Re: [PATCH] drm/mediatek: Fix device/node reference count leaks in mtk_drm_get_all_drm_priv

["CK Hu (胡俊光)" <ck.hu@mediatek.com>]

On Fri, 2025-07-18 at 11:32 +0800, Ma Ke wrote:
> External email : Please do not click links or open attachments until you have verified the sender or the content.
> 
> 
> Using device_find_child() and of_find_device_by_node() to locate
> devices could cause an imbalance in the device's reference count.
> device_find_child() and of_find_device_by_node() both call
> get_device() to increment the reference count of the found device
> before returning the pointer. In mtk_drm_get_all_drm_priv(), these
> references are never released through put_device(), resulting in
> permanent reference count increments. Additionally, the
> for_each_child_of_node() iterator fails to release node references in
> all code paths. This leaks device node references when loop
> termination occurs before reaching MAX_CRTC. These reference count
> leaks may prevent device/node resources from being properly released
> during driver unbind operations.
> 
> As comment of device_find_child() says, 'NOTE: you will need to drop
> the reference with put_device() after use'.
> 
> Found by code review.
> 
> Cc: stable@vger.kernel.org
> Fixes: 1ef7ed48356c ("drm/mediatek: Modify mediatek-drm for mt8195 multi mmsys support")
> Signed-off-by: Ma Ke <make24@iscas.ac.cn>
> ---
>  drivers/gpu/drm/mediatek/mtk_drm_drv.c | 27 +++++++++++++++++---------
>  1 file changed, 18 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/gpu/drm/mediatek/mtk_drm_drv.c b/drivers/gpu/drm/mediatek/mtk_drm_drv.c
> index 7c0c12dde488..c78186debd3e 100644
> --- a/drivers/gpu/drm/mediatek/mtk_drm_drv.c
> +++ b/drivers/gpu/drm/mediatek/mtk_drm_drv.c
> @@ -388,19 +388,24 @@ static bool mtk_drm_get_all_drm_priv(struct device *dev)
> 
>                 of_id = of_match_node(mtk_drm_of_ids, node);
>                 if (!of_id)
> -                       continue;
> +                       goto next;
> 
>                 pdev = of_find_device_by_node(node);
>                 if (!pdev)
> -                       continue;
> +                       goto next;
> 
>                 drm_dev = device_find_child(&pdev->dev, NULL, mtk_drm_match);
> -               if (!drm_dev)
> -                       continue;
> +               if (!drm_dev) {
> +                       put_device(&pdev->dev);
> +                       goto next;

I've define label next_put_device_pdev_dev below, so

goto next_put_device_pdev_dev;

> +               }
> 
>                 temp_drm_priv = dev_get_drvdata(drm_dev);
> -               if (!temp_drm_priv)
> -                       continue;
> +               if (!temp_drm_priv) {
> +                       put_device(drm_dev);
> +                       put_device(&pdev->dev);
> +                       goto next;


I've define label next_put_device_drm_dev below, so

goto next_put_device_drm_dev;

> +               }
> 
>                 if (temp_drm_priv->data->main_len)
>                         all_drm_priv[CRTC_MAIN] = temp_drm_priv;
> @@ -412,10 +417,14 @@ static bool mtk_drm_get_all_drm_priv(struct device *dev)
>                 if (temp_drm_priv->mtk_drm_bound)
>                         cnt++;
> 
> -               if (cnt == MAX_CRTC) {
> -                       of_node_put(node);

next_put_device_drm_dev:

> +               put_device(drm_dev);

next_put_device_pdev_dev:

> +               put_device(&pdev->dev);
> +
> +next:

next_put_node:

Regards,
CK

> +               of_node_put(node);
> +
> +               if (cnt == MAX_CRTC)
>                         break;
> -               }
>         }
> 
>         if (drm_priv->data->mmsys_dev_num == cnt) {
> --
> 2.25.1
> 
>