Re: refcounting drivers' data structures used in sysfs buffers

[Oliver Neukum]

Am Montag, 12. März 2007 15:57 schrieb Alan Stern:probably nece
> 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, I was missing the point. In consequence, drivers must not use
dev_get_drvdata() to get their references to their private data. It's
probably necessary to store it in struct sysfs_buffer and include that
in the store/show callbacks.
(The same does apply to interfaces of course)

> > 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.

That doesn't mean that the method needed to be thrown out.
Sysfs could simply pass through the syscalls for a device, like
it is done in character devices. I am tempted to recommend
such radical surgery.

> (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.)

I will look. Death to all race conditions.

	Regards
		Oliver
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