Dynamic Menu Builder Abstractions to achieve greatness...

Hello folks,

While on the plane back home I decided to write another little blog post. This time I will be showing you a nice class I came up with for x64dbg to manage menu items.

The problem

As with every abstraction it starts with a problem you are trying to solve. In this case the problem was code duplication and general tediousness with the construction of the context (right click) menus in x64dbg.

The general idea of Qt is great. From my understanding, every context menu is a QMenu with a bunch of QAction or other QMenu items in it. When a user right-clicks in the widget a signal will be emitted and the widget can connect to the signal, construct the QMenu and ‘execute’ the menu on the mouse position. Each QAction has a signal called triggered() which you can connect to a slot in your widget to handle the click event.

If there is no variation in the menu everything works perfectly fine. You just create all the actions, menus and connections in the constructor and store the final QMenu item in the class. Then when you need the menu you do mMenu->exec(position) and you are done with it.

In x64dbg the menus are based on the context, so the static approach doesn’t work. What we did was create and connect all the QAction items in the constructor and then dynamically create the menu. What this did was create a lot of fragmentation. All the actions had to be declared in the header, the slots for the actions had to be declared in the header and the source and adding a new action would result in a lot of code that looked exactly like this:

mFollowAddress = new QAction("&Follow in Disassembler", this);
connect(mFollowAddress, SIGNAL(triggered()), this, SLOT(followAddress()));

For actions with a shortcut and an icon it was even worse:

mToggleBreakpoint = new QAction("Toggle Breakpoint", this);
mToggleBreakpoint->setShortcutContext(Qt::WidgetShortcut);
mToggleBreakpoint->setIcon(QIcon(":/images/icons/breakpoint.png"));
addAction(mToggleBreakpoint);
connect(mToggleBreakpoint, SIGNAL(triggered()), this, SLOT(toggleBreakpoint()));

Shortcuts also require setting the actual shortcut in a dedicated slot called refreshShortcutsSlot() which is connected to the settings dialog so shortcuts are updated when the user updates the settings:

void ReferenceView::refreshShortcutsSlot()
{
    mToggleBreakpoint->setShortcut(ConfigShortcut("ActionToggleBreakpoint"));
    mToggleBookmark->setShortcut(ConfigShortcut("ActionToggleBookmark"));
}

Finally the menu is created in contextMenuEvent like this:

if(!DbgMemIsValidReadPtr(addr))
        return;
wMenu->addAction(mFollowAddress);
wMenu->addAction(mFollowDumpAddress);
if(apiAddressFromString(mCurList->getCellContent(mCurList->getInitialSelection(), 1)))
    wMenu->addAction(mFollowApiAddress);
wMenu->addSeparator();
wMenu->addAction(mToggleBreakpoint);
wMenu->addAction(mToggleBookmark);

As you can imagine, adding an action with an icon, a shortcut and some context-dependent behaviour was a very tedious process and this needed to change.

MenuBuilder

Part of the solution is a MenuBuilder class. This is a recursive datatype (like QMenu) but it lazily builds the menu, which allows for proper context-awareness.

To achieve context-awareness, each QAction/QMenu/MenuBuilder you add to a MenuBuilder is paired with an std::function. If the callback returns true, the item is added to the final QMenu, otherwise it is ommitted. This allows for constructs like this:

mBuilder->addAction(followAction, [this](QMenu* menu)
{ //only add followAction if the selected address is readable.
    return DbgMemIsValidReadPtr(this->selectedAddress());
});

The followAction will only be added to the final QMenu if the currently selected address is a valid memory address. This is a huge save in code, the menu creation slot can be replaced with:

QMenu menu;
mBuilder->build(&menu);
menu.exec(pos);

There are some extra features (like using the menu parameter of the lambda to add extra actions to the final QMenu, but if you want more details, read the code here.

Actions

The next problem to solve is the creation of the QAction and QMenu items. The solution was to create a few simple helper methods in the base class (AbstractTableView):

template<typename T>
inline QAction* makeAction(const QString & text, T slot)
{
    return connectAction(new QAction(text, this), slot);
}

inline QAction* connectAction(QAction* action, const char* slot)
{
    connect(action, SIGNAL(triggered(bool)), this, slot);
    return action;
}

inline QAction* connectAction(QAction* action, QActionLambda::TriggerCallback callback)
{
    auto lambda = new QActionLambda(action->parent(), callback);
    connect(action, SIGNAL(triggered(bool)), lambda, SLOT(triggeredSlot()));
    return action;
}

The makeAction uses a template because I added lambda support to the actions. This is not in Qt 4 and rather simple to implemented so I decided to add it:

class QActionLambda : public QObject
{
    Q_OBJECT
public:
    typedef std::function<void()> TriggerCallback;

    QActionLambda(QObject* parent, TriggerCallback callback)
        : QObject(parent),
          _callback(callback)
    {
    }

public slots:
    void triggeredSlot()
    {
        if(_callback)
            _callback();
    }

private:
    TriggerCallback _callback;
};

Now to create an action you’d write:

makeAction("Selection (&No Bytes)", SLOT(copySelectionNoBytesSlot()))

And similarly an action with shortcut and icon:

makeShortcutAction(QIcon(":/icons/images/highlight.png"), "&Highlighting mode", SLOT(enableHighlightingModeSlot()), "ActionHighlightingMode")

Final words

I guess that’s about it for this blog post. If you want to see what the final menu creation code looks like, check out the code here. For reference, the old code is available here, as you can tell it is a great improvement.

Finally, I know reader interaction has been practically non-existent on this blog so far, however it would be nice if you could send me parts of x64dbg you’d like to get insight in development-wise. Any other topics (reversing/programming) related are also welcome!

Greetings,

Duncan