runtime, time: strictly enforce when, period constraints

timer.when must always be positive. addtimer and modtimer already check
that it is non-negative; we expand it to include zero. Also upgrade from
pinning bad values to throwing, as these values shouldn't be possible to
pass (except as below).

timeSleep may overflow timer.nextwhen. This would previously have been
pinned by resetForSleep, now we fix it manually.

runOneTimer may overflow timer.when when adding timer.period. Detect
this and pin to maxWhen.

addtimer is now too strict to allow TestOverflowRuntimeTimer to test an
overflowed timer. Such a timer should not be possible; to help guard
against accidental inclusion siftup / siftdown will check timers as it
goes. This has been replaced with tests for period and sleep overflows.

Change-Id: I17f9739e27ebcb20d87945c635050316fb8e9226
Reviewed-on: https://go-review.googlesource.com/c/go/+/274853
Trust: Michael Pratt <mpratt@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

4 files changed, 60 insertions, 38 deletions

diff --git a/src/runtime/time.go b/src/runtime/time.go
index 83d93c5686..d338705b7c 100644
--- a/src/runtime/time.go
+++ b/src/runtime/time.go
@@ -187,6 +187,9 @@ func timeSleep(ns int64) {
 	t.f = goroutineReady
 	t.arg = gp
 	t.nextwhen = nanotime() + ns
+	if t.nextwhen < 0 { // check for overflow.
+		t.nextwhen = maxWhen
+	}
 	gopark(resetForSleep, unsafe.Pointer(t), waitReasonSleep, traceEvGoSleep, 1)
 }
 
@@ -244,10 +247,14 @@ func goroutineReady(arg interface{}, seq uintptr) {
 // That avoids the risk of changing the when field of a timer in some P's heap,
 // which could cause the heap to become unsorted.
 func addtimer(t *timer) {
-	// when must never be negative; otherwise runtimer will overflow
-	// during its delta calculation and never expire other runtime timers.
-	if t.when < 0 {
-		t.when = maxWhen
+	// when must be positive. A negative value will cause runtimer to
+	// overflow during its delta calculation and never expire other runtime
+	// timers. Zero will cause checkTimers to fail to notice the timer.
+	if t.when <= 0 {
+		throw("timer when must be positive")
+	}
+	if t.period < 0 {
+		throw("timer period must be non-negative")
 	}
 	if t.status != timerNoStatus {
 		throw("addtimer called with initialized timer")
@@ -408,8 +415,11 @@ func dodeltimer0(pp *p) {
 // This is called by the netpoll code or time.Ticker.Reset or time.Timer.Reset.
 // Reports whether the timer was modified before it was run.
 func modtimer(t *timer, when, period int64, f func(interface{}, uintptr), arg interface{}, seq uintptr) bool {
-	if when < 0 {
-		when = maxWhen
+	if when <= 0 {
+		throw("timer when must be positive")
+	}
+	if period < 0 {
+		throw("timer period must be non-negative")
 	}
 
 	status := uint32(timerNoStatus)
@@ -848,6 +858,9 @@ func runOneTimer(pp *p, t *timer, now int64) {
 		// Leave in heap but adjust next time to fire.
 		delta := t.when - now
 		t.when += t.period * (1 + -delta/t.period)
+		if t.when < 0 { // check for overflow.
+			t.when = maxWhen
+		}
 		siftdownTimer(pp.timers, 0)
 		if !atomic.Cas(&t.status, timerRunning, timerWaiting) {
 			badTimer()
@@ -1066,6 +1079,9 @@ func siftupTimer(t []*timer, i int) {
 		badTimer()
 	}
 	when := t[i].when
+	if when <= 0 {
+		badTimer()
+	}
 	tmp := t[i]
 	for i > 0 {
 		p := (i - 1) / 4 // parent
@@ -1086,6 +1102,9 @@ func siftdownTimer(t []*timer, i int) {
 		badTimer()
 	}
 	when := t[i].when
+	if when <= 0 {
+		badTimer()
+	}
 	tmp := t[i]
 	for {
 		c := i*4 + 1 // left child
diff --git a/src/time/internal_test.go b/src/time/internal_test.go
index e70b6f34de..ffe54e47c2 100644
--- a/src/time/internal_test.go
+++ b/src/time/internal_test.go
@@ -33,38 +33,30 @@ var DaysIn = daysIn
 
