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


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