.\"
.\" Copyright (c) 2008-2010 Robert N. M. Watson
.\" Copyright (c) 2012-2013 The FreeBSD Foundation
.\" All rights reserved.
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.\" This software was developed at the University of Cambridge Computer
.\" Laboratory with support from a grant from Google, Inc.
.\"
.\" Portions of this documentation were written by Pawel Jakub Dawidek
.\" under sponsorship from the FreeBSD Foundation.
.\"
.\" Redistribution and use in source and binary forms, with or without
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.\" are met:
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.\" $FreeBSD: releng/12.2/lib/libc/sys/cap_rights_limit.2 364017 2020-08-07 15:11:27Z gbe $
.\"
.Dd May 5, 2020
.Dt CAP_RIGHTS_LIMIT 2
.Os
.Sh NAME
.Nm cap_rights_limit
.Nd limit capability rights
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In sys/capsicum.h
.Ft int
.Fn cap_rights_limit "int fd" "const cap_rights_t *rights"
.Sh DESCRIPTION
When a file descriptor is created by a function such as
.Xr accept 2 ,
.Xr accept4 2 ,
.Xr fhopen 2 ,
.Xr kqueue 2 ,
.Xr mq_open 2 ,
.Xr open 2 ,
.Xr openat 2 ,
.Xr pdfork 2 ,
.Xr pipe 2 ,
.Xr shm_open 2 ,
.Xr socket 2
or
.Xr socketpair 2 ,
it is assigned all capability rights.
Those rights can be reduced (but never expanded) by using the
.Fn cap_rights_limit
system call.
Once capability rights are reduced, operations on the file descriptor will be
limited to those permitted by
.Fa rights .
.Pp
The
.Fa rights
argument should be prepared using
.Xr cap_rights_init 3
family of functions.
.Pp
Capability rights assigned to a file descriptor can be obtained with the
.Xr cap_rights_get 3
function.
.Pp
The complete list of the capability rights can be found in the
.Xr rights 4
manual page.
.Sh RETURN VALUES
.Rv -std
.Sh EXAMPLES
The following example demonstrates how to limit file descriptor capability
rights to allow reading only.
.Bd -literal
cap_rights_t setrights;
char buf[1];
int fd;

fd = open("/tmp/foo", O_RDWR);
if (fd < 0)
	err(1, "open() failed");

if (cap_enter() < 0)
	err(1, "cap_enter() failed");

cap_rights_init(&setrights, CAP_READ);
if (cap_rights_limit(fd, &setrights) < 0)
	err(1, "cap_rights_limit() failed");

buf[0] = 'X';

if (write(fd, buf, sizeof(buf)) > 0)
	errx(1, "write() succeeded!");

if (read(fd, buf, sizeof(buf)) < 0)
	err(1, "read() failed");
.Ed
.Sh ERRORS
.Fn cap_rights_limit
succeeds unless:
.Bl -tag -width Er
.It Bq Er EBADF
The
.Fa fd
argument is not a valid active descriptor.
.It Bq Er EINVAL
An invalid right has been requested in
.Fa rights .
.It Bq Er ENOTCAPABLE
The
.Fa rights
argument contains capability rights not present for the given file descriptor.
Capability rights list can only be reduced, never expanded.
.El
.Sh SEE ALSO
.Xr accept 2 ,
.Xr accept4 2 ,
.Xr cap_enter 2 ,
.Xr fhopen 2 ,
.Xr kqueue 2 ,
.Xr mq_open 2 ,
.Xr open 2 ,
.Xr openat 2 ,
.Xr pdfork 2 ,
.Xr pipe 2 ,
.Xr read 2 ,
.Xr shm_open 2 ,
.Xr socket 2 ,
.Xr socketpair 2 ,
.Xr write 2 ,
.Xr cap_rights_get 3 ,
.Xr cap_rights_init 3 ,
.Xr err 3 ,
.Xr capsicum 4 ,
.Xr rights 4
.Sh HISTORY
The
.Fn cap_rights_limit
function first appeared in
.Fx 8.3 .
Support for capabilities and capabilities mode was developed as part of the
.Tn TrustedBSD
Project.
.Sh AUTHORS
This function was created by
.An Pawel Jakub Dawidek Aq Mt pawel@dawidek.net
under sponsorship of the FreeBSD Foundation.