Re: race in sysfs between sysfs_remove_file() and read()/write() #2

[Maneesh Soni <maneesh@in.ibm.com>]

On Mon, Dec 04, 2006 at 07:38:00AM +0100, Oliver Neukum wrote:
> Am Montag, 4. Dezember 2006 05:43 schrieb Maneesh Soni:
> > On Fri, Dec 01, 2006 at 11:43:06PM +0100, Oliver Neukum wrote:
> > > Hi,
> > >
> > > Alan Stern has discovered a race in sysfs, whereby driver callbacks could be
> > > called after sysfs_remove_file() has run. The attached patch should fix it.
> > >
> > > It introduces a new data structure acting as a collection of all sysfs_buffers
> > > associated with an attribute. Upon removal of an attribute the buffers are
> > > marked orphaned and IO on them returns -ENODEV. Thus sysfs_remove_file()
> > > makes sure that sysfs won't bother a driver after that call, making it safe
> > > to free the associated data structures and to unload the driver.
> > >
> > > 	Regards
> > > 		Oliver
> >
> > Hi Oliver,
> >
> > Thanks for the explaining the patch but some description about the race
> > would also help here. At the least the callpath to the race would be useful.
> >
> >
> > Thanks
> > Maneesh
> 
> We have code like this:
>  static void tv_disconnect(struct usb_interface *interface)
> {
> 	struct trancevibrator *dev;
> 
> 	dev = usb_get_intfdata (interface);
> 	device_remove_file(&interface->dev, &dev_attr_speed);
> 	usb_set_intfdata(interface, NULL);
> 	usb_put_dev(dev->udev);
> 	kfree(dev);
> }
> 
> This has a race:
> 
> CPU A				CPU B
> open sysfs
> 					device_remove_file
> 					kfree
> reading attr
> 
> We cannot do refcounting as sysfs doesn't export open/close. Therefore
> we must be sure that device_remove_file() makes sure that sysfs will
> leave a driver alone after the return of device_remove_file(). Currently
> open will fail, but IO on an already opened file will work. The patch makes
> sure it will fail with -ENODEV without calling into the driver, which may
> indeed be already unloaded.
> 
> 	Regards
> 		Oliver

hmm, I guess Greg has to say the final word. The question is either to fail
the IO (-ENODEV) or fail the file removal (-EBUSY). If we are not going to
fail the removal then your patch is the way to go.

Greg?

Thanks
Maneesh