Re: [PATCH 3/3] drm/bridge: add warning for bridges not using devm_drm_bridge_alloc()

[Maxime Ripard]

Hi Luca,

On Fri, Jun 20, 2025 at 11:32:08AM +0200, 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@xxxxxxxxxxx>
> 
> ---
> 
> 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..7d511c28608f1d8ea8fbb81d00efa9e227b02a13 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)
>  {
> +	if (kref_read(&bridge->refcount) == 0)
> +		DRM_WARN("DRM bridge corrupted or not allocated by devm_drm_bridge_alloc()\n");
> +

I'm fine with it on principle, but I wonder if using bridge->container
is set wouldn't be a more obvious way to check it?

Maxime

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