netdev - Re: [PATCH] neighbour: guarantee the localhost connections be established successfully even the ARP table is full

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Message-ID: <CANn89i+d+NSk5VooFn=AHoby5JOJ3=D1oJ1GSKCzp3ZPM1zaLw@mail.gmail.com>
Date: Thu, 18 Apr 2024 11:45:16 +0200
From: Eric Dumazet <edumazet@...gle.com>
To: Paolo Abeni <pabeni@...hat.com>
Cc: "Li, James Zheng" <James.Z.Li@...l.com>, Zheng Li <lizheng043@...il.com>, 
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>, "bpf@...r.kernel.org" <bpf@...r.kernel.org>, 
	"davem@...emloft.net" <davem@...emloft.net>, "jmorris@...ei.org" <jmorris@...ei.org>, 
	"kuba@...nel.org" <kuba@...nel.org>
Subject: Re: [PATCH] neighbour: guarantee the localhost connections be
 established successfully even the ARP table is full

On Thu, Apr 18, 2024 at 11:33 AM Paolo Abeni <pabeni@...hat.com> wrote:
>
> On Tue, 2024-04-16 at 10:36 +0000, Li, James Zheng wrote:
> > On Tuesday, April 16, 2024 6:02 PM Eric Dumazet <edumazet@...gle.com> wrote:
> > > Hmmm...
> >
> > > Loopback IPv4 can hold 2^24 different addresses, that is 16384 * 1024
> >
> > There is only one Loopback neigh "0.0.0.0 dev lo lladdr 00:00:00:00:00:00 NOARP"
> > existing even you have configured 2^24 different addresses on the loopback device.
>
> Eric, I think James is right, in __ipv4_neigh_lookup_noref():
>
>         if (dev->flags & (IFF_LOOPBACK | IFF_POINTOPOINT))
>                 key = INADDR_ANY;
>
>         return ___neigh_lookup_noref(&arp_tbl, neigh_key_eq32, arp_hashfn, &key, dev);
>
> So there should be at most one neigh entry over the loopback device.
> The patch looks safe to me, am I missing something?

This seems fine, thanks.

It is unfortunate ip command does not seem to display these
neighbours, for some reason.

(I am about to send a series of three patches to remove RTNL from "ip
neighbour show")

Reviewed-by: Eric Dumazet <edumazet@...gle.com>