 func empty(arg interface{}, seq uintptr) {}
 
-// Test that a runtimeTimer with a duration so large it overflows
-// does not cause other timers to hang.
+// Test that a runtimeTimer with a period that would overflow when on
+// expiration does not throw or cause other timers to hang.
 //
 // This test has to be in internal_test.go since it fiddles with
 // unexported data structures.
-func CheckRuntimeTimerOverflow() {
-	// We manually create a runtimeTimer to bypass the overflow
-	// detection logic in NewTimer: we're testing the underlying
-	// runtime.addtimer function.
+func CheckRuntimeTimerPeriodOverflow() {
+	// We manually create a runtimeTimer with huge period, but that expires
+	// immediately. The public Timer interface would require waiting for
+	// the entire period before the first update.
 	r := &runtimeTimer{
-		when: runtimeNano() + (1<<63 - 1),
-		f:    empty,
-		arg:  nil,
+		when:   runtimeNano(),
+		period: 1<<63 - 1,
+		f:      empty,
+		arg:    nil,
 	}
 	startTimer(r)
+	defer stopTimer(r)
 
-	// Start a goroutine that should send on t.C right away.
-	t := NewTimer(1)
-
-	defer func() {
-		stopTimer(r)
-		t.Stop()
-	}()
-
-	// If the test fails, we will hang here until the timeout in the
-	// testing package fires, which is 10 minutes. It would be nice to
-	// catch the problem sooner, but there is no reliable way to guarantee
-	// that timers are run without doing something involving the scheduler.
-	// Previous failed attempts have tried calling runtime.Gosched and
-	// runtime.GC, but neither is reliable. So we fall back to hope:
-	// We hope we don't hang here.
-	<-t.C
+	// If this test fails, we will either throw (when siftdownTimer detects
+	// bad when on update), or other timers will hang (if the timer in a
+	// heap is in a bad state). There is no reliable way to test this, but
+	// we wait on a short timer here as a smoke test (alternatively, timers
+	// in later tests may hang).
+	<-After(25 * Millisecond)
 }
 
 var (
diff --git a/src/time/sleep.go b/src/time/sleep.go
index 22ffd68282..90d8a18a68 100644
--- a/src/time/sleep.go
+++ b/src/time/sleep.go
@@ -31,6 +31,8 @@ func when(d Duration) int64 {
 	}
 	t := runtimeNano() + int64(d)
 	if t < 0 {
+		// N.B. runtimeNano() and d are always positive, so addition
+		// (including overflow) will never result in t == 0.
 		t = 1<<63 - 1 // math.MaxInt64
 	}
 	return t
diff --git a/src/time/sleep_test.go b/src/time/sleep_test.go
index ba0016bf49..084ac33f51 100644
--- a/src/time/sleep_test.go
+++ b/src/time/sleep_test.go
@@ -434,17 +434,29 @@ func TestReset(t *testing.T) {
 	t.Error(err)
 }
 
-// Test that sleeping for an interval so large it overflows does not
-// result in a short sleep duration.
+// Test that sleeping (via Sleep or Timer) for an interval so large it
+// overflows does not result in a short sleep duration. Nor does it interfere
+// with execution of other timers. If it does, timers in this or subsequent
+// tests may not fire.
 func TestOverflowSleep(t *testing.T) {
 	const big = Duration(int64(1<<63 - 1))
+
+	go func() {
+		Sleep(big)
+		// On failure, this may return after the test has completed, so
+		// we need to panic instead.
+		panic("big sleep returned")
+	}()
+
 	select {
 	case <-After(big):
 		t.Fatalf("big timeout fired")
 	case <-After(25 * Millisecond):
 		// OK
 	}
+
 	const neg = Duration(-1 << 63)
+	Sleep(neg) // Returns immediately.
 	select {
 	case <-After(neg):
 		// OK
@@ -473,13 +485,10 @@ func TestIssue5745(t *testing.T) {
 	t.Error("Should be unreachable.")
 }
 
-func TestOverflowRuntimeTimer(t *testing.T) {
-	if testing.Short() {
-		t.Skip("skipping in short mode, see issue 6874")
-	}
+func TestOverflowPeriodRuntimeTimer(t *testing.T) {
 	// This may hang forever if timers are broken. See comment near
 	// the end of CheckRuntimeTimerOverflow in internal_test.go.
-	CheckRuntimeTimerOverflow()
+	CheckRuntimeTimerPeriodOverflow()
 }
 
 func checkZeroPanicString(t *testing.T) {