capget, capset - set/get capabilities of thread(s)
#include <sys/capability.h>
int capget(cap_user_header_t hdrp, cap_user_data_t
datap);
int capset(cap_user_header_t hdrp, const cap_user_data_t
datap);
These two system calls are the raw kernel interface for getting and setting
thread capabilities. Not only are these system calls specific to Linux, but
the kernel API is likely to change and use of these system calls (in
particular the format of the
cap_user_*_t types) is subject to
extension with each kernel revision, but old programs will keep working.
The portable interfaces are
cap_set_proc(3) and
cap_get_proc(3);
if possible, you should use those interfaces in applications.
Now that you have been warned, some current kernel details. The structures are
defined as follows.
#define _LINUX_CAPABILITY_VERSION_1 0x19980330
#define _LINUX_CAPABILITY_U32S_1 1
/* V2 added in Linux 2.6.25; deprecated */
#define _LINUX_CAPABILITY_VERSION_2 0x20071026
#define _LINUX_CAPABILITY_U32S_2 2
/* V3 added in Linux 2.6.26 */
#define _LINUX_CAPABILITY_VERSION_3 0x20080522
#define _LINUX_CAPABILITY_U32S_3 2
typedef struct __user_cap_header_struct {
__u32 version;
int pid;
} *cap_user_header_t;
typedef struct __user_cap_data_struct {
__u32 effective;
__u32 permitted;
__u32 inheritable;
} *cap_user_data_t;
The
effective,
permitted, and
inheritable fields are bit
masks of the capabilities defined in
capabilities(7). Note that the
CAP_* values are bit indexes and need to be bit-shifted before ORing
into the bit fields. To define the structures for passing to the system call,
you have to use the
struct __user_cap_header_struct and
struct
__user_cap_data_struct names because the typedefs are only pointers.
Kernels prior to 2.6.25 prefer 32-bit capabilities with version
_LINUX_CAPABILITY_VERSION_1. Linux 2.6.25 added 64-bit capability sets,
with version
_LINUX_CAPABILITY_VERSION_2. There was, however, an API
glitch, and Linux 2.6.26 added
_LINUX_CAPABILITY_VERSION_3 to fix the
problem.
Note that 64-bit capabilities use
datap[0] and
datap[1], whereas
32-bit capabilities use only
datap[0].
On kernels that support file capabilities (VFS capabilities support), these
system calls behave slightly differently. This support was added as an option
in Linux 2.6.24, and became fixed (nonoptional) in Linux 2.6.33.
For
capget() calls, one can probe the capabilities of any process by
specifying its process ID with the
hdrp->pid field value.
For details on the data, see
capabilities(7).
VFS capabilities employ a file extended attribute (see
xattr(7)) to allow
capabilities to be attached to executables. This privilege model obsoletes
kernel support for one process asynchronously setting the capabilities of
another. That is, on kernels that have VFS capabilities support, when calling
capset(), the only permitted values for
hdrp->pid are 0 or,
equivalently, the value returned by
gettid(2).
On older kernels that do not provide VFS capabilities support
capset()
can, if the caller has the
CAP_SETPCAP capability, be used to change
not only the caller's own capabilities, but also the capabilities of other
threads. The call operates on the capabilities of the thread specified by the
pid field of
hdrp when that is nonzero, or on the capabilities
of the calling thread if
pid is 0. If
pid refers to a
single-threaded process, then
pid can be specified as a traditional
process ID; operating on a thread of a multithreaded process requires a thread
ID of the type returned by
gettid(2). For
capset(),
pid
can also be: -1, meaning perform the change on all threads except the caller
and
init(1); or a value less than -1, in which case the change is
applied to all members of the process group whose ID is -
pid.
On success, zero is returned. On error, -1 is returned, and
errno is set
appropriately.
The calls fail with the error
EINVAL, and set the
version field of
hdrp to the kernel preferred value of
_LINUX_CAPABILITY_VERSION_? when an unsupported
version value is
specified. In this way, one can probe what the current preferred capability
revision is.
- EFAULT
- Bad memory address. hdrp must not be NULL. datap may be NULL
only when the user is trying to determine the preferred capability version
format supported by the kernel.
- EINVAL
- One of the arguments was invalid.
- EPERM
- An attempt was made to add a capability to the permitted set, or to set a
capability in the effective set that is not in the permitted set.
- EPERM
- An attempt was made to add a capability to the inheritable set, and
either:
- *
- that capability was not in the caller's bounding set; or
- *
- the capability was not in the caller's permitted set and the caller lacked
the CAP_SETPCAP capability in its effective set.
- EPERM
- The caller attempted to use capset() to modify the capabilities of
a thread other than itself, but lacked sufficient privilege. For kernels
supporting VFS capabilities, this is never permitted. For kernels lacking
VFS support, the CAP_SETPCAP capability is required. (A bug in
kernels before 2.6.11 meant that this error could also occur if a thread
without this capability tried to change its own capabilities by specifying
the pid field as a nonzero value (i.e., the value returned by
getpid(2)) instead of 0.)
- ESRCH
- No such thread.
These system calls are Linux-specific.
The portable interface to the capability querying and setting functions is
provided by the
libcap library and is available here:
http://git.kernel.org/cgit/linux/kernel/git/morgan/libcap.git
clone(2),
gettid(2),
capabilities(7)