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 functions 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 functions (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
#define _LINUX_CAPABILITY_VERSION_2 0x20071026
#define _LINUX_CAPABILITY_U32S_2 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;
effective, permitted, inheritable are bitmasks of the capabilities
defined in
capability(7). Note 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, and kernels 2.6.25+ prefer 64-bit
capabilities with version
_LINUX_CAPABILITY_VERSION_2. Note, 64-bit
capabilities use
datap[0] and
datap[1], whereas 32-bit
capabilities use only
datap[0].
Another change affecting the behavior of these system calls is kernel support
for file capabilities (VFS capability support). This support is currently a
compile time option (added in kernel 2.6.24).
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
getpid(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(8); 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.
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://www.kernel.org/pub/linux/libs/security/linux-privs
SEE ALSO¶
clone(2),
gettid(2),
capabilities(7)
COLOPHON¶
This page is part of release 3.44 of the Linux
man-pages project. A
description of the project, and information about reporting bugs, can be found
at
http://www.kernel.org/doc/man-pages/.
