Re: [PATCH 1/2] [IPV4] UDP: Always checksum even if without socket filter

[Stephen Hemminger <shemminger@linux-foundation.org>]

David Miller wrote:
> From: Andi Kleen <andi@firstfloor.org>
> Date: Sat, 17 Nov 2007 14:18:46 +0100
>
>   
>> Wang Chen <wangchen@cn.fujitsu.com> writes:
>>
>>     
>>> Herbert Xu said the following on 2007-11-16 12:11:
>>>       
>>>> Wang Chen <wangchen@cn.fujitsu.com> wrote:
>>>>         
>>>>> So, I think the checksum in udp_queue_rcv_skb() actually does
>>>>> the work, not that in udp_recvmsg() and udp_poll().
>>>>>
>>>>> If I am wrong, please point out.
>>>>>           
>>>> We may have a bug in the accounting area.  Check the recent
>>>> patch made to UDP/IPv6 which is probably needed here as well.
>>>>
>>>>         
>>> Like dave said, decrementing the InDataGrams in this case is an
>>> option.
>>> I will check the same place of UDP/IPv6.
>>>       
>> And like Benny pointed out it's probably a bad idea because
>> decrementing counters will be an unexpected ABI change for monitoring 
>> programs who have no other way to detect overflow.
>>     
>
> We could defer the increment until we check the checksum,
> but that is likely to break even more things because people
> (as Wang Chen did initially) will send a packet to some
> port with an app that doesn't eat the packets, and expect the
> InDatagrams counter to increase once the stack eats the packet.
> But it won't until the application does the read.
>
> All of our options suck, we just have to choose the least sucking one
> and right now to me that's decrementing the counter as much as I
> empathize with the SNMP application overflow detection issue.
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
>   
You might be able hide the problem by adding some "defered tick logic" 
so that the
counter is monotonic. Kind of like the NTP clock holding hack.