Using actions in your applications

Now that you’ve learned the basic KAuth concepts and how to register a set of actions into the system, it’s time to see how to actually use KAuth actions inside your application.

Implementing an action helper

From the code point of view, the helper is implemented as a QObject subclass. Every action is implemented by a public slot. In the example/ directory in the source code tree you find a complete example. Let’s look at that. The helper.h file declares the class that implements the helper. It looks like:

#include <KAuth>

using namespace KAuth;

class MyHelper : public QObject
{
    Q_OBJECT
    public Q_SLOTS:
        ActionReply read(const QVariantMap& args);
        ActionReply write(const QVariantMap& args);
        ActionReply longaction(const QVariantMap& args);
};

The slot names are the last part of the action name, without the helper’s ID if it’s a prefix, with all the dots replaced by underscores. In this case, the helper ID is “org.kde.kf5auth.example”, so those three slots implement the actions “org.kde.kf5auth.example.read”, “org.kde.kf5auth.example.write” and “org.kde.kf5auth.example.longaction”. The helper ID doesn’t have to appear at the beginning of the action name, but it’s good practice. If you want to extend MyHelper to implement also a different action like “org.kde.datetime.changetime”, since the helper ID doesn’t match you’ll have to implement a slot called org_kde_datetime_changetime().

The slot’s signature is fixed: the return type is ActionReply, a class that allows you to return results, error codes and custom data to the application when your action has finished to run.

Let’s look at the read action implementation. Its purpose is to read files:

ActionReply MyHelper::read(const QVariantMap& args)
{
    ActionReply reply;
    QString filename = args["filename"].toString();
    QFile file(filename);
    if (!file.open(QIODevice::ReadOnly)) {
       reply = ActionReply::HelperErrorReply();
       reply.setErrorDescription(file.errorString());
       return reply;
    }
    QTextStream stream(&file);
    QString contents;
    stream >> contents;
    reply.addData("contents", contents);
    return reply;
}

First, the code creates a default reply object. The default constructor creates a reply that reports success. Then it gets the filename parameter from the argument QVariantMap, that has previously been set by the application, before calling the helper. If it fails to open the file, it creates an ActionReply object that notifies that some error has happened in the helper, then set the error code to that returned by QFile and returns. If there is no error, it reads the file. The contents are added to the reply.

Because this class will be compiled into a standalone executable, we need a main() function and some code to initialize everything: you don’t have to write it. Instead, you use the KAUTH_HELPER_MAIN() macro that will take care of everything. It’s used like this:

KAUTH_HELPER_MAIN("org.kde.kf5auth.example", MyHelper)

The first parameter is the string containing the helper identifier. Please note that you need to use this same string in the application’s code to tell the library which helper to call, so please stay away from typos, because we don’t have any way to detect them. The second parameter is the name of the helper’s class. Your helper, if complex, can be composed of a lot of source files, but the important thing is to include this macro in at least one of them.

To build the helper, KDE macros provide a function named kauth_install_helper_files(). Use it in your cmake file like this:

add_executable(<helper_target> your sources...)
target_link_libraries(<helper_target> your libraries...)
install(TARGETS <helper_target> DESTINATION ${KAUTH_HELPER_INSTALL_DIR})

kauth_install_helper_files(<helper_target> <helper_id> <user>)

As locale is not inherited, the auth helper will have the text codec explicitly set to use UTF-8.

The first argument is the cmake target name for the helper executable, which you have to build and install separately. Make sure to INSTALL THE HELPER IN ${KAUTH_HELPER_INSTALL_DIR}, otherwise kauth_install_helper_files will not work. The second argument is the helper id. Please be sure to don’t misspell it, and to not quote it. The user parameter is the user that the helper has to be run as. It usually is root, but some actions could require less strict permissions, so you should use the right user where possible (for example the user apache if you have to mess with apache settings). Note that the target created by this macro already links to libkauth and QtCore.

