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



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  applica-
       tions.   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 type-
       defs 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 sup-
       port).  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 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 multi-
       threaded process requires a thread ID of the type returned by gettid(2).  For capset(), pid can also be: -1, meaning per-
       form  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  pre-
              ferred 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 Inherita-
              ble 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  suffi-
              cient  privilege.  For kernels supporting VFS capabilities, this is never permitted.  For kernels lacking VFS sup-
              port, 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
              nonzero 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  avail-
       able here:
       http://www.kernel.org/pub/linux/libs/security/linux-privs

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

COLOPHON
       This  page  is  part of release 3.25 of the Linux man-pages project.  A description of the project, and information about
       reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                                                      2009-01-26                                                  CAPGET(2)