Use mapped device number + topmost inode number for all files in VFS2 overlay.

Before this CL, VFS2's overlayfs uses a single private device number and an
autoincrementing generated inode number for directories; this is consistent
with Linux's overlayfs in the non-samefs non-xino case. However, this breaks
some applications more consistently than on Linux due to more aggressive
caching of Linux overlayfs dentries.

Switch from using mapped device numbers + the topmost layer's inode number for
just non-copied-up non-directory files, to doing so for all files. This still
allows directory dev/ino numbers to change across copy-up, but otherwise keeps
them consistent.

Fixes #5545:
```
$ docker run --runtime=runsc-vfs2-overlay --rm ubuntu:focal bash -c "mkdir -p 1/2/3/4/5/6/7/8 && rm -rf 1 && echo done"
done
```

PiperOrigin-RevId: 359350716

1 files changed, 9 insertions, 15 deletions

diff --git a/pkg/sentry/fsimpl/overlay/filesystem.go b/pkg/sentry/fsimpl/overlay/filesystem.go
index 890b01cdb..f7f795b10 100644
--- a/pkg/sentry/fsimpl/overlay/filesystem.go
+++ b/pkg/sentry/fsimpl/overlay/filesystem.go
@@ -313,22 +313,16 @@ func (fs *filesystem) lookupLocked(ctx context.Context, parent *dentry, name str
 		return nil, topLookupLayer, syserror.ENOENT
 	}
 
-	// Device and inode numbers were copied from the topmost layer above;
-	// override them if necessary.
-	if child.isDir() {
-		child.devMajor = linux.UNNAMED_MAJOR
-		child.devMinor = fs.dirDevMinor
-		child.ino = fs.newDirIno()
-	} else if !child.upperVD.Ok() {
-		childDevMinor, err := fs.getLowerDevMinor(child.devMajor, child.devMinor)
-		if err != nil {
-			ctx.Infof("overlay.filesystem.lookupLocked: failed to map lower layer device number (%d, %d) to an overlay-specific device number: %v", child.devMajor, child.devMinor, err)
-			child.destroyLocked(ctx)
-			return nil, topLookupLayer, err
-		}
-		child.devMajor = linux.UNNAMED_MAJOR
-		child.devMinor = childDevMinor
+	// Device and inode numbers were copied from the topmost layer above. Remap
+	// the device number to an appropriate overlay-private one.
+	childDevMinor, err := fs.getPrivateDevMinor(child.devMajor, child.devMinor)
+	if err != nil {
+		ctx.Infof("overlay.filesystem.lookupLocked: failed to map layer device number (%d, %d) to an overlay-specific device number: %v", child.devMajor, child.devMinor, err)
+		child.destroyLocked(ctx)
+		return nil, topLookupLayer, err
 	}
+	child.devMajor = linux.UNNAMED_MAJOR
+	child.devMinor = childDevMinor
 
 	parent.IncRef()
 	child.parent = parent