Re: [RFC] drm/i915: prevent out of range pt in the PDE macros (take 2)

[Dave Gordon <david.s.gordon@intel.com>]

On 13/06/15 09:28, Chris Wilson wrote:
> On Fri, Jun 12, 2015 at 06:30:56PM -0300, Paulo Zanoni wrote:
>> From: Paulo Zanoni <paulo.r.zanoni@intel.com>
>>
>> We tried to fix this in the following commit:
>>
>> commit fdc454c1484a20e1345cf4e4d7a9feaee814147f
>> Author: Michel Thierry <michel.thierry@intel.com>
>> Date:   Tue Mar 24 15:46:19 2015 +0000
>>     drm/i915: Prevent out of range pt in gen6_for_each_pde
>>
>> but the static analyzer still complains that, just before we break due
>> to "iter < I915_PDES", we do "pt = (pd)->page_table[iter]" with an
>> iter value that is bigger than I915_PDES. Of course, this isn't really
>> a problem since no one uses pt outside the macro. Still, every single
>> new usage of the macro will create a new issue for us to mark as a
>> false possitive.
>>
>> After the commit mentioned above we also created some new versions of
>> the macros, so they carry the same "problem".
>>
>> In order to "solve" this "problem", let's leave the macro with a NULL
>> value for pt. So if somebody uses it, we're more likely to get a big
>> error message instead of some silent failure. I hope the static
>> analyzer won't complain about the new solution (I don't have a way to
>> check this!).
>>
>> I know, the solution looks really ugly. I am hoping the reviewers will
>> help us decide if we prefer this patch or if we prefer to keep marking
>> things as false positives.
>>
>> Cc: Michel Thierry <michel.thierry@intel.com>
>> Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
>> ---
>>  drivers/gpu/drm/i915/i915_gem_gtt.h | 13 +++++++++----
>>  1 file changed, 9 insertions(+), 4 deletions(-)
>>
>> I sent this as an RFC because I really don't know if complicating the
>> macro even more will help us in any way. I won't really be surprised
>> if I see NACKs on this patch, so don't hesitate if you want to.
>>
>> Also, all I did was boot a Kernel with this patch and make sure it
>> shows the desktop. So consider this as untested, possibly broken.
>>
>> diff --git a/drivers/gpu/drm/i915/i915_gem_gtt.h b/drivers/gpu/drm/i915/i915_gem_gtt.h
>> index 0d46dd2..b202ca0 100644
>> --- a/drivers/gpu/drm/i915/i915_gem_gtt.h
>> +++ b/drivers/gpu/drm/i915/i915_gem_gtt.h
>> @@ -352,7 +352,8 @@ struct i915_hw_ppgtt {
>>   */
> 
> Overallocate page_table etc by one and put a NULL sentinel in it.
> 
> for ((iter) = gen6_pde_index(start); \
>      (length) > 0 && (pt = (pd)->page_table[iter]); \
>      (iter)++, \
>      temp = ALIGN(start+1, 1 << GEN6_PDE_SHIFT) - start, \
>      temp = min_t(unsigned, temp, length), \
> 
> -Chris

This might trigger different warnings from some static analysers, as
'pt' doesn't get assigned at all if length == 0.

.Dave.

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