1.\" 2.\" Copyright (c) 1996 Joerg Wunsch 3.\" 4.\" All rights reserved. 5.\" 6.\" This program is free software. 7.\" 8.\" Redistribution and use in source and binary forms, with or without 9.\" modification, are permitted provided that the following conditions 10.\" are met: 11.\" 1. Redistributions of source code must retain the above copyright 12.\" notice, this list of conditions and the following disclaimer. 13.\" 2. Redistributions in binary form must reproduce the above copyright 14.\" notice, this list of conditions and the following disclaimer in the 15.\" documentation and/or other materials provided with the distribution. 16.\" 17.\" THIS SOFTWARE IS PROVIDED BY THE DEVELOPERS ``AS IS'' AND ANY EXPRESS OR 18.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES 19.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. 20.\" IN NO EVENT SHALL THE DEVELOPERS BE LIABLE FOR ANY DIRECT, INDIRECT, 21.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT 22.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, 23.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY 24.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT 25.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF 26.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. 27.\" 28.\" $FreeBSD$ 29.\" 30.Dd October 3, 2004 31.Dt IO 4 i386 32.Os 33.Sh NAME 34.Nm io 35.Nd I/O privilege file 36.Sh SYNOPSIS 37.Cd "device io" 38.Sh DESCRIPTION 39The special file 40.Pa /dev/io 41is a controlled security hole that allows a process to gain I/O 42privileges 43(which are normally reserved for kernel-internal code). 44Any process that holds a file descriptor on 45.Pa /dev/io 46open will get its 47.Em IOPL 48bits in the flag register set, thus allowing it to perform direct 49I/O operations. 50This can be useful in order to write userland 51programs that handle some hardware directly. 52Note that even read-only access will grant the full I/O privileges. 53.Pp 54In addition to any file access permissions on 55.Pa /dev/io , 56the kernel enforces that only the super-user may open this device. 57.Sh FILES 58.Bl -tag -width Pa -compact 59.It Pa /dev/io 60.El 61.Sh SEE ALSO 62.Xr i386_get_ioperm 2 , 63.Xr i386_set_ioperm 2 , 64.Xr mem 4 65.Sh HISTORY 66The 67.Nm 68file appeared in 69.Fx 1.0 . 70