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:

           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.  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);

     cap_close(capsysctl);

RETURN VALUES

     cap_sysctl_limit_init() will return a new limit handle on success or NULL
     on failure, and set errno.  cap_sysctl_limit_mib() and
     cap_sysctl_limit_name() will return the modified limit handle on success
     or NULL on failure and set errno.  After failure, the caller must not
     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 15.1-STABLE-HBSD       December 6, 2023                  CAP_SYSCTL(3)