linux-kernel - Re: [PATCH v2] drm/mediatek: Fix device/node reference count leaks in mtk_drm_get_all_drm

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Message-ID: <CAAOTY_91C=5YDk1qcU3Dir6TqtN17kgjzTzr=XYRNPHGvQwSgA@mail.gmail.com>
Date: Sat, 16 Aug 2025 07:40:42 +0800
From: Chun-Kuang Hu <chunkuang.hu@...nel.org>
To: Ma Ke <make24@...as.ac.cn>
Cc: ck.hu@...iatek.com, chunkuang.hu@...nel.org, p.zabel@...gutronix.de, 
	airlied@...il.com, simona@...ll.ch, matthias.bgg@...il.com, 
	angelogioacchino.delregno@...labora.com, nancy.lin@...iatek.com, 
	dri-devel@...ts.freedesktop.org, linux-mediatek@...ts.infradead.org, 
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org, 
	akpm@...ux-foundation.org, stable@...r.kernel.org
Subject: Re: [PATCH v2] drm/mediatek: Fix device/node reference count leaks in mtk_drm_get_all_drm_priv

Ma Ke <make24@...as.ac.cn> 於 2025年8月12日 週二 下午3:19寫道：
>
> 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.

Applied to mediatek-drm-fixes [1], thanks.

[1] https://git.kernel.org/pub/scm/linux/kernel/git/chunkuang.hu/linux.git/log/?h=mediatek-drm-fixes

Regards,
Chun-Kuang.

>
> Cc: stable@...r.kernel.org
> Fixes: 1ef7ed48356c ("drm/mediatek: Modify mediatek-drm for mt8195 multi mmsys support")
> Signed-off-by: Ma Ke <make24@...as.ac.cn>
> ---
> Changes in v2:
> - added goto labels as suggestions.
> ---
>  drivers/gpu/drm/mediatek/mtk_drm_drv.c | 21 ++++++++++++++-------
>  1 file changed, 14 insertions(+), 7 deletions(-)
>
> diff --git a/drivers/gpu/drm/mediatek/mtk_drm_drv.c b/drivers/gpu/drm/mediatek/mtk_drm_drv.c
> index d5e6bab36414..f8a817689e16 100644
> --- a/drivers/gpu/drm/mediatek/mtk_drm_drv.c
> +++ b/drivers/gpu/drm/mediatek/mtk_drm_drv.c
> @@ -387,19 +387,19 @@ 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_put_node;
>
>                 pdev = of_find_device_by_node(node);
>                 if (!pdev)
> -                       continue;
> +                       goto next_put_node;
>
>                 drm_dev = device_find_child(&pdev->dev, NULL, mtk_drm_match);
>                 if (!drm_dev)
> -                       continue;
> +                       goto next_put_device_pdev_dev;
>
>                 temp_drm_priv = dev_get_drvdata(drm_dev);
>                 if (!temp_drm_priv)
> -                       continue;
> +                       goto next_put_device_drm_dev;
>
>                 if (temp_drm_priv->data->main_len)
>                         all_drm_priv[CRTC_MAIN] = temp_drm_priv;
> @@ -411,10 +411,17 @@ 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_put_node:
> +               of_node_put(node);
> +
> +               if (cnt == MAX_CRTC)
>                         break;
> -               }
>         }
>
>         if (drm_priv->data->mmsys_dev_num == cnt) {
> --
> 2.25.1
>