CAPGET(2)                 Linux Programmer's Manual                CAPGET(2)

NAME         top

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

SYNOPSIS         top

       #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         top

       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         top

       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         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 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         top

       These system calls are Linux-specific.

NOTES         top

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

SEE ALSO         top

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

COLOPHON         top

       This page is part of release 4.10 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at

Linux                            2015-07-23                        CAPGET(2)