plugin-loader: Add optional filter for plugin features

In some cases, the algorithms that have been compiled into a plugin have
to be disabled at runtime. Based on the array returned by the get_features()
function the optionally provided function can strip algorithms or even
callbacks or registrations from a plugin, giving us a handy and powerful way
for runtime feature configuration aside from the plugin list.

Signed-off-by: Thomas Egerer <[email protected]>

src/libstrongswan/plugins/plugin_loader.c

@@ -93,6 +93,12 @@ struct private_plugin_loader_t {
 		/** Number of features in critical plugins that failed to load */
 		int critical;
 	} stats;
+
+	/**
+	 * Fetch features from the given plugin, can optionally be overridden to
+	 * modify feature arrays at loading time
+	 */
+	int (*get_features)(plugin_t *plugin, plugin_feature_t *features[]);
 };
 
 /**
@@ -886,6 +892,14 @@ static void load_features(private_plugin_loader_t *this)
 	enumerator->destroy(enumerator);
 }
 
+/**
+ * Default implementation for plugin feature retrieval
+ */
+static int get_features_default(plugin_t *plugin, plugin_feature_t *features[])
+{
+	return plugin->get_features(plugin, features);
+}
+
 /**
  * Register plugin features provided by the given plugin
  */
@@ -904,7 +918,7 @@ static void register_features(private_plugin_loader_t *this,
 		return;
 	}
 	reg = NULL;
-	count = entry->plugin->get_features(entry->plugin, &feature);
+	count = this->get_features(entry->plugin, &feature);
 	for (i = 0; i < count; i++)
 	{
 		switch (feature->kind)
@@ -1449,8 +1463,14 @@ plugin_loader_t *plugin_loader_create()
 		.features = hashlist_create(
 							(hashtable_hash_t)registered_feature_hash,
 							(hashtable_equals_t)registered_feature_equals, 64),
+		.get_features = dlsym(RTLD_DEFAULT, "plugin_loader_feature_filter"),
 	);
 
+	if (!this->get_features)
+	{
+		this->get_features = get_features_default;
+	}
+
 	return &this->public;
 }