Re: [gtk-vnc-devel] Fix mmap permissions for coroutine stacks

[Anthony Liguori <anthony codemonkey ws>]

Daniel P. Berrange wrote:
> On Wed, Sep 26, 2007 at 04:00:28PM -0500, Anthony Liguori wrote:
>   
> > Daniel P. Berrange wrote:
> >     
> > > Our current code mmaps a region with PROT_EXEC for use as stack space in 
> > > the swapcontext() calls. This is bogus because the stack should not be
> > > executable. On any Fedora / RHEL system with SELinux enabled this results
> > > in the process being terminated with extreme prejudice the moment we do
> > > the swapcontext() call.
> > >
> > > The attach patch removes the use of PROT_EXEC, and it also switches to use
> > > MAP_PRIVATE instead of MAP_SHARED since ther is no reason for stack to be
> > > shared across processes, and finally removes MAP_GROWSDOWN since it is a
> > > broken concept that should not be used according to GLibC guys.
> > >  
> > >       
> > Very interesting patch.   GROWSDOWN seemed like the Right Thing to do 
> > but I'm not surprised if it's broken.  Removing EXEC seems sane to me.  
> > It was just a guess as what the right permissions should be.
> >     
>
> >From my understanding, GORWSDOWN may have been relevant in older kernels 
> (eg on 2.4 era) but today it is not. It is linux specific anyway.
>
>   
> > BTW, if the coroutine stuff ever gets too weird, we can very easily 
> > switch over to lock-step threads.  I know that there's been some 
> > concerns about the portability of coroutines in the context of the gnome 
> > community before (gstreamer initialized used a coroutine mechanism).  
> > I'd still want to stick with lock-step execution of threads b/c I think 
> > it simplifies locking.
> >     
>
> Personally I'd like to avoid threads. The coroutine stuff is a little
> odd if you're not familiar with the idea, but it works well in practice.
> If portability becomes an issue we can deal with it later - perhaps even
> fake the current coroutine API using pthreads on platforms without the
> swapcontext() stuff.
>   

Yeah, that's exactly what I was thinking too.

Regards,

Anthony Liguori

> Dan.
>