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NAME | SYNOPSYS | DESCRIPTION | ERRORS | HISTORY | SEE ALSO | COLOPHON |
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CAP_LAUNCH(3) Linux Programmer's Manual CAP_LAUNCH(3)
cap_new_launcher, cap_func_launcher, cap_launcher_callback,
cap_launcher_set_mode, cap_launcher_set_iab,
cap_launcher_set_chroot, cap_launch, cap_launcher_setuid,
cap_launcher_setgroups - libcap launch functionality
#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.
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.
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.
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.)
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).
This page is part of the libcap (capabilities commands and
library) project. Information about the project can be found at
⟨https://git.kernel.org/pub/scm/libs/libcap/libcap.git/⟩. If you
have a bug report for this manual page, send it to
morgan@kernel.org (please put "libcap" in the Subject line). This
page was obtained from the project's upstream Git repository
⟨https://git.kernel.org/pub/scm/libs/libcap/libcap.git/⟩ on
2025-02-02. (At that time, the date of the most recent commit
that was found in the repository was 2025-02-01.) 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 the page,
or you have corrections or improvements to the information in this
COLOPHON (which is not part of the original manual page), send a
mail to man-pages@man7.org
2021-08-01 CAP_LAUNCH(3)
Pages that refer to this page: capsh(1), cap_iab(3), libcap(3)
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HTML rendering created 2025-02-02 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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