sharedmem: mark as GSO-compatible

Really, any LinkWriter is compatible with gVisor GSO: there's nothing special
to be done. Anything with a WritePackets method is good to go.

PiperOrigin-RevId: 537048401

pkg/tcpip/link/fdbased/endpoint.go

@@ -754,7 +754,7 @@ func (e *endpoint) GSOMaxSize() uint32 {
 	return e.gsoMaxSize
 }
 
-// SupportsHWGSO implements stack.GSOEndpoint.
+// SupportedGSO implements stack.GSOEndpoint.
 func (e *endpoint) SupportedGSO() stack.SupportedGSO {
 	return e.gsoKind
 }

pkg/tcpip/link/sharedmem/sharedmem.go

@@ -133,8 +133,15 @@ type Options struct {
 	// VirtioNetHeaderRequired if true, indicates that all outbound packets should have
 	// a virtio header and inbound packets should have a virtio header as well.
 	VirtioNetHeaderRequired bool
+
+	// GSOMaxSize is the maximum GSO packet size. It is zero if GSO is
+	// disabled. Note that only gVisor GSO is supported, not host GSO.
+	GSOMaxSize uint32
 }
 
+var _ stack.LinkEndpoint = (*endpoint)(nil)
+var _ stack.GSOEndpoint = (*endpoint)(nil)
+
 type endpoint struct {
 	// mtu (maximum transmission unit) is the maximum size of a packet.
 	// mtu is immutable.
@@ -160,6 +167,11 @@ type endpoint struct {
 	// hdrSize is immutable.
 	hdrSize uint32
 
+	// gSOMaxSize is the maximum GSO packet size. It is zero if GSO is
+	// disabled. Note that only gVisor GSO is supported, not host GSO.
+	// gsoMaxSize is immutable.
+	gsoMaxSize uint32
+
 	// virtioNetHeaderRequired if true indicates that a virtio header is expected
 	// in all inbound/outbound packets.
 	virtioNetHeaderRequired bool
@@ -202,6 +214,7 @@ func New(opts Options) (stack.LinkEndpoint, error) {
 		peerFD:                  opts.PeerFD,
 		onClosed:                opts.OnClosed,
 		virtioNetHeaderRequired: opts.VirtioNetHeaderRequired,
+		gsoMaxSize:              opts.GSOMaxSize,
 	}
 
 	if err := e.tx.init(opts.BufferSize, &opts.TX); err != nil {
@@ -480,3 +493,13 @@ func (e *endpoint) dispatchLoop(d stack.NetworkDispatcher) {
 func (*endpoint) ARPHardwareType() header.ARPHardwareType {
 	return header.ARPHardwareEther
 }
+
+// GSOMaxSize implements stack.GSOEndpoint.
+func (e *endpoint) GSOMaxSize() uint32 {
+	return e.gsoMaxSize
+}
+
+// SupportsGSO implements stack.GSOEndpoint.
+func (e *endpoint) SupportedGSO() stack.SupportedGSO {
+	return stack.GvisorGSOSupported
+}