check the return values of the grab functions correctly

CHANGELOG

@@ -4,3 +4,5 @@ Release 0.1:
        Paul Lhussiez.
 Development Version:
 • Enhancement: Report missing libraries when loading via ctypes
+• Security: Check correctly for the result of the
+  xcb_grab_{pointer,keyboard} commands

lib/xcb.py

@@ -412,8 +412,8 @@ def grab_keyboard_sync(conn, owner_events, grab_window, time, ptr_mode,
     """
     Synchronously grab the keyboard.
 
-    Wrapper function for grab_pointer and grab_pointer_reply.
-    Raises ``XCBError`` on error, otherwise returns ``GrabKeyboardReply``.
+    Wrapper function for grab_pointer and grab_pointer_reply. Returns
+    the status field from the reply. Raises ``XCBError`` on error.
     """
     owner_events = 1 if owner_events else 0
 
@@ -424,7 +424,9 @@ def grab_keyboard_sync(conn, owner_events, grab_window, time, ptr_mode,
 
     if error_p:
         raise XCBError(error_p.contents)
-    return kbd_grab
+    status = kbd_grab.contents.status
+    free(kbd_grab)
+    return status
 
 
 grab_pointer = libxcb.xcb_grab_pointer
@@ -449,6 +451,12 @@ def grab_keyboard_sync(conn, owner_events, grab_window, time, ptr_mode,
 ]
 grab_pointer_reply.restype = POINTER(GrabPointerReply)
 
+# constants to interpret grab results
+GrabSuccess = 0
+AlreadyGrabbed = 1
+GrabInvalidTime = 2
+GrabNotViewable = 3
+GrabFrozen = 4
 
 def grab_pointer_sync(conn, owner_events, window, event_mask, ptr_mode,
                       kbd_mode, confine_to, cursor, timestamp):
@@ -466,7 +474,9 @@ def grab_pointer_sync(conn, owner_events, window, event_mask, ptr_mode,
     ptr_grab = grab_pointer_reply(conn, cookie, byref(error_p))
     if error_p:
         raise XCBError(error_p.contents)
-    return ptr_grab
+    status = ptr_grab.contents.status
+    free(ptr_grab)
+    return status
 
 wait_for_event_ = libxcb.xcb_wait_for_event
 wait_for_event_.argtypes = [POINTER(Connection)]

pyxtrlock

@@ -117,9 +117,11 @@ xcb.map_window(conn, window)
 
 # Grab keyboard
 try:
-    kbd_grab = xcb.grab_keyboard_sync(conn, 0, window, xcb.CURRENT_TIME,
-                                      xcb.GRAB_MODE_ASYNC, xcb.GRAB_MODE_ASYNC)
-    xcb.free(kbd_grab)
+    status = xcb.grab_keyboard_sync(conn, 0, window, xcb.CURRENT_TIME,
+                                    xcb.GRAB_MODE_ASYNC, xcb.GRAB_MODE_ASYNC)
+
+    if status != xcb.GrabSuccess:
+        panic("pyxtrlock: Could not get grab keyboard")
 except xcb.XCBError as e:
     print("pyxtrlock: Could not get grab keyboard", file=sys.stderr)
     sys.exit(1)
@@ -133,13 +135,16 @@ except xcb.XCBError as e:
 #  (i.e. after 1s in total), then give up, and emit an error"
 for i in range(100):
     try:
-        ptr_grab = xcb.grab_pointer_sync(conn, False, window, 0,
-                                         xcb.GRAB_MODE_ASYNC,
-                                         xcb.GRAB_MODE_ASYNC,
-                                         xcb.WINDOW_NONE, cursor,
-                                         xcb.CURRENT_TIME)
-        xcb.free(ptr_grab)
-        break
+        status = xcb.grab_pointer_sync(conn, False, window, 0,
+                                       xcb.GRAB_MODE_ASYNC,
+                                       xcb.GRAB_MODE_ASYNC,
+                                       xcb.WINDOW_NONE, cursor,
+                                       xcb.CURRENT_TIME)
+
+        if status == xcb.GrabSuccess:
+            break
+        else:
+            time.sleep(0.01)
     except xcb.XCBError as e:
         time.sleep(0.01)
 else: