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

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Date:   Wed, 17 Aug 2022 20:34:53 +0200
From:   Florian Westphal <fw@...len.de>
To:     Daniel Xu <dxu@...uu.xyz>
Cc:     Florian Westphal <fw@...len.de>,
        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

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".