linux-kernel - Re: [PATCH] neighbour: purge nf_bridged skb from foreign device neigh

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Date: Wed, 10 Jan 2024 19:16:35 +0800
From: Pavel Tikhomirov <ptikhomirov@...tuozzo.com>
To: Florian Westphal <fw@...len.de>
Cc: "David S. Miller" <davem@...emloft.net>,
 Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>,
 Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org,
 linux-kernel@...r.kernel.org, kernel@...nvz.org
Subject: Re: [PATCH] neighbour: purge nf_bridged skb from foreign device neigh

Here is the new version 
https://lore.kernel.org/netdev/20240110110451.5473-1-ptikhomirov@virtuozzo.com/

On 09/01/2024 19:12, Florian Westphal wrote:
> Pavel Tikhomirov <ptikhomirov@...tuozzo.com> wrote:
>> index f980edfdd2783..105fbdb029261 100644
>> --- a/include/linux/netfilter_bridge.h
>> +++ b/include/linux/netfilter_bridge.h
>> @@ -56,11 +56,15 @@ static inline int nf_bridge_get_physoutif(const struct
>> sk_buff *skb)
>>   }
>>
>>   static inline struct net_device *
>> -nf_bridge_get_physindev(const struct sk_buff *skb)
>> +nf_bridge_get_physindev_rcu(const struct sk_buff *skb)
>>   {
>>          const struct nf_bridge_info *nf_bridge = nf_bridge_info_get(skb);
>> +       struct net_device *dev;
>>
>> -       return nf_bridge ? nf_bridge->physindev : NULL;
>> +       if (!nf_bridge || !skb->dev)
>> +               return 0;
>> +
>> +       return dev_get_by_index_rcu(skb->dev->net, nf_bridge->physindev_if);
> 
> You could use dev_net(skb->dev), yes.

In br_nf_pre_routing_finish_bridge_slow I had to use dev_net(skb->dev).

> 
> Or create a preparation patch that does:
> 
> -nf_bridge_get_physindev(const struct sk_buff *skb)
> +nf_bridge_get_physindev(const struct sk_buff *skb, struct net *net)
> 
> (all callers have a struct net available).

For all other cases I did the prep-patch propagating net.

> 
> No need to rename the function, see below.
> 
>> -       br_indev = nf_bridge_get_physindev(oldskb);
>> +       rcu_read_lock_bh();
>> +       br_indev = nf_bridge_get_physindev_rcu(oldskb);
> 
> No need for rcu read lock, all netfilter hooks run inside
> rcu_read_lock().

Thanks for this hint! I have checked all those tons of cases and 
actually proved to myself that all cases have rcu_read_lock =)

> 
>> Does it sound good?
> 
> Yes, seems ok to me.
> 
>> Or maybe instead we can have extra physindev_if field in addition to
>> existing physindev to only do dev_get_by_index_rcu inside
>> br_nf_pre_routing_finish_bridge_slow to doublecheck the ->physindev link?
>>
>> Sorry in advance if I'm missing anything obvious.
> 
> Alternative would be to add a 'br_nf_unreg_serno' that gets incremented
> from brnf_device_event(), then store that in nf_bridge_info struct and
> compare to current value before net_device deref. If not equal, toss skb.
> 
> Problem is that we'd need some indirection to retrieve the current
> value, otherwise places like nfnetlink_log() gain a module dependency on
> br_netfilter :-(
> 
> We'd likely need
> const atomic_t *br_nf_unreg_serno __read_mostly;
> EXPORT_SYMBOL_GPL(br_nf_unreg_serno);
> 
> in net/netfilter/core.c for this, then set/clear the
> pointer from br_netfilter_hooks.c.
> 
> I can't say/don't know which of the two options is better/worse.
> 
> s/struct net_device */int// has the benefit of shrinking nf_bridge_info,
> so I'd try that first.

Ok, did s/struct net_device */int// variant.

-- 
Best regards, Tikhomirov Pavel
Senior Software Developer, Virtuozzo.