netdev - Re: [PATCH bpf-next 2/3] bpf: Add support for writing to nf

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Date:   Mon, 15 Aug 2022 15:47:46 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Florian Westphal <fw@...len.de>
Cc:     Toke Høiland-Jørgensen <toke@...nel.org>,
        Daniel Xu <dxu@...uu.xyz>, bpf <bpf@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Kumar Kartikeya Dwivedi <memxor@...il.com>,
        Pablo Neira Ayuso <pablo@...filter.org>,
        netfilter-devel <netfilter-devel@...r.kernel.org>,
        Network Development <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH bpf-next 2/3] bpf: Add support for writing to nf_conn:mark

On Mon, Aug 15, 2022 at 3:40 PM Florian Westphal <fw@...len.de> wrote:
>
> Toke Høiland-Jørgensen <toke@...nel.org> wrote:
> > > 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.
> > >
> > > One example use case would be if a bpf prog is responsible for advanced
> > > packet classification and iptables/nftables is later used for routing
> > > due to pre-existing/legacy code.
> > >
> > > Signed-off-by: Daniel Xu <dxu@...uu.xyz>
> >
> > Didn't we agree the last time around that all field access should be
> > using helper kfuncs instead of allowing direct writes to struct nf_conn?
>
> I don't see why ct->mark needs special handling.
>
> It might be possible we need to change accesses on nf/tc side to use
> READ/WRITE_ONCE though.

+1
I don't think we need to have a hard rule.
If fields is safe to access directly than it's faster
to let bpf prog read/write it.
There are no backward compat concerns. If conntrack side decides
to make that field special we can disallow direct writes in
the same kernel version. These accesses, just like kfuncs, are unstable.