drm/i915: clarify reasoning for the access_ok call

This clarifies the comment above the access_ok check so a missing
VERIFY_READ doesn't alarm anyone.

v2:
 - rewrote comment, thanks to Chris Wilson
Signed-off-by: Kees Cook <keescook@chromium.org>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
[danvet: add patch history log to commit message.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_gem_execbuffer.c

@@ -747,7 +747,11 @@ validate_exec_list(struct drm_i915_gem_exec_object2 *exec,
 
 		length = exec[i].relocation_count *
 			sizeof(struct drm_i915_gem_relocation_entry);
-		/* we may also need to update the presumed offsets */
+		/*
+		 * We must check that the entire relocation array is safe
+		 * to read, but since we may need to update the presumed
+		 * offsets during execution, check for full write access.
+		 */
 		if (!access_ok(VERIFY_WRITE, ptr, length))
 			return -EFAULT;