netdev - Re: [PATCH net] net: bridge: clear bridge's private skb space on xmit

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Message-ID: <181931fb-dc60-7db6-60ac-b8ff1402efec@cumulusnetworks.com>
Date:   Fri, 31 Jul 2020 20:37:50 +0300
From:   Nikolay Aleksandrov <nikolay@...ulusnetworks.com>
To:     David Ahern <dsahern@...il.com>, netdev@...r.kernel.org
Cc:     bridge@...ts.linux-foundation.org, roopa@...ulusnetworks.com,
        davem@...emloft.net
Subject: Re: [PATCH net] net: bridge: clear bridge's private skb space on xmit

On 31/07/2020 20:27, David Ahern wrote:
> On 7/31/20 10:26 AM, Nikolay Aleksandrov wrote:
>> We need to clear all of the bridge private skb variables as they can be
>> stale due to the packet being recirculated through the stack and then
>> transmitted through the bridge device. Similar memset is already done on
>> bridge's input. We've seen cases where proxyarp_replied was 1 on routed
>> multicast packets transmitted through the bridge to ports with neigh
>> suppress which were getting dropped. Same thing can in theory happen with
>> the port isolation bit as well.
>>
>> Fixes: 821f1b21cabb ("bridge: add new BR_NEIGH_SUPPRESS port flag to suppress arp and nd flood")
>> Signed-off-by: Nikolay Aleksandrov <nikolay@...ulusnetworks.com>
>> ---
>>  net/bridge/br_device.c | 2 ++
>>  1 file changed, 2 insertions(+)
>>
>> diff --git a/net/bridge/br_device.c b/net/bridge/br_device.c
>> index 8c7b78f8bc23..9a2fb4aa1a10 100644
>> --- a/net/bridge/br_device.c
>> +++ b/net/bridge/br_device.c
>> @@ -36,6 +36,8 @@ netdev_tx_t br_dev_xmit(struct sk_buff *skb, struct net_device *dev)
>>  	const unsigned char *dest;
>>  	u16 vid = 0;
>>  
>> +	memset(skb->cb, 0, sizeof(struct br_input_skb_cb));
>> +
>>  	rcu_read_lock();
>>  	nf_ops = rcu_dereference(nf_br_ops);
>>  	if (nf_ops && nf_ops->br_dev_xmit_hook(skb)) {
>>
> 
> What's the performance hit of doing this on every packet?
> 
> Can you just set a flag that tells the code to reset on recirculation?
> Seems like br_input_skb_cb has space for that.
> 

Virtually non-existent, we had a patch that turned that field into a 16 byte
field so that is really 2 8 byte stores. It is already cache hot, we could
initialize each individual field separately as br_input does.

I don't want to waste flags on such thing, this makes it future-proof 
and I'll remove the individual field zeroing later which will alleviate
the cost further.