linux-kernel - Re: [PATCH v9 3/3] drm/bridge: add warning for bridges not using devm_drm_bridge

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Message-ID: <a506cff0-6a0f-44b2-93e5-0488aa041fa2@oss.qualcomm.com>
Date: Mon, 23 Jun 2025 14:06:43 +0300
From: Dmitry Baryshkov <dmitry.baryshkov@....qualcomm.com>
To: Luca Ceresoli <luca.ceresoli@...tlin.com>,
        Andrzej Hajda <andrzej.hajda@...el.com>,
        Neil Armstrong <neil.armstrong@...aro.org>,
        Robert Foss <rfoss@...nel.org>,
        Laurent Pinchart <Laurent.pinchart@...asonboard.com>,
        Jonas Karlman <jonas@...boo.se>,
        Jernej Skrabec <jernej.skrabec@...il.com>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>
Cc: Anusha Srivatsa <asrivats@...hat.com>, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org, Hui Pu <Hui.Pu@...ealthcare.com>,
        Thomas Petazzoni <thomas.petazzoni@...tlin.com>
Subject: Re: [PATCH v9 3/3] drm/bridge: add warning for bridges not using
 devm_drm_bridge_alloc()

On 20/06/2025 18:59, Luca Ceresoli wrote:
> To the best of my knowledge, all drivers in the mainline kernel adding a
> DRM bridge are now converted to using devm_drm_bridge_alloc() for
> allocation and initialization. Among others this ensures initialization of
> the bridge refcount, allowing dynamic allocation lifetime.
> 
> devm_drm_bridge_alloc() is now mandatory for all new bridges. Code using
> the old pattern ([devm_]kzalloc + filling the struct fields +
> drm_bridge_add) is not allowed anymore.
> 
> Any drivers that might have been missed during the conversion, patches in
> flight towards mainline and out-of-tre drivers still using the old pattern
> will already be caught by a warning looking like:
> 
>    ------------[ cut here ]------------
>    refcount_t: addition on 0; use-after-free.
>    WARNING: CPU: 2 PID: 83 at lib/refcount.c:25 refcount_warn_saturate+0x120/0x148
>    [...]
>    Call trace:
>     refcount_warn_saturate+0x120/0x148 (P)
>     drm_bridge_get.part.0+0x70/0x98 [drm]
>     drm_bridge_add+0x34/0x108 [drm]
>     sn65dsi83_probe+0x200/0x480 [ti_sn65dsi83]
>     [...]
> 
> This warning comes from the refcount code and happens because
> drm_bridge_add() is increasing the refcount, which is uninitialized and
> thus initially zero.
> 
> Having a warning and the corresponding stack trace is surely useful, but
> the warning text does not clarify the root problem nor how to fix it.
> 
> Add a DRM_WARN() just before increasing the refcount, so the log will be
> much more readable:
> 
>    [drm] DRM bridge corrupted or not allocated by devm_drm_bridge_alloc()
>    ------------[ cut here ]------------
>    refcount_t: addition on 0; use-after-free.
>    [...etc...]
> 
> A DRM_WARN is used because drm_warn and drm_WARN require a struct
> drm_device pointer which is not yet available when adding a bridge.
> 
> Do not print the dev_name() in the warning because struct drm_bridge has no
> pointer to the struct device. The affected driver should be easy to catch
> based on the following stack trace however.
> 
> Signed-off-by: Luca Ceresoli <luca.ceresoli@...tlin.com>
> 
> ---
> 
> Changes in v9:
> - change warning trigger from "refcount != 1" to "container not NULL"
> 
> This patch was added in v8
> ---
>   drivers/gpu/drm/drm_bridge.c | 3 +++
>   1 file changed, 3 insertions(+)
> 
> diff --git a/drivers/gpu/drm/drm_bridge.c b/drivers/gpu/drm/drm_bridge.c
> index f001bbe95559aabf0aac9f25f89250ad4e1ad9c8..0b450b334afd82e0460f18fdd248f79d0a2b153d 100644
> --- a/drivers/gpu/drm/drm_bridge.c
> +++ b/drivers/gpu/drm/drm_bridge.c
> @@ -295,6 +295,9 @@ EXPORT_SYMBOL(__devm_drm_bridge_alloc);
>    */
>   void drm_bridge_add(struct drm_bridge *bridge)

Can we rename this function, making sure that noone can call it 
directly? E.g. adding two underscores might point out that is is an 
internal API.

>   {
> +	if (!bridge->container)
> +		DRM_WARN("DRM bridge corrupted or not allocated by devm_drm_bridge_alloc()\n");
> +
>   	drm_bridge_get(bridge);
>   
>   	mutex_init(&bridge->hpd_mutex);
> 


-- 
With best wishes
Dmitry