CAPGET(2)                 (2020-02-09)                  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 get-
          ting 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 work-
          ing.

          The portable interfaces are cap_set_proc(3) and
          cap_get_proc(3); if possible, you should use those inter-
          faces 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;

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          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 ver-
          sion _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 executa-
          bles.  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 sup-
          port capset() can, if the caller has the CAP_SETPCAP capa-
          bility, be used to change not only the caller's own capabil-
          ities, 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 capabili-
          ties of the calling thread if pid is 0.  If pid refers to a
          single-threaded process, then pid can be specified as a tra-
          ditional 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
          perform the change on all threads except the caller and
          init(1); or a value less than -1, in which case the change

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          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 permit-
               ted 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 inheri-
               table 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 capa-
               bilities, 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

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               No such thread.

     CONFORMING TO
          These system calls are Linux-specific.

     NOTES
          The portable interface to the capability querying and set-
          ting functions is provided by the libcap library and is
          available here:

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

     COLOPHON
          This page is part of release 5.10 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/.

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