Subject: Re: [PATCH 1/7] uio: add module owner to prevent inappropriate module unloading

From: Sebastian Andrzej Siewior <[email-masked]>
Date: Thu, 15 Aug 2013 18:03:44 +0200

On 08/15/2013 05:55 PM, Greg Kroah-Hartman wrote:
> But that's a "platform" device, for a resource that is described as not
> going away.
> 
> If this is really a mfd device, then make your uio driver a mfd driver,
> not a platform driver for a resource that isn't under your control.

As you described it later yourself: You have the same problem if you
manually unbind the platform_device from the driver while the device
node is open.

>> If you look now at uio_write() then you will notice that it will
>> deference idev->info->irqcontrol but once the device is gone the memory
>> starting at info is gone, not to mention the code behind irqcontrol.
> 
> It sounds like the wrong uio driver is binding to this device, fix the
> uio driver and you should be fine, right?

For this to happen you would need a refcount in uio-core which learns
that the device is gone and does not invoke any callbacks because the
device is gone. Something like you have in USB where you return 0 on
reads from ttyUSB after someone pulled the cable.

> A module reference count will not "save" you from a device going away,
> only a code chunk going away.  That is why no other subsystem has this
> type of thing.  If you dynamically remove the mfd device, but not remove
> the module (i.e. through the sysfs files to do that), then you would
> still have this same problem, right?

Yes, I think so.

> There's a reason the driver core doesn't deal with module reference
> counts, it's not the proper thing for devices.  So I'm not willing to
> add it to the UIO code either, as it's not the correct thing for it.

okay.

> thanks,
> greg k-h

Sebastian