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

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Date:   Wed, 17 Aug 2022 12:28:55 -0600
From:   "Daniel Xu" <dxu@...uu.xyz>
To:     "Florian Westphal" <fw@...len.de>,
        Toke Høiland-Jørgensen <toke@...nel.org>
Cc:     "bpf@...r.kernel.org" <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@...filter.org,
        netfilter-devel@...r.kernel.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH bpf-next 2/3] bpf: Add support for writing to nf_conn:mark

Hi Florian,

On Mon, Aug 15, 2022, at 4:40 PM, Florian Westphal 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.

I reviewed some of the LKMM literature and I would concur that
READ/WRITE_ONCE() is necessary. Especially after this patchset.

However, it's unclear to me if this is a latent issue. IOW: is reading
ct->mark protected by a lock? I only briefly looked but it doesn't
seem like it.

I'll do some more digging.

In the meantime, I'll send out a v2 on this patchset and I'll plan on
sending out a followup patchset for adding READ/WRITE_ONCE()
to ct->mark accesses.

Thanks,
Daniel