8 files changed, 177 insertions, 95 deletions
diff --git a/pkg/tcpip/BUILD b/pkg/tcpip/BUILD
index aa30cfc85..e96ba50ae 100644
--- a/pkg/tcpip/BUILD
+++ b/pkg/tcpip/BUILD
@@ -22,8 +22,9 @@ go_library(
         "errors.go",
         "sock_err_list.go",
         "socketops.go",
+        "stdclock.go",
+        "stdclock_state.go",
         "tcpip.go",
-        "time_unsafe.go",
         "timer.go",
     ],
     visibility = ["//visibility:public"],
diff --git a/pkg/tcpip/stack/neighbor_cache_test.go b/pkg/tcpip/stack/neighbor_cache_test.go
index 48bb75e2f..9821a18d3 100644
--- a/pkg/tcpip/stack/neighbor_cache_test.go
+++ b/pkg/tcpip/stack/neighbor_cache_test.go
@@ -1556,7 +1556,7 @@ func TestNeighborCacheRetryResolution(t *testing.T) {
 func BenchmarkCacheClear(b *testing.B) {
 	b.StopTimer()
 	config := DefaultNUDConfigurations()
-	clock := &tcpip.StdClock{}
+	clock := tcpip.NewStdClock()
 	linkRes := newTestNeighborResolver(nil, config, clock)
 	linkRes.delay = 0
 
diff --git a/pkg/tcpip/stack/stack.go b/pkg/tcpip/stack/stack.go
index 843118b13..436392f23 100644
--- a/pkg/tcpip/stack/stack.go
+++ b/pkg/tcpip/stack/stack.go
@@ -322,7 +322,7 @@ func (*TransportEndpointInfo) IsEndpointInfo() {}
 func New(opts Options) *Stack {
 	clock := opts.Clock
 	if clock == nil {
-		clock = &tcpip.StdClock{}
+		clock = tcpip.NewStdClock()
 	}
 
 	if opts.UniqueID == nil {
diff --git a/pkg/tcpip/stdclock.go b/pkg/tcpip/stdclock.go
new file mode 100644
index 000000000..7ce43a68e
--- /dev/null
+++ b/pkg/tcpip/stdclock.go
@@ -0,0 +1,130 @@
+// Copyright 2018 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 tcpip
+
+import (
+	"fmt"
+	"time"
+
+	"gvisor.dev/gvisor/pkg/sync"
+)
+
+// stdClock implements Clock with the time package.
+//
+// +stateify savable
+type stdClock struct {
+	// baseTime holds the time when the clock was constructed.
+	//
+	// This value is used to calculate the monotonic time from the time package.
+	// As per https://golang.org/pkg/time/#hdr-Monotonic_Clocks,
+	//
+	//   Operating systems provide both a “wall clock,” which is subject to
+	//   changes for clock synchronization, and a “monotonic clock,” which is not.
+	//   The general rule is that the wall clock is for telling time and the
+	//   monotonic clock is for measuring time. Rather than split the API, in this
+	//   package the Time returned by time.Now contains both a wall clock reading
+	//   and a monotonic clock reading; later time-telling operations use the wall
+	//   clock reading, but later time-measuring operations, specifically
+	//   comparisons and subtractions, use the monotonic clock reading.
+	//
+	//   ...
+	//
+	//   If Times t and u both contain monotonic clock readings, the operations
+	//   t.After(u), t.Before(u), t.Equal(u), and t.Sub(u) are carried out using
+	//   the monotonic clock readings alone, ignoring the wall clock readings. If
+	//   either t or u contains no monotonic clock reading, these operations fall
+	//   back to using the wall clock readings.
+	//
+	// Given the above, we can safely conclude that time.Since(baseTime) will
+	// return monotonically increasing values if we use time.Now() to set baseTime
+	// at the time of clock construction.
+	//
+	// Note that time.Since(t) is shorthand for time.Now().Sub(t), as per
+	// https://golang.org/pkg/time/#Since.
+	baseTime time.Time `state:"nosave"`
+
+	// monotonicOffset is the offset applied to the calculated monotonic time.
+	//
+	// monotonicOffset is assigned maxMonotonic after restore so that the
+	// monotonic time will continue from where it "left off" before saving as part
+	// of S/R.
+	monotonicOffset int64 `state:"nosave"`
+
+	// monotonicMU protects maxMonotonic.
+	monotonicMU  sync.Mutex `state:"nosave"`
+	maxMonotonic int64
+}
+
+// NewStdClock returns an instance of a clock that uses the time package.
+func NewStdClock() Clock {
+	return &stdClock{
+		baseTime: time.Now(),
+	}
+}
+
+var _ Clock = (*stdClock)(nil)
+
+// NowNanoseconds implements Clock.NowNanoseconds.
+func (*stdClock) NowNanoseconds() int64 {
+	return time.Now().UnixNano()
+}
+
+// NowMonotonic implements Clock.NowMonotonic.
+func (s *stdClock) NowMonotonic() int64 {
+	sinceBase := time.Since(s.baseTime)
+	if sinceBase < 0 {
+		panic(fmt.Sprintf("got negative duration = %s since base time = %s", sinceBase, s.baseTime))
+	}
+
+	monotonicValue := sinceBase.Nanoseconds() + s.monotonicOffset
+
+	s.monotonicMU.Lock()
+	defer s.monotonicMU.Unlock()
+
+	// Monotonic time values must never decrease.
+	if monotonicValue > s.maxMonotonic {
+		s.maxMonotonic = monotonicValue
+	}
+
+	return s.maxMonotonic
+}
+
+// AfterFunc implements Clock.AfterFunc.
+func (*stdClock) AfterFunc(d time.Duration, f func()) Timer {
+	return &stdTimer{
+		t: time.AfterFunc(d, f),
+	}
+}
+
+type stdTimer struct {
+	t *time.Timer
+}
+
+var _ Timer = (*stdTimer)(nil)
+
+// Stop implements Timer.Stop.
+func (st *stdTimer) Stop() bool {
+	return st.t.Stop()
+}
+
+// Reset implements Timer.Reset.
+func (st *stdTimer) Reset(d time.Duration) {
+	st.t.Reset(d)
+}
+
+// NewStdTimer returns a Timer implemented with the time package.
+func NewStdTimer(t *time.Timer) Timer {
+	return &stdTimer{t: t}
+}
diff --git a/pkg/tcpip/stdclock_state.go b/pkg/tcpip/stdclock_state.go
new file mode 100644
index 000000000..795db9181
--- /dev/null
+++ b/pkg/tcpip/stdclock_state.go
@@ -0,0 +1,26 @@
+// Copyright 2021 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 tcpip
+
+import "time"
+
+// afterLoad is invoked by stateify.
+func (s *stdClock) afterLoad() {
+	s.baseTime = time.Now()
+
+	s.monotonicMU.Lock()
+	defer s.monotonicMU.Unlock()
+	s.monotonicOffset = s.maxMonotonic
+}
diff --git a/pkg/tcpip/tcpip.go b/pkg/tcpip/tcpip.go
index 2e426ffe7..d8a10065d 100644
--- a/pkg/tcpip/tcpip.go
+++ b/pkg/tcpip/tcpip.go
@@ -73,7 +73,7 @@ type Clock interface {
 	// nanoseconds since the Unix epoch.
 	NowNanoseconds() int64
 
