.\" Copyright (c) 2018 Mariusz Zaborski <oshogbo@FreeBSD.org>
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.Dd April 6, 2020
.Dt CAP_SYSCTL 3
.Os
.Sh NAME
.Nm cap_sysctl
.Nd "library for getting or setting system information in capability mode"
.Sh LIBRARY
.Lb libcap_sysctl
.Sh SYNOPSIS
.In libcasper.h
.In casper/cap_sysctl.h
.Ft int
.Fn cap_sysctl "cap_channel_t *chan" "const int *name" "u_int namelen" "void *oldp" "size_t *oldlenp" "const void *newp" "size_t newlen"
.Ft int
.Fn cap_sysctlbyname "cap_channel_t *chan" "const char *name" "void *oldp" "size_t *oldlenp" "const void *newp" "size_t newlen"
.Ft int
.Fn cap_sysctlnametomib "cap_channel_t *chan" "const char *name" "int *mibp" "size_t *sizep"
.Ft void *
.Fn cap_sysctl_limit_init "cap_channel_t *chan"
.Ft void *
.Fn cap_sysctl_limit_name "void *limit" "const char *name" "int flags"
.Ft void *
.Fn cap_sysctl_limit_mib "void *limit" "const int *mibp" "u_int miblen" "int flags"
.Ft int
.Fn cap_sysctl_limit "void *limit"
.Sh DESCRIPTION
The
.Fn cap_sysctl ,
.Fn cap_sysctlbyname
and
.Fn cap_sysctlnametomib
functions are equivalent to
.Xr sysctl 3 ,
.Xr sysctlbyname 3
and
.Xr sysctlnametomib 3 ,
except that they are implemented by the
.Ql system.sysctl
.Xr libcasper 3
service and require a corresponding
.Xr libcasper 3
capability.
.Sh LIMITS
By default, the
.Nm
capability provides unrestricted access to the sysctl namespace.
Applications typically only require access to a small number of sysctl
variables; the
.Fn cap_sysctl_limit
interface can be used to restrict the sysctls that can be accessed using
the
.Nm
capability.
.Fn cap_sysctl_limit_init
returns an opaque limit handle used to store a list of permitted sysctls
and access rights.
Rights are encoded using the following flags:
.Pp
.Bd -literal -offset indent -compact
CAP_SYSCTL_READ		allow reads of the sysctl variable
CAP_SYSCTL_WRITE        allow writes of the sysctl variable
CAP_SYSCTL_RDWR         allow reads and writes of the sysctl variable
CAP_RECURSIVE           permit access to any child of the sysctl variable
.Ed
.Pp
The
.Fn cap_sysctl_limit_name
function adds the sysctl identified by
.Ar name
to the limit list, and
.Fn cap_sysctl_limit_mib
function adds the sysctl identified by
.Ar mibp
to the limit list.
The access rights for the sysctl are specified in the
.Ar flags
parameter; at least one of
.Dv CAP_SYSCTL_READ ,
.Dv CAP_SYSCTL_WRITE
and
.Dv CAP_SYSCTL_RDWR
must be specified.
.Fn cap_sysctl_limit
applies a set of sysctl limits to the capability, denying access to sysctl
variables not belonging to the set.
.Pp
Once a set of limits is applied, subsequent calls to
.Fn cap_sysctl_limit
will fail unless the new set is a subset of the current set.
.Pp
.Fn cap_sysctlnametomib
will succeed so long as the named sysctl variable is present in the limit set,
regardless of its access rights.
When a sysctl variable name is added to a limit set, its MIB identifier is
automatically added to the set.
.Sh EXAMPLES
The following example first opens a capability to casper, uses this
capability to create the
.Nm system.sysctl
casper service, and then uses the
.Nm
capability to get the value of
.Dv kern.trap_enotcap .
.Bd -literal
cap_channel_t *capcas, *capsysctl;
const char *name = "kern.trap_enotcap";
void *limit;
int value;
size_t size;

/* Open capability to Casper. */
capcas = cap_init();
if (capcas == NULL)
	err(1, "Unable to contact Casper");

/* Enter capability mode sandbox. */
if (cap_enter() < 0 && errno != ENOSYS)
	err(1, "Unable to enter capability mode");

/* Use Casper capability to create capability to the system.sysctl service. */
capsysctl = cap_service_open(capcas, "system.sysctl");
if (capsysctl == NULL)
	err(1, "Unable to open system.sysctl service");

/* Close Casper capability, we don't need it anymore. */
cap_close(capcas);

/* Create limit for one MIB with read access only. */
limit = cap_sysctl_limit_init(capsysctl);
(void)cap_sysctl_limit_name(limit, name, CAP_SYSCTL_READ);

/* Limit system.sysctl. */
if (cap_sysctl_limit(limit) < 0)
	err(1, "Unable to set limits");

/* Fetch value. */
if (cap_sysctlbyname(capsysctl, name, &value, &size, NULL, 0) < 0)
	err(1, "Unable to get value of sysctl");

printf("The value of %s is %d.\\n", name, value);

cap_close(capsysctl);
.Ed
.Sh SEE ALSO
.Xr cap_enter 2 ,
.Xr err 3 ,
.Xr sysctl 3 ,
.Xr sysctlbyname 3 ,
.Xr sysctlnametomib 3 ,
.Xr capsicum 4 ,
.Xr nv 9
.Sh AUTHORS
The
.Nm cap_sysctl
service was implemented by
.An Pawel Jakub Dawidek Aq Mt pawel@dawidek.net
under sponsorship from the FreeBSD Foundation.
.Pp
This manual page was written by
.An Mariusz Zaborski Aq Mt oshogbo@FreeBSD.org .