capset manpage

Search topic Section

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);

       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 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_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 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 gettid(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(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 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 4.10 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				  2015-07-23			     CAPGET(2)