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, 11 insertions, 0 deletions

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