Re: [PATCH] drm/bochs: Fix NULL dereference on atomic_disable helper

Re: [PATCH] drm/bochs: Fix NULL dereference on atomic_disable helper

[Gerd Hoffmann]

On Sun, Mar 10, 2019 at 06:21:50PM -0300, Rodrigo Siqueira wrote:
> When the subtest basic-plain-flip from IGT project was executed on Bochs
> driver, the following bug appears:
> 
> BUG: unable to handle kernel NULL pointer dereference at 0000000000000000

>  ? drm_atomic_helper_commit_modeset_disables+0x396/0x3e0 [drm_kms_helper]

> +static void  bochs_crtc_atomic_disable(struct drm_crtc *crtc,
> +				       struct drm_crtc_state *old_state)
> +{
> +}
> +
>  static void bochs_crtc_atomic_flush(struct drm_crtc *crtc,
>  				    struct drm_crtc_state *old_crtc_state)
>  {
> @@ -66,6 +71,7 @@ static const struct drm_crtc_funcs bochs_crtc_funcs = {
>  static const struct drm_crtc_helper_funcs bochs_helper_funcs = {
>  	.mode_set_nofb = bochs_crtc_mode_set_nofb,
>  	.atomic_enable = bochs_crtc_atomic_enable,
> +	.atomic_disable = bochs_crtc_atomic_disable,

Shouldn't we make the callback optional instead of adding empty dummy
functions to drivers?

cheers,
  Gerd

Re: [PATCH] drm/bochs: Fix NULL dereference on atomic_disable helper

[Gerd Hoffmann]

  Hi,

> > >  static void bochs_crtc_atomic_flush(struct drm_crtc *crtc,
> > >  				    struct drm_crtc_state *old_crtc_state)
> > >  {
> > > @@ -66,6 +71,7 @@ static const struct drm_crtc_funcs bochs_crtc_funcs = {
> > >  static const struct drm_crtc_helper_funcs bochs_helper_funcs = {
> > >  	.mode_set_nofb = bochs_crtc_mode_set_nofb,
> > >  	.atomic_enable = bochs_crtc_atomic_enable,
> > > +	.atomic_disable = bochs_crtc_atomic_disable,
> > 
> > Shouldn't we make the callback optional instead of adding empty dummy
> > functions to drivers?
> 
> Hi Gerd,
> 
> I agree, and I can work in this issue.
> Just one question, should we make atomic_enable optional as well?

IIRC the drm code checks for the atomic_enable callback presence to
figure whenever it should take the atomic or legacy code paths.

So, I think that will not work.

cheers,
  Gerd

Re: [PATCH] drm/bochs: Fix NULL dereference on atomic_disable helper

[Daniel Vetter]

On Mon, Mar 11, 2019 at 02:07:16PM +0100, Gerd Hoffmann wrote:
>   Hi,
> 
> > > >  static void bochs_crtc_atomic_flush(struct drm_crtc *crtc,
> > > >  				    struct drm_crtc_state *old_crtc_state)
> > > >  {
> > > > @@ -66,6 +71,7 @@ static const struct drm_crtc_funcs bochs_crtc_funcs = {
> > > >  static const struct drm_crtc_helper_funcs bochs_helper_funcs = {
> > > >  	.mode_set_nofb = bochs_crtc_mode_set_nofb,
> > > >  	.atomic_enable = bochs_crtc_atomic_enable,
> > > > +	.atomic_disable = bochs_crtc_atomic_disable,
> > > 
> > > Shouldn't we make the callback optional instead of adding empty dummy
> > > functions to drivers?
> > 
> > Hi Gerd,
> > 
> > I agree, and I can work in this issue.
> > Just one question, should we make atomic_enable optional as well?
> 
> IIRC the drm code checks for the atomic_enable callback presence to
> figure whenever it should take the atomic or legacy code paths.

It should check for drm_driver->mode_config.funcs.atomic_commit for that,
see drm_drv_uses_atomic_modeset(). Anything else should be a bug.

Or do you mean the fallback to the old crtc helper prepare/commit
callbacks? We'd need to make all of them optional ofc, with atomic_
variants being preferred ofc.
-Daniel


> 
> So, I think that will not work.
> 
> cheers,
>   Gerd
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch

Re: [PATCH] drm/bochs: Fix NULL dereference on atomic_disable helper

[Gerd Hoffmann]

  Hi,

> > IIRC the drm code checks for the atomic_enable callback presence to
> > figure whenever it should take the atomic or legacy code paths.
> 
> It should check for drm_driver->mode_config.funcs.atomic_commit for that,
> see drm_drv_uses_atomic_modeset(). Anything else should be a bug.
> 
> Or do you mean the fallback to the old crtc helper prepare/commit
> callbacks?

Probably the later.  There was some reason why I've left in the empty
bochs_crtc_atomic_enable() callback ...

cheers,
  Gerd

Re: [PATCH] drm/bochs: Fix NULL dereference on atomic_disable helper

[Daniel Vetter]

On Mon, Mar 11, 2019 at 02:49:58PM -0300, Rodrigo Siqueira wrote:
> On 03/11, Gerd Hoffmann wrote:
> >   Hi,
> > 
> > > > IIRC the drm code checks for the atomic_enable callback presence to
> > > > figure whenever it should take the atomic or legacy code paths.
> > > 
> > > It should check for drm_driver->mode_config.funcs.atomic_commit for that,
> > > see drm_drv_uses_atomic_modeset(). Anything else should be a bug.
> > > 
> > > Or do you mean the fallback to the old crtc helper prepare/commit
> > > callbacks?
> > 
> > Probably the later.  There was some reason why I've left in the empty
> > bochs_crtc_atomic_enable() callback ...
> 
> Just for checking before I start to work in this patch:
> The correct solution should be made atomic_enable and atomic_disable
> optional, right? I should do it, and check if Bochs driver really needs
> bochs_crtc_atomic_enable after my change, right?

Yup. I just tried to remember why we haven't done this yet, but I think
that was a patch to make crtc->helper_funcs optional. And that doesn't
make sense imo, since if your crtc doesn't do anything then you don't
really have an atomic driver :-) And if there's ever a legit use case for
this, then that drive probably shouldn't use the atomic helpers ...

But making crtc_helper_funcs->atomic_enable/disable optional sounds like a
good idea.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch