErrUnknownProtocolOption
 	}