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

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Message-ID: <20190312110044.GA2665@phenom.ffwll.local>
Date:   Tue, 12 Mar 2019 12:00:44 +0100
From:   Daniel Vetter <daniel@...ll.ch>
To:     Rodrigo Siqueira <rodrigosiqueiramelo@...il.com>
Cc:     Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <maxime.ripard@...tlin.com>,
        Sean Paul <sean@...rly.run>, David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        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

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.
> ---
>  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.
-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