capget(2) System Calls Manual capget(2)
NAME
capget, capset - set/get capabilities of thread(s)
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#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).
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 sys-
tem calls (in particular the format of the cap_user_*_t types) is sub-
ject 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 struc-
tures 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 al-
low capabilities to be attached to executables. This privilege model
obsoletes kernel support for one process asynchronously setting the ca-
pabilities 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 ca-
pabilities of the calling thread if pid is 0. If pid refers to a sin-
gle-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 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.
ERRORS
EFAULT Bad memory address. hdrp must not be NULL. datap may be NULL
only when the user is trying to determine the preferred capabil-
ity 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 per-
mitted 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 permit-
ted. For kernels lacking VFS support, the CAP_SETPCAP capabil-
ity is required. (A bug in kernels before Linux 2.6.11 meant
that this error could also occur if a thread without this capa-
bility tried to change its own capabilities by specifying the
pid field as a nonzero value (i.e., the value returned by get-
pid(2)) instead of 0.)
ESRCH No such thread.
STANDARDS
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:
⟨http://git.kernel.org/cgit/linux/kernel/git/morgan/libcap.git⟩
SEE ALSO
clone(2), gettid(2), capabilities(7)
Linux man-pages 6.03 2023-02-05 capget(2)
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