NAME
cap_from_text, cap_to_text, cap_to_name, cap_from_name - capability
state textual representation translation
SYNOPSIS
#include <sys/capability.h>
cap_t cap_from_text(const char *buf_p);
char *cap_to_text(cap_t caps, ssize_t *length_p);
int cap_from_name(const char *name, cap_value_t *cap_p);
char *cap_to_name(cap_value_t cap);
Link with -lcap.
DESCRIPTION
These functions translate a capability state between an internal
representation and a textual one. The internal representation is
managed by the capability functions in working storage. The textual
representation is a structured, human-readable string suitable for
display.
cap_from_text() allocates and initializes a capability state in working
storage. It then sets the contents of this newly created capability
state to the state represented by a human-readable, nul-terminated
character string pointed to by buf_p. It returns a pointer to the
newly created capability state. When the capability state in working
storage is no longer required, the caller should free any releasable
memory by calling cap_free() with cap_t as an argument. The function
returns an error if it cannot parse the contents of the string pointed
to by buf_p or does not recognize any capability_name or flag character
as valid. The function also returns an error if any flag is both set
and cleared within a single clause.
cap_to_text() converts the capability state in working storage
identified by cap_p into a nul-terminated human-readable string. This
function allocates any memory necessary to contain the string, and
returns a pointer to the string. If the pointer len_p is not NULL, the
function shall also return the full length of the string (not including
the nul terminator) in the location pointed to by len_p. The
capability state in working storage, identified by cap_p, is completely
represented in the character string. When the capability state in
working storage is no longer required, the caller should free any
releasable memory by calling cap_free() with the returned string
pointer as an argument.
cap_from_name() converts a text representation of a capability, such as
"cap_chown", to its numerical representation (CAP_CHOWN=0), writing the
decoded value into *cap_p. If cap_p is NULL no result is written, but
the return code of the function indicates whether or not the specified
capability can be represented by the library.
cap_to_name() converts a capability index value, cap, to a libcap-
allocated textual string. This string should be deallocated with
cap_free().
TEXTUAL REPRESENTATION
A textual representation of capability sets consists of one or more
whitespace-separated clauses. Each clause specifies some operations on
a capability set; the set starts out with all capabilities lowered, and
the meaning of the string is the state of the capability set after all
the clauses have been applied in order.
Each clause consists of a list of comma-separated capability names (or
the word `all'), followed by an action-list. An action-list consists
of a sequence of operator flag pairs. Legal operators are: `=', '+',
and `-'. Legal flags are: `e', `i', and `p'. These flags are case-
sensitive and specify the Effective, Inheritable and Permitted sets
respectively.
In the capability name lists, all names are case-insensitive. The
special name `all' specifies all capabilities; it is equivalent to a
list naming every capability individually.
Unnamed capabilities can also be specified by number. This feature
ensures that libcap can support capabilities that were not allocated at
the time libcap was compiled. However, generally upgrading libcap will
add names for recently allocated capabilities.
The `=' operator indicates that the listed capabilities are first reset
in all three capability sets. The subsequent flags (which are optional
when associated with this operator) indicate that the listed
capabilities for the corresponding set are to be raised. For example:
"all=p" means lower every capability in the Effective and Inheritable
sets but raise all of the Permitted capabilities; or, "cap_fowner=ep"
means raise the Effective and Permitted override-file-ownership
capability, while lowering this Inheritable capability.
In the case that the leading operator is `=', and no list of
capabilities is provided, the action-list is assumed to refer to `all'
capabilities. For example, the following three clauses are equivalent
to each other (and indicate a completely empty capability set): "all=";
"="; "cap_chown,<every-other-capability>=".
The operators, `+' and `-' both require an explicit preceding
capability list and one or more explicit trailing flags. The `+'
operator will raise all of the listed capabilities in the flagged
capability sets. The `-' operator will lower all of the listed
capabilities in the flagged capability sets. For example: "all+p" will
raise all of the Permitted capabilities; "cap_fowner+p-i" will raise
the override-file-ownership capability in the Permitted capability set
and lower this Inheritable capability; "cap_fowner+pe-i" and
"cap_fowner=+pe" are equivalent.
RETURN VALUE
cap_from_text(), cap_to_text() and cap_to_name() return a non-NULL
value on success, and NULL on failure. cap_from_name() returns 0 for
success, and -1 on failure (unknown capability).
On failure, errno is set to EINVAL, or ENOMEM.
CONFORMING TO
cap_from_text() and cap_to_text() are specified by the withdrawn
POSIX.1e draft specification. cap_from_name() and cap_to_name() are
Linux extensions.
EXAMPLE
The example program below demonstrates the use of cap_from_text() and
cap_to_text(). The following shell session shows a some example runs:
$ ./a.out "cap_chown=p cap_chown+e"
caps_to_text() returned "= cap_chown+ep"
$ ./a.out "all=pe cap_chown-e cap_kill-pe"
caps_to_text() returned "=ep cap_chown-e cap_kill-ep"
The source code of the program is as follows:
#include <stdlib.h>
#include <stdio.h>
#include <sys/capability.h>
#define handle_error(msg) \
do { perror(msg); exit(EXIT_FAILURE); } while (0)
int
main(int argc, char *argv[])
{
cap_t caps;
char *txt_caps;
if (argc != 2) {
fprintf(stderr, "%s <textual-cap-set>\n", argv[0]);
exit(EXIT_FAILURE);
}
caps = cap_from_text(argv[1]);
if (caps == NULL)
handle_error("cap_from_text");
txt_caps = cap_to_text(caps, NULL);
if (txt_caps == NULL)
handle_error("cap_to_text");
printf("caps_to_text() returned \"%s\"\n", txt_caps);
if (cap_free(txt_caps) != 0 || cap_free(caps) != 0)
handle_error("cap_free");
exit(EXIT_SUCCESS);
}
SEE ALSO
libcap(3), cap_clear(3), cap_compare(3), cap_copy_ext(3),
cap_get_file(3), cap_get_proc(3), cap_init(3), capabilities(7)
2008-05-10 CAP_FROM_TEXT(3)