netdev - Re: [PATCH bpf-next v2 3/4] bpf: Add support for writing to nf

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Message-ID: <87wnb4tmc0.fsf@toke.dk>
Date:   Fri, 19 Aug 2022 15:05:51 +0200
From:   Toke Høiland-Jørgensen <toke@...nel.org>
To:     Daniel Xu <dxu@...uu.xyz>
Cc:     bpf@...r.kernel.org, ast@...nel.org, daniel@...earbox.net,
        andrii@...nel.org, memxor@...il.com, pablo@...filter.org,
        fw@...len.de, netfilter-devel@...r.kernel.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH bpf-next v2 3/4] bpf: Add support for writing to
 nf_conn:mark

Daniel Xu <dxu@...uu.xyz> writes:

> Hi Toke,
>
> On Thu, Aug 18, 2022 at 09:52:08PM +0200, Toke Høiland-Jørgensen wrote:
>> Daniel Xu <dxu@...uu.xyz> writes:
>> 
>> > Support direct writes to nf_conn:mark from TC and XDP prog types. This
>> > is useful when applications want to store per-connection metadata. This
>> > is also particularly useful for applications that run both bpf and
>> > iptables/nftables because the latter can trivially access this
>> > metadata.
>> 
>> Looking closer at the nf_conn definition, the mark field (and possibly
>> secmark) seems to be the only field that is likely to be feasible to
>> support direct writes to, as everything else either requires special
>> handling (like status and timeout), or they are composite field that
>> will require helpers anyway to use correctly.
>> 
>> Which means we're in the process of creating an API where users have to
>> call helpers to fill in all fields *except* this one field that happens
>> to be directly writable. That seems like a really confusing and
>> inconsistent API, so IMO it strengthens the case for just making a
>> helper for this field as well, even though it adds a bit of overhead
>> (and then solving the overhead issue in a more generic way such as by
>> supporting clever inlining).
>> 
>> -Toke
>
> I don't particularly have a strong opinion here. But to play devil's
> advocate:
>
> * It may be confusing now, but over time I expect to see more direct
>   write support via BTF, especially b/c there is support for unstable
>   helpers now. So perhaps in the future it will seem more sensible.

Right, sure, for other structs. My point was that it doesn't look like
this particular one (nf_conn) is likely to grow any other members we can
access directly, so it'll be a weird one-off for that single field...

> * The unstable helpers do not have external documentation. Nor should
>   they in my opinion as their unstableness + stale docs may lead to
>   undesirable outcomes. So users of the unstable API already have to
>   splunk through kernel code and/or selftests to figure out how to wield
>   the APIs. All this to say there may not be an argument for
>   discoverability.

This I don't buy at all. Just because it's (supposedly) "unstable" is no
excuse to design a bad API, or make it actively user-hostile by hiding
things so users have to go browse kernel code to know how to use it. So
in any case, we should definitely document everything.

> * Direct writes are slightly more ergnomic than using a helper.

This is true, and that's the main argument for doing it this way. The
point of my previous email was that since it's only a single field,
consistency weighs heavier than ergonomics :)

-Toke