[Auditbeat/FIM/fsnotify]: remove time window where a child file operation of a directory can be lost

CHANGELOG.next.asciidoc

@@ -96,6 +96,9 @@ https://github.com/elastic/beats/compare/v8.8.1\...main[Check the HEAD diff]
 *Auditbeat*
 - Set field types to correctly match ECS in sessionmd processor {issue}38955[38955] {pull}38994[38994]
 
+- Prevent scenario of losing children-related file events in a directory for recursive fsnotify backend of auditbeat file integrity module {pull}39133[39133]
+
+
 *Filebeat*
 
 - [Gcs Input] - Added missing locks for safe concurrency {pull}34914[34914]

auditbeat/module/file_integrity/monitor/monitor_test.go

@@ -144,6 +144,11 @@ func TestRecursiveSubdirPermissions(t *testing.T) {
 		t.Skip("Skipping permissions test on Windows")
 	}
 
+	if os.Getuid() == 0 {
+		t.Skip("skipping as root can access every file and thus this unittest will fail")
+		return
+	}
+
 	// Create dir to be watched
 
 	dir, err := os.MkdirTemp("", "monitor")
@@ -202,7 +207,7 @@ func TestRecursiveSubdirPermissions(t *testing.T) {
 	for {
 		// No event is received
 		ev, err := readTimeout(t, watcher)
-		if err == errReadTimeout {
+		if errors.Is(err, errReadTimeout) {
 			break
 		}
 		assertNoError(t, err)

auditbeat/module/file_integrity/monitor/recursive.go

@@ -84,37 +84,60 @@ func (watcher *recursiveWatcher) ErrorChannel() <-chan error {
 	return watcher.inner.Errors
 }
 
+func (watcher *recursiveWatcher) watchFile(path string, info os.FileInfo) error {
+	var err error
+	if info == nil {
+		info, err = os.Lstat(path)
+		if err != nil {
+			return err
+		}
+	}
+
+	if info.IsDir() {
+		if err = watcher.tree.AddDir(path); err == nil {
+			if err = watcher.inner.Add(path); err != nil {
+				return fmt.Errorf("failed adding watcher to '%s': %w", path, err)
+			}
+		}
+	} else {
+		err = watcher.tree.AddFile(path)
+	}
+	return err
+}
+
 func (watcher *recursiveWatcher) addRecursive(path string) error {
 	if watcher.isExcludedPath(path) {
 		return nil
 	}
 
+	if err := watcher.watchFile(path, nil); err != nil {
+		return fmt.Errorf("failed adding watcher to '%s': %w", path, err)
+	}
+
 	var errs multierror.Errors
-	err := filepath.Walk(path, func(path string, info os.FileInfo, fnErr error) error {
-		if watcher.isExcludedPath(path) {
+	err := filepath.Walk(path, func(walkPath string, info os.FileInfo, fnErr error) error {
+		if walkPath == path {
+			return nil
+		}
+
+		if watcher.isExcludedPath(walkPath) {
 			return nil
 		}
 
 		if fnErr != nil {
-			errs = append(errs, fmt.Errorf("error walking path '%s': %w", path, fnErr))
+			errs = append(errs, fmt.Errorf("error walking path '%s': %w", walkPath, fnErr))
 			// If FileInfo is not nil, the directory entry can be processed
 			// even if there was some error
 			if info == nil {
 				return nil
 			}
 		}
-		var err error
-		if info.IsDir() {
-			if err = watcher.tree.AddDir(path); err == nil {
-				if err = watcher.inner.Add(path); err != nil {
-					errs = append(errs, fmt.Errorf("failed adding watcher to '%s': %w", path, err))
-					return nil
-				}
-			}
-		} else {
-			err = watcher.tree.AddFile(path)
+
+		if err := watcher.watchFile(walkPath, info); err != nil {
+			errs = append(errs, fmt.Errorf("failed adding watcher to '%s': %w", walkPath, err))
 		}
-		return err
+
+		return nil
 	})
 	watcher.log.Debugw("Added recursive watch", "path", path)