linux-kernel - Re: [PATCH net-next] net: bpf: add hook for close of tcp timewait sock

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Date:   Thu, 30 Dec 2021 10:04:08 +0800
From:   Menglong Dong <menglong8.dong@...il.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Daniel Borkmann <daniel@...earbox.net>,
        Alexei Starovoitov <ast@...nel.org>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        David Ahern <dsahern@...nel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Network Development <netdev@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>,
        Menglong Dong <imagedong@...cent.com>
Subject: Re: [PATCH net-next] net: bpf: add hook for close of tcp timewait sock

On Thu, Dec 30, 2021 at 12:46 AM Alexei Starovoitov
<alexei.starovoitov@...il.com> wrote:
>
> On Wed, Dec 29, 2021 at 3:33 AM <menglong8.dong@...il.com> wrote:
> >
> > From: Menglong Dong <imagedong@...cent.com>
> >
> > The cgroup eBPF attach type 'CGROUP_SOCK_OPS' is able to monitor the
> > state change of a tcp connect with 'BPF_SOCK_OPS_STATE_CB' ops.
> >
> > However, it can't trace the whole state change of a tcp connect. While
> > a connect becomes 'TCP_TIME_WAIT' state, this sock will be release and
> > a tw sock will be created. While tcp sock release, 'TCP_CLOSE' state
> > change will be passed to eBPF program. Howeven, the real state of this
> > connect is 'TCP_TIME_WAIT'.
> >
> > To make eBPF get the real state change of a tcp connect, add
> > 'CGROUP_TWSK_CLOSE' cgroup attach type, which will be called when
> > tw sock release and tcp connect become CLOSE.
>
> The use case is not explained.

Sorry for the absence of use cases and selftests. In my case, it is for NAT of
a docker container.

Simply speaking, I'll add an element to a hash map during sys_connect() with
'BPF_SOCK_OPS_TCP_CONNECT_CB' ops of 'BPF_CGROUP_SOCK_OPS'
cgroup attach type. Therefore, the received packet of the host can do DNAT
according to the hash map.

I need to release the element in the hashmap when the connection closes.
With the help of 'BPF_SOCK_OPS_STATE_CB', I can monitor the TCP_CLOSE
of the connection. However, as I mentioned above, it doesn't work well when
it comes to tw sock. When the connect become 'FIN_WAIT2' or 'TIME_WAIT',
the state of the tcp sock becomes 'TCP_CLOSE', which doesn't match the connect
state. Therefore, the 'fin' packet that the host received can't be DNAT, as the
element is already removed.

In this patch, BPF_SOCK_OPS_TW_CLOSE_FLAG is introduced, which is used
make 'BPF_SOCK_OPS_STATE_CB' not called when this sock becomes
TCP_CLOSE if it is being replaced with a tw sock.

> Why bpf tracing cannot be used to achieve the same?

En...do you mean kprobe based eBPF trace? It can work, but I don't think it's
high-performance, especially for network NAT. Strictly speaking, attach types,
such as 'CGROUP_INET_SOCK_RELEASE', can be replaced by bpf tracing, but
they exist out of performance.

Thanks!
Menglong Dong

>
> Also there are no selftests.