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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  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
       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
       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; 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 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 capabilities 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

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