Re: [PATCH net-next V2 3/3] tun: add eBPF based queue selection method

[Jason Wang]

On 2017å11æ01æ 21:59, Michael S. Tsirkin wrote:
> On Wed, Nov 01, 2017 at 09:02:03PM +0800, Jason Wang wrote:
> > On 2017å11æ01æ 00:45, Michael S. Tsirkin wrote:
> > > > +static void __tun_set_steering_ebpf(struct tun_struct *tun,
> > > > +				    struct bpf_prog *new)
> > > > +{
> > > > +	struct bpf_prog *old;
> > > > +
> > > > +	old = rtnl_dereference(tun->steering_prog);
> > > > +	rcu_assign_pointer(tun->steering_prog, new);
> > > > +
> > > > +	if (old) {
> > > > +		synchronize_net();
> > > > +		bpf_prog_destroy(old);
> > > > +	}
> > > > +}
> > > > +
> > > Is this really called under rtnl?
> > Yes it is __tun_chr_ioctl() will call rtnl_lock().
> Is the call from tun_free_netdev under rtnl too?

Looks not, need hold rtnl_lock() in tun_free_netdev in next version.

> > > If no then rtnl_dereference
> > > is wrong. If yes I'm not sure you can call synchronize_net
> > > under rtnl.
> > Are you worrying about the long wait? Looking at synchronize_net(), it does:
> >
> > void synchronize_net(void)
> > {
> >  ÂÂÂ might_sleep();
> >  ÂÂÂ if (rtnl_is_locked())
> >  ÂÂÂ ÂÂÂ synchronize_rcu_expedited();
> >  ÂÂÂ else
> >  ÂÂÂ ÂÂÂ synchronize_rcu();
> > }
> > EXPORT_SYMBOL(synchronize_net);
> >
> > Thanks
> Not the wait - expedited is not a good thing to allow unpriveledged
> userspace to do, it interrupts all VMs running on the same box.

Good point.

> We could use a callback though the docs warn userspace can use that
> to cause a DOS and needs to be limited.

Will do this in next version.

Thanks