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



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

SYNOPSIS
       #undef _POSIX_SOURCE
       #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  exer-
       cise.  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 inter-
       faces 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 only use datap[0].

       Another change affecting the behavior of these system calls is kernel support for file capabilities (VFS  capa-
       bility 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 non-zero, 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; oper-
       ating 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 only be NULL 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 capa-
              bilities by specifying the pid field as a non-zero 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
       This page is part of release 3.22 of the Linux man-pages project.  A description of the project,  and  informa-
       tion about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                             2009-01-26                         CAPGET(2)