[cairo] pixman crash from setting clip region on image surface (patch)

Vladimir Vukicevic vladimir at pobox.com
Sat May 8 01:59:02 PDT 2004

Hi Carl,

Carl Worth wrote:
> On Wed, 05 May 2004 20:21:37 -0700, Vladimir Vukicevic wrote:
>>The attached patch fixes this; just a patch to icimage.c to check if the 
>>region is not NULL before setting a new clipping region.
> Thanks. This is committed.

More problems on this front, but I'm not sure what the fix is.  When 
pixman gets a clip region set via pixman_image_set_clip_region, it takes 
ownership of this region and later destroys it in 
pixman_image_destroyClip when the clip region is changed.

However, cairo also keeps track of the clip region in 
gstate->clip.region, and calls pixman_region_destroy on it in 
_cairo_gstate_fini, among others.  So, given that cairo creates a region 
in _cairo_gstate_clip, and then assigns it to gstate->clip.region and 
calls _cairo_surface_set_clip_region() (which in turn calls 
pixman_image_set_clip_region) using the same pointer, we end up with a 
double free in many places where pixman_region_destroy() is called in 

One solution is to always create a copy of the region before calling 
_cairo_surface_set_clip_region (or that this function creates a copy 
before calling the back end implementation); however, this would mean 
that all the back ends would have to agree to clean up that region if 
they don't use pixman_image_set_clip_region().

Any thoughts on how to fix this?

	- Vlad

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