linux-kernel - Re: [PATCH] drm/of: free the iterator object on failure

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Message-ID: <YOxyvIoJcZFAgUz5@pendragon.ideasonboard.com>
Date:   Mon, 12 Jul 2021 19:50:04 +0300
From:   Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:     Steven Price <steven.price@....com>
Cc:     Daniel Vetter <daniel@...ll.ch>, David Airlie <airlied@...ux.ie>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        Biju Das <biju.das.jz@...renesas.com>
Subject: Re: [PATCH] drm/of: free the iterator object on failure

Hi Steven,

Thank you for the patch.

On Mon, Jul 12, 2021 at 04:57:58PM +0100, Steven Price wrote:
> When bailing out due to the sanity check the iterator value needs to be
> freed because the early return prevents for_each_child_of_node() from
> doing the dereference itself.
> 
> Fixes: 4ee48cc5586b ("drm: of: Fix double-free bug")

I don't think the Fixes tag is correct, the issue was already present
before 4ee48cc5586b. The fix looks right though.

> Signed-off-by: Steven Price <steven.price@....com>
> ---
>  drivers/gpu/drm/drm_of.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> Daniel's email[1] made me take a look at this function and it appears
> that for_each_child_of_node()'s interface had caused a bad bug fix due
> to the hidden reference counting in the iterator.
> 
> [1] https://lore.kernel.org/r/YOxQ5TbkNrqCGBDJ%40phenom.ffwll.local
> 
> diff --git a/drivers/gpu/drm/drm_of.c b/drivers/gpu/drm/drm_of.c
> index 197c57477344..997b8827fed2 100644
> --- a/drivers/gpu/drm/drm_of.c
> +++ b/drivers/gpu/drm/drm_of.c
> @@ -331,8 +331,10 @@ static int drm_of_lvds_get_remote_pixels_type(
>  		 * configurations by passing the endpoints explicitly to
>  		 * drm_of_lvds_get_dual_link_pixel_order().
>  		 */
> -		if (!current_pt || pixels_type != current_pt)
> +		if (!current_pt || pixels_type != current_pt) {
> +			of_node_put(endpoint);
>  			return -EINVAL;
> +		}
>  	}
>  
>  	return pixels_type;

-- 
Regards,

Laurent Pinchart