linux-kernel - Re: [PATCH] tilcdc: tilcdc_external: fix an incorrect NULL check on list iterator

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Date:   Mon, 28 Mar 2022 01:13:21 +0300
From:   Jyri Sarha <jyri.sarha@....fi>
To:     Xiaomeng Tong <xiam0nd.tong@...il.com>
Cc:     tomba@...nel.org, airlied@...ux.ie, daniel@...ll.ch,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org
Subject: Re: [PATCH] tilcdc: tilcdc_external: fix an incorrect NULL check on
 list iterator

On 2022-03-27 9:15, Xiaomeng Tong wrote:
> The bug is here:
> 	if (!encoder) {
> 
> The list iterator value 'encoder' will *always* be set and non-NULL
> by list_for_each_entry(), so it is incorrect to assume that the
> iterator value will be NULL if the list is empty or no element
> is found.
> 
> To fix the bug, use a new variable 'iter' as the list iterator,
> while use the original variable 'encoder' as a dedicated pointer
> to point to the found element.
> 
> Cc: stable@...r.kernel.org
> Fixes: ec9eab097a500 ("drm/tilcdc: Add drm bridge support for
> attaching drm bridge drivers")
> Signed-off-by: Xiaomeng Tong <xiam0nd.tong@...il.com>

Thanks for the patch. Good catch.

Reviewed-by: Jyri Sarha <jyri.sarha@....fi>
Tested-by: Jyri Sarha <jyri.sarha@....fi>

I'll apply this to drm-misc-next in couple of days.

Best regards,
Jyri

> ---
>  drivers/gpu/drm/tilcdc/tilcdc_external.c | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/gpu/drm/tilcdc/tilcdc_external.c
> b/drivers/gpu/drm/tilcdc/tilcdc_external.c
> index 7594cf6e186e..3b86d002ef62 100644
> --- a/drivers/gpu/drm/tilcdc/tilcdc_external.c
> +++ b/drivers/gpu/drm/tilcdc/tilcdc_external.c
> @@ -60,11 +60,13 @@ struct drm_connector
> *tilcdc_encoder_find_connector(struct drm_device *ddev,
>  int tilcdc_add_component_encoder(struct drm_device *ddev)
>  {
>  	struct tilcdc_drm_private *priv = ddev->dev_private;
> -	struct drm_encoder *encoder;
> +	struct drm_encoder *encoder = NULL, *iter;
> 
> -	list_for_each_entry(encoder, &ddev->mode_config.encoder_list, head)
> -		if (encoder->possible_crtcs & (1 << priv->crtc->index))
> +	list_for_each_entry(iter, &ddev->mode_config.encoder_list, head)
> +		if (iter->possible_crtcs & (1 << priv->crtc->index)) {
> +			encoder = iter;
>  			break;
> +		}
> 
>  	if (!encoder) {
>  		dev_err(ddev->dev, "%s: No suitable encoder found\n", __func__);