[Mesa-dev] glXMakeCurrent crashes (was: Re: How to obtain OpenGL implementation/driver information?)

Benoit Jacob bjacob at mozilla.com
Fri Feb 4 14:21:52 PST 2011

----- Original Message -----
> Benoit Jacob wrote:
> > ----- Original Message -----
> >> On Thu, Feb 3, 2011 at 4:37 PM, Benoit Jacob <bjacob at mozilla.com>
> >> wrote:
> >>> I'm trying to see how to implement selective
> >>> whitelisting/blacklisting of driver versions on X11 (my use case
> >>> is
> >>> to whitelist drivers for Firefox). The naive approach consists in
> >>> creating an OpenGL context and calling glGetString(), however that
> >>> is not optimal for me, for these reasons:
> >>>  * This has been enough to trigger crashes in the past.
> >>>
> >>> Ideally I want to be able to know the driver name, driver version,
> >>> Mesa version, and any other thing that you think may be relevant.
> >>> I
> >>> need to get that information in a fast and safe way.
> >>>
> >> There is no other way than glGetString if you ever experienced
> >> crash
> >> with it, it would be because you are doing something terribly wrong
> >> like using it without current context.
> >
> > It's not glGetString that's crashing, it's glXMakeCurrent.
> >
> > I forwarded a bug report from a user, though he's not been able to
> > reproduce since:
> >
> >   https://bugs.freedesktop.org/show_bug.cgi?id=32238
> >
> > A search in Mesa's bugzilla confirms that I'm not alone:
> >
> >   https://bugs.freedesktop.org/show_bug.cgi?id=30557
> This latter bug looks like an i915 driver bug, as opposed to a
> MakeCurrent bug.
> > Since the glGetString way will at best be slow, especially if we
> > have
> > to XSync and check for errors, could you consider exposing this
> > information as new glXGetServerString / glXGetClientString strings?
> ? I don't understand the logic here.
> You're hitting a bug in glXCreateContext or MakeCurrent or something
> like that. So you'd like to add an entire new way to query the same
> information a driver already provides, just to provide an alternate
> path
> that hopefully doesn't exhibit the bug?
> Just fix the bug! There's no reason for glX extensions to add new
> functions here.

My point is just that bugs exist.

Since bugs exist, I am trying to implement a driver blacklist.

My problem is that with GLX it's tricky because I can't get answer to the question "should I avoid creating GL contexts on this driver" without creating a GL context.

I proposed to allow handling this in glXQuery(Server|Client)String because these functions are known to be non-crashy.

> > It seems that such crashes happen only after a X error in
> > glxCreateNewContext. So it may be possible to avoid them by doing
> > XSync and checking for errors. But that makes the process even
> > slower.
> I don't think you need to XSync. Just do:
> glXCreateContext(...)
> glGetString(...)

After I create a GL context, don't I need to make it current? Are you saying that glXCreateContext makes the newly created context current?

> glXDestroyContext(...)
> in your code. If that crashes, there's a bug in the implementation, so
> report it and let's get it fixed!

This is for Web browsers. This would be considered a security flaw.


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