netdev - Re: [PATCH net-next 2/4] packet: add eBPF fanout mode

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Message-ID: <CA+FuTSeJ8krk3b8R==BwvdLH8Cw6k3f9mQT=CMKm70Sh4dsX3w@mail.gmail.com>
Date:	Fri, 14 Aug 2015 22:28:52 -0400
From:	Willem de Bruijn <willemb@...gle.com>
To:	Daniel Borkmann <daniel@...earbox.net>
Cc:	Alexei Starovoitov <ast@...mgrid.com>,
	Network Development <netdev@...r.kernel.org>,
	David Miller <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>
Subject: Re: [PATCH net-next 2/4] packet: add eBPF fanout mode

On Fri, Aug 14, 2015 at 3:46 PM, Daniel Borkmann <daniel@...earbox.net> wrote:
> On 08/14/2015 09:27 PM, Willem de Bruijn wrote:
> ...
>>>
>>> Btw, in case someone sets sock_flag(sk, SOCK_FILTER_LOCKED),
>>> perhaps we should also apply it on fanout?
>>
>>
>> Good point. With classic bpf, packet access control is fully
>> enforced in per-socket filters, but playing with load balancing
>> filters could allow an adversary to infer some information
>> about the dropped packets*. With eBPF and maps, access
>> is even more direct. Let's support locking of fanout filters in
>> place.
>
>
> Right, a process could share a map between the fanout lb filter
> and actual sk filter, i.e. to look up how much actually passed
> through on the later sk level filter, and use that information
> in addition for its lb decisions.
>
>> I intend to test the existing socket flag. No need to add a
>> separate flag for the fanout group, as far as I can see.
>
>
> Agreed, should be okay.

Great. Thanks for the suggestion, Daniel! I'll send a v2 the
three suggested changes in a minute.

>
> Thanks Willem!
>
>> (*) I noticed that a similar unintended effect also causes the
>> PACKET_FANOUT_LB selftest to be flaky: filters on the
>> sockets ensure that the test only reads expected packets.
>> But, all traffic makes it through packet_rcv_fanout. Packets
>> that are later dropped by sk_filter have already incremented
>> rr_cur. Worst case, with 2 sockets and each accepted packet
>> interleaved with a dropped packet, all packets are queued on
>> only one socket. Test flakiness is fixed, e.g., by running in a
>> private network namespace. The implementation behavior
>> may be unexpected in other, production, environments.
>> --
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>
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