linux-kernel - Re: [PATCH v2] media: v4l2-dev: fix error handling in __video_register

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Message-ID: <Z9lmKWC-JlVo6Biw@kekkonen.localdomain>
Date: Tue, 18 Mar 2025 12:25:13 +0000
From: Sakari Ailus <sakari.ailus@...ux.intel.com>
To: Ma Ke <make24@...as.ac.cn>
Cc: mchehab@...nel.org, hverkuil@...all.nl, sebastian.fricke@...labora.com,
	ribalda@...omium.org, benjamin.gaignard@...labora.com,
	viro@...iv.linux.org.uk, bartosz.golaszewski@...aro.org,
	hljunggr@...co.com, linux-media@...r.kernel.org,
	linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH v2] media: v4l2-dev: fix error handling in
 __video_register_device()

Hi Ma,

Thanks for the update.

On Tue, Mar 18, 2025 at 05:09:45PM +0800, Ma Ke wrote:
> Once device_register() failed, we should call put_device() to
> decrement reference count for cleanup. Or it could cause memory leak.
> And move callback function before put_device().
> 
> As comment of device_register() says, 'NOTE: _Never_ directly free
> @dev after calling this function, even if it returned an error! Always
> use put_device() to give up the reference initialized in this function
> instead.'
> 
> Found by code review.
> 
> Cc: stable@...r.kernel.org
> Fixes: baa057e29b58 ("media: v4l2-dev: use pr_foo() for printing messages")
> Signed-off-by: Ma Ke <make24@...as.ac.cn>
> ---
> Changes in v2:
> - modified the patch as no callback function before put_device().
> ---
>  drivers/media/v4l2-core/v4l2-dev.c | 7 +++----
>  1 file changed, 3 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/media/v4l2-core/v4l2-dev.c b/drivers/media/v4l2-core/v4l2-dev.c
> index 5bcaeeba4d09..4a8fdf8115c0 100644
> --- a/drivers/media/v4l2-core/v4l2-dev.c
> +++ b/drivers/media/v4l2-core/v4l2-dev.c
> @@ -1054,17 +1054,16 @@ int __video_register_device(struct video_device *vdev,
>  	vdev->dev.class = &video_class;
>  	vdev->dev.devt = MKDEV(VIDEO_MAJOR, vdev->minor);
>  	vdev->dev.parent = vdev->dev_parent;
> +	vdev->dev.release = v4l2_device_release;
>  	dev_set_name(&vdev->dev, "%s%d", name_base, vdev->num);
>  	mutex_lock(&videodev_lock);
>  	ret = device_register(&vdev->dev);
>  	if (ret < 0) {
>  		mutex_unlock(&videodev_lock);
>  		pr_err("%s: device_register failed\n", __func__);
> -		goto cleanup;
> +		put_device(&vdev->dev);
> +		return ret;
>  	}
> -	/* Register the release callback that will be called when the last
> -	   reference to the device goes away. */
> -	vdev->dev.release = v4l2_device_release;
>  
>  	if (nr != -1 && nr != vdev->num && warn_if_nr_in_use)
>  		pr_warn("%s: requested %s%d, got %s\n", __func__,

I think this still needs some work. E.g. v4l2_device_get() hasn't been
called yet here, but still v4l2_device_release() will call
v4l2_device_put().

-- 
Regards,

Sakari Ailus