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

[Kuniyuki Iwashima <kuniyu@amazon.com>]

From: David Laight <david.laight.linux@gmail.com>
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.

---8<---
commit dab78a1745ab3c6001e1e4d50a9d09efef8e260d
Author: Philo Lu <lulie@linux.alibaba.com>
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.

> 
> Signed-off-by: David Laight <david.laight.linux@gmail.com>
> ---
>  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
>