[bug report] drm/ttm: Fix dummy res NULL ptr deref bug

[Dan Carpenter]

On Thu, Aug 11, 2022 at 04:36:33PM +0530, Arunpravin Paneer Selvam wrote:
> Hi Dan,
> 
> drm-misc-fixes doesn't have the updated ttm_bo.c file, we have the updated
> ttm_bo.c version in
> drm-misc-next branch. Please find below for the line number 907.
> 
> On 8/11/2022 3:25 PM, Dan Carpenter wrote:
> > Hello Arunpravin Paneer Selvam,
> > 
> > This is a semi-automatic email about new static checker warnings.
> > 
> > The patch cf4b7387c0a8: "drm/ttm: Fix dummy res NULL ptr deref bug"
> > from Aug 9, 2022, leads to the following Smatch complaint:
> > 
> >      drivers/gpu/drm/ttm/ttm_bo.c:915 ttm_bo_validate()
> >      warn: variable dereferenced before check 'bo->resource' (see line 907)
> > 
> > drivers/gpu/drm/ttm/ttm_bo.c
> >     906		 */
> >     907		if (!ttm_resource_compat(bo->resource, placement)) {
> >                                           ^^^^^^^^^^^^
> > Unchecked dereference here inside the function.
> 
> |if (!bo->resource || !ttm_resource_compat(bo->resource, placement)) { we
> have this version in drm-misc-next Regards, Arun |
> 

Huh...  That's very interesting.  It appears there was a bug in
drm-misc-next, we applied the fix to the wrong tree, and now both trees
are wrong.  The drm-misc-next tree still has the bug and the other tree
has a static checker warning about nonsensical NULL checks.

Eventually drm-misc-next will get merged and everything will work.  Is
it too late to remove the bogus "CC: stable at vger.kernel.org"?

This could have been avoided if the NULL dereference fix had a Fixes tag.

regards,
dan carpenter