capget

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 system
       calls (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.

   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

                   /* 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 allow
       capabilities to be attached to executables.  This privilege model
       obsoletes kernel support for one process asynchronously setting the
       capabilities 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 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(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 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 set that is not in the
              permitted 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 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
              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 available here:
       ⟨http://git.kernel.org/cgit/linux/kernel/git/morgan/libcap.gitSEE ALSO
       clone(2), gettid(2), capabilities(7)

COLOPHON
       This page is part of release 5.09 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
       https://www.kernel.org/doc/man-pages/.



Linux                              2020-02-09                          CAPGET(2)