Do not resolve routes immediately

When a response needs to be sent to an incoming packet, the stack should
consult its neighbour table to determine the remote address's link
address.

When an entry does not exist in the stack's neighbor table, the stack
should queue the packet while link resolution completes. See comments.

PiperOrigin-RevId: 336185457

1 files changed, 133 insertions, 0 deletions

diff --git a/pkg/tcpip/stack/pending_packets.go b/pkg/tcpip/stack/pending_packets.go
new file mode 100644
index 000000000..f838eda8d
--- /dev/null
+++ b/pkg/tcpip/stack/pending_packets.go
@@ -0,0 +1,133 @@
+// 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 stack
+
+import (
+	"fmt"
+
+	"gvisor.dev/gvisor/pkg/sync"
+	"gvisor.dev/gvisor/pkg/tcpip"
+)
+
+const (
+	// maxPendingResolutions is the maximum number of pending link-address
+	// resolutions.
+	maxPendingResolutions          = 64
+	maxPendingPacketsPerResolution = 256
+)
+
+type pendingPacket struct {
+	route *Route
+	proto tcpip.NetworkProtocolNumber
+	pkt   *PacketBuffer
+}
+
+// packetsPendingLinkResolution is a queue of packets pending link resolution.
+//
+// Once link resolution completes successfully, the packets will be written.
+type packetsPendingLinkResolution struct {
+	sync.Mutex
+
+	// The packets to send once the resolver completes.
+	packets map[<-chan struct{}][]pendingPacket
+
+	// FIFO of channels used to cancel the oldest goroutine waiting for
+	// link-address resolution.
+	cancelChans []chan struct{}
+}
+
+func (f *packetsPendingLinkResolution) init() {
+	f.Lock()
+	defer f.Unlock()
+	f.packets = make(map[<-chan struct{}][]pendingPacket)
+}
+
+func (f *packetsPendingLinkResolution) enqueue(ch <-chan struct{}, r *Route, proto tcpip.NetworkProtocolNumber, pkt *PacketBuffer) {
+	f.Lock()
+	defer f.Unlock()
+
+	packets, ok := f.packets[ch]
+	if len(packets) == maxPendingPacketsPerResolution {
+		p := packets[0]
+		packets[0] = pendingPacket{}
+		packets = packets[1:]
+		p.route.Stats().IP.OutgoingPacketErrors.Increment()
+		p.route.Release()
+	}
+
+	if l := len(packets); l >= maxPendingPacketsPerResolution {
+		panic(fmt.Sprintf("max pending packets for resolution reached; got %d packets, max = %d", l, maxPendingPacketsPerResolution))
+	}
+
+	f.packets[ch] = append(packets, pendingPacket{
+		route: r,
+		proto: proto,
+		pkt:   pkt,
+	})
+
+	if ok {
+		return
+	}
+
+	// Wait for the link-address resolution to complete.
+	cancel := f.newCancelChannelLocked()
+	go func() {
+		cancelled := false
+		select {
+		case <-ch:
+		case <-cancel:
+			cancelled = true
+		}
+
+		f.Lock()
+		packets, ok := f.packets[ch]
+		delete(f.packets, ch)
+		f.Unlock()
+
+		if !ok {
+			panic(fmt.Sprintf("link-resolution goroutine woke up but no entry exists in the queue of packets"))
+		}
+
+		for _, p := range packets {
+			if cancelled {
+				p.route.Stats().IP.OutgoingPacketErrors.Increment()
+			} else if _, err := p.route.Resolve(nil); err != nil {
+				p.route.Stats().IP.OutgoingPacketErrors.Increment()
+			} else {
+				p.route.nic.writePacket(p.route, nil /* gso */, p.proto, p.pkt)
+			}
+			p.route.Release()
+		}
+	}()
+}
+
+// newCancelChannel creates a channel that can cancel a pending forwarding
+// activity. The oldest channel is closed if the number of open channels would
+// exceed maxPendingResolutions.
+func (f *packetsPendingLinkResolution) newCancelChannelLocked() chan struct{} {
+	if len(f.cancelChans) == maxPendingResolutions {
+		ch := f.cancelChans[0]
+		f.cancelChans[0] = nil
+		f.cancelChans = f.cancelChans[1:]
+		close(ch)
+	}
+	if l := len(f.cancelChans); l >= maxPendingResolutions {
+		panic(fmt.Sprintf("max pending resolutions reached; got %d active resolutions, max = %d", l, maxPendingResolutions))
+	}
+
+	ch := make(chan struct{})
+	f.cancelChans = append(f.cancelChans, ch)
+	return ch
+}