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

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

       #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);

       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  effec-
       tive  capabilities  identifying which capabilities (if any) it may cur-
       rently exercise.  Each thread also has a set of  inheritable  capabili-
       ties that may be passed through an execve(2) call, and a set of permit-
       ted capabilities that it can make effective or inheritable.

       These two functions are the raw kernel interface for getting  and  set-
       ting  thread capabilities.  Not only are these system calls specific to
       Linux, but the kernel API is likely to change and use  of  these  func-
       tions  (in  particular the format of the cap_user_*_t types) is subject
       to extension with each kernel revision,  but  old  programs  will  keep

       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 struc-
       tures 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 capabil-
       ities   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 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 capa-
       bilities 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 equiva-

   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 get-
       tid(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 specified.  In this way, one can  probe
       what the current preferred capability revision is.

       EFAULT Bad  memory  address.  hdrp must not be NULL.  datap may only be
              NULL when the user is trying to determine the preferred capabil-
              ity 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 permit-
              ted.  For kernels lacking VFS support, the CAP_SETPCAP  capabil-
              ity  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.

       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 3.25 of the Linux  man-pages  project.   A
       description  of  the project, and information about reporting bugs, can
       be found at http://www.kernel.org/doc/man-pages/.

Linux                             2009-01-26                         CAPGET(2)

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