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: <920b1b9f566471efc2438c259dca683e02c87943.camel@mediatek.com>
Date: Fri, 15 Aug 2025 03:50:22 +0000
From: CK Hu (胡俊光) <ck.hu@...iatek.com>
To: "chunkuang.hu@...nel.org" <chunkuang.hu@...nel.org>, "make24@...as.ac.cn"
	<make24@...as.ac.cn>, AngeloGioacchino Del Regno
	<angelogioacchino.delregno@...labora.com>,
	Nancy Lin (林欣螢) <Nancy.Lin@...iatek.com>,
	"airlied@...il.com" <airlied@...il.com>, "simona@...ll.ch" <simona@...ll.ch>,
	"p.zabel@...gutronix.de" <p.zabel@...gutronix.de>, "matthias.bgg@...il.com"
	<matthias.bgg@...il.com>
CC: "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
	"linux-mediatek@...ts.infradead.org" <linux-mediatek@...ts.infradead.org>,
	"stable@...r.kernel.org" <stable@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org"
	<linux-arm-kernel@...ts.infradead.org>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "akpm@...ux-foundation.org"
	<akpm@...ux-foundation.org>
Subject: Re: [PATCH v2] drm/mediatek: Fix device/node reference count leaks in
 mtk_drm_get_all_drm_priv

On Tue, 2025-08-12 at 15:19 +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.

Reviewed-by: CK Hu <ck.hu@...iatek.com>

> 
> 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
>