netdev - Re: [PATCH net-next] neighbor: fix proxy

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Date:   Wed, 25 Jan 2023 14:50:25 -0500
From:   Brian Haley <haleyb.dev@...il.com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     davem@...emloft.net, edumazet@...gle.com, pabeni@...hat.com,
        netdev@...r.kernel.org
Subject: Re: [PATCH net-next] neighbor: fix proxy_delay usage when it is zero

Jakub - thanks for the review.

On 1/24/23 11:30 PM, Jakub Kicinski wrote:
> On Mon, 23 Jan 2023 13:58:29 -0500 Brian Haley wrote:
>> When set to zero, the neighbor sysctl proxy_delay value
>> does not cause an immediate reply for ARP/ND requests
>> as expected, it instead causes a random delay between
>> [0, U32_MAX]. Looking at this comment from
>> __get_random_u32_below() explains the reason:
>>
>> /*
>>   * This function is technically undefined for ceil == 0, and in fact
>>   * for the non-underscored constant version in the header, we build bug
>>   * on that. But for the non-constant case, it's convenient to have that
>>   * evaluate to being a straight call to get_random_u32(), so that
>>   * get_random_u32_inclusive() can work over its whole range without
>>   * undefined behavior.
>>   */
>>
>> Added helper function that does not call get_random_u32_below()
>> if proxy_delay is zero and just uses the current value of
>> jiffies instead, causing pneigh_enqueue() to respond
>> immediately.
>>
>> Also added definition of proxy_delay to ip-sysctl.txt since
>> it was missing.
> 
> Sounds like this never worked, until commit a533b70a657c ("net:
> neighbor: fix a crash caused by mod zero") it crashed, now it
> does something silly. Can we instead reject 0 as invalid input
> during configuration?

To me, proxy_delay==0 implies respond immediately, so I think zero is 
valid input.

>> diff --git a/Documentation/networking/ip-sysctl.rst b/Documentation/networking/ip-sysctl.rst
>> index 7fbd060d6047..34183fb38b20 100644
>> --- a/Documentation/networking/ip-sysctl.rst
>> +++ b/Documentation/networking/ip-sysctl.rst
>> @@ -1589,6 +1589,12 @@ proxy_arp_pvlan - BOOLEAN
>>   	  Hewlett-Packard call it Source-Port filtering or port-isolation.
>>   	  Ericsson call it MAC-Forced Forwarding (RFC Draft).
>>   
>> +proxy_delay - INTEGER
>> +	Delay proxy response.
>> +
>> +	The maximum number of jiffies to delay a response to a neighbor
>> +	solicitation when proxy_arp or proxy_ndp is enabled. Defaults to 80.
> 
> Is there a better way of expressing the fact that we always
> choose a value lower than proxy_delay ? Maximum sounds a bit
> like we'd do:
> 
> 	when = jiffies + random() % (proxy_delay + 1);

I think technically it's in the range of [0, proxy_delay] but I'll 
change the text to make that more obvious.

>>   shared_media - BOOLEAN
>>   	Send(router) or accept(host) RFC1620 shared media redirects.
>>   	Overrides secure_redirects.
>> diff --git a/net/core/neighbour.c b/net/core/neighbour.c
>> index f00a79fc301b..8bd8aaae6d5e 100644
>> --- a/net/core/neighbour.c
>> +++ b/net/core/neighbour.c
>> @@ -1662,11 +1662,22 @@ static void neigh_proxy_process(struct timer_list *t)
>>   	spin_unlock(&tbl->proxy_queue.lock);
>>   }
>>   
>> +static __inline__ unsigned long neigh_proxy_delay(struct neigh_parms *p)
> 
> Drop the inline please, it's pointless for a tiny static function

JayV mentioned the same in private, guess I owe him a beer.

>> +{
>> +	/*
> 
> did you run checkpatch?

No, my kernel-fu is a little rusty, will do on v2.

> 
>> +	 * If proxy_delay is zero, do not call get_random_u32_below()
>> +	 * as it is undefined behavior.
>> +	 */
>> +	unsigned long proxy_delay = NEIGH_VAR(p, PROXY_DELAY);
> 
> empty line here
> 
>> +	return proxy_delay ?
>> +	       jiffies + get_random_u32_below(NEIGH_VAR(p, PROXY_DELAY)) :
> 
> also - since you have proxy_delay in a local variable why not use it

Ack.

Thanks,

-Brian