ErrDTypeNotSupported denotes that the backing filesystem doesn't support d_type.

GenerateID creates a new random string identifier with the given length.

GetOverlayXattr combines the overlay module's xattr class with the named xattr -- `user` when mounted inside a user namespace, and `trusted` when mounted in the 'root' namespace.

NeedsUserXAttr returns whether overlayfs should be mounted with the "userxattr" mount option.

SupportsOverlay checks if the system supports overlay filesystem by performing an actual overlay mount.