Re: [PATCH 2/3] drm/display/dp_mst: Fix drm_atomic_get_mst_topology_state()

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Thu, Jun 02, 2022 at 04:17:56PM -0400, Lyude Paul wrote:
> I noticed a rather surprising issue here while working on removing all of
> the non-atomic MST code: drm_atomic_get_mst_topology_state() doesn't check
> the return value of drm_atomic_get_private_obj_state() and instead just
> passes it directly to to_dp_mst_topology_state(). This means that if we
> hit a deadlock or something else which would return an error code pointer,
> we'll likely segfault the kernel.
> 
> This is definitely another one of those fixes where I'm astonished we
> somehow managed never to discover this issue until now…

It has been discussed before.

struct drm_dp_mst_topology_state {
	struct drm_private_state base;
	...
}

so offsetof(base)==0.

> 
> Signed-off-by: Lyude Paul <lyude@redhat.com>
> Fixes: a4370c777406 ("drm/atomic: Make private objs proper objects")
> Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
> Cc: <stable@vger.kernel.org> # v4.14+
> ---
>  drivers/gpu/drm/display/drm_dp_mst_topology.c | 2 +-
>  include/drm/display/drm_dp_mst_helper.h       | 2 ++
>  2 files changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/display/drm_dp_mst_topology.c b/drivers/gpu/drm/display/drm_dp_mst_topology.c
> index d84673b3294b..d6e595b95f07 100644
> --- a/drivers/gpu/drm/display/drm_dp_mst_topology.c
> +++ b/drivers/gpu/drm/display/drm_dp_mst_topology.c
> @@ -5468,7 +5468,7 @@ EXPORT_SYMBOL(drm_dp_mst_topology_state_funcs);
>  struct drm_dp_mst_topology_state *drm_atomic_get_mst_topology_state(struct drm_atomic_state *state,
>  								    struct drm_dp_mst_topology_mgr *mgr)
>  {
> -	return to_dp_mst_topology_state(drm_atomic_get_private_obj_state(state, &mgr->base));
> +	return to_dp_mst_topology_state_safe(drm_atomic_get_private_obj_state(state, &mgr->base));
>  }
>  EXPORT_SYMBOL(drm_atomic_get_mst_topology_state);
>  
> diff --git a/include/drm/display/drm_dp_mst_helper.h b/include/drm/display/drm_dp_mst_helper.h
> index 10adec068b7f..fe7577e7f305 100644
> --- a/include/drm/display/drm_dp_mst_helper.h
> +++ b/include/drm/display/drm_dp_mst_helper.h
> @@ -541,6 +541,8 @@ struct drm_dp_payload {
>  };
>  
>  #define to_dp_mst_topology_state(x) container_of(x, struct drm_dp_mst_topology_state, base)
> +#define to_dp_mst_topology_state_safe(x) \
> +	container_of_safe(x, struct drm_dp_mst_topology_state, base)

Wasn't aware of container_of_safe(). I suppose no real harm 
in using it. Not sure why we'd even keep the non-safe version
around?

Though the use of container_of_safe() everywhere won't help
when "casting" the other way (&foo->base, when foo==NULL/errptr).
In order to make that work for non-zero offsets we'd have to
introduce a casting macro for that direction as well.

>  
>  struct drm_dp_vcpi_allocation {
>  	struct drm_dp_mst_port *port;
> -- 
> 2.35.3

-- 
Ville Syrjälä
Intel