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

[Menglong Dong]

On Thu, Dec 30, 2021 at 10:12 AM Alexei Starovoitov
<alexei.starovoitov@xxxxxxxxx> wrote:
>
> On Wed, Dec 29, 2021 at 6:07 PM Menglong Dong <menglong8.dong@xxxxxxxxx> wrote:
> >
> > On Thu, Dec 30, 2021 at 12:46 AM Alexei Starovoitov
> > <alexei.starovoitov@xxxxxxxxx> wrote:
> > >
> > > On Wed, Dec 29, 2021 at 3:33 AM <menglong8.dong@xxxxxxxxx> wrote:
> > > >
> > > > From: Menglong Dong <imagedong@xxxxxxxxxxx>
> > > >
> > > > 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.
>
> kprobe at the entry is pretty fast.
> fentry is even faster. It's actually faster than cgroup based hook.

Really? Doesn't it already consider the data copy of bpf_probe_read()?
After all,
sock based eBPF can read sock data directly.

What's more, I'm not sure bpf tracing is enough, because I do NAT not for all
the docker containers, which means not all tcp connect close should call my
eBPF program, and that's the advantage of cgroup based eBPF.

Thanks!
Menglong Dong

> Give it a shot.