-	// NowMonotonic returns a monotonic time value.
+	// NowMonotonic returns a monotonic time value at nanosecond resolution.
 	NowMonotonic() int64
 
 	// AfterFunc waits for the duration to elapse and then calls f in its own
diff --git a/pkg/tcpip/time_unsafe.go b/pkg/tcpip/time_unsafe.go
deleted file mode 100644
index eeea97b12..000000000
--- a/pkg/tcpip/time_unsafe.go
+++ /dev/null
@@ -1,75 +0,0 @@
-// Copyright 2018 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.
-
-// +build go1.9
-// +build !go1.18
-
-// Check go:linkname function signatures when updating Go version.
-
-package tcpip
-
-import (
-	"time"     // Used with go:linkname.
-	_ "unsafe" // Required for go:linkname.
-)
-
-// StdClock implements Clock with the time package.
-//
-// +stateify savable
-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
-}
-
-// AfterFunc implements Clock.AfterFunc.
-func (*StdClock) AfterFunc(d time.Duration, f func()) Timer {
-	return &stdTimer{
-		t: time.AfterFunc(d, f),
-	}
-}
-
-type stdTimer struct {
-	t *time.Timer
-}
-
-var _ Timer = (*stdTimer)(nil)
-
-// Stop implements Timer.Stop.
-func (st *stdTimer) Stop() bool {
-	return st.t.Stop()
-}
-
-// Reset implements Timer.Reset.
-func (st *stdTimer) Reset(d time.Duration) {
-	st.t.Reset(d)
-}
-
-// NewStdTimer returns a Timer implemented with the time package.
-func NewStdTimer(t *time.Timer) Timer {
-	return &stdTimer{t: t}
-}
diff --git a/pkg/tcpip/timer_test.go b/pkg/tcpip/timer_test.go
index a82384c49..1633d0aeb 100644
--- a/pkg/tcpip/timer_test.go
+++ b/pkg/tcpip/timer_test.go
@@ -29,7 +29,7 @@ const (
 )
 
 func TestJobReschedule(t *testing.T) {
-	var clock tcpip.StdClock
+	clock := tcpip.NewStdClock()
 	var wg sync.WaitGroup
 	var lock sync.Mutex
 
