Plumb monotonic time to netstack

Netstack needs to be portable, so this seems to be preferable to using raw
system calls.

PiperOrigin-RevId: 212917409
Change-Id: I7b2073e7db4b4bf75300717ca23aea4c15be944c

pkg/sentry/kernel/kernel.go

@@ -943,6 +943,8 @@ func (k *Kernel) SetExitError(err error) {
 	}
 }
 
+var _ tcpip.Clock = (*Kernel)(nil)
+
 // NowNanoseconds implements tcpip.Clock.NowNanoseconds.
 func (k *Kernel) NowNanoseconds() int64 {
 	now, err := k.timekeeper.GetTime(sentrytime.Realtime)
@@ -952,6 +954,15 @@ func (k *Kernel) NowNanoseconds() int64 {
 	return now
 }
 
+// NowMonotonic implements tcpip.Clock.NowMonotonic.
+func (k *Kernel) NowMonotonic() int64 {
+	now, err := k.timekeeper.GetTime(sentrytime.Monotonic)
+	if err != nil {
+		panic("Kernel.NowMonotonic: " + err.Error())
+	}
+	return now
+}
+
 // SupervisorContext returns a Context with maximum privileges in k. It should
 // only be used by goroutines outside the control of the emulated kernel
 // defined by e.

pkg/tcpip/BUILD

@@ -4,7 +4,10 @@ load("//tools/go_stateify:defs.bzl", "go_library", "go_test")
 
 go_library(
     name = "tcpip",
-    srcs = ["tcpip.go"],
+    srcs = [
+        "tcpip.go",
+        "time_unsafe.go",
+    ],
     importpath = "gvisor.googlesource.com/gvisor/pkg/tcpip",
     visibility = ["//visibility:public"],
     deps = [

pkg/tcpip/tcpip.go

@@ -123,16 +123,11 @@ func (e ErrSaveRejection) Error() string {
 // time, but never for netstack internal timekeeping.
 type Clock interface {
 	// NowNanoseconds returns the current real time as a number of
-	// nanoseconds since some epoch.
+	// nanoseconds since the Unix epoch.
 	NowNanoseconds() int64
-}
 
-// StdClock implements Clock with the time package.
-type StdClock struct{}
-
-// NowNanoseconds implements Clock.NowNanoseconds.
-func (*StdClock) NowNanoseconds() int64 {
-	return time.Now().UnixNano()
+	// NowMonotonic returns a monotonic time value.
+	NowMonotonic() int64
 }
 
 // Address is a byte slice cast as a string that represents the address of a

pkg/tcpip/time_unsafe.go

@@ -0,0 +1,43 @@
+// Copyright 2018 Google Inc.
+//
+// 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.
+
+// +build go1.9
+// +build !go1.12
+
+package tcpip
+
+import (
+	_ "time"   // Used with go:linkname.
+	_ "unsafe" // Required for go:linkname.
+)
+
+// StdClock implements Clock with the time package.
+type StdClock struct{}
+
+var _ Clock = (*StdClock)(nil)
+
+//go:linkname now time.now
+func now() (sec int64, nsec int32, mono int64)
+
+// NowNanoseconds implements Clock.NowNanoseconds.
+func (*StdClock) NowNanoseconds() int64 {
+	sec, nsec, _ := now()
+	return sec*1e9 + int64(nsec)
+}
+
+// NowMonotonic implements Clock.NowMonotonic.
+func (*StdClock) NowMonotonic() int64 {
+	_, _, mono := now()
+	return mono
+}