Large coordinates in the X server

[Robert Morell]

On Fri, Nov 14, 2014 at 08:29:11AM -0800, Robert Morell wrote:
> Hi,
> 
> I have been experimenting with root window sizes that meet the coordinate space
> limits of the X11 protocol -- 32767 for both width and height.  Unsurprisingly,
> this doesn't work as well as one might hope, even for very simple tests.  One
> example is 'xlogo'.  With a 32767x40 root window, running:
> `xlogo -geometry 40x40+32726+0`
> works as you'd expect.  But running:
> `xlogo -geometry 40x40+32727+0`
> appears corrupt.
> 
> It looks like what's happening is that xlogo [1] is making a bunch of
> FillPolygon requests, and some of the coordinates extend beyond the window's
> clip region.  Normally this is fine, and the polygon is just partially clipped
> away.  miFillPolygon [2] will translate the points by the window's origin, but
> in this case, the translated point overflows DDXPoint's 'INT16' type, and the
> polygon ends up getting rendered from the far negative end of the coordinate
> space rather than the far positive end.
> 
> I'm sure that this is only the tip of the iceberg.  There are a few different
> ways that I can think of to deal with this:
> a) Carefully check for overflow everywhere that translation (or other
>    coordinate manipulation) is performed
> b) Change the X server's DDXPointRec type from 2xINT16 to 2xINT32; Convert
>    16-bit values from the protocol to 32-bit values as soon as we get them from
>    the wire, and do most internal processing of coordinates with 32-bit numbers
> c) Assert that this is a bug in the application and don't fix it; instead,
>    (attempt to) fix applications as necessary
> 
> Of those options, it seems to me that:
> a) is very difficult to get right everywhere, and would introduce a lot more
>    processing overhead than is necessary
> b) is an unfortunately intrusive change, but hopefully would "just work" once
>    everything is updated
> c) isn't really an option; besides being too much code to realistically change,
>    dragging a window off the end of the root window could still result in
>    translated coordinates which legitimately extend beyond the end of the
>    coordinate system.
> 
> So my vote would be on b).  Does anyone have any objections to this, or better
> suggestions?

Since this has been met with crickets, I'm going to forge ahead with
option b) for now.  If you have any objections to this, please bring
them up sooner rather than later.

- Robert

> [1] xlogo calls Xmu to actually draw the logo:
>     http://cgit.freedesktop.org/xorg/lib/libXmu/tree/src/DrawLogo.c
> [2] http://cgit.freedesktop.org/xorg/xserver/tree/mi/mipoly.c
> 
> 
> Thanks,
> Robert
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