The special file
is a controlled security hole that allows a process to gain I/O
(which are normally reserved for kernel-internal code).
Any process that holds a file descriptor on
open will get its
bits in the flag register set, thus allowing it to perform direct
This can be useful in order to write userland
programs that handle some hardware directly.
Note that even read-only access will grant the full I/O privileges.
In addition to any file access permissions on
the kernel enforces that only the super-user may open this device.