cap_sysctl.3

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.Dd December 6, 2023
.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 cap_sysctl_limit_t *
.Fn cap_sysctl_limit_init "cap_channel_t *chan"
.Ft cap_sysctl_limit_t *
.Fn cap_sysctl_limit_name "cap_sysctl_limit_t *limit" "const char *name" "int flags"
.Ft cap_sysctl_limit_t *
.Fn cap_sysctl_limit_mib "cap_sysctl_limit_t *limit" "const int *mibp" "u_int miblen" "int flags"
.Ft int
.Fn cap_sysctl_limit "cap_sysctl_limit_t *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.
.Pp
All of these functions, with the exceptions of
.Fn cap_sysctl_limit_init
and
.Fn cap_sysctl_limit_mib ,
are reentrant but not thread-safe.
That is, they may be called from separate threads only with different
.Vt cap_channel_t
arguments or with synchronization.
.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.
It consumes the limit handle.
After either success or failure, the user must not access the handle again.
.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;
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);
.Ed
.Sh RETURN VALUES
.Fn cap_sysctl_limit_init
will return a new limit handle on success or
.Dv NULL
on failure, and set
.Va errno .
.Fn cap_sysctl_limit_mib
and
.Fn cap_sysctl_limit_name
will return the modified limit handle on success or
.Dv NULL
on failure and set
.Va errno .
After failure, the caller must not access the limit handle again.
.Fn cap_sysctl_limit
will return
.Dv -1
on failure and set
.Va errno .
.Fn cap_sysctl ,
.Fn cap_sysctlbyname ,
and
.Fn cap_sysctlnametomib
have the same return values as their non-capability-mode equivalents as
documented in
.Xr sysctl 3 .
.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 HISTORY
The
.Nm cap_sysctl
service first appeared in
.Fx 10.3 .
.Sh AUTHORS
The
.Nm cap_sysctl
service was implemented by
.An Pawel Jakub Dawidek Aq Mt [email protected]
under sponsorship from the FreeBSD Foundation.
.Pp
This manual page was written by
.An Mariusz Zaborski Aq Mt [email protected] .