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

[Jason Wang <jasowang@redhat.com>]

On 2017年11月02日 03:12, Alexei Starovoitov wrote:
> 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?
>

I'm not sure I get the question. eBPF here is used to do queue 
selection, so we could not reuse socket filter (tun_detach_filter use 
call_rcu()). cBPF could be used here, but I'm not quite sure it's worth 
to support it. And I agree we should use call_rcu() here.

Hope this answer your question.

Thanks