Provided by: libcap-dev_2.66-3ubuntu2_amd64 
NAME
#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));
int cap_launcher_callback(cap_launch_t attr,
int (callback_fn)(void *detail));
int 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);
int 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);
int cap_launcher_setuid(cap_launch_t attr, uid_t uid);
int cap_launcher_setgroups(cap_launch_t attr, gid_t gid,
int ngroups, const gid_t *groups);
Link with -lcap.
DESCRIPTION
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 application.
If the callback function returns anything other than zero, a cap_launch() will be aborted.
If detail of the failure is important to the caller, it should be communicated via the
detail argument.
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):
cap_launcher_callback() can be used to install or replace the currently installed callback
function of the launcher.
cap_launcher_set_mode() can be used to ensure that the libcap-mode of the launched program
is the desired one.
cap_launcher_set_iab() 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. This iab value is locked to the laucher and cannot be modified while associated with
the launcher. 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.
cap_launcher_set_chroot() This function causes the launched program executable to be
invoked inside a chroot root directory.
cap_launcher_setuid() This function causes the launched program executable to be invoked
with the specified user identifier (uid_t).
cap_launcher_setgroups() 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
A return of NULL for a cap_launch_t should be considered an error.
cap_launch() returns -1 in the case of an error.
In all such cases a return value of 0 implies success. In other cases, consult errno(3)
for further details.
HISTORY
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
https://sites.google.com/site/fullycapable/who-ordered-libpsx for an explanation.)
SEE ALSO
libpsx(3), psx_syscall(3), libcap(3), cap_mode(3), cap_iab(3), capabilities(7), errno(3),
fork(2), mmap(2), chroot(2), and munmap(2).
2021-08-01 CAP_LAUNCH(3)