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)
#undef _POSIX_SOURCE
#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 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).
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 only use 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.
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.
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 non-zero, 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).
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 only be NULL
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 non-zero 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://www.kernel.org/pub/linux/libs/security/linux-privs
clone(2), gettid(2), capabilities(7)
This page is part of release 3.21 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/.
Linux 2009-01-26 CAPGET(2)