CAPGET
Section: Linux Programmer's Manual (2)
Updated: 2020-02-09
Page Index
NAME
capget, capset - set/get capabilities of thread(s)
SYNOPSIS
#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);
DESCRIPTION
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.
Current details
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).
With VFS capabilities support
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).
Without VFS capabilities support
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.
RETURN VALUE
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.
ERRORS
- 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.
CONFORMING TO
These system calls are Linux-specific.
NOTES
The portable interface to the capability querying and setting
functions is provided by the
libcap
library and is available here:
SEE ALSO
clone(2),
gettid(2),
capabilities(7)
COLOPHON
This page is part of release 5.10 of the Linux
man-pages
project.
A description of the project,
information about reporting bugs,
and the latest version of this page,
can be found at
https://www.kernel.org/doc/man-pages/.