linux-kernel - Re: [PATCH net v3 1/2] net: tcp: lookup the best matched listen socket

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Message-ID: <CAL+tcoD80feEhPA_1L7D55UqkRuLSnZ-Kmmdab5J2v9K6uCzTA@mail.gmail.com>
Date: Sun, 3 Aug 2025 10:53:33 +0800
From: Jason Xing <kerneljasonxing@...il.com>
To: Menglong Dong <menglong8.dong@...il.com>
Cc: edumazet@...gle.com, kuniyu@...gle.com, ncardwell@...gle.com, 
	davem@...emloft.net, dsahern@...nel.org, kuba@...nel.org, pabeni@...hat.com, 
	horms@...nel.org, shuah@...nel.org, kraig@...gle.com, 
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org, 
	linux-kselftest@...r.kernel.org
Subject: Re: [PATCH net v3 1/2] net: tcp: lookup the best matched listen socket

On Sun, Aug 3, 2025 at 9:59 AM Menglong Dong <menglong8.dong@...il.com> wrote:
>
> On Sat, Aug 2, 2025 at 9:06 PM Jason Xing <kerneljasonxing@...il.com> wrote:
> >
> > Hi Menglong,
> >
> > On Sat, Aug 2, 2025 at 5:28 PM Menglong Dong <menglong8.dong@...il.com> wrote:
> > >
> > > For now, the tcp socket lookup will terminate if the socket is reuse port
> > > in inet_lhash2_lookup(), which makes the socket is not the best match.
> > >
> > > For example, we have socket1 and socket2 both listen on "0.0.0.0:1234",
> > > but socket1 bind on "eth0". We create socket1 first, and then socket2.
> > > Then, all connections will goto socket2, which is not expected, as socket1
> > > has higher priority.
> > >
> > > This can cause unexpected behavior if TCP MD5 keys is used, as described
> > > in Documentation/networking/vrf.rst -> Applications.
> > >
> > > Therefor, we lookup the best matched socket first, and then do the reuse
> >
> > s/Therefor/Therefore
> >
> > > port logic. This can increase some overhead if there are many reuse port
> > > socket :/
> > >
> > > Fixes: c125e80b8868 ("soreuseport: fast reuseport TCP socket selection")
> > > Signed-off-by: Menglong Dong <dongml2@...natelecom.cn>
> > > ---
> > > v3:
> > > * use the approach in V1
> > > * add the Fixes tag
> > > ---
> > >  net/ipv4/inet_hashtables.c  | 13 +++++++------
> > >  net/ipv6/inet6_hashtables.c | 13 +++++++------
> > >  2 files changed, 14 insertions(+), 12 deletions(-)
> > >
> > > diff --git a/net/ipv4/inet_hashtables.c b/net/ipv4/inet_hashtables.c
> > > index ceeeec9b7290..51751337f394 100644
> > > --- a/net/ipv4/inet_hashtables.c
> > > +++ b/net/ipv4/inet_hashtables.c
> > > @@ -389,17 +389,18 @@ static struct sock *inet_lhash2_lookup(const struct net *net,
> > >         sk_nulls_for_each_rcu(sk, node, &ilb2->nulls_head) {
> > >                 score = compute_score(sk, net, hnum, daddr, dif, sdif);
> > >                 if (score > hiscore) {
> > > -                       result = inet_lookup_reuseport(net, sk, skb, doff,
> > > -                                                      saddr, sport, daddr, hnum, inet_ehashfn);
> > > -                       if (result)
> > > -                               return result;
> > > -
> > >                         result = sk;
> > >                         hiscore = score;
> > >                 }
> > >         }
> > >
> > > -       return result;
> > > +       if (!result)
> > > +               return NULL;
> > > +
> > > +       sk = inet_lookup_reuseport(net, result, skb, doff,
> > > +                                  saddr, sport, daddr, hnum, inet_ehashfn);
> > > +
> > > +       return sk ? sk : result;
> > >  }
> >
> > IMHO, I don't see it as a bugfix. So can you elaborate on what the exact
> > side effect you're faced with is when the algorithm finally prefers
> > socket2 (without
> > this patch)?
>
> Hi, Jason. The case is that the user has several NIC,
> and there are some sockets that are binded to them,
> who listen on TCP port 6666. And a global socket doesn't
> bind any NIC and listens on TCP port 6666.
>
> In theory, the connection request from the NIC will goto
> the listen socket that is binded on it. When on socket
> is binded on the NIC, it goto the global socket. However,
> the connection request always goto the global socket, which
> is not expected.
>
> What's worse is that when TCP MD5 is used on the socket,
> the connection will fail :/

I'm trying to picture what the usage can be in the userland as you
pointed out in the MD5 case. As to the client side, it seems weird
since it cannot detect and know the priority of the other side where a
few sockets listen on the same address and port.

I'm not saying the priority problem doesn't exist, just not knowing
how severe the case could be. It doesn't look that bad at least until
now. Only the selection policy itself matters more to the server side
than to the client side.

Thanks,
Jason