Re: [PATCH 3/3] eventfd: add internal reference counting to fix notifier race conditions

Rusty Russell
2009-06-25T11:42:36Z

On Thu, 25 Jun 2009 08:15:11 am Davide Libenzi wrote:
> On Wed, 24 Jun 2009, Rusty Russell wrote:
> > On Tue, 23 Jun 2009 03:33:22 am Davide Libenzi wrote:
> > > What you're doing there, is setting up a kernel-to-kernel (since
> > > userspace only role is to create the eventfd) communication, using a
> > > file* as accessory. That IMO is plain wrong.
> >
> > The most sensible is that userspace can use these fds; an in-kernel
> > variant is possible too, but not primary IMHO.
> >
> > It's nice that userspace create the fds; it can then use the same fd for
> > multiple event sources.
> >
> > But I didn't see anything wrong with the way eventfd used to work: you
> > have a kvm ioctl to say "attach this eventfd to this guest notification"
> > and that does the eventfd_fget. A detach ioctl does the fput (as does
> > release of the kvm fd).
> >
> > If they close the eventfd and don't do the detach ioctl, it's their
> > problem.
>
> Some components would like to know if userspace dropped the fd, and take
> proper action accordingly (release resources, drop module instances,
> etc...).

Like to know? Possibly. Need to know? Not anything I've seen so far.

If userspace creates the fd, component grab a ref and if userspace wants that
fd completely freed must close the fd *and* tell component. Simple, race free
and explicit. All wins.

As this discussion shows, doing some kind of implies non-reference is hard,
complex and racy.

> Another thing that comes in my mind (that for some components might not
> matter) is considering the effect of userspace doing things like:
>
> for (;;) {
> fd = eventfd(...);
> ioctl(xfd, XXX_ADD, fd);
> close(fd);
> }
>
> That might lead to unprivileged users drawing kernel memory w/out any
> userspace accountability, if not properly handled.

No, fget_eventfd covers this exactly as expected. Don't doubt your ability to
design sane kernel interfaces; eventfd is nice! All lguest needed was a
couple of EXPORT_SYMBOLS and it fitted in beautifully.

Thanks,
Rusty.