netdev - Re: [PATCH nf-next v5 0/6] Netfilter egress hook

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Date:   Thu, 30 Sep 2021 09:33:23 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Pablo Neira Ayuso <pablo@...filter.org>
Cc:     netfilter-devel@...r.kernel.org, davem@...emloft.net,
        netdev@...r.kernel.org, kuba@...nel.org, lukas@...ner.de,
        kadlec@...filter.org, fw@...len.de, ast@...nel.org,
        edumazet@...gle.com, tgraf@...g.ch, nevola@...il.com,
        john.fastabend@...il.com, willemb@...gle.com
Subject: Re: [PATCH nf-next v5 0/6] Netfilter egress hook

On 9/30/21 9:19 AM, Pablo Neira Ayuso wrote:
> On Thu, Sep 30, 2021 at 08:08:53AM +0200, Daniel Borkmann wrote:
>> On 9/28/21 11:55 AM, Pablo Neira Ayuso wrote:
>>> Hi,
>>>
>>> This patchset v5 that re-adds the Netfilter egress:
>>>
>>> 1) Rename linux/netfilter_ingress.h to linux/netfilter_netdev.h
>>>      from Lukas Wunner.
>>>
>>> 2) Generalize ingress hook file to accomodate egress support,
>>>      from Lukas Wunner.
>>>
>>> 3) Modularize Netfilter ingress hook into nf_tables_netdev: Daniel
>>>      Borkmann is requesting for a mechanism to allow to blacklist
>>>      Netfilter, this allows users to blacklist this new module that
>>>      includes ingress chain and the new egress chain for the netdev
>>>      family. There is no other in-tree user of the ingress and egress
>>>      hooks than this which might interfer with his matter.
>>>
>>> 4) Place the egress hook again before the tc egress hook as requested
>>>      by Daniel Borkmann. Patch to add egress hook from Lukas Wunner.
>>>      The Netfilter egress hook remains behind the static key, if unused
>>>      performance degradation is negligible.
>>>
>>> 5) Add netfilter egress handling to af_packet.
>>>
>>> Arguably, distributors might decide to compile nf_tables_netdev
>>> built-in. Traditionally, distributors have compiled their kernels using
>>> the default configuration that Netfilter Kconfig provides (ie. use
>>> modules whenever possible). In any case, I consider that distributor
>>> policy is out of scope in this discussion, providing a mechanism to
>>> allow Daniel to prevent Netfilter ingress and egress chains to be loaded
>>> should be sufficient IMHO.
>>
>> Hm, so in the case of SRv6 users were running into a similar issue and commit
>> 7a3f5b0de364 ("netfilter: add netfilter hooks to SRv6 data plane") [0] added
>> a new hook along with a sysctl which defaults the new hook to off.
>>
>> The rationale for it was given as "the hooks are enabled via nf_hooks_lwtunnel
>> sysctl to make sure existing netfilter rulesets do not break." [0,1]
>>
>> If the suggestion to flag the skb [2] one way or another from the tc forwarding
>> path (e.g. skb bit or per-cpu marker) is not technically feasible, then why not
>> do a sysctl toggle like in the SRv6 case?
> 
> I am already providing a global toggle to disable netdev
> ingress/egress hooks?
> 
> In the SRv6 case that is not possible.
> 
> Why do you need you need a sysctl knob when my proposal is already
> addressing your needs?

Well, it's not addressing anything ... you even mention it yourself "arguably,
distributors might decide to compile nf_tables_netdev built-in".