drm/i915/gem: Comment on inability to check args.pad for MMAP_OFFSET

Since we didn't check and insist that args.pad must be zero for MMAP_GTT
historically, we cannot insert a check now as old userspace may be
feeding in garbage. As such the lack of check is enshrined into the ABI,
so add a comment to remind us we cannot add the check later.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191207222644.2830129-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/gem/i915_gem_mman.c

@@ -620,6 +620,15 @@ i915_gem_mmap_offset_ioctl(struct drm_device *dev, void *data,
 	enum i915_mmap_type type;
 	int err;
 
+	/*
+	 * Historically we failed to check args.pad and args.offset
+	 * and so we cannot use those fields for user input and we cannot
+	 * add -EINVAL for them as the ABI is fixed, i.e. old userspace
+	 * may be feeding in garbage in those fields.
+	 *
+	 * if (args->pad) return -EINVAL; is verbotten!
+	 */
+
 	err = i915_user_extensions(u64_to_user_ptr(args->extensions),
 				   NULL, 0, NULL);
 	if (err)