apparmor/libraries/libapparmor/doc/aa_features.pod

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=pod

=head1 NAME

aa_features - an opaque object representing a set of AppArmor kernel features

aa_features_new - create a new aa_features object based on a path

aa_features_new_from_string - create a new aa_features object based on a string

aa_features_new_from_kernel - create a new aa_features object based on the current kernel

aa_features_ref - increments the ref count of an aa_features object

aa_features_unref - decrements the ref count and frees the aa_features object when 0

aa_features_write_to_file - write a string representation of an aa_features object to a file

aa_features_is_equal - equality test for two aa_features objects

aa_features_supports - provides aa_features object support status

aa_features_id - provides unique identifier for an aa_features object

aa_features_value - the value associated with a given feature.

=head1 SYNOPSIS

B<#include E<lt>sys/apparmor.hE<gt>>

B<typedef struct aa_features aa_features;>

B<int aa_features_new(aa_features **features, int dirfd, const char *path);>

B<int aa_features_new_from_file(aa_features **features, int fd);>

B<int aa_features_new_from_string(aa_features **features, const char *string, size_t size);>

B<int aa_features_new_from_kernel(aa_features **features);>

B<aa_features *aa_features_ref(aa_features *features);>

B<void aa_features_unref(aa_features *features);>

B<int aa_features_write_to_file(aa_features *features, int dirfd, const char *path);>

B<bool aa_features_is_equal(aa_features *features1, aa_features *features2);>

B<bool aa_features_supports(aa_features *features, const char *str);>

B<char *aa_features_id(aa_features *features);>

B<char *aa_features_value(aa_features *features, const char *str, size_t *len);>

Link with B<-lapparmor> when compiling.

=head1 DESCRIPTION

The I<aa_features> object contains information about the AppArmor features
supported by a kernel. The feature support information is based upon the files
AppArmor represents in securityfs, which is typically found at
/sys/kernel/security/apparmor/features/. That information may be parsed and
turned into a string or flat file in order to represent a set of features of a
kernel that is not currently running.

The aa_features_new() function creates an I<aa_features> object based upon a
directory file descriptor and path. The I<path> can point to a file or
directory. See the openat(2) man page for examples of I<dirfd> and I<path>. The
allocated I<features> object must be freed using aa_features_unref().

The aa_features_new_from_file() function is similar except that it
accepts an open file as the argument. The allocated I<features> object
must be freed using aa_features_unref().

The aa_features_new_from_string() function is similar except that it accepts a
NUL-terminated string representation of the AppArmor features as the I<string>
argument. The length of the features string, not counting the NUL-terminator,
must be specified as the I<size> argument. The allocated I<features> object
must be freed using aa_features_unref().

The aa_features_new_from_kernel() function creates an I<aa_features> object
from the current running kernel. The allocated I<features> object must be freed
using aa_features_unref().

aa_features_ref() increments the reference count on the I<features> object.

aa_features_unref() decrements the reference count on the I<features> object
and releases all corresponding resources when the reference count reaches zero.

The aa_features_write_to_file() function writes a string representation of the
I<features> object to the file specified by the I<dirfd> and I<path>
combination.

aa_features_is_equal() can be used to detect if the I<features1> and
I<features2> objects are equal. The definition of equality is private to
libapparmor and may be changed in ways that do not break backward
compatibility.

The aa_features_supports() function can be used to query the I<features> object
to determine if a feature is supported. The I<str> argument should be equal to
the path, relative to the "apparmor/features/" directory of securityfs, of the
feature to query. For example, to test if policy version 6 is supported, I<str>
would be "policy/versions/v6".

The aa_features_id() function returns a string representation of an
identifier that can be used to uniquely identify an I<aa_features> object.
The mechanism for generating the string representation is internal to
libapparmor and subject to change but an example implementation is
applying a hash function to the features string.

=head1 RETURN VALUE

The aa_features_new() family of functions return 0 on success and I<*features>
will point to an I<aa_features> object that must be freed by
aa_features_unref(). -1 is returned on error, with errno set appropriately, and
I<*features> will be set to NULL.

aa_features_ref() returns the value of I<features>.

aa_features_write_to_file() returns 0 on success. -1 is returned on error, with
errno set appropriately.

aa_features_is_equal() returns true if I<features1> and I<features2> are equal
and false if they are not equal.

aa_features_supports() returns true if the feature represented by I<str> is
supported and false if it is not supported.

aa_features_id() returns a string identifying I<features> which must be
freed by the caller. NULL is returned on error, with errno set
appropriately.

aa_features_value() returns a null terminated string with is
associated length which must be freed by the caller. NULL is returned
on error, with errno set to ENOENT if the feature was not found,
ENODIR if the specified feature does not resolve to a leaf feature.

=head1 ERRORS

The errno value will be set according to the underlying error in the
I<aa_features> family of functions that return -1 or NULL on error.

=head1 NOTES

The aa_features_id() function can be found in libapparmor version
2.13. All the other aa_feature functions described above are present
in libapparmor version 2.10.

aa_features_unref() saves the value of errno when called and restores errno
before exiting in libapparmor version 2.12 and newer.

=head1 BUGS

None known. If you find any, please report them at
L<https://gitlab.com/apparmor/apparmor/-/issues>.

=head1 SEE ALSO

openat(2) and L<https://wiki.apparmor.net>.

=cut