Action registration

To be able to authorize the actions, they have to be added to the policy database. To do this in a cross-platform way, we provide a cmake macro. It looks like:

kauth_install_actions(<helper_id> <actions definition file>)

The action definition file describes which actions are implemented by your code and which default security options they should have. It is a common text file in ini format, with one section for each action and some parameters. The definition for the read action is:

[org.kde.kf5auth.example.read]
Name=Read action
Description=Read action description
Policy=auth_admin
Persistence=session

The name parameter is a text describing the action for who reads the file. The description parameter is the message shown to the user in the authentication dialog. It should be a finite phrase. The policy attribute specify the default rule that the user must satisfy to be authorized. Possible values are:

  • yes: the action should be always allowed
  • no: the action should be always denied
  • auth_self: the user should authenticate as itself
  • auth_admin: the user should authenticate as an administrator user

The persistence attribute is optional. It says how long an authorization should be retained for that action. The values could be:

  • session: the authorization persists until the user logs-out
  • always: the authorization will persist indefinitely

If this attribute is missing, the authorization will be queried every time.

Calling the helper from the application

Once the helper is ready, we need to call it from the main application. In examples/client.cpp you can see how this is done. To create a reference to an action, an object of type Action has to be created. Every Action object refers to an action by its action id. Two objects with the same action id will act on the same action. With an Action object, you can authorize and execute the action. To execute an action you need to retrieve an ExecuteJob, which is a standard KJob that you can run synchronously or asynchronously. See the KJob documentation (from KCoreAddons) for more details.

The piece of code that calls the action of the previous example is:

    QVariantMap args;
    args["filename"] = filename;
    Action readAction("org.kde.kf5auth.example.read");
    readAction.setHelperId("org.kde.kf5auth.example");
    readAction.setArguments(args);
    ExecuteJob *job = readAction.execute();
    if (!job->exec()) {
       qDebug() << "KAuth returned an error code:" << job->error();
    } else {
       QString contents = job->data()["contents"].toString();
    }

First of all, it creates the action object specifying the action id. Then it loads the filename (we want to read a forbidden file) into the arguments() QVariantMap, which will be directly passed to the helper in the read() slot’s parameter. This example code uses a synchronous call to execute the action and retrieve the reply. If the reply succeeded, the reply data is retrieved from the returned QVariantMap object. Please note that you have to explicitly set the helper ID to the action: this is done for added safety, to prevent the caller from accidentally invoking a helper, and also because KAuth actions may be used without a helper attached (the default).

Please note that if your application is calling the helper multiple times it must do so from the same thread.

Asynchronous calls, data reporting, and action termination

For a more advanced example, we look at the action “org.kde.kf5auth.example.longaction” in the example helper. This is an action that takes a long time to execute, so we need some features:

  • The helper needs to regularly send data to the application, to inform about the execution status.
  • The application needs to be able to stop the action execution if the user stops it or close the application. The example code follows:
ActionReply MyHelper::longaction(const QVariantMap&)
{
    for (int i = 1; i <= 100; i++) {
       if (HelperSupport::isStopped())
          break;
       HelperSupport::progressStep(i);
       QThread::usleep(250000);
    }
    return ActionReply::SuccessReply();
}

In this example, the action is only waiting a “long” time using a loop, but we can see some interesting line. The progress status is sent to the application using the HelperSupport::progressStep() method. When this method is called, the HelperProxy associated with this action will emit the progressStep() signal, reporting back the data to the application. There are two overloads of these methods and corresponding signals. The one used here takes an integer. Its meaning is application dependent, so you can use it as a sort of percentage. The other overload takes a QVariantMap object that is directly passed to the app. In this way, you can report to the application all the custom data you want.

In this example code, the loop exits when the HelperSupport::isStopped() returns true. This happens when the application calls the HelperProxy::stopAction() method on the correponding action object. The stopAction() method, this way, asks the helper to stop the action execution. It’s up to the helper to obbey to this request, and if it does so, it should return from the slot, not exit.

Other features

It doesn’t happen very frequently that you code something that doesn’t require some debugging, and you’ll need some tool, even a basic one, to debug your helper code as well. For this reason, the KDE Authorization library provides a message handler for the Qt debugging system. This means that every call to qDebug() & co. will be reported to the application, and printed using the same qt debugging system, with the same debug level. If, in the helper code, you write something like:

qDebug() << "I'm in the helper";

You’ll see something like this in the application’s output:

Debug message from the helper: I'm in the helper

Remember that the debug level is preserved, so if you use qFatal() you won’t only abort the helper (which isn’t suggested anyway), but also the application.

These code examples are licencied under the LGPL-2.1-or-later licence.