Expose some kernel hooks.

- Add Kernel.IsPaused() which indicates whether the kernel is currently paused.
- Add TaskSet.ForEachThreadGroup() which allows callers to iterate through all
  thread groups in the kernel.
- Export FDTable.ForEach() which allows other packages to iterate over all FDs.

PiperOrigin-RevId: 640760723

pkg/sentry/kernel/fd_table.go

@@ -93,7 +93,7 @@ func (f *FDTable) saveDescriptorTable() map[int32]descriptor {
 	m := make(map[int32]descriptor)
 	f.mu.Lock()
 	defer f.mu.Unlock()
-	f.forEach(context.Background(), func(fd int32, file *vfs.FileDescription, flags FDFlags) bool {
+	f.ForEach(context.Background(), func(fd int32, file *vfs.FileDescription, flags FDFlags) bool {
 		m[fd] = descriptor{
 			file:  file,
 			flags: flags,
@@ -172,10 +172,10 @@ func (f *FDTable) forEachUpTo(ctx context.Context, maxFd int32, fn func(fd int32
 	})
 }
 
-// forEach iterates over all non-nil files upto maxFd in sorted order.
+// ForEach iterates over all non-nil files upto maxFd in sorted order.
 //
 // It is the caller's responsibility to acquire an appropriate lock.
-func (f *FDTable) forEach(ctx context.Context, fn func(fd int32, file *vfs.FileDescription, flags FDFlags) bool) {
+func (f *FDTable) ForEach(ctx context.Context, fn func(fd int32, file *vfs.FileDescription, flags FDFlags) bool) {
 	f.forEachUpTo(ctx, MaxFdLimit, fn)
 }
 
@@ -187,7 +187,7 @@ func (f *FDTable) String() string {
 	f.mu.Lock()
 	// Can't release f.mu from defer, because vfsObj.PathnameWithDeleted
 	// should not be called under the fdtable mutex.
-	f.forEach(ctx, func(fd int32, file *vfs.FileDescription, flags FDFlags) bool {
+	f.ForEach(ctx, func(fd int32, file *vfs.FileDescription, flags FDFlags) bool {
 		if file != nil {
 			file.IncRef()
 			files[fd] = file
@@ -424,7 +424,7 @@ func (f *FDTable) GetFDs(ctx context.Context) []int32 {
 	f.mu.Lock()
 	defer f.mu.Unlock()
 	fds := make([]int32, 0, int(f.fdBitmap.GetNumOnes()))
-	f.forEach(ctx, func(fd int32, _ *vfs.FileDescription, _ FDFlags) bool {
+	f.ForEach(ctx, func(fd int32, _ *vfs.FileDescription, _ FDFlags) bool {
 		fds = append(fds, fd)
 		return true
 	})
@@ -486,7 +486,7 @@ func (f *FDTable) RemoveIf(ctx context.Context, cond func(*vfs.FileDescription,
 	var files []*vfs.FileDescription
 
 	f.mu.Lock()
-	f.forEach(ctx, func(fd int32, file *vfs.FileDescription, flags FDFlags) bool {
+	f.ForEach(ctx, func(fd int32, file *vfs.FileDescription, flags FDFlags) bool {
 		if cond(file, flags) {
 			// Clear from table.
 			if df := f.set(fd, nil, FDFlags{}); df != nil {

pkg/sentry/kernel/kernel.go

@@ -1215,7 +1215,7 @@ func (k *Kernel) pauseTimeLocked(ctx context.Context) {
 		// This means we'll iterate FDTables shared by multiple tasks repeatedly,
 		// but ktime.Timer.Pause is idempotent so this is harmless.
 		if t.fdTable != nil {
-			t.fdTable.forEach(ctx, func(_ int32, fd *vfs.FileDescription, _ FDFlags) bool {
+			t.fdTable.ForEach(ctx, func(_ int32, fd *vfs.FileDescription, _ FDFlags) bool {
 				if tfd, ok := fd.Impl().(*timerfd.TimerFileDescription); ok {
 					tfd.PauseTimer()
 				}
@@ -1246,7 +1246,7 @@ func (k *Kernel) resumeTimeLocked(ctx context.Context) {
 			}
 		}
 		if t.fdTable != nil {
-			t.fdTable.forEach(ctx, func(_ int32, fd *vfs.FileDescription, _ FDFlags) bool {
+			t.fdTable.ForEach(ctx, func(_ int32, fd *vfs.FileDescription, _ FDFlags) bool {
 				if tfd, ok := fd.Impl().(*timerfd.TimerFileDescription); ok {
 					tfd.ResumeTimer()
 				}
@@ -1354,6 +1354,11 @@ func (k *Kernel) Pause() {
 	k.tasks.aioGoroutines.Wait()
 }
 
+// IsPaused returns true if the kernel is currently paused.
+func (k *Kernel) IsPaused() bool {
+	return k.tasks.isExternallyStopped()
+}
+
 // ReceiveTaskStates receives full states for all tasks.
 func (k *Kernel) ReceiveTaskStates() {
 	k.extMu.Lock()

pkg/sentry/kernel/task_stop.go

@@ -241,3 +241,11 @@ func (ts *TaskSet) EndExternalStop() {
 		t.tg.signalHandlers.mu.Unlock()
 	}
 }
+
+// isExternallyStopped returns true if BeginExternalStop() has been called on
+// this TaskSet, without a corresponding call to EndExternalStop().
+func (ts *TaskSet) isExternallyStopped() bool {
+	ts.mu.Lock()
+	defer ts.mu.Unlock()
+	return ts.stopCount > 0
+}

pkg/sentry/kernel/threads.go

@@ -106,6 +106,13 @@ func newTaskSet(pidns *PIDNamespace) *TaskSet {
 	return ts
 }
 
+// ForEachThreadGroup applies f to each thread group in ts.
+func (ts *TaskSet) ForEachThreadGroup(f func(tg *ThreadGroup)) {
+	ts.mu.RLock()
+	defer ts.mu.RUnlock()
+	ts.forEachThreadGroupLocked(f)
+}
+
 // forEachThreadGroupLocked applies f to each thread group in ts.
 //
 // Preconditions: ts.mu must be locked (for reading or writing).