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

3 files changed, 267 insertions, 0 deletions

diff --git a/pkg/tcpip/link/nested/BUILD b/pkg/tcpip/link/nested/BUILD
new file mode 100644
index 000000000..bdd5276ad
--- /dev/null
+++ b/pkg/tcpip/link/nested/BUILD
@@ -0,0 +1,31 @@
+load("//tools:defs.bzl", "go_library", "go_test")
+
+package(licenses = ["notice"])
+
+go_library(
+    name = "nested",
+    srcs = [
+        "nested.go",
+    ],
+    visibility = ["//visibility:public"],
+    deps = [
+        "//pkg/sync",
+        "//pkg/tcpip",
+        "//pkg/tcpip/buffer",
+        "//pkg/tcpip/stack",
+    ],
+)
+
+go_test(
+    name = "nested_test",
+    size = "small",
+    srcs = [
+        "nested_test.go",
+    ],
+    deps = [
+        "//pkg/tcpip",
+        "//pkg/tcpip/header",
+        "//pkg/tcpip/link/nested",
+        "//pkg/tcpip/stack",
+    ],
+)
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
+}
diff --git a/pkg/tcpip/link/nested/nested_test.go b/pkg/tcpip/link/nested/nested_test.go
new file mode 100644
index 000000000..c1a219f02
--- /dev/null
+++ b/pkg/tcpip/link/nested/nested_test.go
@@ -0,0 +1,105 @@
+// 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_test
+
+import (
+	"testing"
+
+	"gvisor.dev/gvisor/pkg/tcpip"
+	"gvisor.dev/gvisor/pkg/tcpip/header"
+	"gvisor.dev/gvisor/pkg/tcpip/link/nested"
+	"gvisor.dev/gvisor/pkg/tcpip/stack"
+)
+
+type parentEndpoint struct {
+	nested.Endpoint
+}
+
+var _ stack.LinkEndpoint = (*parentEndpoint)(nil)
+var _ stack.NetworkDispatcher = (*parentEndpoint)(nil)
+
+type childEndpoint struct {
+	stack.LinkEndpoint
+	dispatcher stack.NetworkDispatcher
+}
+
+var _ stack.LinkEndpoint = (*childEndpoint)(nil)
+
+func (c *childEndpoint) Attach(dispatcher stack.NetworkDispatcher) {
+	c.dispatcher = dispatcher
+}
+
+func (c *childEndpoint) IsAttached() bool {
+	return c.dispatcher != nil
+}
+
+type counterDispatcher struct {
+	count int
+}
+
+var _ stack.NetworkDispatcher = (*counterDispatcher)(nil)
+
+func (d *counterDispatcher) DeliverNetworkPacket(tcpip.LinkAddress, tcpip.LinkAddress, tcpip.NetworkProtocolNumber, *stack.PacketBuffer) {
+	d.count++
+}
+
+func TestNestedLinkEndpoint(t *testing.T) {
+	const emptyAddress = tcpip.LinkAddress("")
+
+	var (
+		childEP  childEndpoint
+		nestedEP parentEndpoint
+		disp     counterDispatcher
+	)
+	nestedEP.Endpoint.Init(&childEP, &nestedEP)
+
+	if childEP.IsAttached() {
+		t.Error("On init, childEP.IsAttached() = true, want = false")
+	}
+	if nestedEP.IsAttached() {
+		t.Error("On init, nestedEP.IsAttached() = true, want = false")
+	}
+
+	nestedEP.Attach(&disp)
+	if disp.count != 0 {
+		t.Fatalf("After attach, got disp.count = %d, want = 0", disp.count)
+	}
+	if !childEP.IsAttached() {
+		t.Error("After attach, childEP.IsAttached() = false, want = true")
+	}
+	if !nestedEP.IsAttached() {
+		t.Error("After attach, nestedEP.IsAttached() = false, want = true")
+	}
+
+	nestedEP.DeliverNetworkPacket(emptyAddress, emptyAddress, header.IPv4ProtocolNumber, &stack.PacketBuffer{})
+	if disp.count != 1 {
+		t.Errorf("After first packet with dispatcher attached, got disp.count = %d, want = 1", disp.count)
+	}
+
+	nestedEP.Attach(nil)
+	if childEP.IsAttached() {
+		t.Error("After detach, childEP.IsAttached() = true, want = false")
+	}
+	if nestedEP.IsAttached() {
+		t.Error("After detach, nestedEP.IsAttached() = true, want = false")
+	}
+
+	disp.count = 0
+	nestedEP.DeliverNetworkPacket(emptyAddress, emptyAddress, header.IPv4ProtocolNumber, &stack.PacketBuffer{})
+	if disp.count != 0 {
+		t.Errorf("After second packet with dispatcher detached, got disp.count = %d, want = 0", disp.count)
+	}
+
+}