Enable SACK in runsc

SACK is disabled by default and needs to be manually enabled. It not only
improves performance, but also fixes hangs downloading files from certain
websites.

PiperOrigin-RevId: 207906742
Change-Id: I4fb7277b67bfdf83ac8195f1b9c38265a0d51e8b

3 files changed, 19 insertions, 7 deletions

diff --git a/pkg/sentry/socket/hostinet/stack.go b/pkg/sentry/socket/hostinet/stack.go
index 44c3b9a3f..f64809d39 100644
--- a/pkg/sentry/socket/hostinet/stack.go
+++ b/pkg/sentry/socket/hostinet/stack.go
@@ -84,11 +84,13 @@ func (s *Stack) Configure() error {
 		log.Warningf("Failed to read TCP send buffer size, using default values")
 	}
 
-	s.tcpSACKEnabled = false
+	// SACK is important for performance and even compatibility, assume it's
+	// enabled if we can't find the actual value.
+	s.tcpSACKEnabled = true
 	if sack, err := ioutil.ReadFile("/proc/sys/net/ipv4/tcp_sack"); err == nil {
 		s.tcpSACKEnabled = strings.TrimSpace(string(sack)) != "0"
 	} else {
-		log.Warningf("Failed to read if TCP SACK if enabled, setting to false")
+		log.Warningf("Failed to read if TCP SACK if enabled, setting to true")
 	}
 
 	return nil
diff --git a/runsc/boot/controller.go b/runsc/boot/controller.go
index c6e934e66..fc6ea326a 100644
--- a/runsc/boot/controller.go
+++ b/runsc/boot/controller.go
@@ -285,7 +285,10 @@ func (cm *containerManager) Restore(o *RestoreOpts, _ *struct{}) error {
 	fs.SetRestoreEnvironment(*renv)
 
 	// Prepare to load from the state file.
-	networkStack := newEmptyNetworkStack(cm.l.conf, k)
+	networkStack, err := newEmptyNetworkStack(cm.l.conf, k)
+	if err != nil {
+		return fmt.Errorf("failed to create network: %v", err)
+	}
 	info, err := o.FilePayload.Files[0].Stat()
 	if err != nil {
 		return err
diff --git a/runsc/boot/loader.go b/runsc/boot/loader.go
index 2f976cd52..f6c7bf223 100644
--- a/runsc/boot/loader.go
+++ b/runsc/boot/loader.go
@@ -174,7 +174,10 @@ func New(spec *specs.Spec, conf *Config, controllerFD int, ioFDs []int, console
 	// this point. Netns is configured before Run() is called. Netstack is
 	// configured using a control uRPC message. Host network is configured inside
 	// Run().
-	networkStack := newEmptyNetworkStack(conf, k)
+	networkStack, err := newEmptyNetworkStack(conf, k)
+	if err != nil {
+		return nil, fmt.Errorf("failed to create network: %v", err)
+	}
 
 	// Initiate the Kernel object, which is required by the Context passed
 	// to createVFS in order to mount (among other things) procfs.
@@ -525,16 +528,20 @@ func (l *Loader) WaitExit() kernel.ExitStatus {
 	return l.k.GlobalInit().ExitStatus()
 }
 
-func newEmptyNetworkStack(conf *Config, clock tcpip.Clock) inet.Stack {
+func newEmptyNetworkStack(conf *Config, clock tcpip.Clock) (inet.Stack, error) {
 	switch conf.Network {
 	case NetworkHost:
-		return hostinet.NewStack()
+		return hostinet.NewStack(), nil
 
 	case NetworkNone, NetworkSandbox:
 		// NetworkNone sets up loopback using netstack.
 		netProtos := []string{ipv4.ProtocolName, ipv6.ProtocolName, arp.ProtocolName}
 		protoNames := []string{tcp.ProtocolName, udp.ProtocolName, ping.ProtocolName4}
-		return &epsocket.Stack{stack.New(netProtos, protoNames, stack.Options{Clock: clock})}
+		s := &epsocket.Stack{stack.New(netProtos, protoNames, stack.Options{Clock: clock})}
+		if err := s.Stack.SetTransportProtocolOption(tcp.ProtocolNumber, tcp.SACKEnabled(true)); err != nil {
+			return nil, fmt.Errorf("failed to enable SACK: %v", err)
+		}
+		return s, nil
 
 	default:
 		panic(fmt.Sprintf("invalid network configuration: %v", conf.Network))