Subject: Re: [PATCH net 1/2] udp4: Rescan udp hash chains if cross-linked.

From: David Laight <[email-masked]>
Date: Mon, 27 Jan 2025 21:26:11 +0000

On Mon, 27 Jan 2025 12:33:04 -0800
Kuniyuki Iwashima <[email-masked]> wrote:

> From: David Laight <[email-masked]>
> Date: Mon, 27 Jan 2025 19:40:23 +0000
> > udp_lib_rehash() can get called at any time and will move a
> > socket to a different hash2 chain.
> > This can cause udp4_lib_lookup2() (processing incoming UDP) to
> > fail to find a socket and an ICMP port unreachable be sent.
> > 
> > Prior to ca065d0cf80fa the lookup used 'hlist_nulls' and checked
> > that the 'end if list' marker was on the correct list.  
> 
> I think we should use hlist_nulls for hash2 as hash4.

From what I remember when I first wrote this patch (mid 2023) using
hlist_nulls doesn't make much difference.
The code just did a rescan when the 'wrong NULL' was found rather than
when the last item wasn't on the starting hash chain.
ISTR it was removed to simplify other code paths.

> 
> ---8<---
> commit dab78a1745ab3c6001e1e4d50a9d09efef8e260d
> Author: Philo Lu <[email-masked]>
> Date:   Thu Nov 14 18:52:05 2024 +0800
> 
>     net/udp: Add 4-tuple hash list basis
> ...
>     hash4 uses hlist_nulls to avoid moving wrongly onto another hlist due to
>     concurrent rehash, because rehash() can happen with lookup().
> ---8<---
> 
> 
> Also, Fixes: tag is missing in both patches.

Semi-deliberate to stop it being immediately backported.

While I think we have a system/test that fails it is running Ubuntu on a Dell
server and I don't think the raid controller driver is in the main kernel tree.
(We're definitely seeing unexpected ICMP on localhost - hard to get otherwise.)

	David

> 
> > 
> > Signed-off-by: David Laight <[email-masked]>
> > ---
> >  net/ipv4/udp.c | 19 +++++++++++++++++--
> >  1 file changed, 17 insertions(+), 2 deletions(-)
> > 
> > diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
> > index 86d282618515..a8e2b431d348 100644
> > --- a/net/ipv4/udp.c
> > +++ b/net/ipv4/udp.c
> > @@ -425,16 +425,21 @@ static struct sock *udp4_lib_lookup2(const struct net *net,
> >  				     __be32 saddr, __be16 sport,
> >  				     __be32 daddr, unsigned int hnum,
> >  				     int dif, int sdif,
> > +				     unsigned int hash2, unsigned int mask,
> >  				     struct udp_hslot *hslot2,
> >  				     struct sk_buff *skb)
> >  {
> > +	unsigned int hash2_rescan;
> >  	struct sock *sk, *result;
> >  	int score, badness;
> >  	bool need_rescore;
> >  
> > +rescan:
> > +	hash2_rescan = hash2;
> >  	result = NULL;
> >  	badness = 0;
> >  	udp_portaddr_for_each_entry_rcu(sk, &hslot2->head) {
> > +		hash2_rescan = udp_sk(sk)->udp_portaddr_hash;
> >  		need_rescore = false;
> >  rescore:
> >  		score = compute_score(need_rescore ? result : sk, net, saddr,
> > @@ -475,6 +480,16 @@ static struct sock *udp4_lib_lookup2(const struct net *net,
> >  			goto rescore;
> >  		}
> >  	}
> > +
> > +	/* udp sockets can get moved to a different hash chain.
> > +	 * If the chains have got crossed then rescan.
> > +	 */                         
> 
> nit: trailing spaces here ^^^^^^^^
> 
> 
> > +	if ((hash2_rescan ^ hash2) & mask) {
> > +		/* Ensure hslot2->head is reread */
> > +		barrier();
> > +		goto rescan;
> > +	}
> > +
> >  	return result;
> >  }
> >  
> > @@ -654,7 +669,7 @@ struct sock *__udp4_lib_lookup(const struct net *net, __be32 saddr,
> >  	/* Lookup connected or non-wildcard socket */
> >  	result = udp4_lib_lookup2(net, saddr, sport,
> >  				  daddr, hnum, dif, sdif,
> > -				  hslot2, skb);
> > +				  hash2, udptable->mask, hslot2, skb);
> >  	if (!IS_ERR_OR_NULL(result) && result->sk_state == TCP_ESTABLISHED)
> >  		goto done;
> >  
> > @@ -680,7 +695,7 @@ struct sock *__udp4_lib_lookup(const struct net *net, __be32 saddr,
> >  
> >  	result = udp4_lib_lookup2(net, saddr, sport,
> >  				  htonl(INADDR_ANY), hnum, dif, sdif,
> > -				  hslot2, skb);
> > +				  hash2, udptable->mask, hslot2, skb);
> >  done:
> >  	if (IS_ERR(result))
> >  		return NULL;
> > -- 
> > 2.39.5
> >