cap_iab(3) — Linux manual page


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

NAME         top

       #include <sys/capability.h>

       cap_iab_t cap_iab_init(void);

       cap_iab_t cap_iab_get_proc(void);

       int cap_iab_set_proc(cap_iab_t iab);

       char *cap_iab_to_text(cap_iab_t iab);

       cap_iab_t cap_iab_from_text(const char *text);

       cap_flag_value_t cap_iab_get_vector(cap_iab_t iab, cap_iab_vector_t vec,
           cap_value_t val);

       int cap_iab_set_vector(cap_iab_t iab, cap_iab_vector_t vec, cap_value_t val,
           cap_flag_value_t enable);

       int cap_iab_fill(cap_iab_t iab, cap_iab_vector_t vec,
           cap_t set, cap_flag_t flag);

       Link with -lcap.

DESCRIPTION         top

       The functions defined in this man page concern the three naively
       inheritable process capability vectors: Inh, Amb and Bound. This
       IAB 3-tuple of capability vectors, captured in type cap_iab_t
       combine to pass capabilities from one process to another through
       execve(2) system calls. The convolution rules using the IAB set
       are a fail over set of rules when the executed file has no
       configured file-capabilities.

       There are some constraints enforced by the kernel with respect to
       the three components of an IAB set and the Permitted process
       capability flag. They are: the Inh vector is entirely equal to
       the process Inheritable flag at all times; the the Amb vector
       contains no more capability values than the intersection of the
       Inh vector and the Permitted flag for the process; no Amb value
       blocked in the Bound Vector will survive execve(2); and the Bound
       (or blocked) vector is the twos-complement of the process
       bounding set.

       In some environments, it is considered desireable to naively
       inherit capabilities. That is pass capabilities, independent of
       the status of the executed binary, from parent to child through
       exec* system calls. The surviving capabilities become the
       Permitted flag for the post-exec process. This method of
       inheritance differs significantly from the handshake inheritance
       between pre-exec* process and file-capability bestowed executable
       of the traditional capability mechanism.

       The convolution rules for IAB style inheritance are: I'=I; A'= A
       & ~B; P'=A & ~B. Where P etc are the pre-exec values and P' etc
       are the post-exec values.

       With an understanding of these convolution rules, we can explain
       how libcap(3) support for the IAB set is managed: the IAB API.

       cap_iab_init() returns an empty IAB value. That is a mostly-
       harmless tuple. It will not block and capabilities through exec,
       but it won't bestow any either. The returned cap_iab_t should be
       freed with cap_free(3).

       cap_iab_get_proc() returns a copy of the IAB value for the
       current process.  The returned cap_iab_t should be freed with

       cap_iab_set_proc() can be used to set the IAB value carried by
       the current process. Such a setting will fail if the process is
       insufficiently capable. The process requires CAP_SETPCAP and a
       superset of P values over the A and I vectors.

       cap_iab_to_text() will convert an IAB set to a canonical text
       representation. The representation is slightly redundant but
       libcap will try to generate as short a representation as it is

       cap_iab_from_text() generates an IAB set from a text string
       (likely generated by the previous function). The returned IAB set
       should be freed with cap_free(3).

       The text format accepted by cap_iab_from_text() is a comma
       separated list of capability values. Each capability is prefixed
       by nothing (or %) (Inh); ! (Bound); ^ (Amb). Or, some combination
       thereof. Since the Amb vector is constrained to be no greater
       than the Inh set, ^ is eqivalent to %^. Further, unless B is non-
       zero, % can be omitted. The following are legal text
       representations: "!%cap_chown" (Bound but Inh),
       "!cap_setuid,^cap_chown" (Bound, Inh+Amb).
       "cap_setuid,!cap_chown" (Inh, Bound). As noted above, this text
       representation is the syntax for the config file.

       cap_iab_get_vector() can be used to determine the specific
       capability value of an IAB vector.

       cap_iab_set_vector() can be used to set a specific vector value
       to the enable setting.  cap_iab_fill() can be used to wholesale
       copy a cap_t flag value into the vec vector of the IAB set.
       Copying into Amb in this way may implicitly raise Inh values in
       the IAB set. Similarly copying into the Inh vector may implicitly
       lower Amb values that are not present in the resulting Inh

ERRORS         top

       The functions returning cap_iab_t values or allocated memory in
       the form of a string return NULL on error.

       Integer return values are -1 on error and 0 on success.

       In the case of error consult errno.

NOTES         top

       Unlike the traditional cap_t capability set, the IAB set, taken
       together, is incompatible with filesystem capabilities created
       via tools like setcap(8).  That is, the Amb vector of the IAB set
       is rendered moot when an executable with a file capability is

       Further, there are libcap cap_mode(3)s that render the Amb vector
       and its method of process inheritance disabled.

HISTORY         top

       The IAB format for inheritable variants of capabilities was first
       developed as the configuration syntax for the Linux-
       PAM module in libcap-2.29. It was introduced to extend the simple
       comma separated list of process Inheritable capabilities, that
       the module could besow on an authenticated process tree, to
       include enforced limits on the Bounding set and introduce support
       for the Amibient set of capability bits.

       While the Inheritable and Bounding sets were anticipated by the
       POSIX.1e draft that introduced capabilities, the Ambient set is a
       Linux invention, and incompatible with the POSIX.1e file
       capability model. As such, it was felt that trying to meld
       together all of the 5 capability vectors into one text
       representation was not going to work. Instead the
       config syntax was generalized into a whole set of libcap
       functions for bundling together all three naively inheritable
       capabilities: the IAB set. The support for this debuted in

SEE ALSO         top

       libcap(3), cap_launch(3), cap_init(3), capabilities(7) and

COLOPHON         top

       This page is part of the libcap (capabilities commands and
       library) project.  Information about the project can be found at
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       This page was obtained from the project's upstream Git repository
       ⟨⟩ on
       2021-04-01.  (At that time, the date of the most recent commit
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                               2021-03-10                     CAP_IAB(3)

Pages that refer to this page: cap_launch(3)