CAP_SYSCTL(3)

NAME
cap_sysctl - library for getting or setting system information in capability mode

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);
void *
cap_sysctl_limit_init(cap_channel_t *chan);
void *
cap_sysctl_limit_name(void *limit, const char *name, int flags);
void *
cap_sysctl_limit_mib(void *limit, const int *mibp, u_int miblen, int flags);
int
cap_sysctl_limit(void *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.

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:

    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

The cap_sysctl_limit_name() function adds the sysctl identified by name to the limit list, and cap_sysctl_limit_mib() function adds the sysctl 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.

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);
    cap_close(capsysctl);

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>.