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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | NOTES | SEE ALSO | COLOPHON |
CAPGET(2) Linux Programmer's Manual CAPGET(2)
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);
As of Linux 2.2, the power of the superuser (root) has been
partitioned into a set of discrete capabilities. Each thread has a
set of effective capabilities identifying which capabilities (if any)
it may currently exercise. Each thread also has a set of inheritable
capabilities that may be passed through an execve(2) call, and a set
of permitted capabilities that it can make effective or inheritable.
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. If you
wish to use the Linux extensions in applications, you should use the
easier-to-use interfaces capsetp(3) and capgetp(3).
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 capability 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.
With VFS capability support
VFS Capability support creates a file-attribute method for adding
capabilities to privileged executables. This privilege model
obsoletes kernel support for one process asynchronously setting the
capabilities of another. That is, with VFS support, for capset()
calls the only permitted values for hdrp->pid are 0 or gettid(2),
which are equivalent.
Without VFS capability support
When the kernel does not support VFS capabilities, capset() calls can
operate 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.
For details on the data, see capabilities(7).
On success, zero is returned. On error, -1 is returned, and errno is
set appropriately.
The calls will 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 or Inheritable sets
that is not in the Permitted 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)
This page is part of release 4.12 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/.
Linux 2015-07-23 CAPGET(2)
Pages that refer to this page: gettid(2), syscalls(2), capng_apply(3), capabilities(7)