@@ -43,7 +43,7 @@ func TestJobReschedule(t *testing.T) {
 			// that has an active timer (even if it has been stopped as a stopped
 			// timer may be blocked on a lock before it can check if it has been
 			// stopped while another goroutine holds the same lock).
-			job := tcpip.NewJob(&clock, &lock, func() {
+			job := tcpip.NewJob(clock, &lock, func() {
 				wg.Done()
 			})
 			job.Schedule(shortDuration)
@@ -56,11 +56,11 @@ func TestJobReschedule(t *testing.T) {
 func TestJobExecution(t *testing.T) {
 	t.Parallel()
 
-	var clock tcpip.StdClock
+	clock := tcpip.NewStdClock()
 	var lock sync.Mutex
 	ch := make(chan struct{})
 
-	job := tcpip.NewJob(&clock, &lock, func() {
+	job := tcpip.NewJob(clock, &lock, func() {
 		ch <- struct{}{}
 	})
 	job.Schedule(shortDuration)
@@ -83,11 +83,11 @@ func TestJobExecution(t *testing.T) {
 func TestCancellableTimerResetFromLongDuration(t *testing.T) {
 	t.Parallel()
 
-	var clock tcpip.StdClock
+	clock := tcpip.NewStdClock()
 	var lock sync.Mutex
 	ch := make(chan struct{})
 
-	job := tcpip.NewJob(&clock, &lock, func() { ch <- struct{}{} })
+	job := tcpip.NewJob(clock, &lock, func() { ch <- struct{}{} })
 	job.Schedule(middleDuration)
 
 	lock.Lock()
@@ -114,12 +114,12 @@ func TestCancellableTimerResetFromLongDuration(t *testing.T) {
 func TestJobRescheduleFromShortDuration(t *testing.T) {
 	t.Parallel()
 
-	var clock tcpip.StdClock
+	clock := tcpip.NewStdClock()
 	var lock sync.Mutex
 	ch := make(chan struct{})
 
 	lock.Lock()
-	job := tcpip.NewJob(&clock, &lock, func() { ch <- struct{}{} })
+	job := tcpip.NewJob(clock, &lock, func() { ch <- struct{}{} })
 	job.Schedule(shortDuration)
 	job.Cancel()
 	lock.Unlock()
@@ -151,13 +151,13 @@ func TestJobRescheduleFromShortDuration(t *testing.T) {
 func TestJobImmediatelyCancel(t *testing.T) {
 	t.Parallel()
 
-	var clock tcpip.StdClock
+	clock := tcpip.NewStdClock()
 	var lock sync.Mutex
 	ch := make(chan struct{})
 
 	for i := 0; i < 1000; i++ {
 		lock.Lock()
-		job := tcpip.NewJob(&clock, &lock, func() { ch <- struct{}{} })
+		job := tcpip.NewJob(clock, &lock, func() { ch <- struct{}{} })
 		job.Schedule(shortDuration)
 		job.Cancel()
 		lock.Unlock()
@@ -174,12 +174,12 @@ func TestJobImmediatelyCancel(t *testing.T) {
 func TestJobCancelledRescheduleWithoutLock(t *testing.T) {
 	t.Parallel()
 
-	var clock tcpip.StdClock
+	clock := tcpip.NewStdClock()
 	var lock sync.Mutex
 	ch := make(chan struct{})
 
 	lock.Lock()
-	job := tcpip.NewJob(&clock, &lock, func() { ch <- struct{}{} })
+	job := tcpip.NewJob(clock, &lock, func() { ch <- struct{}{} })
 	job.Schedule(shortDuration)
 	job.Cancel()
 	lock.Unlock()
@@ -206,12 +206,12 @@ func TestJobCancelledRescheduleWithoutLock(t *testing.T) {
 func TestManyCancellableTimerResetAfterBlockedOnLock(t *testing.T) {
 	t.Parallel()
 
-	var clock tcpip.StdClock
+	clock := tcpip.NewStdClock()
 	var lock sync.Mutex
 	ch := make(chan struct{})
 
 	lock.Lock()
-	job := tcpip.NewJob(&clock, &lock, func() { ch <- struct{}{} })
+	job := tcpip.NewJob(clock, &lock, func() { ch <- struct{}{} })
 	job.Schedule(shortDuration)
 	for i := 0; i < 10; i++ {
 		// Sleep until the timer fires and gets blocked trying to take the lock.
@@ -239,12 +239,12 @@ func TestManyCancellableTimerResetAfterBlockedOnLock(t *testing.T) {
 func TestManyJobReschedulesUnderLock(t *testing.T) {
 	t.Parallel()
 
-	var clock tcpip.StdClock
+	clock := tcpip.NewStdClock()
 	var lock sync.Mutex
 	ch := make(chan struct{})
 
 	lock.Lock()
-	job := tcpip.NewJob(&clock, &lock, func() { ch <- struct{}{} })
+	job := tcpip.NewJob(clock, &lock, func() { ch <- struct{}{} })
 	job.Schedule(shortDuration)
 	for i := 0; i < 10; i++ {
 		job.Cancel()