linux-kernel - Re: [PATCH] drm/i915: avoid potential uninitialized variable use

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Date:   Thu, 5 Oct 2017 15:32:50 +0200
From:   Daniel Vetter <daniel@...ll.ch>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Jani Nikula <jani.nikula@...ux.intel.com>,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        David Airlie <airlied@...ux.ie>,
        Ville Syrjälä 
        <ville.syrjala@...ux.intel.com>,
        Ander Conselvan de Oliveira 
        <ander.conselvan.de.oliveira@...el.com>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Tvrtko Ursulin <tvrtko.ursulin@...el.com>,
        Shashank Sharma <shashank.sharma@...el.com>,
        Paulo Zanoni <paulo.r.zanoni@...el.com>,
        intel-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drm/i915: avoid potential uninitialized variable use

On Thu, Oct 05, 2017 at 02:08:26PM +0200, Arnd Bergmann wrote:
> One of the recent changes introduced a warning about
> undefined behavior in the sanity checking:
> 
> drivers/gpu/drm/i915/intel_ddi.c: In function 'intel_ddi_hdmi_level':
> drivers/gpu/drm/i915/intel_ddi.c:654:6: error: 'n_hdmi_entries' may be used uninitialized in this function [-Werror=maybe-uninitialized]
> 
> It seems that the new cnl specific get_buf_trans functions
> can return uninitialized data if the voltage level is set
> to an unexpected value. This changes the code to always return
> '1' in that error case, which seems like the safest choice
> as we use one less than the number as an array index later on.
> 
> Fixes: cc9cabfdec38 ("drm/i915/cnl: Move voltage check into ddi buf trans functions.")
> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> ---
>  drivers/gpu/drm/i915/intel_ddi.c | 12 +++++++++---
>  1 file changed, 9 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/intel_ddi.c b/drivers/gpu/drm/i915/intel_ddi.c
> index 93cbbcbbc193..d0b786078bea 100644
> --- a/drivers/gpu/drm/i915/intel_ddi.c
> +++ b/drivers/gpu/drm/i915/intel_ddi.c
> @@ -602,8 +602,10 @@ cnl_get_buf_trans_hdmi(struct drm_i915_private *dev_priv, int *n_entries)
>  	} else if (voltage == VOLTAGE_INFO_1_05V) {
>  		*n_entries = ARRAY_SIZE(cnl_ddi_translations_hdmi_1_05V);
>  		return cnl_ddi_translations_hdmi_1_05V;
> -	} else
> +	} else {
> +		*n_entries = 1;
>  		MISSING_CASE(voltage);
> +	}

Somewhat meh on this, so added a /* shut up gcc */ comment and merged.

Thanks, Daniel

>  	return NULL;
>  }
>  
> @@ -621,8 +623,10 @@ cnl_get_buf_trans_dp(struct drm_i915_private *dev_priv, int *n_entries)
>  	} else if (voltage == VOLTAGE_INFO_1_05V) {
>  		*n_entries = ARRAY_SIZE(cnl_ddi_translations_dp_1_05V);
>  		return cnl_ddi_translations_dp_1_05V;
> -	} else
> +	} else {
> +		*n_entries = 1;
>  		MISSING_CASE(voltage);
> +	}
>  	return NULL;
>  }
>  
> @@ -641,8 +645,10 @@ cnl_get_buf_trans_edp(struct drm_i915_private *dev_priv, int *n_entries)
>  		} else if (voltage == VOLTAGE_INFO_1_05V) {
>  			*n_entries = ARRAY_SIZE(cnl_ddi_translations_edp_1_05V);
>  			return cnl_ddi_translations_edp_1_05V;
> -		} else
> +		} else {
> +			*n_entries = 1;
>  			MISSING_CASE(voltage);
> +		}
>  		return NULL;
>  	} else {
>  		return cnl_get_buf_trans_dp(dev_priv, n_entries);
> -- 
> 2.9.0
> 

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