sentry: Add simple SIOCGIFFLAGS support (IFF_RUNNING and IFF_PROMIS).

Establishes a way of communicating interface flags between netstack and
epsocket. More flags can be added over time.

PiperOrigin-RevId: 197616669
Change-Id: I230448c5fb5b7d2e8d69b41a451eb4e1096a0e30

2 files changed, 37 insertions, 0 deletions

diff --git a/pkg/tcpip/stack/registration.go b/pkg/tcpip/stack/registration.go
index e7e6381ac..15b2418ad 100644
--- a/pkg/tcpip/stack/registration.go
+++ b/pkg/tcpip/stack/registration.go
@@ -224,6 +224,10 @@ type LinkEndpoint interface {
 	// Attach attaches the data link layer endpoint to the network-layer
 	// dispatcher of the stack.
 	Attach(dispatcher NetworkDispatcher)
+
+	// IsAttached returns whether a NetworkDispatcher is attached to the
+	// endpoint.
+	IsAttached() bool
 }
 
 // A LinkAddressResolver is an extension to a NetworkProtocol that
diff --git a/pkg/tcpip/stack/stack.go b/pkg/tcpip/stack/stack.go
index f0fbd8aad..3976f585c 100644
--- a/pkg/tcpip/stack/stack.go
+++ b/pkg/tcpip/stack/stack.go
@@ -541,6 +541,39 @@ func (s *Stack) NICInfo() map[tcpip.NICID]NICInfo {
 	return nics
 }
 
+// NICStateFlags holds information about the state of an NIC.
+type NICStateFlags struct {
+	// Up indicates whether the interface is running.
+	Up bool
+
+	// Running indicates whether resources are allocated.
+	Running bool
+
+	// Promiscuous indicates whether the interface is in promiscuous mode.
+	Promiscuous bool
+}
+
+// NICFlags returns flags about the state of the NIC. It returns an error if
+// the NIC corresponding to id cannot be found.
+func (s *Stack) NICFlags(id tcpip.NICID) (NICStateFlags, *tcpip.Error) {
+	s.mu.RLock()
+	defer s.mu.RUnlock()
+
+	nic := s.nics[id]
+	if nic == nil {
+		return NICStateFlags{}, tcpip.ErrUnknownNICID
+	}
+
+	ret := NICStateFlags{
+		// Netstack interfaces are always up.
+		Up: true,
+
+		Running:     nic.linkEP.IsAttached(),
+		Promiscuous: nic.promiscuous,
+	}
+	return ret, nil
+}
+
 // AddAddress adds a new network-layer address to the specified NIC.
 func (s *Stack) AddAddress(id tcpip.NICID, protocol tcpip.NetworkProtocolNumber, addr tcpip.Address) *tcpip.Error {
 	s.mu.RLock()