Xlib contract for XEvent up/down-casting

[Po Lu]

Jeremy Huddleston Sequoia <jeremyhu at apple.com> writes:

> I've been running XQuartz with ASan+UBSan to try to catch some issues
> some users have reported, and I stumbled across something below GLUT
> (specifically, freeglut 2.8.1), which does:
>
>     XConfigureEvent fakeEvent = {0};
>     ...
>     XPutBackEvent(fgDisplay.Display, (XEvent*)&fakeEvent);
>
> and XPutBackEvent eventually does:
>
>     XEvent store = *event;
>
> which overflows the stack on read because:
>
>     sizeof(XConfigureEvent) == 88
>     sizeof(XEvent) == 192
>
> So the problem is clear, but I'm not sure which side needs to change.
>
> What is the contract for Xlib's APIs that take XEvent *?  Is Xlib
> expected to handle any XEvent "subtype", or does it need to be exactly
> an XEvent (ie: is it the client's responsibility to pad it)?

It needs to be an XEvent, since the event ends up back on the event
queue.  The client is supposed to pad it.