Re: refcounting drivers' data structures used in sysfs buffers

Alan Stern
2007-03-12T14:57:38Z

On Mon, 12 Mar 2007, Oliver Neukum wrote:

> > > Why? What's wrong with simply calling kref_get/put?
> >
> > It's the same old problem: the race between unbind and sysfs I/O. What
> > good does holding a reference to the private data structure do if the
> > show/store method gets called after the driver has been unbound from the
> > device? dev_get_drvdata() will no longer provide a valid pointer to the
> > private data, so the method will have no way to access it. Hence the
> > method needs another argument.
>
> It does half the job. You can make sure the driver is not asked to access
> freed memory.
> It is true that a driver will have to mark that device "disconnected"
> and return errors if that device's attributes are referenced, but this can
> be done internally.

No, you're missing the point. Let's say driver A's disconnect() is
called, so the driver marks its private data structure as "disconnected"
and does dev_set_drvdata(NULL). Then driver B is probed and bound to the
device, and it does its own dev_set_drvdata(). Then a user still holding
an open sysfs file reference for driver A calls a show() or store()
method. The method will do dev_get_drvdata(), receiving the pointer to
driver B's private data. Now you're in trouble, because A's method will
think it owns B's private data!

> Yes, this is a bit more complicated.
> {rant mode}
> Who came up with the idea of making life simpler by adding a code path?
> All these problems were already solved for device nodes. Ioctl is ugly, but
> at least a known code path.
> {rant off}

I'll let Greg give the complete answer. :-) Bear in mind, however, that
the aim was probably to make life simpler for userspace -- which does not
mean making life simpler for the kernel.

(Incidentally, I'm not so sure that all these problems really were solved
by ioctl on device nodes. I bet you could find plenty of cases where
ioctl races with disconnect if you looked.)

Alan Stern