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

6 files changed, 129 insertions, 11 deletions

diff --git a/pkg/tcpip/stack/nic.go b/pkg/tcpip/stack/nic.go
index 586ca873e..43d7c2ec4 100644
--- a/pkg/tcpip/stack/nic.go
+++ b/pkg/tcpip/stack/nic.go
@@ -399,6 +399,21 @@ func (n *NIC) DeliverNetworkPacket(linkEP LinkEndpoint, remote, _ tcpip.LinkAddr
 
 	src, dst := netProto.ParseAddresses(vv.First())
 
+	// If the packet is destined to the IPv4 Broadcast address, then make a
+	// route to each IPv4 network endpoint and let each endpoint handle the
+	// packet.
+	if dst == header.IPv4Broadcast {
+		for _, ref := range n.endpoints {
+			if ref.protocol == header.IPv4ProtocolNumber && ref.tryIncRef() {
+				r := makeRoute(protocol, dst, src, linkEP.LinkAddress(), ref)
+				r.RemoteLinkAddress = remote
+				ref.ep.HandlePacket(&r, vv)
+				ref.decRef()
+			}
+		}
+		return
+	}
+
 	if ref := n.getRef(protocol, dst); ref != nil {
 		r := makeRoute(protocol, dst, src, linkEP.LinkAddress(), ref)
 		r.RemoteLinkAddress = remote
diff --git a/pkg/tcpip/stack/transport_demuxer.go b/pkg/tcpip/stack/transport_demuxer.go
index a5ff2159a..c18208dc0 100644
--- a/pkg/tcpip/stack/transport_demuxer.go
+++ b/pkg/tcpip/stack/transport_demuxer.go
@@ -132,7 +132,22 @@ func (ep *multiPortEndpoint) selectEndpoint(id TransportEndpointID) TransportEnd
 // HandlePacket is called by the stack when new packets arrive to this transport
 // endpoint.
 func (ep *multiPortEndpoint) HandlePacket(r *Route, id TransportEndpointID, vv buffer.VectorisedView) {
-	ep.selectEndpoint(id).HandlePacket(r, id, vv)
+	// If this is a broadcast datagram, deliver the datagram to all endpoints
+	// managed by ep.
+	if id.LocalAddress == header.IPv4Broadcast {
+		for i, endpoint := range ep.endpointsArr {
+			// HandlePacket modifies vv, so each endpoint needs its own copy.
+			if i == len(ep.endpointsArr)-1 {
+				endpoint.HandlePacket(r, id, vv)
+				break
+			}
+			vvCopy := buffer.NewView(vv.Size())
+			copy(vvCopy, vv.ToView())
+			endpoint.HandlePacket(r, id, vvCopy.ToVectorisedView())
+		}
+	} else {
+		ep.selectEndpoint(id).HandlePacket(r, id, vv)
+	}
 }
 
 // HandleControlPacket implements stack.TransportEndpoint.HandleControlPacket.
@@ -224,20 +239,47 @@ func (d *transportDemuxer) unregisterEndpoint(netProtos []tcpip.NetworkProtocolN
 	}
 }
 
-// deliverPacket attempts to deliver the given packet. Returns true if it found
-// an endpoint, false otherwise.
+var loopbackSubnet = func() tcpip.Subnet {
+	sn, err := tcpip.NewSubnet("\x7f\x00\x00\x00", "\xff\x00\x00\x00")
+	if err != nil {
+		panic(err)
+	}
+	return sn
+}()
+
+// deliverPacket attempts to find one or more matching transport endpoints, and
+// then, if matches are found, delivers the packet to them. Returns true if it
+// found one or more endpoints, false otherwise.
 func (d *transportDemuxer) deliverPacket(r *Route, protocol tcpip.TransportProtocolNumber, vv buffer.VectorisedView, id TransportEndpointID) bool {
 	eps, ok := d.protocol[protocolIDs{r.NetProto, protocol}]
 	if !ok {
 		return false
 	}
 
+	// If a sender bound to the Loopback interface sends a broadcast,
+	// that broadcast must not be delivered to the sender.
+	if loopbackSubnet.Contains(r.RemoteAddress) && r.LocalAddress == header.IPv4Broadcast && id.LocalPort == id.RemotePort {
+		return false
+	}
+
+	// If the packet is a broadcast, then find all matching transport endpoints.
+	// Otherwise, try to find a single matching transport endpoint.
+	destEps := make([]TransportEndpoint, 0, 1)
 	eps.mu.RLock()
-	ep := d.findEndpointLocked(eps, vv, id)
+
+	if protocol == header.UDPProtocolNumber && id.LocalAddress == header.IPv4Broadcast {
+		for epID, endpoint := range eps.endpoints {
+			if epID.LocalPort == id.LocalPort {
+				destEps = append(destEps, endpoint)
+			}
+		}
+	} else if ep := d.findEndpointLocked(eps, vv, id); ep != nil {
+		destEps = append(destEps, ep)
+	}
 	eps.mu.RUnlock()
 
-	// Fail if we didn't find one.
-	if ep == nil {
+	// Fail if we didn't find at least one matching transport endpoint.
+	if len(destEps) == 0 {
 		// UDP packet could not be delivered to an unknown destination port.
 		if protocol == header.UDPProtocolNumber {
 			r.Stats().UDP.UnknownPortErrors.Increment()
@@ -246,7 +288,9 @@ func (d *transportDemuxer) deliverPacket(r *Route, protocol tcpip.TransportProto
 	}
 
