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

[Chris Wilson]

On Fri, Jun 12, 2015 at 06:30:56PM -0300, Paulo Zanoni wrote:
> From: Paulo Zanoni <paulo.r.zanoni at intel.com>
> 
> We tried to fix this in the following commit:
> 
> commit fdc454c1484a20e1345cf4e4d7a9feaee814147f
> Author: Michel Thierry <michel.thierry at 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 at intel.com>
> Signed-off-by: Paulo Zanoni <paulo.r.zanoni at 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

-- 
Chris Wilson, Intel Open Source Technology Centre