cap_launch(3) — Linux manual page


CAP_LAUNCH(3)           Linux Programmer's Manual          CAP_LAUNCH(3)

NAME         top

       #include <sys/capability.h>

       cap_launch_t cap_new_launcher(const char *arg0, const char * const *argv,
           const char * const *envp);

       cap_launch_t cap_func_launcher(int (callback_fn)(void *detail));

       void cap_launcher_callback(cap_launch_t attr,
           int (callback_fn)(void *detail));
       void cap_launcher_set_mode(cap_launch_t attr, cap_mode_t flavor);
       cap_iab_t cap_launcher_set_iab(cap_launch_t attr, cap_iab_t iab);
       void cap_launcher_set_chroot(cap_launch_t attr, const char *chroot);

       #include <sys/types.h>

       pid_t cap_launch(cap_launch_t attr, void *detail);
       void cap_launcher_setuid(cap_launch_t attr, uid_t uid);
       void cap_launcher_setgroups(cap_launch_t attr, gid_t gid,

       Link with -lcap.

DESCRIPTION         top

       A launcher provides a mechanism for code to execute a callback
       function and/or a program executable in an environment with a
       modified security context. Essentially it provides a mechanism
       for a program to fork(2) a new context from that of the main
       program manipulate capability and other privileged state in that
       fork(2)d process before (optionally) execve(2)ing a new program.
       When the application links to -lpsx this support is needed to
       robustly execute the launched code without modifying the privilge
       of the whole (POSIX semantics honoring) main application.

       A launcher is defined by one of these two functions:
       cap_new_launcher() or cap_func_launcher().  The return value
       being of opaque type cap_launch_t a return value of NULL implies
       an error has occurred.

       Once defined, a cap_launch_t value can be used with cap_launch()
       to execute that launcher. In such cases, a non-negative return
       value indicates success: zero meaning success without a program
       being invoked; non-zero being equal to the process ID (pid_t) of
       the newly launched program.

       A cap_launch_t occupies allocated memory and should be freed with
       cap_free(3).  Before being cap_free(3)d a cap_value_t value may
       be reused for multiple independent launches. The detail argument
       to cap_launch(), in conjunction with the launcher's callback
       function, can be used to customize the invocation of the launch.
       Care must be taken to leverage custom shared memory (see mmap(2))
       or some other IPC to return values to the main program via detail
       since the callback and any subsequent program execution will
       occur outside the main process of the calling application. An
       example of this would be to allocate detail as follows:

          const *char[] args = { "echo", "hello", NULL };
          cap_launch_t cmd = cap_new_launcher("/usr/bin/echo", args, NULL);
          int *detail = mmap(NULL, sizeof(int), PROT_READ | PROT_WRITE,
                             MAP_SHARED | MAP_ANONYMOUS, -1, 0);
          cap_launcher_callback(cmd, &answer_detail_fn);
          *detail = 41;
          pid_t pid = cap_launch(cmd, detail);
          printf("launcher callback set detail to %d\n", *detail);
          munmap(detail, sizeof(int));

       Unless modified by the callback function, the launched code will
       execute with the capability and other security context of the

       If the callback function returns anything other than zero, a
       will be aborted. If detail of the failure is important to the caller,
       it should be communicated via the

       The following functions can be used to instruct the launcher to modify
       the security state of the invoked program without altering the state
       of the calling program. Such modifications must be performed prior to
       calling cap_launch() if they are to have the desired
       effect. Further, they are only invoked after any installed callback
       has completed. For example, one can drop or modify capabilities,
       just for executing a file.

       The following functions instruct the launcher to do some common tasks
       of this sort (note some require permitted capability bits to succeed):

       can be used to install or replace the currently installed callback
       function of the launcher.

       can be used to ensure that the libcap-mode of the launched program is
       the desired one.

       This function returns the cap_iab_t previously associated with
       the launcher. Calling this function with an IAB value of NULL will
       configure the launcher to not set an IAB value (the default).  See
       cap_iab(3) for details on the IAB set. Note, the launcher is
       associated directly with the supplied iab value, and does not
       make a copy of it. Set with NULL to regain control over the memory
       associated with that IAB value, otherwise the IAB value will be
       cap_free()'d when the launcher is.

       This function causes the launched program executable to be invoked
       inside a chroot root directory.

       This function causes the launched program executable to be invoked
       with the specified user identifier (uid_t).

       This function causes the launched program executable to be invoked
       with the specified primary and supplementary group IDs.

       Note, if any of the launcher enhancements made by the above functions
       should fail to take effect (typically for a lack of sufficient
       privilege), the launch will fail and return -1.

ERRORS         top

       A return of NULL for a cap_launch_t should be considered an

       cap_launch() returns -1 in the case of an error.

       In all such cases consult errno(3) for further details.

HISTORY         top

       The cap_launch() family of functions were introduced in libcap
       2.33. It primarily addresses a complexity with -lpsx linked
       pthreads(7) applications that use capabilities but also honor
       POSIX semantics.

       Using -lcap and -lpthread together without the POSIX semantics
       support from -lpsx introduces a subtle class of exposure to
       privilege escalation. (See for
       an explanation.)

SEE ALSO         top

       libpsx(3), psx_syscall(3), libcap(3), cap_mode(3), cap_iab(3),
       capabilities(7), errno(3), fork(2), mmap(2), chroot(2), and

COLOPHON         top

       This page is part of the libcap (capabilities commands and
       library) project.  Information about the project can be found at
       ⟨⟩.  If you
       have a bug report for this manual page, send it to (please put "libcap" in the Subject line).
       This page was obtained from the project's upstream Git repository
       ⟨⟩ on
       2021-08-27.  (At that time, the date of the most recent commit
       that was found in the repository was 2021-08-25.)  If you
       discover any rendering problems in this HTML version of the page,
       or you believe there is a better or more up-to-date source for
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       manual page), send a mail to

                               2021-08-01                  CAP_LAUNCH(3)

Pages that refer to this page: cap_iab(3)