linux-kernel - Re: [PATCH 2/2] fbdev: Make fb_release() return -ENODEV if fbdev was unregistered

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Date:   Wed, 4 May 2022 11:47:57 +0200
From:   Daniel Vetter <daniel@...ll.ch>
To:     Javier Martinez Canillas <javierm@...hat.com>
Cc:     linux-kernel@...r.kernel.org, Maxime Ripard <maxime@...no.tech>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Alex Deucher <alexander.deucher@....com>,
        Changcheng Deng <deng.changcheng@....com.cn>,
        Daniel Vetter <daniel@...ll.ch>,
        Guenter Roeck <linux@...ck-us.net>,
        Helge Deller <deller@....de>, Sam Ravnborg <sam@...nborg.org>,
        Zhen Lei <thunder.leizhen@...wei.com>,
        Zheyu Ma <zheyuma97@...il.com>,
        dri-devel@...ts.freedesktop.org, linux-fbdev@...r.kernel.org
Subject: Re: [PATCH 2/2] fbdev: Make fb_release() return -ENODEV if fbdev was
 unregistered

On Mon, May 02, 2022 at 03:09:44PM +0200, Javier Martinez Canillas wrote:
> A reference to the framebuffer device struct fb_info is stored in the file
> private data, but this reference could no longer be valid and must not be
> accessed directly. Instead, the file_fb_info() accessor function must be
> used since it does sanity checking to make sure that the fb_info is valid.
> 
> This can happen for example if the fbdev driver was one that is using a
> framebuffer provided by the system firmware. In that case, the fbdev core
> could unregister the framebuffer device if a real video driver is probed.
> 
> Reported-by: Maxime Ripard <maxime@...no.tech>
> Signed-off-by: Javier Martinez Canillas <javierm@...hat.com>

Doesn't this mean we just leak the references? Also anything the driver
might refcount in fb_open would be leaked too.

I'm not sure what exactly you're trying to fix here, but this looks a bit
wrong.

Maybe stepping back what fbdev would need, but doesn't have (see the
commit reference I dropped on the previous version) is drm_dev_enter/exit
around hw access. the file_fb_info check essentially provides that, but
with races and everything.

But drm_dev_enter/exit should not disable sw side code, especially not
refcount cleanup like fb_release does here.
-Daniel

> ---
> 
>  drivers/video/fbdev/core/fbmem.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/video/fbdev/core/fbmem.c b/drivers/video/fbdev/core/fbmem.c
> index 20d8929df79f..d68097105f93 100644
> --- a/drivers/video/fbdev/core/fbmem.c
> +++ b/drivers/video/fbdev/core/fbmem.c
> @@ -1439,7 +1439,10 @@ fb_release(struct inode *inode, struct file *file)
>  __acquires(&info->lock)
>  __releases(&info->lock)
>  {
> -	struct fb_info * const info = file->private_data;
> +	struct fb_info * const info = file_fb_info(file);
> +
> +	if (!info)
> +		return -ENODEV;
>  
>  	lock_fb_info(info);
>  	if (info->fbops->fb_release)
> -- 
> 2.35.1
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch