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@bootlin.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)
 {
+	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);

-- 
2.49.0

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@bootlin.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

Hello Dmitry,

thanks for reviewing!

On Mon, 23 Jun 2025 14:06:43 +0300
Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com> wrote:

> 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@bootlin.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.

I'm afraid I don't think this would be correct. Every bridge driver is
expected to call drm_bridge_add() at the end of probe/initialization,
to make the DRM subsystem aware that this bridge is ready for being
used.

The point of this patch, which is a completion to [1], is to ensure
that all drivers use the "new" process:

 1. bridge = devm_drm_bridge_alloc()
 2. drm_bridge_add(bridge)

and there are no users of the old process:

 1. bridge = [devm_]kzalloc()
 2. drm_bridge_add(bridge)

Does this addresses your concern?

[1] https://cgit.freedesktop.org/drm/drm-misc/commit/?id=b3f13e00a8de351832df7d628a15ca4db49ca94f

Luca

-- 
Luca Ceresoli, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com

On Fri, 20 Jun 2025 17:59:55 +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.
> 
> 
> [ ... ]

Reviewed-by: Maxime Ripard <mripard@kernel.org>

Thanks!
Maxime