netdev - Re: [PATCH net-next] bpf, net: Support redirecting to ifb with bpf

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Message-ID: <874jpj2682.fsf@toke.dk>
Date:   Thu, 13 Apr 2023 16:43:09 +0200
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     Daniel Borkmann <daniel@...earbox.net>,
        Yafang Shao <laoar.shao@...il.com>, davem@...emloft.net,
        edumazet@...gle.com, kuba@...nel.org, pabeni@...hat.com,
        ast@...nel.org, hawk@...nel.org, john.fastabend@...il.com
Cc:     netdev@...r.kernel.org, bpf@...r.kernel.org,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Tonghao Zhang <xiangxia.m.yue@...il.com>, martin.lau@...ux.dev
Subject: Re: [PATCH net-next] bpf, net: Support redirecting to ifb with bpf

Daniel Borkmann <daniel@...earbox.net> writes:

>> 2). We can't redirect ingress packet to ifb with bpf
>> By trying to analyze if it is possible to redirect the ingress packet to
>> ifb with a bpf program, we find that the ifb device is not supported by
>> bpf redirect yet.
>
> You actually can: Just let BPF program return TC_ACT_UNSPEC for this
> case and then add a matchall with higher prio (so it runs after bpf)
> that contains an action with mirred egress redirect that pushes to ifb
> dev - there is no change needed.

I wasn't aware that BPF couldn't redirect directly to an IFB; any reason
why we shouldn't merge this patch in any case?

>> This patch tries to resolve it by supporting redirecting to ifb with bpf
>> program.
>> 
>> Ingress bandwidth limit is useful in some scenarios, for example, for the
>> TCP-based service, there may be lots of clients connecting it, so it is
>> not wise to limit the clients' egress. After limiting the server-side's
>> ingress, it will lower the send rate of the client by lowering the TCP
>> cwnd if the ingress bandwidth limit is reached. If we don't limit it,
>> the clients will continue sending requests at a high rate.
>
> Adding artificial queueing for the inbound traffic, aren't you worried
> about DoS'ing your node?

Just as an aside, the ingress filter -> ifb -> qdisc on the ifb
interface does work surprisingly well, and we've been using that over in
OpenWrt land for years[0]. It does have some overhead associated with it,
but I wouldn't expect it to be a source of self-DoS in itself (assuming
well-behaved TCP traffic).

-Toke

[0] https://openwrt.org/docs/guide-user/network/traffic-shaping/sqm