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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);


     Since 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 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  sys-
     tem  calls (in particular the format of the cap_user_*_t types) is sub-
     ject 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
                 /*  V2  added  in  Linux  2.6.25;  deprecated  */   #define
         _LINUX_CAPABILITY_VERSION_2    0x20071026  #define  _LINUX_CAPABIL-
         ITY_U32S_2     2
                 /* V3 added in  Linux  2.6.26  */  #define  _LINUX_CAPABIL-
         ITY_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 capabilities 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 capabilities support
     VFS  capabilities  employ  a  file extended attribute (see xattr(7)) to
     allow capabilities to be attached to executables.  This privilege model
     obsoletes  kernel  support  for  one process asynchronously setting the
     capabilities of another.  That is, on kernels that have  VFS  capabili-
     ties  support,  when  calling  capset(),  the only permitted values for
     hdrp->pid are 0 or, equivalently, the value returned by gettid(2).
 Without VFS capabilities support
     On older kernels that do not provide VFS capabilities support  capset()
     can,  if  the  caller has the CAP_SETPCAP capability, be used to change
     not only the caller's own capabilities, but also  the  capabilities  of
     other  threads.   The  call  operates 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 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 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 4.16 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

Linux 2017-09-15 CAPGET(2)

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