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Date: Thu, 2 Nov 2017 11:24:16 +0800 From: Jason Wang <jasowang@...hat.com> To: Alexei Starovoitov <alexei.starovoitov@...il.com>, "Michael S. Tsirkin" <mst@...hat.com> Cc: netdev@...r.kernel.org, linux-kernel@...r.kernel.org, willemdebruijn.kernel@...il.com, tom@...bertland.com, Daniel Borkmann <daniel@...earbox.net> Subject: Re: [PATCH net-next V2 3/3] tun: add eBPF based queue selection method 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
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