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