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From:   Joe Stringer <joe@cilium.io>
Date:   Thu, 25 May 2023 22:56:50 -0700
Message-ID: <CADa=RywoZZ9cAVPqa88mRNc2g1gQF743oEiSw2vnVHEFrN956g@mail.gmail.com>
Subject: Re: [PATCH bpf-next 1/2] bpf, net: Support SO_REUSEPORT sockets with bpf_sk_assign
To:     Lorenz Bauer <lmb@isovalent.com>
Cc:     "David S. Miller" <davem@davemloft.net>,
        Eric Dumazet <edumazet@google.com>,
        Jakub Kicinski <kuba@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Andrii Nakryiko <andrii@kernel.org>,
        Martin KaFai Lau <martin.lau@linux.dev>,
        Song Liu <song@kernel.org>, Yonghong Song <yhs@fb.com>,
        John Fastabend <john.fastabend@gmail.com>,
        KP Singh <kpsingh@kernel.org>,
        Stanislav Fomichev <sdf@google.com>,
        Hao Luo <haoluo@google.com>, Jiri Olsa <jolsa@kernel.org>,
        David Ahern <dsahern@kernel.org>,
        Willem de Bruijn <willemdebruijn.kernel@gmail.com>,
        Joe Stringer <joe@wand.net.nz>, Joe Stringer <joe@cilium.io>,
        Martin KaFai Lau <kafai@fb.com>, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org, bpf@vger.kernel.org
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On Thu, May 25, 2023 at 1:19=E2=80=AFAM Lorenz Bauer <lmb@isovalent.com> wr=
ote:
>
> Currently the bpf_sk_assign helper in tc BPF context refuses SO_REUSEPORT
> sockets. This means we can't use the helper to steer traffic to Envoy, wh=
ich
> configures SO_REUSEPORT on its sockets. In turn, we're blocked from remov=
ing
> TPROXY from our setup.
>
> The reason that bpf_sk_assign refuses such sockets is that the bpf_sk_loo=
kup
> helpers don't execute SK_REUSEPORT programs. Instead, one of the
> reuseport sockets is selected by hash. This could cause dispatch to the
> "wrong" socket:
>
>     sk =3D bpf_sk_lookup_tcp(...) // select SO_REUSEPORT by hash
>     bpf_sk_assign(skb, sk) // SK_REUSEPORT wasn't executed
>
> Fixing this isn't as simple as invoking SK_REUSEPORT from the lookup
> helpers unfortunately. In the tc context, L2 headers are at the start
> of the skb, while SK_REUSEPORT expects L3 headers instead.
>
> Instead, we execute the SK_REUSEPORT program when the assigned socket
> is pulled out of the skb, further up the stack. This creates some
> trickiness with regards to refcounting as bpf_sk_assign will put both
> refcounted and RCU freed sockets in skb->sk. reuseport sockets are RCU
> freed. We can infer that the sk_assigned socket is RCU freed if the
> reuseport lookup succeeds, but convincing yourself of this fact isn't
> straight forward. Therefore we defensively check refcounting on the
> sk_assign sock even though it's probably not required in practice.
>
> Fixes: 8e368dc ("bpf: Fix use of sk->sk_reuseport from sk_assign")
> Fixes: cf7fbe6 ("bpf: Add socket assign support")
> Co-developed-by: Daniel Borkmann <daniel@iogearbox.net>
> Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
> Signed-off-by: Lorenz Bauer <lmb@isovalent.com>
> Cc: Joe Stringer <joe@cilium.io>
> Link: https://lore.kernel.org/bpf/CACAyw98+qycmpQzKupquhkxbvWK4OFyDuuLMBN=
ROnfWMZxUWeA@mail.gmail.com/

Nice approach to fix this issue, wish I'd thought of it :)

I pulled this and tested out in a little-vm-helper environment with
kind and Cilium's examples/kubernetes/connectivity-check proxy suite,
as well as cilium-cli's connectivity tests and the L7 features seem to
be working as expected with SO_REUSEPORT.

Tested-by: Joe Stringer <joe@cilium.io>

I also glanced through the commit, and the various protocols seem to
be handled consistently at the very least, though I agree it'd be
simpler for review and bisecting if broken down into more incremental
changes.