.\"
.\" Copyright (c) 1996 Joerg Wunsch
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.\" $FreeBSD: releng/12.2/share/man/man4/io.4 236596 2012-06-05 03:14:39Z eadler $
.\"
.Dd June 1, 2010
.Dt IO 4
.Os
.Sh NAME
.Nm io
.Nd I/O privilege file
.Sh SYNOPSIS
.Cd "device io"
.Pp
.In sys/types.h
.In sys/ioctl.h
.In dev/io/iodev.h
.In machine/iodev.h
.Bd -literal
struct iodev_pio_req {
	u_int access;
	u_int port;
	u_int width;
	u_int val;
};
.Ed
.Sh DESCRIPTION
The special file
.Pa /dev/io
is a controlled security hole that allows a process to gain I/O
privileges
(which are normally reserved for kernel-internal code).
This can be useful in order to write userland
programs that handle some hardware directly.
.Pp
The usual operations on the device are to open it via the
.Xr open 2
interface and to send I/O requests to the file descriptor using the
.Xr ioctl 2
syscall.
.Pp
The
.Xr ioctl 2
requests available for
.Pa /dev/io
are mostly platform dependent, but there are also some in common between
all of them.
The
.Dv IODEV_PIO
is used by all the architectures in order to request that an I/O operation
be performed.
It takes a 'struct iodev_pio_req' argument that must be previously setup.
.Pp
The
.Fa access
member specifies the type of operation requested.
It may be:
.Bl -tag -width IODEV_PIO_WRITE
.It Dv IODEV_PIO_READ
The operation is an "in" type.
A value will be read from the specified port
(retrieved from the
.Fa port
member) and the result will be stored in the
.Fa val
member.
.It Dv IODEV_PIO_WRITE
The operation is a "out" type.
The value will be fetched from the
.Fa val
member and will be written out to the specified port (defined as the
.Fa port
member).
.El
.Pp
Finally, the
.Fa width
member specifies the size of the operand to be read/written, expressed
in bytes.
.Pp
In addition to any file access permissions on
.Pa /dev/io ,
the kernel enforces that only the super-user may open this device.
.Sh LEGACY
The
.Pa /dev/io
interface used to be very i386 specific and worked differently.
The initial implementation simply raised the
.Em IOPL
of the current thread when
.Xr open 2
was called on the device.
This behaviour is retained in the current implementation as legacy
support for both i386 and amd64 architectures.
.Sh SEE ALSO
.Xr close 2 ,
.Xr i386_get_ioperm 2 ,
.Xr i386_set_ioperm 2 ,
.Xr ioctl 2 ,
.Xr open 2 ,
.Xr mem 4
.Sh HISTORY
The
.Nm
file appeared in
.Fx 1.0 .