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       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  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 capability 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

       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  gettid(2),  which  are

   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(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).


       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 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.


       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)


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