• 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 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 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 typedefs 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 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 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 non-zero, 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) .
  
   

• 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 preferred
  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 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 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 capabilities by specifying the
  pid
  field as a non-zero 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 available 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 3.19 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/.
  

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