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

["Daniel P. Berrange" <berrange redhat com>]

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.

Dan.
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