linux-kernel - Re: Gem GTT mmaps..

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Date:	Wed, 04 Feb 2009 15:42:00 -0800
From:	Eric Anholt <eric@...olt.net>
To:	Jesse Barnes <jbarnes@...tuousgeek.org>
Cc:	Thomas Hellström <thomas@...pmail.org>,
	DRI <dri-devel@...ts.sourceforge.net>,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: Gem GTT mmaps..

On Wed, 2009-02-04 at 15:02 -0800, Jesse Barnes wrote:
> On Wednesday, February 4, 2009 2:32 pm Thomas Hellström wrote:
> > Jesse,
> >
> > I have some concerns about the GEM GTT mmap functionality.
> 
> Thanks for looking it over again; you would know since some of this code came 
> from you in the first place. :)
> 
> > First, a gem object pointer is copied to map->offset and then to the
> > vma->vm_private_data without proper reference counting. This pointer is
> > used in i915_gem_fault() to access the gem object. However if the gem
> > object is destroyed and a process then tries to access data in a vma
> > mapping the (now destroyed) object, it would dereference a stale pointer
> > into kernel space? Shouldn't those pointers be reference counted, and to
> > account for fork(), a vm open and close would be needed to  reference
> > count corresponding pointers of newly created and destroyed vmas?
> 
> Yeah looks like we don't protect against vm_private_data pointing at a freed 
> or other object.  But rather than refcounting the pointers I wonder if we 
> could make the private data use the GEM object name instead, then do the 
> lookup in the fault handler?

The object doesn't necessarily have a public name.  You do need to
refcount the objects.

-- 
Eric Anholt
eric@...olt.net                         eric.anholt@...el.com



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