QGestureManager: fix UB in filterEvent()

The code infers from the presence of an address in a
QHash<QGesture *, ...> that the address belongs to a
QGesture. So far that is fine enough.

But in order to perform the lookup, it static_cast<>s
the QObject* argument to a QGesture* for the QHash::
contains() call. Even though the pointer is not
dereferenced, the cast is UB. Says UBSan:

  qgesturemanager.cpp:558:73: runtime error: downcast of address 0x2ab83364f3a0 which does not point to an object of type 'QGesture'
  0x2ab83364f3a0: note: object is of type 'QDBusConnectionManager'

which is a particularly hideous error message because
of the constantly-changing completely-unrelated actual
type in the second line of the message:

     52 QDBusConnectionManager
     19 QSocketNotifier
     14 QFusionStyle
     13 QAction
      6 QApplication
      3 QGraphicsWidget
      1 Window
      1 TestRunnable
      1 RectWidget
      1 QTimer
      1 QSingleShotTimer
      1 QOffscreenSurface
      1 QGraphicsProxyWidget
      1 QDefaultAnimationDriver
      1 QDBusPendingCallWatcherHelper

This error is also _very_ common, triggered 116 times
in a single run of make -C tests/auto check.

Fix by using qobject_cast first and then doing the
lookup only when the cast succeeded.

Depending on the performance of qobject_cast<>, this
may actually perform better, too.

Change-Id: I884ec7d885711acc3c1d004ce93c628268d8fc18
Reviewed-by: Olivier Goffart (Woboq GmbH) <ogoffart@woboq.com>

1 files changed, 2 insertions, 2 deletions

diff --git a/src/widgets/kernel/qgesturemanager.cpp b/src/widgets/kernel/qgesturemanager.cpp
index fb2914d53a..967ef6b40c 100644
--- a/src/widgets/kernel/qgesturemanager.cpp
+++ b/src/widgets/kernel/qgesturemanager.cpp
@@ -548,9 +548,9 @@ bool QGestureManager::filterEvent(QObject *receiver, QEvent *event)
     if (widgetWindow)
         return filterEvent(widgetWindow->widget(), event);
 
-    if (!m_gestureToRecognizer.contains(static_cast<QGesture *>(receiver)))
+    QGesture *state = qobject_cast<QGesture *>(receiver);
+    if (!state || !m_gestureToRecognizer.contains(state))
         return false;
-    QGesture *state = static_cast<QGesture *>(receiver);
     QMultiMap<QObject *, Qt::GestureType> contexts;
     contexts.insert(state, state->gestureType());
     return filterEventThroughContexts(contexts, event);