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

[Alexei Starovoitov <alexei.starovoitov@gmail.com>]

On Wed, Nov 01, 2017 at 03:59:48PM +0200, Michael S. Tsirkin wrote:
> On Wed, Nov 01, 2017 at 09:02:03PM +0800, Jason Wang wrote:
> > 
> > 
> > On 2017年11月01日 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?
> 
> > > 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)
> > {
> >     might_sleep();
> >     if (rtnl_is_locked())
> >         synchronize_rcu_expedited();
> >     else
> >         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.
> 
> We could use a callback though the docs warn userspace can use that
> to cause a DOS and needs to be limited.

the whole __tun_set_steering_ebpf() looks odd to me.
There is tun_attach_filter/tun_detach_filter pattern
that works for classic BPF. Why for eBPF this strange
synchronize_net() is there?