Re: [PATCH][next] drm/amd/display: Fix Wstringop-overflow warnings in dc_link_dp.c

["Gustavo A. R. Silva" <gustavoars@kernel.org>]

On Thu, Mar 03, 2022 at 12:19:57PM -0600, Gustavo A. R. Silva wrote:
> On Thu, Mar 03, 2022 at 09:43:28AM -0800, Kees Cook wrote:
> > On Thu, Mar 03, 2022 at 11:25:03AM -0600, Gustavo A. R. Silva wrote:
> > > Fix the following Wstringop-overflow warnings when building with GCC-11:
> > > 
> > > drivers/gpu/drm/amd/amdgpu/../display/dc/core/dc_link_dpia.c:493:17: warning: ‘dp_decide_lane_settings’ accessing 4 bytes in a region of size 1 [-Wstringop-overflow=]
> > 
> > Can you "show your work" a little more here? I don't actually see the
> > what is getting fixed:
> > 
> > enum dc_lane_count {
> > 	...
> >         LANE_COUNT_FOUR = 4,
> > 	...
> >         LANE_COUNT_DP_MAX = LANE_COUNT_FOUR
> > };
> > 
> > struct link_training_settings {
> > 	...
> >         union dpcd_training_lane dpcd_lane_settings[LANE_COUNT_DP_MAX];
> > };
> > 
> > void dp_hw_to_dpcd_lane_settings(
> > 		...
> > 		union dpcd_training_lane dpcd_lane_settings[LANE_COUNT_DP_MAX])
> > {
> > 	...
> > }
> > 
> > static enum link_training_result dpia_training_cr_transparent(
> > 		...
> >                 struct link_training_settings *lt_settings)
> > {
> > 	...
> >                 dp_decide_lane_settings(lt_settings, dpcd_lane_adjust,
> >                                 lt_settings->hw_lane_settings, lt_settings->dpcd_lane_settings);
> > 	...
> > }
> > 
> > Everything looks to be the correct size?
> 
> Yep; this fix is similar to the one for intel_pm.c in this
> 
> 	commit e7c6e405e171fb33990a12ecfd14e6500d9e5cf2
> 
> where the array size of 8 seems to be fine for all the
> struct members related (pri_latency, spr_latency, cur_latency
> and skl_latency):
> 
> drivers/gpu/drm/i915/i915_drv.h:465:struct drm_i915_private {
> ...
> 
> drivers/gpu/drm/i915/i915_drv.h-739-    struct {
> 
> ...
> drivers/gpu/drm/i915/i915_drv.h-745-            /* primary */
> drivers/gpu/drm/i915/i915_drv.h-746-            u16 pri_latency[5];
> drivers/gpu/drm/i915/i915_drv.h-747-            /* sprite */
> drivers/gpu/drm/i915/i915_drv.h-748-            u16 spr_latency[5];
> drivers/gpu/drm/i915/i915_drv.h-749-            /* cursor */
> drivers/gpu/drm/i915/i915_drv.h-750-            u16 cur_latency[5];
> drivers/gpu/drm/i915/i915_drv.h-751-            /*
> drivers/gpu/drm/i915/i915_drv.h-752-             * Raw watermark memory latency values
> drivers/gpu/drm/i915/i915_drv.h-753-             * for SKL for all 8 levels
> drivers/gpu/drm/i915/i915_drv.h-754-             * in 1us units.
> drivers/gpu/drm/i915/i915_drv.h-755-             */
> drivers/gpu/drm/i915/i915_drv.h-756-            u16 skl_latency[8];
> 
> ...
> drivers/gpu/drm/i915/i915_drv.h-773-    } wm;
> ...
> }

and in this case the ilk_wm_max_level() returns the right maximum size for the
corresponding 'struct wm' member:

drivers/gpu/drm/i915/intel_pm.c:2993:int ilk_wm_max_level(const struct drm_i915_private *dev_priv)
drivers/gpu/drm/i915/intel_pm.c-2994-{
drivers/gpu/drm/i915/intel_pm.c-2995-   /* how many WM levels are we expecting */
drivers/gpu/drm/i915/intel_pm.c-2996-   if (HAS_HW_SAGV_WM(dev_priv))
drivers/gpu/drm/i915/intel_pm.c-2997-           return 5;
drivers/gpu/drm/i915/intel_pm.c-2998-   else if (DISPLAY_VER(dev_priv) >= 9)
drivers/gpu/drm/i915/intel_pm.c-2999-           return 7;
drivers/gpu/drm/i915/intel_pm.c-3000-   else if (IS_HASWELL(dev_priv) || IS_BROADWELL(dev_priv))
drivers/gpu/drm/i915/intel_pm.c-3001-           return 4;
drivers/gpu/drm/i915/intel_pm.c-3002-   else if (DISPLAY_VER(dev_priv) >= 6)
drivers/gpu/drm/i915/intel_pm.c-3003-           return 3;
drivers/gpu/drm/i915/intel_pm.c-3004-   else
drivers/gpu/drm/i915/intel_pm.c-3005-           return 2;
drivers/gpu/drm/i915/intel_pm.c-3006-}

drivers/gpu/drm/i915/intel_pm.c:3009:static void intel_print_wm_latency(struct drm_i915_private *dev_priv,
drivers/gpu/drm/i915/intel_pm.c-3010-                              const char *name,
drivers/gpu/drm/i915/intel_pm.c-3011-                              const u16 wm[])
drivers/gpu/drm/i915/intel_pm.c-3012-{
drivers/gpu/drm/i915/intel_pm.c-3013-   int level, max_level = ilk_wm_max_level(dev_priv);
drivers/gpu/drm/i915/intel_pm.c-3014-
drivers/gpu/drm/i915/intel_pm.c-3015-   for (level = 0; level <= max_level; level++) {
drivers/gpu/drm/i915/intel_pm.c-3016-           unsigned int latency = wm[level];
drivers/gpu/drm/i915/intel_pm.c-3017-
...
}

still GCC warns about this with Wstringop-overread, as it is explained
in commit e7c6e405e171.

--
Gustavo

> 
> however GCC warns about accessing bytes beyond the limits, and turning the
> argument declarations into pointers (removing the over-specified array
> size from the argument declaration) silence the warnings.
> 
> --
> Gustavo