linux-kernel - Re: [PATCH] drm/atomic-helper: Validate pointer before dereference

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Message-ID: <20190312115007.6k65cqwvfcs2tewk@smtp.gmail.com>
Date:   Tue, 12 Mar 2019 08:50:07 -0300
From:   Rodrigo Siqueira <rodrigosiqueiramelo@...il.com>
To:     Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <maxime.ripard@...tlin.com>,
        Sean Paul <sean@...rly.run>, David Airlie <airlied@...ux.ie>,
        Gerd Hoffmann <kraxel@...hat.com>,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drm/atomic-helper: Validate pointer before dereference

Hi,

First of all, thanks for the feedback.

I will fix all the problems pointed out in the review.

I just have two inline questions.

On 03/12, Daniel Vetter wrote:
> On Mon, Mar 11, 2019 at 06:01:20PM -0300, Rodrigo Siqueira wrote:
> > The function disable_outputs() and
> > drm_atomic_helper_commit_modeset_enables() tries to retrieve
> > helper_private from the target CRTC, for dereferencing some operations.
> > However, the current implementation does not check whether
> > helper_private is null and, if not, if it has a valid pointer to a dpms
> > and commit functions. This commit adds pointer validations before
> > trying to dereference the dpms and commit function.
> > 
> > Signed-off-by: Rodrigo Siqueira <rodrigosiqueiramelo@...il.com>
> 
> Please also adjust the kerneldoc for these functions. And I think the
> patch subject can be improved, e.g. "Make ->atomic_enable/disable crtc
> callbacks optional". Describe what you're trying to achieve in the
> summary, not how you achieve it.

Do I need to add information which says that both functions are
optional? I'm asking because the description related to the affected
functions and struct looks good for me [1,2,3].

1. Documentation for drm_crtc_helper_funcs:
https://dri.freedesktop.org/docs/drm/gpu/drm-kms-helpers.html?highlight=drm_crtc_helper_funcs#c.drm_crtc_helper_funcs

2. Documentation for drm_atomic_helper_commit_modeset_enables():
https://dri.freedesktop.org/docs/drm/gpu/drm-kms-helpers.html?highlight=drm_atomic_helper_commit_modeset_enables#c.drm_atomic_helper_commit_modeset_enables

3. Documentation for drm_atomic_helper_commit_modeset_disables():
https://dri.freedesktop.org/docs/drm/gpu/drm-kms-helpers.html?highlight=drm_atomic_helper_commit_modeset_disables#c.drm_atomic_helper_commit_modeset_disables

> > ---
> >  drivers/gpu/drm/drm_atomic_helper.c | 30 ++++++++++++++++-------------
> >  1 file changed, 17 insertions(+), 13 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/drm_atomic_helper.c b/drivers/gpu/drm/drm_atomic_helper.c
> > index 540a77a2ade9..fbeef7c461fc 100644
> > --- a/drivers/gpu/drm/drm_atomic_helper.c
> > +++ b/drivers/gpu/drm/drm_atomic_helper.c
> > @@ -1028,14 +1028,16 @@ disable_outputs(struct drm_device *dev, struct drm_atomic_state *old_state)
> >  
> >  
> >  		/* Right function depends upon target state. */
> > -		if (new_crtc_state->enable && funcs->prepare)
> > -			funcs->prepare(crtc);
> > -		else if (funcs->atomic_disable)
> > -			funcs->atomic_disable(crtc, old_crtc_state);
> > -		else if (funcs->disable)
> > -			funcs->disable(crtc);
> > -		else
> > -			funcs->dpms(crtc, DRM_MODE_DPMS_OFF);
> > +		if (funcs) {
> 
> I don't think making funcs optional is a good idea. If you have a crtc
> with no functions implemented, it's not terribly useful.
> 
> Also making functions optional just here is not going to help if we still
> require it everywhere else.

Should I remove the other occurrence of "if (funcs)" inside
disable_outputs() and drm_atomic_helper_commit_modeset_enables()? Both
functions, already had this before.

Best Regards

> -Daniel
> 
> > +			if (new_crtc_state->enable && funcs->prepare)
> > +				funcs->prepare(crtc);
> > +			else if (funcs->atomic_disable)
> > +				funcs->atomic_disable(crtc, old_crtc_state);
> > +			else if (funcs->disable)
> > +				funcs->disable(crtc);
> > +			else if (funcs->dpms)
> > +				funcs->dpms(crtc, DRM_MODE_DPMS_OFF);
> > +		}
> >  
> >  		if (!(dev->irq_enabled && dev->num_crtcs))
> >  			continue;
> > @@ -1277,11 +1279,13 @@ void drm_atomic_helper_commit_modeset_enables(struct drm_device *dev,
> >  		if (new_crtc_state->enable) {
> >  			DRM_DEBUG_ATOMIC("enabling [CRTC:%d:%s]\n",
> >  					 crtc->base.id, crtc->name);
> > -
> > -			if (funcs->atomic_enable)
> > -				funcs->atomic_enable(crtc, old_crtc_state);
> > -			else
> > -				funcs->commit(crtc);
> > +			if (funcs) {
> > +				if (funcs->atomic_enable)
> > +					funcs->atomic_enable(crtc,
> > +							     old_crtc_state);
> > +				else if (funcs->atomic_enable)
> > +					funcs->commit(crtc);
> > +			}
> >  		}
> >  	}
> >  
> > -- 
> > 2.21.0
> 
> -- 
> Daniel Vetter
> Software Engineer, Intel Corporation
> http://blog.ffwll.ch

-- 
Rodrigo Siqueira
https://siqueira.tech
Graduate Student
Department of Computer Science
University of São Paulo