Extract common nested LinkEndpoint pattern

... and unify logic for detached netsted endpoints.

sniffer.go caused crashes if a packet delivery is attempted when the dispatcher
is nil.

Extracted the endpoint nesting logic into a common composable type so it can be
used by the Fuchsia Netstack (the pattern is widespread there).

PiperOrigin-RevId: 317682842

1 files changed, 131 insertions, 0 deletions

diff --git a/pkg/tcpip/link/nested/nested.go b/pkg/tcpip/link/nested/nested.go
new file mode 100644
index 000000000..2998f9c4f
--- /dev/null
+++ b/pkg/tcpip/link/nested/nested.go
@@ -0,0 +1,131 @@
+// Copyright 2020 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// Package nested provides helpers to implement the pattern of nested
+// stack.LinkEndpoints.
+package nested
+
+import (
+	"gvisor.dev/gvisor/pkg/sync"
+	"gvisor.dev/gvisor/pkg/tcpip"
+	"gvisor.dev/gvisor/pkg/tcpip/buffer"
+	"gvisor.dev/gvisor/pkg/tcpip/stack"
+)
+
+// Endpoint is a wrapper around stack.LinkEndpoint and stack.NetworkDispatcher
+// that can be used to implement nesting safely by providing lifecycle
+// concurrency guards.
+//
+// See the tests in this package for example usage.
+type Endpoint struct {
+	child    stack.LinkEndpoint
+	embedder stack.NetworkDispatcher
+
+	// mu protects dispatcher.
+	mu         sync.RWMutex
+	dispatcher stack.NetworkDispatcher
+}
+
+var _ stack.GSOEndpoint = (*Endpoint)(nil)
+var _ stack.LinkEndpoint = (*Endpoint)(nil)
+var _ stack.NetworkDispatcher = (*Endpoint)(nil)
+
+// Init initializes a nested.Endpoint that uses embedder as the dispatcher for
+// child on Attach.
+//
+// See the tests in this package for example usage.
+func (e *Endpoint) Init(child stack.LinkEndpoint, embedder stack.NetworkDispatcher) {
+	e.child = child
+	e.embedder = embedder
+}
+
+// DeliverNetworkPacket implements stack.NetworkDispatcher.
+func (e *Endpoint) DeliverNetworkPacket(remote, local tcpip.LinkAddress, protocol tcpip.NetworkProtocolNumber, pkt *stack.PacketBuffer) {
+	e.mu.RLock()
+	d := e.dispatcher
+	e.mu.RUnlock()
+	if d != nil {
+		d.DeliverNetworkPacket(remote, local, protocol, pkt)
+	}
+}
+
+// Attach implements stack.LinkEndpoint.
+func (e *Endpoint) Attach(dispatcher stack.NetworkDispatcher) {
+	e.mu.Lock()
+	e.dispatcher = dispatcher
+	e.mu.Unlock()
+	// If we're attaching to a valid dispatcher, pass embedder as the dispatcher
+	// to our child, otherwise detach the child by giving it a nil dispatcher.
+	var pass stack.NetworkDispatcher
+	if dispatcher != nil {
+		pass = e.embedder
+	}
+	e.child.Attach(pass)
+}
+
+// IsAttached implements stack.LinkEndpoint.
+func (e *Endpoint) IsAttached() bool {
+	e.mu.RLock()
+	isAttached := e.dispatcher != nil
+	e.mu.RUnlock()
+	return isAttached
+}
+
+// MTU implements stack.LinkEndpoint.
+func (e *Endpoint) MTU() uint32 {
+	return e.child.MTU()
+}
+
+// Capabilities implements stack.LinkEndpoint.
+func (e *Endpoint) Capabilities() stack.LinkEndpointCapabilities {
+	return e.child.Capabilities()
+}
+
+// MaxHeaderLength implements stack.LinkEndpoint.
+func (e *Endpoint) MaxHeaderLength() uint16 {
+	return e.child.MaxHeaderLength()
+}
+
+// LinkAddress implements stack.LinkEndpoint.
+func (e *Endpoint) LinkAddress() tcpip.LinkAddress {
+	return e.child.LinkAddress()
+}
+
+// WritePacket implements stack.LinkEndpoint.
+func (e *Endpoint) WritePacket(r *stack.Route, gso *stack.GSO, protocol tcpip.NetworkProtocolNumber, pkt *stack.PacketBuffer) *tcpip.Error {
+	return e.child.WritePacket(r, gso, protocol, pkt)
+}
+
+// WritePackets implements stack.LinkEndpoint.
+func (e *Endpoint) WritePackets(r *stack.Route, gso *stack.GSO, pkts stack.PacketBufferList, protocol tcpip.NetworkProtocolNumber) (int, *tcpip.Error) {
+	return e.child.WritePackets(r, gso, pkts, protocol)
+}
+
+// WriteRawPacket implements stack.LinkEndpoint.
+func (e *Endpoint) WriteRawPacket(vv buffer.VectorisedView) *tcpip.Error {
+	return e.child.WriteRawPacket(vv)
+}
+
+// Wait implements stack.LinkEndpoint.
+func (e *Endpoint) Wait() {
+	e.child.Wait()
+}
+
+// GSOMaxSize implements stack.GSOEndpoint.
+func (e *Endpoint) GSOMaxSize() uint32 {
+	if e, ok := e.child.(stack.GSOEndpoint); ok {
+		return e.GSOMaxSize()
+	}
+	return 0
+}