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

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

       #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);

       As of Linux 2.2, the power of the superuser (root) has been partitioned into a set of dis‐
       crete 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 inherita‐
       ble 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 capa‐
       bilities.  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 work‐

       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 applica‐
       tions, 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

           #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;

       The effective, permitted, and inheritable fields are bit masks of the capabilities defined
       in  capabilities(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_VER‐
       SION_1, and kernels 2.6.25+ prefer 64-bit capabilities with version _LINUX_CAPABILITY_VER‐
       SION_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  privi‐
       leged  executables.   This  privilege model obsoletes kernel support for one process asyn‐
       chronously 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 capa‐
       bilities 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).

       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 spec‐
       ified.  In this way, one can probe what the current preferred capability revision is.

       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  capabil‐
              ity 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 capabili‐
              ties,  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 capa‐
              bilities by specifying the pid field as a nonzero value (i.e., the  value  returned
              by getpid(2)) instead of 0.)

       ESRCH  No such thread.

       These system calls are Linux-specific.

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

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

       This page is part of release 3.74 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 http://www.kernel.org/doc/man-pages/.

Linux                                       2013-03-11                                  CAPGET(2)

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