Ubuntu Manpage: capget, capset - set/get capabilities of thread(s)

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 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; see NOTES.

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 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 allow capabilities to
be attached to executables. This privilege model obsoletes kernel support for one process
asynchronously setting the capabilities 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 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.

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 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 set that is not in the permitted 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  permitted.   For  kernels  lacking VFS support, the
              CAP_SETPCAP capability is required.  (A bug in kernels before  Linux  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.

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⟩