linux-kernel - Re: [PATCH v3 1/4] fbdev: Prevent possible use-after-free in fb

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Message-ID: <35ffd96d-3cbe-12dd-c1ea-878299ec173c@redhat.com>
Date:   Tue, 10 May 2022 11:06:17 +0200
From:   Javier Martinez Canillas <javierm@...hat.com>
To:     Thomas Zimmermann <tzimmermann@...e.de>,
        Andrzej Hajda <andrzej.hajda@...el.com>,
        linux-kernel@...r.kernel.org
Cc:     linux-fbdev@...r.kernel.org,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        Helge Deller <deller@....de>, dri-devel@...ts.freedesktop.org,
        Daniel Vetter <daniel.vetter@...el.com>
Subject: Re: [PATCH v3 1/4] fbdev: Prevent possible use-after-free in
 fb_release()

Hello Thomas,

On 5/10/22 10:50, Thomas Zimmermann wrote:

[snip]

>>> Drivers shouldn't really explicitly call this helper in my opinion.
> 
> One more stupid question: does armada actually use 
> drm_fbdev_fb_destroy()? It's supposed to be a callback for struct 
> fb_ops. Armada uses it's own instance of fb_ops, which apparently 
> doesn't contain fb_destroy. [1]
>

No stupid question at all. You are correct on this. So I guess we still
need this call in the drivers that don't provide a .fb_destroy() handler.

I see many options here:

1) Document in drm_fb_helper_alloc_fbi() that drivers only need to call
   drm_fb_helper_fini() explicitly if they are not setting up a fbdev
   with drm_fbdev_generic_setup(), otherwise is not needed.

2) Make drm_fbdev_fb_destroy() an exported symbol so drivers that have
   custom fb_ops can use it.

3) Set .fb_destroy to drm_fbdev_fb_destroy() if isn't set by drivers when
   they call drm_fb_helper_initial_config() or drm_fb_helper_fill_info().

I'm leaning towards option (3). Then the fb_info release will be automatic
whether drivers are using the generic setup or a custom one.

-- 
Best regards,

Javier Martinez Canillas
Linux Engineering
Red Hat