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NAME | SYNOPSIS | DESCRIPTION | ERRORS | NOTES | HISTORY | REPORTING BUGS | SEE ALSO | COLOPHON |
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CAP_IAB(3) Linux Programmer's Manual CAP_IAB(3)
cap_iab_init, cap_iab_dup, cap_iab_get_proc, cap_iab_get_pid,
cap_iab_set_proc, cap_iab_to_text, cap_iab_from_text,
cap_iab_get_vector, cap_iab_compare, cap_iab_set_vector,
cap_iab_fill, cap_proc_root - inheritable IAB tuple support
functions
#include <sys/capability.h>
cap_iab_t cap_iab_init(void);
cap_iab_t cap_iab_dup(cap_iab_t iab);
cap_iab_t cap_iab_get_proc(void);
cap_iab_t cap_iab_get_pid(pid_t pid);
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_compare(cap_iab_t a, cap_iab_t b);
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);
char *cap_proc_root(const char *root);
Link with -lcap.
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 tuple
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 tuple and the Permitted process
capability flag. They are: the Inh vector is entirely equal to
the process Inheritable flag at all times; the Amb vector
contains no more capability values than the intersection of the
Inh vector and the Permitted flag for the process; and the Bound
(or blocked) vector is the twos-complement of the process
bounding vector.
In some environments, it is considered desirable 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 a pre-exec* process and a file-capability bestowed
executable of the traditional (POSIX.1e) capability mechanism.
The convolution rules for IAB style inheritance are: I'=I;
A'=A&I; P'=A&I&P. 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 tuple is managed: the IAB API.
cap_iab_init() returns an empty IAB value. That is a mostly-
harmless tuple. It will not block any Permitted file capabilities
through exec, but it won't bestow any either. The returned
cap_iab_t should be freed with cap_free(3).
cap_iab_dup() returns a copy of the specified IAB value. 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_free(3).
cap_iab_get_pid() returns a copy of the IAB value for the
specified process. The returned cap_iab_t should be freed with
cap_free(3). This function defaults to searching
/proc/<PID>/status for the IAB information, but that location can
be overridden using the cap_proc_root() function.
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 raised
in the E flag and a superset of P and I values over those in the
A vectors.
cap_iab_to_text() will convert an IAB tuple to a canonical text
representation. The representation is slightly redundant but
libcap will try to generate as short a representation as it is
able.
cap_iab_from_text() generates an IAB tuple from a text string
(likely generated by the previous function). The returned IAB
tuple 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, but think Blocked); ^ (Amb).
Or, some combination thereof. Since the Amb vector is
constrained to be no greater than the Inh vector, ^ is equivalent
to %^. Further, unless B is non-zero, % can be omitted. The
following are legal text representations: "!%cap_chown" (Bound
but Inh), "!cap_chown,^cap_chown" (Bound, Inh+Amb).
"cap_setuid,!cap_chown" (Inh, Bound). As noted above, this text
representation is the syntax for the pam_cap.so config file.
cap_iab_get_vector() can be used to determine the specific
capability value of an IAB vector.
cap_iab_compare() can be used to compare two cap_iab_t tuples.
When the return value is non-zero, the macro
CAP_IAB_DIFFERS(status, vector) evaluates to non-zero if the
returned status differs in its vector components.
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 tuple. Copying into Amb in this
way may implicitly raise Inh values in the IAB tuple. Similarly
copying into the Inh vector may implicitly lower Amb values that
are not present in the resulting Inh vector.
cap_proc_root() can be used to determine the current location
queried by cap_iab_get_pid(). Returned values should be released
with cap_free(3). If the argument to cap_proc_root() is not
NULL, a copy of it will become the replacement for /proc. Note,
this function is not thread safe with respect to concurrent calls
to cap_iab_get_pid().
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.
Unlike the traditional cap_t capability set, the IAB tuple, taken
together, is incompatible with filesystem capabilities created
via tools like setcap(8). That is, the Amb vector of the IAB
tuple is rendered moot when an executable with a file capability
is executed.
Further, there are libcap cap_mode(3)s that render the Amb vector
and its method of process inheritance disabled.
The IAB format for inheritable variants of capabilities was first
developed as the configuration syntax for the pam_cap.so 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 vector and introduce
support for the Amibient vector of capability bits.
While the Inheritable and Bounding vectors were anticipated by
the POSIX.1e draft that introduced capabilities, the Ambient
vector 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 pam_cap.so
config syntax was generalized into a whole set of libcap
functions for bundling together all three naively inheritable
capabilities: the IAB tuple. The support for this debuted in
libcap-2.33.
Please report bugs via:
https://bugzilla.kernel.org/buglist.cgi?component=libcap&list_id=1090757
libcap(3), cap_launch(3), cap_init(3), capabilities(7) and
errno(3).
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
[email protected] (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
2024-06-14. (At that time, the date of the most recent commit
that was found in the repository was 2024-05-18.) 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 [email protected]
2022-10-16 CAP_IAB(3)
Pages that refer to this page: capsh(1), cap_launch(3), libcap(3), capability.conf(5), captree(8), getpcaps(8)
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HTML rendering created 2024-06-26 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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