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CAP_SYSCTL(3) FreeBSD Library Functions Manual CAP_SYSCTL(3)
NAME
cap_sysctl - library for getting or setting system information in
capability mode
LIBRARY
library "libcap_sysctl"
SYNOPSIS
#include <libcasper.h>
#include <casper/cap_sysctl.h>
int
cap_sysctl(cap_channel_t *chan, const int *name, u_int namelen,
void *oldp, size_t *oldlenp, const void *newp, size_t newlen);
int
cap_sysctlbyname(cap_channel_t *chan, const char *name, void *oldp,
size_t *oldlenp, const void *newp, size_t newlen);
int
cap_sysctlnametomib(cap_channel_t *chan, const char *name, int *mibp,
size_t *sizep);
cap_sysctl_limit_t *
cap_sysctl_limit_init(cap_channel_t *chan);
cap_sysctl_limit_t *
cap_sysctl_limit_name(cap_sysctl_limit_t *limit, const char *name,
int flags);
cap_sysctl_limit_t *
cap_sysctl_limit_mib(cap_sysctl_limit_t *limit, const int *mibp,
u_int miblen, int flags);
int
cap_sysctl_limit(cap_sysctl_limit_t *limit);
DESCRIPTION
The cap_sysctl(), cap_sysctlbyname() and cap_sysctlnametomib() functions
are equivalent to sysctl(3), sysctlbyname(3) and sysctlnametomib(3),
except that they are implemented by the `system.sysctl' libcasper(3)
service and require a corresponding libcasper(3) capability.
All of these functions, with the exceptions of cap_sysctl_limit_init()
and cap_sysctl_limit_mib(), are reentrant but not thread-safe. That is,
they may be called from separate threads only with different
cap_channel_t arguments or with synchronization.
LIMITS
By default, the cap_sysctl capability provides unrestricted access to the
sysctl namespace. Applications typically only require access to a small
number of sysctl variables; the cap_sysctl_limit() interface can be used
to restrict the sysctls that can be accessed using the cap_sysctl
capability. 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:
identified by mibp to the limit list. The access rights for the sysctl
are specified in the flags parameter; at least one of CAP_SYSCTL_READ,
CAP_SYSCTL_WRITE and CAP_SYSCTL_RDWR must be specified.
cap_sysctl_limit() applies a set of sysctl limits to the capability,
denying access to sysctl variables not belonging to the set. It consumes
the limit handle. After either success or failure, the user must not
access the handle again.
Once a set of limits is applied, subsequent calls to cap_sysctl_limit()
will fail unless the new set is a subset of the current set.
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.
EXAMPLES
The following example first opens a capability to casper, uses this
capability to create the system.sysctl casper service, and then uses the
cap_sysctl capability to get the value of kern.trap_enotcap.
cap_channel_t *capcas, *capsysctl;
const char *name = "kern.trap_enotcap";
void *limit;
size_t size;
bool value;
/* 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. */
size = sizeof(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);
access the limit handle again. cap_sysctl_limit() will return -1 on
failure and set errno. cap_sysctl(), cap_sysctlbyname(), and
cap_sysctlnametomib() have the same return values as their non-
capability-mode equivalents as documented in sysctl(3).
SEE ALSO
cap_enter(2), err(3), sysctl(3), sysctlbyname(3), sysctlnametomib(3),
capsicum(4), nv(9)
HISTORY
The cap_sysctl service first appeared in FreeBSD 10.3.
AUTHORS
The cap_sysctl service was implemented by Pawel Jakub Dawidek
<pawel@dawidek.net> under sponsorship from the FreeBSD Foundation.
This manual page was written by
Mariusz Zaborski <oshogbo@FreeBSD.org>.
FreeBSD 14.2-RELEASE December 6, 2023 FreeBSD 14.2-RELEASE