runtime: fix GoroutineProfile stacks not getting null terminated

Fix a regression introduced in CL 572396 causing goroutine stacks not
getting null terminated.

This bug impacts callers that reuse the []StackRecord slice for multiple
calls to GoroutineProfile. See felixge/fgprof#33
for an example of the problem.

Add a test case to prevent similar regressions in the future. Use null
padding instead of null termination to be consistent with other profile
types and because it's less code to implement. Also fix the
ThreadCreateProfile code path.

Fixes #69243

Change-Id: I0b9414f6c694c304bc03a5682586f619e9bf0588
Reviewed-on: https://go-review.googlesource.com/c/go/+/609815
Reviewed-by: Tim King <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Michael Pratt <[email protected]>

Author: felixge

src/runtime/mprof.go

@@ -1277,7 +1277,8 @@ func pprof_mutexProfileInternal(p []profilerecord.BlockProfileRecord) (n int, ok
 // of calling ThreadCreateProfile directly.
 func ThreadCreateProfile(p []StackRecord) (n int, ok bool) {
 	return threadCreateProfileInternal(len(p), func(r profilerecord.StackRecord) {
-		copy(p[0].Stack0[:], r.Stack)
+		i := copy(p[0].Stack0[:], r.Stack)
+		clear(p[0].Stack0[i:])
 		p = p[1:]
 	})
 }
@@ -1656,7 +1657,8 @@ func GoroutineProfile(p []StackRecord) (n int, ok bool) {
 		return
 	}
 	for i, mr := range records[0:n] {
-		copy(p[i].Stack0[:], mr.Stack)
+		l := copy(p[i].Stack0[:], mr.Stack)
+		clear(p[i].Stack0[l:])
 	}
 	return
 }

src/runtime/pprof/pprof_test.go

@@ -2471,16 +2471,7 @@ func TestTimeVDSO(t *testing.T) {
 }
 
 func TestProfilerStackDepth(t *testing.T) {
-	// Disable sampling, otherwise it's difficult to assert anything.
-	oldMemRate := runtime.MemProfileRate
-	runtime.MemProfileRate = 1
-	runtime.SetBlockProfileRate(1)
-	oldMutexRate := runtime.SetMutexProfileFraction(1)
-	t.Cleanup(func() {
-		runtime.MemProfileRate = oldMemRate
-		runtime.SetBlockProfileRate(0)
-		runtime.SetMutexProfileFraction(oldMutexRate)
-	})
+	t.Cleanup(disableSampling())
 
 	const depth = 128
 	go produceProfileEvents(t, depth)
@@ -2772,3 +2763,84 @@ runtime/pprof.inlineA`,
 		})
 	}
 }
+
+func TestProfileRecordNullPadding(t *testing.T) {
+	// Produce events for the different profile types.
+	t.Cleanup(disableSampling())
+	memSink = make([]byte, 1)      // MemProfile
+	<-time.After(time.Millisecond) // BlockProfile
+	blockMutex(t)                  // MutexProfile
+	runtime.GC()
+
+	// Test that all profile records are null padded.
+	testProfileRecordNullPadding(t, "MutexProfile", runtime.MutexProfile)
+	testProfileRecordNullPadding(t, "GoroutineProfile", runtime.GoroutineProfile)
+	testProfileRecordNullPadding(t, "BlockProfile", runtime.BlockProfile)
+	testProfileRecordNullPadding(t, "MemProfile/inUseZero=true", func(p []runtime.MemProfileRecord) (int, bool) {
+		return runtime.MemProfile(p, true)
+	})
+	testProfileRecordNullPadding(t, "MemProfile/inUseZero=false", func(p []runtime.MemProfileRecord) (int, bool) {
+		return runtime.MemProfile(p, false)
+	})
+	// Not testing ThreadCreateProfile because it is broken, see issue 6104.
+}
+
+func testProfileRecordNullPadding[T runtime.StackRecord | runtime.MemProfileRecord | runtime.BlockProfileRecord](t *testing.T, name string, fn func([]T) (int, bool)) {
+	stack0 := func(sr *T) *[32]uintptr {
+		switch t := any(sr).(type) {
+		case *runtime.StackRecord:
+			return &t.Stack0
+		case *runtime.MemProfileRecord:
+			return &t.Stack0
+		case *runtime.BlockProfileRecord:
+			return &t.Stack0
+		default:
+			panic(fmt.Sprintf("unexpected type %T", sr))
+		}
+	}
+
+	t.Run(name, func(t *testing.T) {
+		var p []T
+		for {
+			n, ok := fn(p)
+			if ok {
+				p = p[:n]
+				break
+			}
+			p = make([]T, n*2)
+			for i := range p {
+				s0 := stack0(&p[i])
+				for j := range s0 {
+					// Poison the Stack0 array to identify lack of zero padding
+					s0[j] = ^uintptr(0)
+				}
+			}
+		}
+
+		if len(p) == 0 {
+			t.Fatal("no records found")
+		}
+
+		for _, sr := range p {
+			for i, v := range stack0(&sr) {
+				if v == ^uintptr(0) {
+					t.Fatalf("record p[%d].Stack0 is not null padded: %+v", i, sr)
+				}
+			}
+		}
+	})
+}
+
+// disableSampling configures the profilers to capture all events, otherwise
+// it's difficult to assert anything.
+func disableSampling() func() {
+	oldMemRate := runtime.MemProfileRate
+	runtime.MemProfileRate = 1
+	runtime.SetBlockProfileRate(1)
+	oldMutexRate := runtime.SetMutexProfileFraction(1)
+	return func() {
+		runtime.MemProfileRate = oldMemRate
+		runtime.SetBlockProfileRate(0)
+		runtime.SetMutexProfileFraction(oldMutexRate)
+	}
+}