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-Device Whitelist Controller
-Implement a cgroup to track and enforce open and mknod restrictions
-on device files. A device cgroup associates a device access
-whitelist with each cgroup. A whitelist entry has 4 fields.
-'type' is a (all), c (char), or b (block). 'all' means it applies
-to all types and all major and minor numbers. Major and minor are
-either an integer or * for all. Access is a composition of r
-(read), w (write), and m (mknod).
-The root device cgroup starts with rwm to 'all'. A child device
-cgroup gets a copy of the parent. Administrators can then remove
-devices from the whitelist or add new entries. A child cgroup can
-never receive a device access which is denied by its parent. However
-when a device access is removed from a parent it will not also be
-removed from the child(ren).
-2. User Interface
-An entry is added using devices.allow, and removed using
-devices.deny. For instance
- echo 'c 1:3 mr' > /sys/fs/cgroup/1/devices.allow
-allows cgroup 1 to read and mknod the device usually known as
- echo a > /sys/fs/cgroup/1/devices.deny
-will remove the default 'a *:* rwm' entry. Doing
- echo a > /sys/fs/cgroup/1/devices.allow
-will add the 'a *:* rwm' entry to the whitelist.
-Any task can move itself between cgroups. This clearly won't
-suffice, but we can decide the best way to adequately restrict
-movement as people get some experience with this. We may just want
-to require CAP_SYS_ADMIN, which at least is a separate bit from
-CAP_MKNOD. We may want to just refuse moving to a cgroup which
-isn't a descendant of the current one. Or we may want to use
-CAP_MAC_ADMIN, since we really are trying to lock down root.
-CAP_SYS_ADMIN is needed to modify the whitelist or move another
-task to a new cgroup. (Again we'll probably want to change that).
-A cgroup may not be granted more permissions than the cgroup's