fs: Allow overlays to revalidate files from the upper fs.

Previously, an overlay would panic if either the upper or lower fs required
revalidation for a given Dirent. Now, we allow revalidation from the upper
file, but not the lower.

If a cached overlay inode does need revalidation (because the upper needs
revalidation), then the entire overlay Inode will be discarded and a new
overlay Inode will be built with a fresh copy of the upper file.

As a side effect of this change, Revalidate must take an Inode instead of a
Dirent, since an overlay needs to revalidate individual Inodes.

PiperOrigin-RevId: 208293638
Change-Id: Ic8f8d1ffdc09114721745661a09522b54420c5f1

1 files changed, 6 insertions, 2 deletions

diff --git a/pkg/sentry/fs/overlay.go b/pkg/sentry/fs/overlay.go
index af13dc8c7..5a30af419 100644
--- a/pkg/sentry/fs/overlay.go
+++ b/pkg/sentry/fs/overlay.go
@@ -88,10 +88,11 @@ func isXattrOverlay(name string) bool {
 // Preconditions:
 //
 // - upper and lower must be non-nil.
+// - upper must not be an overlay.
 // - lower should not expose character devices, pipes, or sockets, because
 //   copying up these types of files is not supported.
-// - upper and lower must not require that file objects be revalidated.
-// - upper and lower must not have dynamic file/directory content.
+// - lower must not require that file objects be revalidated.
+// - lower must not have dynamic file/directory content.
 func NewOverlayRoot(ctx context.Context, upper *Inode, lower *Inode, flags MountSourceFlags) (*Inode, error) {
 	if !IsDir(upper.StableAttr) {
 		return nil, fmt.Errorf("upper Inode is not a directory")
@@ -99,6 +100,9 @@ func NewOverlayRoot(ctx context.Context, upper *Inode, lower *Inode, flags Mount
 	if !IsDir(lower.StableAttr) {
 		return nil, fmt.Errorf("lower Inode is not a directory")
 	}
+	if upper.overlay != nil {
+		return nil, fmt.Errorf("cannot nest overlay in upper file of another overlay")
+	}
 
 	msrc := newOverlayMountSource(upper.MountSource, lower.MountSource, flags)
 	overlay, err := newOverlayEntry(ctx, upper, lower, true)