netdev - Re: [PATCH net-next 7/8] net/mlx5e: XDP TX forwarding support

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Date:   Tue, 20 Sep 2016 12:21:54 -0700
From:   Tom Herbert <tom@...bertland.com>
To:     Jesper Dangaard Brouer <brouer@...hat.com>
Cc:     Eric Dumazet <eric.dumazet@...il.com>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Tariq Toukan <ttoukan.linux@...il.com>,
        Tariq Toukan <tariqt@...lanox.com>,
        "David S. Miller" <davem@...emloft.net>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Eran Ben Elisha <eranbe@...lanox.com>,
        Saeed Mahameed <saeedm@...lanox.com>,
        Rana Shahout <ranas@...lanox.com>
Subject: Re: [PATCH net-next 7/8] net/mlx5e: XDP TX forwarding support

On Tue, Sep 20, 2016 at 11:59 AM, Jesper Dangaard Brouer
<brouer@...hat.com> wrote:
> On Tue, 20 Sep 2016 10:39:20 -0700
> Eric Dumazet <eric.dumazet@...il.com> wrote:
>
>> On Tue, 2016-09-20 at 09:45 -0700, Alexei Starovoitov wrote:
>>
>> > because 'div by zero' is an abnormal situation that shouldn't be exploited.
>> > Meaning if xdp program is doing DoS prevention and it has a bug that
>> > attacker can now exploit by sending a crafted packet that causes
>> > 'div by zero' and kernel will warn then attack got successful.
>> > Therefore it has to be silent drop.
>>
>> A silent drop means a genuine error in a BPF program might be never
>> caught, since a tracepoint might never be enabled.
>
> I do see your point. But we can document our way out of it.
>
>> > tracpoint in such case is great, since the user can do debugging with it
>> > and even monitoring 24/7 and if suddenly the control plan sees a lot
>> > of such trace_xdp_abotred events, it can disable that tracepoint to avoid
>> > spam and adjust the program or act on attack some other way.
>> > Hardcoded warnings and counters are not generic enough for all
>> > the use cases people want to throw at XDP.
>> > The tracepoints idea is awesome, in a sense that it's optional.
>>
>>
>> Note that tracepoints are optional in a kernel.
>
> Well, that is a good thing, as it can be compiled out (as that provides
> an option for zero cost).
>
>
>> Many existing supervision infrastructures collect device snmp
>> counters, and run as unprivileged programs.
>
> A supervision infrastructures is a valid use-case. It again indicate
> that such XDP stats need to structured, not just a random driver
> specific ethtool counter, to make it easy for such collection daemons.
>
I am currently adding a structure to define an XDP hook (plan to post
patches shortly). Counters can be added to that in a uniform fashion
that doesn't need code in every driver.

Tom

>
>> tracepoints might not fit the need here, compared to a mere
>> tx_ring->tx_drops++
>
> I do see your point.  I really liked the tracepoint idea, but now I'm
> uncertain again...
>
>
> I do have a use-case where I want to use the NIC HW-RX-ingress-overflow
> and TX-overflow drop indicators, but I don't want to tie it into this
> discussion.  The abort and error indicators a not relevant for that
> use-case.
>
> --
> Best regards,
>   Jesper Dangaard Brouer
>   MSc.CS, Principal Kernel Engineer at Red Hat
>   Author of http://www.iptv-analyzer.org
>   LinkedIn: http://www.linkedin.com/in/brouer