linux-kernel - Re: [PATCH] drm/atomic_helper: Add missing NULL check for drm_plane_helper_funcs.atomic

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Message-ID: <htfplghwrowt4oihykcj53orgaeudo7a664ysyybint2oib3u5@lcyhfss3nyja>
Date: Mon, 30 Sep 2024 09:01:58 +0200
From: Maxime Ripard <mripard@...nel.org>
To: Lyude Paul <lyude@...hat.com>
Cc: dri-devel@...ts.freedesktop.org, stable@...r.kernel.org, 
	Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>, Thomas Zimmermann <tzimmermann@...e.de>, 
	David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>, 
	Sean Paul <seanpaul@...omium.org>, open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] drm/atomic_helper: Add missing NULL check for
 drm_plane_helper_funcs.atomic_update

Hi,

On Fri, Sep 27, 2024 at 04:46:16PM GMT, Lyude Paul wrote:
> Something I discovered while writing rvkms since some versions of the
> driver didn't have a filled out atomic_update function - we mention that
> this callback is "optional", but we don't actually check whether it's NULL
> or not before calling it. As a result, we'll segfault if it's not filled
> in.
> 
>   rvkms rvkms.0: [drm:drm_atomic_helper_commit_modeset_disables] modeset on [ENCODER:36:Virtual-36]
>   BUG: kernel NULL pointer dereference, address: 0000000000000000
>   PGD 0 P4D 0
>   Oops: Oops: 0010 [#1] PREEMPT SMP NOPTI
>   Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS edk2-20240813-1.fc40 08/13/2024
>   RIP: 0010:0x0
> 
> So, let's fix that.
> 
> Signed-off-by: Lyude Paul <lyude@...hat.com>
> Fixes: c2fcd274bce5 ("drm: Add atomic/plane helpers")
> Cc: dri-devel@...ts.freedesktop.org
> Cc: <stable@...r.kernel.org> # v3.19+

So we had kind of a similar argument with drm_connector_init early this
year, but I do agree we shouldn't fault if we're missing a callback.

I do wonder how we can implement a plane without atomic_update though?
Do we have drivers in such a case?

If not, a better solution would be to make it mandatory and check it
when registering.

Maxime

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