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

       Since  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 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.

   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.

       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 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.

CONFORMING TO

       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⟩

SEE ALSO

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

COLOPHON

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