Re: [PATCH 1/4] drm/i915/dp_mst: Fix connector initialization in intel_dp_add_mst_connector()

[Imre Deak <imre.deak@intel.com>]

On Fri, Nov 15, 2024 at 03:20:58PM -0500, Rodrigo Vivi wrote:
> On Fri, Nov 15, 2024 at 06:41:56PM +0200, Imre Deak wrote:
> > The connector initialization in intel_dp_add_mst_connector() depends on
> > the device pointer in connector to be valid, at least by connector
> > debug printing. The device pointer is initialized by drm_connector_init(),
> > however that function also exposes the connector to in-kernel users,
> > which can't be done before the connector is fully initialized. For now
> > make sure the device pointer is valid before it's used, until a
> > follow-up change moving this to DRM core.
> > 
> > This issue was revealed by the commit in the Fixes: line below, before
> > which the above debug printing checked and handled a NULL device pointer
> > gracefully in DRM core.
> > 
> > Cc: Jani Nikula <jani.nikula@intel.com>
> > Fixes: 529798bd786a ("drm/i915/mst: convert to struct intel_display")
> 
> This is awkward. This patch actually removes callers of base.dev.
> I don't see how that it could be causing this new null dereference.

It adds

struct intel_display *display = to_intel_display(connector);

which will be NULL since connector->base.dev is NULL and later display
is dereferenced.

> > Closes: https://gitlab.freedesktop.org/drm/i915/kernel/-/issues/12799
> 
> But well, trusting more the tests then my eyes, let's move forward.
> 
> Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
> 
> > Signed-off-by: Imre Deak <imre.deak@intel.com>
> > ---
> >  drivers/gpu/drm/i915/display/intel_dp_mst.c | 10 ++++++++++
> >  1 file changed, 10 insertions(+)
> > 
> > diff --git a/drivers/gpu/drm/i915/display/intel_dp_mst.c b/drivers/gpu/drm/i915/display/intel_dp_mst.c
> > index df7edcfe885b6..f058360a26413 100644
> > --- a/drivers/gpu/drm/i915/display/intel_dp_mst.c
> > +++ b/drivers/gpu/drm/i915/display/intel_dp_mst.c
> > @@ -1727,6 +1727,16 @@ static struct drm_connector *intel_dp_add_mst_connector(struct drm_dp_mst_topolo
> >  
> >  	intel_dp_init_modeset_retry_work(intel_connector);
> >  
> > +	/*
> > +	 * TODO: The following drm_connector specific initialization belongs
> > +	 * to DRM core, however it happens atm too late in
> > +	 * drm_connector_init(). That function will also expose the connector
> > +	 * to in-kernel users, so it can't be called until the connector is
> > +	 * sufficiently initialized; init the device pointer used by the
> > +	 * following DSC setup, until a fix moving this to DRM core.
> > +	 */
> > +	intel_connector->base.dev = mgr->dev;
> > +
> >  	intel_connector->dp.dsc_decompression_aux = drm_dp_mst_dsc_aux_for_port(port);
> >  	intel_dp_mst_read_decompression_port_dsc_caps(intel_dp, intel_connector);
> >  	intel_connector->dp.dsc_hblank_expansion_quirk =
> > -- 
> > 2.44.2
> >