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: src/share/man/man4/man4.i386/io.4,v 1.6.2.4 2001/09/22 19:58:01 iedowse Exp $ 29.\" 30.Dd January 1, 1996 31.Dt IO 4 32.Os 33.Sh NAME 34.Nm io 35.Nd I/O privilege file 36.Sh DESCRIPTION 37The special file 38.Pa /dev/io 39is a controlled security hole that allows a process to gain I/O 40privileges 41(which are normally reserved for kernel-internal code). 42Any process that holds a file descriptor on 43.Pa /dev/io 44open will get its 45.Em IOPL 46bits in the flag register set, thus allowing it to perform direct 47I/O operations. This can be useful in order to write userland 48programs that handle some hardware directly. 49Note that even read-only access will grant the full I/O privileges. 50.Pp 51In addition to any file access permissions on 52.Pa /dev/io , 53the kernel enforces that only the super-user may open this device. 54.Sh FILES 55.Bl -tag -width Pa -compact 56.It Pa /dev/io 57.El 58.Sh SEE ALSO 59.Xr mem 4 60.Sh HISTORY 61The 62.Nm 63file appeared in 64.Fx 1.0 . 65