capset

CAPGET(2)                  Linux Programmer's Manual                 CAPGET(2)

NAME
       capget, capset - set/get capabilities of thread(s)

SYNOPSIS
       #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
       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
       working.

       The portable interfaces are  cap_set_proc(3)  and  cap_get_proc(3);  if
       possible, you should use those interfaces in applications.

   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  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
       2.6.33.

       For  capget()  calls,  one can probe the capabilities of any process by
       specifying its process ID with the hdrp->pid field value.

       For details on the data, see capabilities(7).

   With VFS capabilities support
       VFS capabilities employ a file extended attribute (see xattr(7)) to al-
       low  capabilities  to be attached to executables.  This privilege model
       obsoletes kernel support for one process asynchronously setting the ca-
       pabilities  of another.  That is, on kernels that have VFS capabilities
       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 ca-
       pabilities of the calling thread if pid is 0.  If pid refers to a  sin-
       gle-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.

RETURN VALUE
       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.

ERRORS
       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 set that is not in the per-
              mitted set.

       EPERM  An attempt was made to add a capability to the inheritable  set,
              and either:

              *  that capability was not in the caller's bounding set; or

              *  the  capability was not in the caller's permitted set and the
                 caller lacked the CAP_SETPCAP  capability  in  its  effective
                 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))  in-
              stead 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://git.kernel.org/cgit/linux/kernel/git/morgan/libcap.git>

SEE ALSO
       clone(2), gettid(2), capabilities(7)

COLOPHON
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Linux                             2020-02-09                         CAPGET(2)