label: Introduce LabelOps to do pre/post labelling operations

By default, label_ops is initialized with a NULL pointer which translates
to noop labelling operations. In mac_selinux_init() and the new mac_smack_init(),
we initialize label_ops with a MAC specific LabelOps pointer.

We also introduce mac_init() to initialize any configured MACs and replace all
usages of mac_selinux_init() with mac_init().

src/basic/label.c

@@ -0,0 +1,30 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+
+#include <errno.h>
+#include <stddef.h>
+
+#include "label.h"
+
+static const LabelOps *label_ops = NULL;
+
+int label_ops_set(const LabelOps *ops) {
+        if (label_ops)
+                return -EBUSY;
+
+        label_ops = ops;
+        return 0;
+}
+
+int label_ops_pre(int dir_fd, const char *path, mode_t mode) {
+        if (!label_ops || !label_ops->pre)
+                return 0;
+
+        return label_ops->pre(dir_fd, path, mode);
+}
+
+int label_ops_post(int dir_fd, const char *path) {
+        if (!label_ops || !label_ops->post)
+                return 0;
+
+        return label_ops->post(dir_fd, path);
+}

src/basic/label.h

@@ -0,0 +1,14 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+#pragma once
+
+#include <sys/types.h>
+
+typedef struct LabelOps {
+        int (*pre)(int dir_fd, const char *path, mode_t mode);
+        int (*post)(int dir_fd, const char *path);
+} LabelOps;
+
+int label_ops_set(const LabelOps *label_ops);
+
+int label_ops_pre(int dir_fd, const char *path, mode_t mode);
+int label_ops_post(int dir_fd, const char *path);

src/basic/meson.build

@@ -44,6 +44,7 @@ basic_sources = files(
         'inotify-util.c',
         'io-util.c',
         'ioprio-util.c',
+        'label.c',
         'limits-util.c',
         'locale-util.c',
         'lock-util.c',