[$] User namespaces + file capabilities = trouble

User namespaces are a key part of thekernel's toolkit for assembling containers; processes within a usernamespace can appear to have full root access to the system while gainingno extra privileges in the wider system. At least, that is the intent. Inpractice, keeping privilege contained within a user namespace has oftenturned out to be difficult. As an example, consider thissmall patch series from Josef Bacik, which is intended to address somesituations where a process within a user namespace can elevate theprivileges of processes outside of it.