xcb: prevent crash with pixmap cursors on XRender-less X servers

We were using xcb_render_* APIs without checking if the server even
supports this extension. Attempting to use an extension which is
not present will always result in a crash. This patch adds the required
guards and refactors how we detect presence of XRender extension.

Also instead of falling back to some odd-looking bitmapped version
just leave the current cursor unchanged. That is how we did it in
Qt4 AFAICT.

Task-number: QTBUG-66935
Change-Id: I4f27f1d65a77563ec34f3e0e94492c9236d7f9a6
Reviewed-by: Laszlo Agocs <laszlo.agocs@qt.io>

3 files changed, 28 insertions, 8 deletions

diff --git a/src/plugins/platforms/xcb/qxcbconnection.cpp b/src/plugins/platforms/xcb/qxcbconnection.cpp
index 2431f99ab8..444e3a7669 100644
--- a/src/plugins/platforms/xcb/qxcbconnection.cpp
+++ b/src/plugins/platforms/xcb/qxcbconnection.cpp
@@ -2136,17 +2136,22 @@ void QXcbConnection::initializeXRender()
 {
 #if QT_CONFIG(xcb_render)
     const xcb_query_extension_reply_t *reply = xcb_get_extension_data(m_connection, &xcb_render_id);
-    if (!reply || !reply->present)
+    if (!reply || !reply->present) {
+        qCDebug(lcQpaXcb, "XRender extension not present on the X server");
         return;
+    }
 
     auto xrender_query = Q_XCB_REPLY(xcb_render_query_version, m_connection,
                                      XCB_RENDER_MAJOR_VERSION,
                                      XCB_RENDER_MINOR_VERSION);
-    if (!xrender_query || (xrender_query->major_version == 0 && xrender_query->minor_version < 5)) {
-        qWarning("QXcbConnection: Failed to initialize XRender");
+    if (!xrender_query) {
+        qCWarning(lcQpaXcb, "xcb_render_query_version failed");
         return;
     }
+
     has_render_extension = true;
+    m_xrenderVersion.first = xrender_query->major_version;
+    m_xrenderVersion.second = xrender_query->minor_version;
 #endif
 }
 
diff --git a/src/plugins/platforms/xcb/qxcbconnection.h b/src/plugins/platforms/xcb/qxcbconnection.h
index ced5208c81..d9321d94d0 100644
--- a/src/plugins/platforms/xcb/qxcbconnection.h
+++ b/src/plugins/platforms/xcb/qxcbconnection.h
@@ -478,7 +478,13 @@ public:
     bool hasXRandr() const { return has_randr_extension; }
     bool hasInputShape() const { return has_input_shape; }
     bool hasXKB() const { return has_xkb; }
-    bool hasXRender() const { return has_render_extension; }
+    bool hasXRender(int major = -1, int minor = -1) const
+    {
+        if (has_render_extension && major != -1 && minor != -1)
+            return m_xrenderVersion >= qMakePair(major, minor);
+
+        return has_render_extension;
+    }
     bool hasXInput2() const { return m_xi2Enabled; }
     bool hasShm() const { return has_shm; }
     bool hasShmFd() const { return has_shm_fd; }
@@ -707,6 +713,8 @@ private:
     bool has_shm = false;
     bool has_shm_fd = false;
 
+    QPair<int, int> m_xrenderVersion;
+
     Qt::MouseButtons m_buttonState = 0;
     Qt::MouseButton m_button = Qt::NoButton;
 
diff --git a/src/plugins/platforms/xcb/qxcbcursor.cpp b/src/plugins/platforms/xcb/qxcbcursor.cpp
index 70138abede..8d151b760b 100644
--- a/src/plugins/platforms/xcb/qxcbcursor.cpp
+++ b/src/plugins/platforms/xcb/qxcbcursor.cpp
@@ -601,12 +601,19 @@ xcb_cursor_t QXcbCursor::createFontCursor(int cshape)
 
 xcb_cursor_t QXcbCursor::createBitmapCursor(QCursor *cursor)
 {
-    xcb_connection_t *conn = xcb_connection();
     QPoint spot = cursor->hotSpot();
     xcb_cursor_t c = XCB_NONE;
-    if (cursor->pixmap().depth() > 1)
-        c = qt_xcb_createCursorXRender(m_screen, cursor->pixmap().toImage(), spot);
-    if (!c) {
+    if (cursor->pixmap().depth() > 1) {
+#if QT_CONFIG(xcb_render)
+        if (connection()->hasXRender(0, 5))
+            c = qt_xcb_createCursorXRender(m_screen, cursor->pixmap().toImage(), spot);
+        else
+            qCWarning(lcQpaXcb, "xrender >= 0.5 required to create pixmap cursors");
+#else
+        qCWarning(lcQpaXcb, "This build of Qt does not support pixmap cursors");
+#endif
+    } else {
+        xcb_connection_t *conn = xcb_connection();
         xcb_pixmap_t cp = qt_xcb_XPixmapFromBitmap(m_screen, cursor->bitmap()->toImage());
         xcb_pixmap_t mp = qt_xcb_XPixmapFromBitmap(m_screen, cursor->mask()->toImage());
         c = xcb_generate_id(conn);