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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

       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).

   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

           #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 use only 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.

   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 getpid(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(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).

RETURN VALUE

       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

       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://www.kernel.org/pub/linux/libs/security/linux-privs

SEE ALSO

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

COLOPHON

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