 	// Deliver the packet.
-	ep.HandlePacket(r, id, vv)
+	for _, ep := range destEps {
+		ep.HandlePacket(r, id, vv)
+	}
 
 	return true
 }
@@ -277,7 +321,7 @@ func (d *transportDemuxer) deliverControlPacket(net tcpip.NetworkProtocolNumber,
 
 func (d *transportDemuxer) findEndpointLocked(eps *transportEndpoints, vv buffer.VectorisedView, id TransportEndpointID) TransportEndpoint {
 	// Try to find a match with the id as provided.
-	if ep := eps.endpoints[id]; ep != nil {
+	if ep, ok := eps.endpoints[id]; ok {
 		return ep
 	}
 
@@ -285,7 +329,7 @@ func (d *transportDemuxer) findEndpointLocked(eps *transportEndpoints, vv buffer
 	nid := id
 
 	nid.LocalAddress = ""
-	if ep := eps.endpoints[nid]; ep != nil {
+	if ep, ok := eps.endpoints[nid]; ok {
 		return ep
 	}
 
@@ -293,11 +337,15 @@ func (d *transportDemuxer) findEndpointLocked(eps *transportEndpoints, vv buffer
 	nid.LocalAddress = id.LocalAddress
 	nid.RemoteAddress = ""
 	nid.RemotePort = 0
-	if ep := eps.endpoints[nid]; ep != nil {
+	if ep, ok := eps.endpoints[nid]; ok {
 		return ep
 	}
 
 	// Try to find a match with only the local port.
 	nid.LocalAddress = ""
-	return eps.endpoints[nid]
+	if ep, ok := eps.endpoints[nid]; ok {
+		return ep
+	}
+
+	return nil
 }
diff --git a/pkg/tcpip/tcpip.go b/pkg/tcpip/tcpip.go
index a6e47397a..89e9d6741 100644
--- a/pkg/tcpip/tcpip.go
+++ b/pkg/tcpip/tcpip.go
@@ -100,6 +100,7 @@ var (
 	ErrNetworkUnreachable    = &Error{msg: "network is unreachable"}
 	ErrMessageTooLong        = &Error{msg: "message too long"}
 	ErrNoBufferSpace         = &Error{msg: "no buffer space available"}
+	ErrBroadcastDisabled     = &Error{msg: "broadcast socket option disabled"}
 )
 
 // Errors related to Subnet
@@ -502,6 +503,10 @@ type RemoveMembershipOption MembershipOption
 // TCP out-of-band data is delivered along with the normal in-band data.
 type OutOfBandInlineOption int
 
+// BroadcastOption is used by SetSockOpt/GetSockOpt to specify whether
+// datagram sockets are allowed to send packets to a broadcast address.
+type BroadcastOption int
+
 // Route is a row in the routing table. It specifies through which NIC (and
 // gateway) sets of packets should be routed. A row is considered viable if the
 // masked target address matches the destination adddress in the row.
@@ -527,6 +532,12 @@ func (r *Route) Match(addr Address) bool {
 		return false
 	}
 
+	// Using header.Ipv4Broadcast would introduce an import cycle, so
+	// we'll use a literal instead.
+	if addr == "\xff\xff\xff\xff" {
+		return true
+	}
+
 	for i := 0; i < len(r.Destination); i++ {
 		if (addr[i] & r.Mask[i]) != r.Destination[i] {
 			return false
diff --git a/pkg/tcpip/transport/tcp/endpoint.go b/pkg/tcpip/transport/tcp/endpoint.go
index 1ee9f8d25..aa31a78af 100644
--- a/pkg/tcpip/transport/tcp/endpoint.go
+++ b/pkg/tcpip/transport/tcp/endpoint.go
@@ -116,6 +116,9 @@ type endpoint struct {
 	route             stack.Route `state:"manual"`
 	v6only            bool
 	isConnectNotified bool
+	// TCP should never broadcast but Linux nevertheless supports enabling/
+	// disabling SO_BROADCAST, albeit as a NOOP.
+	broadcast bool
 
 	// effectiveNetProtos contains the network protocols actually in use. In
 	// most cases it will only contain "netProto", but in cases like IPv6
@@ -813,6 +816,12 @@ func (e *endpoint) SetSockOpt(opt interface{}) *tcpip.Error {
 		e.notifyProtocolGoroutine(notifyKeepaliveChanged)
 		return nil
 
+	case tcpip.BroadcastOption:
+		e.mu.Lock()
+		e.broadcast = v != 0
+		e.mu.Unlock()
+		return nil
+
 	default:
 		return nil
 	}
@@ -971,6 +980,17 @@ func (e *endpoint) GetSockOpt(opt interface{}) *tcpip.Error {
 		*o = 1
 		return nil
 
+	case *tcpip.BroadcastOption:
+		e.mu.Lock()
+		v := e.broadcast
+		e.mu.Unlock()
+
+		*o = 0
+		if v {
+			*o = 1
+		}
+		return nil
+
 	default:
 		return tcpip.ErrUnknownProtocolOption
 	}
diff --git a/pkg/tcpip/transport/tcp/endpoint_state.go b/pkg/tcpip/transport/tcp/endpoint_state.go
index 4891c7941..a07cd9011 100644
--- a/pkg/tcpip/transport/tcp/endpoint_state.go
+++ b/pkg/tcpip/transport/tcp/endpoint_state.go
@@ -336,6 +336,7 @@ func loadError(s string) *tcpip.Error {
 			tcpip.ErrNetworkUnreachable,
 			tcpip.ErrMessageTooLong,
 			tcpip.ErrNoBufferSpace,
+			tcpip.ErrBroadcastDisabled,
 		}
 
 		messageToError = make(map[string]*tcpip.Error)
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
 	}