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

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Message-Id: <668406b9-714f-4ade-889d-051cf42ceefc@www.fastmail.com>
Date:   Wed, 17 Aug 2022 12:41:12 -0600
From:   "Daniel Xu" <dxu@...uu.xyz>
To:     "Florian Westphal" <fw@...len.de>
Cc:     Toke Høiland-Jørgensen <toke@...nel.org>,
        "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

On Wed, Aug 17, 2022, at 12:34 PM, Florian Westphal wrote:
> Daniel Xu <dxu@...uu.xyz> wrote:
>> 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.
>
> No, its not protected by a lock.  READ/WRITE_ONCE is unrelated to your
> patchset, this is a pre-existing "bug".

Thanks for confirming. Since it's pre-existing I will send out a followup
patchset then.

Thanks,
Daniel