linux-kernel - Re: refcounting drivers' data structures used in sysfs buffers

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Date:	Mon, 12 Mar 2007 10:57:33 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Oliver Neukum <oneukum@...e.de>
cc:	Dmitry Torokhov <dmitry.torokhov@...il.com>,
	Maneesh Soni <maneesh@...ibm.com>, <gregkh@...e.de>,
	<linux-kernel@...r.kernel.org>
Subject: Re: refcounting drivers' data structures used in sysfs buffers

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

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