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

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Message-ID: <87pmgxuy6v.fsf@toke.dk>
Date:   Thu, 18 Aug 2022 21:52:08 +0200
From:   Toke Høiland-Jørgensen <toke@...nel.org>
To:     Daniel Xu <dxu@...uu.xyz>, bpf@...r.kernel.org, ast@...nel.org,
        daniel@...earbox.net, andrii@...nel.org, memxor@...il.com
Cc:     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:

> 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