capget(2) — Linux manual page


capget(2)                  System Calls Manual                 capget(2)

NAME         top

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

LIBRARY         top

       Standard C library (libc, -lc)

SYNOPSIS         top

       #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         top

       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

   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         top

       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         top

       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

       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;

              •  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         top


NOTES         top

       The portable interface to the capability querying and setting
       functions is provided by the libcap library and is available

SEE ALSO         top

       clone(2), gettid(2), capabilities(7)

Linux man-pages (unreleased)     (date)                        capget(2)

Pages that refer to this page: gettid(2)syscalls(2)cap_get_proc(3)capng_apply(3)capabilities(7)