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?