[PATCH xinit 2/2] startx: Make startx auto display select work with per user /tmp dirs

[Hans de Goede]

Hi,

On 25-03-15 22:45, Julien Cristau wrote:
> On Wed, Mar 25, 2015 at 09:15:12 -0400, Ray Strode wrote:
>
>> Hi,
>>
>>> Now, I've tried to avoid anything xauth-related, but from the little I know:
>>> to support displayfd in startx you'd have to communicate back to startx
>>> about the $DISPLAY and do the xauth dance before continuing with the xinit
>>> initial client connection. AFAICT, that's the tricky bit about -displayfd
>>> support in startx. Does that make sense or am I way off here?
>> Sending $DISPLAY back to startx isn't actually an option since, $DISPLAY comes
>> from the X server, and the auth file has to be prepared before
>> starting the X server.
>> If you start the X server without the auth file then the X server will
>> get started wide
>> open to anyone on the host. Sure you could lock it down at that point,
>> but then there's
>> a race where anyone could open the display and snoop from then on.
>>
> I ran into this when trying to make our xvfb-run wrapper use
> -displayfd, would be nice if we can fix this.  Maybe by making the
> server consider empty -auth file as "nobody allowed" instead of
> "everybody allowed"?

As discussed already the proper fix here is to teach xinit to write
out the xauth file, and then have xinit pass both
-displayfd and -xauth <xinit-generated-file> to the server.

See example the gdm code for launching the server for how to do this.

Regards,

Hans