Date:   Tue, 24 Apr 2018 12:14:46 +0200
From:   Peter Rosin <peda@...ntia.se>
To:     Russell King - ARM Linux <linux@...linux.org.uk>
Cc:     linux-kernel@...r.kernel.org, David Airlie <airlied@...ux.ie>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Nicolas Ferre <nicolas.ferre@...rochip.com>,
        Alexandre Belloni <alexandre.belloni@...tlin.com>,
        Boris Brezillon <boris.brezillon@...e-electrons.com>,
        Jyri Sarha <jsarha@...com>,
        Tomi Valkeinen <tomi.valkeinen@...com>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        Jacopo Mondi <jacopo+renesas@...ndi.org>,
        dri-devel@...ts.freedesktop.org, devicetree@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v4 7/8] drm/i2c: tda998x: register as a drm bridge

On 2018-04-24 10:08, Russell King - ARM Linux wrote:
> On Tue, Apr 24, 2018 at 08:58:42AM +0200, Peter Rosin wrote:
>> On 2018-04-23 18:08, Russell King - ARM Linux wrote:
>>> On Mon, Apr 23, 2018 at 09:23:00AM +0200, Peter Rosin wrote:
>>>>  static int tda998x_remove(struct i2c_client *client)
>>>>  {
>>>> -	component_del(&client->dev, &tda998x_ops);
>>>> +	struct device *dev = &client->dev;
>>>> +	struct tda998x_bridge *bridge = dev_get_drvdata(dev);
>>>> +
>>>> +	drm_bridge_remove(&bridge->bridge);
>>>> +	component_del(dev, &tda998x_ops);
>>>> +
>>>
>>> I'd like to ask a rather fundamental question about DRM bridge support,
>>> because I suspect that there's a major fsckup here.
>>>
>>> The above is the function that deals with the TDA998x device being
>>> unbound from the driver.  With the component API, this results in the
>>> DRM device correctly being torn down, because one of the hardware
>>> devices has gone.
>>>
>>> With DRM bridge, the bridge is merely removed from the list of
>>> bridges:
>>>
>>> void drm_bridge_remove(struct drm_bridge *bridge)
>>> {
>>>         mutex_lock(&bridge_lock);
>>>         list_del_init(&bridge->list);
>>>         mutex_unlock(&bridge_lock);
>>> }
>>> EXPORT_SYMBOL(drm_bridge_remove);
>>>
>>> and the memory backing the "struct tda998x_bridge" (which contains
>>> the struct drm_bridge) will be freed by the devm subsystem.
>>>
>>> However, there is no notification into the rest of the DRM subsystem
>>> that the device has gone away.  Worse, the memory that is still in
>>> use by DRM has now been freed, so further use of the DRM device
>>> results in a use-after-free bug.
>>>
>>> This is really not good, and to me looks like a fundamental problem
>>> with the DRM bridge code.  I see nothing in the DRM bridge code that
>>> deals with the lifetime of a "DRM bridge" or indeed the lifetime of
>>> the actual device itself.
>>>
>>> So, from what I can see, there seems to be a fundamental lifetime
>>> issue with the design of the DRM bridge code.  This needs to be
>>> fixed.
>>
>> Oh crap. A gigantic can of worms...
> 
> Yes, it's especially annoying for me, having put the effort in to
> the component helper to cover all these cases.
> 
>> Would a patch (completely untested btw) along this line of thinking make
>> any difference whatsoever?
> 
> It looks interesting - from what I can see of the device links code,
> it would have the effect of unbinding the DRM device just before
> TDA998x is unbound, so that's an improvement.
> 
> However, from what I can see, the link vanishes at that point (as
> DL_FLAG_AUTOREMOVE is set), and re-binding the TDA998x device results
> in nothing further happening - the link will be recreated, but there
> appears to be nothing that triggers the "consumer" to rebind at that
> point.  Maybe I've missed something?

Right, auto-remove is a no-go. So, improving on the previous...

(I think drm_panel might suffer from this issue too?)

Cheers,
Peter

diff --git a/drivers/gpu/drm/drm_bridge.c b/drivers/gpu/drm/drm_bridge.c
index 1638bfe9627c..b1365cfee445 100644
--- a/drivers/gpu/drm/drm_bridge.c
+++ b/drivers/gpu/drm/drm_bridge.c
@@ -121,12 +121,17 @@ int drm_bridge_attach(struct drm_encoder *encoder, struct drm_bridge *bridge,
 	if (bridge->dev)
 		return -EBUSY;
 
+	bridge->link = device_link_add(encoder->dev->dev, bridge->owner, 0);
+	if (!bridge->link)
+		return -EINVAL;
+
 	bridge->dev = encoder->dev;
 	bridge->encoder = encoder;
 
 	if (bridge->funcs->attach) {
 		ret = bridge->funcs->attach(bridge);
 		if (ret < 0) {
+			device_link_del(bridge->link);
 			bridge->dev = NULL;
 			bridge->encoder = NULL;
 			return ret;
@@ -153,6 +158,8 @@ void drm_bridge_detach(struct drm_bridge *bridge)
 	if (bridge->funcs->detach)
 		bridge->funcs->detach(bridge);
 
+	device_link_del(bridge->link);
+
 	bridge->dev = NULL;
 }
 
diff --git a/drivers/gpu/drm/i2c/tda998x_drv.c b/drivers/gpu/drm/i2c/tda998x_drv.c
index b8cb6237a38b..29eba4e9a39d 100644
--- a/drivers/gpu/drm/i2c/tda998x_drv.c
+++ b/drivers/gpu/drm/i2c/tda998x_drv.c
@@ -1857,6 +1857,7 @@ tda998x_probe(struct i2c_client *client, const struct i2c_device_id *id)
 	bridge->dev = dev;
 	dev_set_drvdata(dev, bridge);
 
+	bridge->bridge.owner = dev;
 	bridge->bridge.funcs = &tda998x_bridge_funcs;
 #ifdef CONFIG_OF
 	bridge->bridge.of_node = dev->of_node;
diff --git a/include/drm/drm_bridge.h b/include/drm/drm_bridge.h
index 682d01ba920c..b8f33aba3216 100644
--- a/include/drm/drm_bridge.h
+++ b/include/drm/drm_bridge.h
@@ -224,6 +224,8 @@ struct drm_bridge_funcs {
 
 /**
  * struct drm_bridge - central DRM bridge control structure
+ * @owner: device that owns the bridge
+ * @link: drm consumer <-> bridge supplier
  * @dev: DRM device this bridge belongs to
  * @encoder: encoder to which this bridge is connected
  * @next: the next bridge in the encoder chain
@@ -233,6 +235,8 @@ struct drm_bridge_funcs {
  * @driver_private: pointer to the bridge driver's internal context
  */
 struct drm_bridge {
+	struct device *owner;
+	struct device_link *link;
 	struct drm_device *dev;
 	struct drm_encoder *encoder;
 	struct drm_bridge *next;

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