Q*Application: fix UB caused by accessing QGuiApplication from QCoreApplication ctor

As reported by ubsan:

  src/gui/kernel/qplatformintegration.cpp:463:10: runtime error: downcast of address 0x7ffdc2942490 which does not point to an object of type 'QGuiApplication'
  0x7ffdc2942490: note: object is of type 'QCoreApplication'

  src/gui/kernel/qplatformintegration.cpp:466:14: runtime error: downcast of address 0x7ffdc2942490 which does not point to an object of type 'QGuiApplication'
  0x7ffdc2942490: note: object is of type 'QCoreApplication'

  src/gui/kernel/qplatformintegration.cpp:466:43: runtime error: member call on address 0x7ffdc2942490 which does not point to an object of type 'QGuiApplication'
  0x7ffdc2942490: note: object is of type 'QCoreApplication'

to name just a few which are reported when running gui and widget
auto-tests; there're definitely more where these came from.

This is caused by QCoreApplication::init() being called from the
QCoreApplication ctor, calling virtual functions on Q*AppPrivate,
which happen to attempt, in this case, to emit QGuiApp signals.
At that point in time, the QGuiApplication ctor has not entered
the constructor body, ergo the object is still a QCoreApplication,
and calling the signal, as a member function on the derived class,
invokes UB.

Fix by cleaning up the wild mix of initialization functions used in
this hierarchy. The cleanup restores the

  1. Q*ApplicationPrivate::Q*ApplicationPrivate()
  2. Q*ApplicationPrivate::init(), calling each base class'
     init() as the first thing

two-stage construction pattern commonly used elsewhere in Qt to make
sure that the public class' object is fully constructed by the time
each level's Private::init() is called.

Change-Id: I290402b3232315d7ed687c97e740bfbdbd3ecd1a
Reviewed-by: Lars Knoll <lars.knoll@theqtcompany.com>

2 files changed, 8 insertions, 3 deletions

diff --git a/src/widgets/kernel/qapplication.cpp b/src/widgets/kernel/qapplication.cpp
index f7d4139ed8..b7de0d7a7e 100644
--- a/src/widgets/kernel/qapplication.cpp
+++ b/src/widgets/kernel/qapplication.cpp
@@ -562,13 +562,18 @@ QApplication::QApplication(int &argc, char **argv)
 QApplication::QApplication(int &argc, char **argv, int _internal)
 #endif
     : QGuiApplication(*new QApplicationPrivate(argc, argv, _internal))
-{ Q_D(QApplication); d->construct(); }
+{
+    Q_D(QApplication);
+    d->init();
+}
 
 /*!
     \internal
 */
-void QApplicationPrivate::construct()
+void QApplicationPrivate::init()
 {
+    QGuiApplicationPrivate::init();
+
     initResources();
 
     qt_is_gui_used = (application_type != QApplicationPrivate::Tty);
diff --git a/src/widgets/kernel/qapplication_p.h b/src/widgets/kernel/qapplication_p.h
index cb158011f0..832d37a329 100644
--- a/src/widgets/kernel/qapplication_p.h
+++ b/src/widgets/kernel/qapplication_p.h
@@ -150,7 +150,7 @@ public:
 
     bool notify_helper(QObject *receiver, QEvent * e);
 
-    void construct(
+    void init(
 #ifdef Q_DEAD_CODE_FROM_QT4_X11
                    Display *dpy = 0, Qt::HANDLE visual = 0, Qt::HANDLE cmap = 0
 #endif