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Linux 2.6 - man page for capset (linux section 2)

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 dis-
       crete 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 inherita-
       ble 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 capabili-
       ties.  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 applica-
       tions, 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

	   #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 capabil-
       ity(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_VER-
       SION_1, and kernels 2.6.25+ prefer 64-bit capabilities with version _LINUX_CAPABILITY_VER-
       SION_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 privi-
       leged executables.  This privilege model obsoletes kernel support for  one  process  asyn-
       chronously  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 capa-
       bilities  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(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 spec-
       ified.  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 capability version  format  supported  by  the

       EINVAL One of the arguments was invalid.

       EPERM  An  attempt was made to add a capability to the Permitted set, or to set a capabil-
	      ity 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 capabili-
	      ties, 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  capa-
	      bilities	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 3.27 of the Linux man-pages project.  A description of the
       project,  and  information  about  reporting  bugs,  can  be  found   at   http://www.ker-

Linux					    2010-09-20					CAPGET(2)

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