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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | STANDARDS | NOTES | SEE ALSO | COLOPHON |
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capget(2) System Calls Manual capget(2)
capget, capset - set/get capabilities of thread(s)
Standard C library (libc, -lc)
#include <linux/capability.h> /* Definition of CAP_* and
_LINUX_CAPABILITY_* constants */
#include <sys/syscall.h> /* Definition of SYS_* constants */
#include <unistd.h>
int syscall(SYS_capget, cap_user_header_t hdrp,
cap_user_data_t datap);
int syscall(SYS_capset, cap_user_header_t hdrp,
const cap_user_data_t datap);
Note: glibc provides no wrappers for these system calls,
necessitating the use of syscall(2).
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; see
NOTES.
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 Linux 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.
On success, zero is returned. On error, -1 is returned, and
errno is set to indicate the error.
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 Linux 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.
Linux.
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)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.9.1.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2024-06-26. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
[email protected]
Linux man-pages 6.9.1 2024-05-02 capget(2)
Pages that refer to this page: gettid(2), syscalls(2), cap_get_proc(3), capng_apply(3), capabilities(7)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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