netdev - Re: [PATCH net-next 2/2] datagram, udp: Set local address and rehash socket atomically against lookup

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Message-ID: <Z1Ip9Ij8_JpoFu8c@zatzit>
Date: Fri, 6 Dec 2024 09:32:20 +1100
From: David Gibson <david@...son.dropbear.id.au>
To: Eric Dumazet <edumazet@...gle.com>
Cc: Stefano Brivio <sbrivio@...hat.com>,
	Willem de Bruijn <willemdebruijn.kernel@...il.com>,
	netdev@...r.kernel.org, Kuniyuki Iwashima <kuniyu@...zon.com>,
	Mike Manning <mvrmanning@...il.com>,
	Paul Holzinger <pholzing@...hat.com>,
	Philo Lu <lulie@...ux.alibaba.com>,
	Cambda Zhu <cambda@...ux.alibaba.com>,
	Fred Chen <fred.cc@...baba-inc.com>,
	Yubing Qiu <yubing.qiuyubing@...baba-inc.com>
Subject: Re: [PATCH net-next 2/2] datagram, udp: Set local address and rehash
 socket atomically against lookup

On Thu, Dec 05, 2024 at 05:35:52PM +0100, Eric Dumazet wrote:
> On Wed, Dec 4, 2024 at 11:12 PM Stefano Brivio <sbrivio@...hat.com> wrote:
[snip]
> > diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
> > index 6a01905d379f..8490408f6009 100644
> > --- a/net/ipv4/udp.c
> > +++ b/net/ipv4/udp.c
> > @@ -639,18 +639,21 @@ struct sock *__udp4_lib_lookup(const struct net *net, __be32 saddr,
> >                 int sdif, struct udp_table *udptable, struct sk_buff *skb)
> >  {
> >         unsigned short hnum = ntohs(dport);
> > -       struct udp_hslot *hslot2;
> > +       struct udp_hslot *hslot, *hslot2;
> >         struct sock *result, *sk;
> >         unsigned int hash2;
> >
> > +       hslot = udp_hashslot(udptable, net, hnum);
> > +       spin_lock_bh(&hslot->lock);
> 
> This is not acceptable.
> UDP is best effort, packets can be dropped.
> Please fix user application expectations.

The packets aren't merely dropped, they're rejected with an ICMP Port
Unreachable.

-- 
David Gibson (he or they)	| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you, not the other way
				| around.
http://www.ozlabs.org/~dgibson

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