QRegion: fix potential UB in QBasicAtomic initialization

Until C++17 (inclusive), a default-constructed std::atomic object can,
officially, only be initialized with a call to std::atomic_init, for
which QBasicAtomic doesn't have API. It is even unclear whether
zero-initialization of static and thread-local objects will cause the
object to be initialized.

QRegion is using QtPrivate::RefCount, but that's just another wrapper
around QBasicAtomic, so it has the same problems: it must always be
initialized.

So don't default-construct and then storeRelaxed() (via
initializeOwned()), use NSDMI with (newly-added)
Q_REFCOUNT_INITIALIZE_OWNED to avoid this dark language corner.

Task-number: QTBUG-137465
Pick-to: 6.10 6.9 6.8 6.5
Change-Id: I4b765aed329211984c35c40fbc5648bf104990ce
Reviewed-by: Thiago Macieira <[email protected]>

3 files changed, 2 insertions, 4 deletions

diff --git a/src/corelib/tools/qrefcount.h b/src/corelib/tools/qrefcount.h
index 9472716a723..da0942903d0 100644
--- a/src/corelib/tools/qrefcount.h
+++ b/src/corelib/tools/qrefcount.h
@@ -50,6 +50,7 @@ public:
 }
 
 #define Q_REFCOUNT_INITIALIZE_STATIC { Q_BASIC_ATOMIC_INITIALIZER(-1) }
+#define Q_REFCOUNT_INITIALIZE_OWNED { Q_BASIC_ATOMIC_INITIALIZER(1) }
 
 QT_END_NAMESPACE
 
diff --git a/src/gui/painting/qregion.cpp b/src/gui/painting/qregion.cpp
index 0d39ee05437..2b8c86ad84a 100644
--- a/src/gui/painting/qregion.cpp
+++ b/src/gui/painting/qregion.cpp
@@ -3812,7 +3812,6 @@ QRegion::QRegion(const QRect &r, RegionType t)
         d = const_cast<QRegionData*>(&shared_empty);
     } else {
         d = new QRegionData;
-        d->ref.initializeOwned();
         if (t == Rectangle) {
             d->qt_rgn = new QRegionPrivate(r);
         } else if (t == Ellipse) {
@@ -3831,7 +3830,6 @@ QRegion::QRegion(const QPolygon &a, Qt::FillRule fillRule)
                                                fillRule == Qt::WindingFill ? WindingRule : EvenOddRule);
         if (qt_rgn) {
             d =  new QRegionData;
-            d->ref.initializeOwned();
             d->qt_rgn = qt_rgn;
         } else {
             d = const_cast<QRegionData*>(&shared_empty);
@@ -3854,7 +3852,6 @@ QRegion::QRegion(const QBitmap &bm)
         d = const_cast<QRegionData*>(&shared_empty);
     } else {
         d = new QRegionData;
-        d->ref.initializeOwned();
         d->qt_rgn = qt_bitmapToRegion(bm);
     }
 }
diff --git a/src/gui/painting/qregion.h b/src/gui/painting/qregion.h
index 4b852815f32..f7f35a8725b 100644
--- a/src/gui/painting/qregion.h
+++ b/src/gui/painting/qregion.h
@@ -121,7 +121,7 @@ Q_GUI_EXPORT
     void exec(const QByteArray &ba, int ver = 0, QDataStream::ByteOrder byteOrder = QDataStream::BigEndian);
 #endif
     struct QRegionData {
-        QtPrivate::RefCount ref;
+        QtPrivate::RefCount ref = Q_REFCOUNT_INITIALIZE_OWNED;
         QRegionPrivate *qt_rgn;
     };
     struct QRegionData *d;