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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
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).
RETURN VALUE
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.
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 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.
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:
⟨http://git.kernel.org/cgit/linux/kernel/git/morgan/libcap.git⟩
SEE ALSO
clone(2), gettid(2), capabilities(7)
COLOPHON
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