[PATCH] drm/kms: Catch mode_object lifetime errors

[Daniel Vetter]

On Fri, Jun 28, 2019 at 7:24 PM Sean Paul <sean at poorly.run> wrote:
>
> On Fri, Jun 14, 2019 at 08:17:23AM +0200, Daniel Vetter wrote:
> > Only dynamic mode objects, i.e. those which are refcounted and have a free
> > callback, can be added while the overall drm_device is visible to
> > userspace. All others must be added before drm_dev_register and
> > removed after drm_dev_unregister.
> >
> > Small issue around drivers still using the load/unload callbacks, we
> > need to make sure we set dev->registered so that load/unload code in
> > these callbacks doesn't trigger false warnings. Only a small
> > adjustement in drm_dev_register was needed.
> >
> > Motivated by some irc discussions about object ids of dynamic objects
> > like blobs become invalid, and me going on a bit an audit spree.
> >
>
> Seems like a very worthwhile change, any idea how many drivers are going
> to be sad after this change?

None I think/hope, really just defense WARN_ON just in case. The main
ones that would be sad are all the ones that have a ->load callback,
but I'm taking care of them. Everyone else does things correctly and
calls drm_dev_register last in their probe function (or around where
they set up fbdev, which is also register the driver at least to the
fbdev world, so really the same).

> > Signed-off-by: Daniel Vetter <daniel.vetter at intel.com>
> > ---
> >  drivers/gpu/drm/drm_drv.c         | 4 ++--
> >  drivers/gpu/drm/drm_mode_object.c | 4 ++++
> >  2 files changed, 6 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/drm_drv.c b/drivers/gpu/drm/drm_drv.c
> > index cb6f0245de7c..48c84e3e1931 100644
> > --- a/drivers/gpu/drm/drm_drv.c
> > +++ b/drivers/gpu/drm/drm_drv.c
> > @@ -997,14 +997,14 @@ int drm_dev_register(struct drm_device *dev, unsigned long flags)
> >       if (ret)
> >               goto err_minors;
> >
> > -     dev->registered = true;
> > -
> >       if (dev->driver->load) {
> >               ret = dev->driver->load(dev, flags);
> >               if (ret)
> >                       goto err_minors;
> >       }
> >
> > +     dev->registered = true;
> > +
> >       if (drm_core_check_feature(dev, DRIVER_MODESET))
> >               drm_modeset_register_all(dev);
> >
> > diff --git a/drivers/gpu/drm/drm_mode_object.c b/drivers/gpu/drm/drm_mode_object.c
> > index 1c6e51135962..c355ba8e6d5d 100644
> > --- a/drivers/gpu/drm/drm_mode_object.c
> > +++ b/drivers/gpu/drm/drm_mode_object.c
> > @@ -42,6 +42,8 @@ int __drm_mode_object_add(struct drm_device *dev, struct drm_mode_object *obj,
> >  {
> >       int ret;
> >
> > +     WARN_ON(dev->registered && !obj_free_cb);
>
> These should probably have a comment above them giving some guidance to the
> driver developer.
>
> With some comments, this is:

What comment do you expect here? drm_dev_register explains what you
should do already, and I expect driver developers to find that one
pretty quickly. From there: "This should be done last in the device
initialization sequence to make sure userspace can't access an
inconsistent state."
-Daniel

> Reviewed-by: Sean Paul <sean at poorly.run>
>
>
> > +
> >       mutex_lock(&dev->mode_config.idr_mutex);
> >       ret = idr_alloc(&dev->mode_config.object_idr, register_obj ? obj : NULL,
> >                       1, 0, GFP_KERNEL);
> > @@ -102,6 +104,8 @@ void drm_mode_object_register(struct drm_device *dev,
> >  void drm_mode_object_unregister(struct drm_device *dev,
> >                               struct drm_mode_object *object)
> >  {
> > +     WARN_ON(dev->registered && !object->free_cb);
> > +
> >       mutex_lock(&dev->mode_config.idr_mutex);
> >       if (object->id) {
> >               idr_remove(&dev->mode_config.object_idr, object->id);
> > --
> > 2.20.1
> >
> > _______________________________________________
> > dri-devel mailing list
> > dri-devel at lists.freedesktop.org
> > https://lists.freedesktop.org/mailman/listinfo/dri-devel
>
> --
> Sean Paul, Software Engineer, Google / Chromium OS



-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch