Short-circuit Readdir calls on overlay files when the dirent is frozen.

If we have an overlay file whose corresponding Dirent is frozen, then we should
not bother calling Readdir on the upper or lower files, since DirentReaddir
will calculate children based on the frozen Dirent tree.

A test was added that fails without this change.

PiperOrigin-RevId: 213531215
Change-Id: I4d6c98f1416541a476a34418f664ba58f936a81d

1 files changed, 15 insertions, 7 deletions

diff --git a/pkg/sentry/fs/file_overlay.go b/pkg/sentry/fs/file_overlay.go
index 113962368..41e646ee8 100644
--- a/pkg/sentry/fs/file_overlay.go
+++ b/pkg/sentry/fs/file_overlay.go
@@ -163,6 +163,21 @@ func (f *overlayFileOperations) Seek(ctx context.Context, file *File, whence See
 
 // Readdir implements FileOperations.Readdir.
 func (f *overlayFileOperations) Readdir(ctx context.Context, file *File, serializer DentrySerializer) (int64, error) {
+	root := RootFromContext(ctx)
+	defer root.DecRef()
+	dirCtx := &DirCtx{
+		Serializer: serializer,
+		DirCursor:  &f.dirCursor,
+	}
+
+	// If the directory dirent is frozen, then DirentReaddir will calculate
+	// the children based off the frozen dirent tree. There is no need to
+	// call readdir on the upper/lower layers.
+	if file.Dirent.frozen {
+		return DirentReaddir(ctx, file.Dirent, f, root, dirCtx, file.Offset())
+	}
+
+	// Otherwise proceed with usual overlay readdir.
 	o := file.Dirent.Inode.overlay
 
 	o.copyMu.RLock()
@@ -174,13 +189,6 @@ func (f *overlayFileOperations) Readdir(ctx context.Context, file *File, seriali
 		return file.Offset(), err
 	}
 
-	root := RootFromContext(ctx)
-	defer root.DecRef()
-
-	dirCtx := &DirCtx{
-		Serializer: serializer,
-		DirCursor:  &f.dirCursor,
-	}
 	return DirentReaddir(ctx, file.Dirent, f, root, dirCtx, file.Offset())
 }