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?
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