In the Linux kernel, the following vulnerability has been resolved:ovl: don't allow datadir onlyIn theory overlayfs could support upper layer directly referring to a data
layer, but there's no current use case for this.Originally, when data-only layers were introduced, this wasn't allowed,
only introduced by the "datadir+" feature, but without actually handling
this case, resulting in an Oops.Fix by disallowing datadir without lowerdir.