IO(4)

NAME
io - I/O privilege file

DESCRIPTION
The special file /dev/io is a controlled security hole that allows a process to gain I/O privileges (which are normally reserved for kernel-internal code). Any process that holds a file descriptor on /dev/io open will get its IOPL bits in the flag register set, thus allowing it to perform direct I/O operations. This can be useful in order to write userland programs that handle some hardware directly.

The entire access control is handled by the file access permissions of /dev/io, so care should be taken in granting rights for this device. Note that even read/only access will grant the full I/O privileges.

FILES
/dev/io

SEE ALSO
mem(4)

HISTORY
The io file appeared in FreeBSD 1.0.