Conceptual bug on SoundWire probe/remove?

[Greg Kroah-Hartman]

On Fri, Mar 25, 2022 at 10:51:51AM -0500, Pierre-Louis Bossart wrote:
> 
> 
> On 3/24/22 06:15, Greg Kroah-Hartman wrote:
> > On Wed, Mar 23, 2022 at 02:45:59PM -0500, Pierre-Louis Bossart wrote:
> > > Hi,
> > > I could use feedback/guidance on a possible conceptual bug in the SoundWire
> > > probe and bus handling.
> > > 
> > > When we probe a driver, the code does this:
> > > 
> > > static int sdw_drv_probe(struct device *dev)
> > > {
> > > 	struct sdw_slave *slave = dev_to_sdw_dev(dev);
> > > 	struct sdw_driver *drv = drv_to_sdw_driver(dev->driver);
> > > 	const struct sdw_device_id *id;
> > > 	const char *name;
> > > 	int ret;
> > > 
> > > 	/*
> > > 	 * fw description is mandatory to bind
> > > 	 */
> > > 	if (!dev->fwnode)
> > > 		return -ENODEV;
> > > 
> > > 	if (!IS_ENABLED(CONFIG_ACPI) && !dev->of_node)
> > > 		return -ENODEV;
> > > 
> > > 	id = sdw_get_device_id(slave, drv);
> > > 	if (!id)
> > > 		return -ENODEV;
> > > 
> > > 	slave->ops = drv->ops;
> > 
> > That is wrong and should never happen as you lost all reference
> > counting.  Please don't do that.
> 
> ok, so I think we all agree on the issue. It's not new code, it's been there
> since December 2017 and 4.16

It's hard to notice that in code review :(

> > > The last line is the problematic one. If at some point, the user does an
> > > rmmod and unbinds the SoundWire codec driver, the .remove will be called and
> > > the 'drv' will no longer be valid, but we will still have a reference to
> > > drv->ops and use that pointer in the bus code, e.g.
> > > 
> > > 		/* Update the Slave driver */
> > > 		if (slave_notify && slave->ops &&
> > > 		    slave->ops->interrupt_callback) {
> > > 			slave_intr.sdca_cascade = sdca_cascade;
> > > 			slave_intr.control_port = clear;
> > > 			memcpy(slave_intr.port, &port_status,
> > > 			       sizeof(slave_intr.port));
> > > 
> > > 			slave->ops->interrupt_callback(slave,
> > > &slave_intr);
> > > 		}
> > > 
> > > I noodled with a potential fix in
> > > https://github.com/thesofproject/linux/pull/3534/commits/82d64fb0fd39b532263f060a8ec86e47e9ab305b
> > > 
> > > where I force-reset this slave->ops pointer, but it is likely to be very
> > > racy.
> > 
> > Just properly reference count things and this should be ok.  You can
> > NEVER just save off a pointer to a random thing that you do not own
> > without increasing the reference count, otherwise bad things will
> > happen.  It's always been this way.
> 
> but I am not following the reference count recommendation in that specific
> case. If we increase a reference count in the probe, wouldn't that prevent
> the unbind/remove?

Depends on what you are increasing the reference count of :)

bind/unbind has nothing to do with the reference count of objects, it
only prevents the devices/whatever from being removed from the system
(hopefully.)

When doing "give me the driver ops of that driver over there" you have
to be VERY careful.  It's a very uncommon pattern that I can't think of
anyone else doing outside of a bus logic.  I don't recommend it, instead
grab references to the devices themselves and go through the device and
what is bound/unbound to that.  Also keep things in sync when you grab
those other devices, that can also get messy.

How about just not doing this at all?