netdev - Re: [Patch net] net: invert the check of detecting hardware RX checksum fault

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Date:   Fri, 16 Nov 2018 13:33:35 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Cong Wang <xiyou.wangcong@...il.com>,
        Eric Dumazet <eric.dumazet@...il.com>
Cc:     Herbert Xu <herbert@...dor.apana.org.au>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Tom Herbert <tom@...bertland.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Saeed Mahameed <saeedm@...lanox.com>
Subject: Re: [Patch net] net: invert the check of detecting hardware RX
 checksum fault



On 11/16/2018 12:15 PM, Cong Wang wrote:
> On Thu, Nov 15, 2018 at 8:52 PM Eric Dumazet <eric.dumazet@...il.com> wrote:
>>
>> It is very possible NIC provides an incorrect CHECKSUM_COMPLETE, in the
>> case non zero trailer bytes were added by a buggy switch (or host)
>>
>> Saeed can comment/confirm, but the theory is that the NIC does header analysis and
>> computes a checksum only on the bytes of the IP frame, not including the tail bytes
>> that were added by a switch.
> 
> 
> This theory seems can't explain why Pawel saw this warning so often,
> which is beyond the probability of a buggy switch. I don't know.

Well the bug here would be the receiver NIC, not really respecting CHECKSUM_COMPLETE premise
(provide a checksum over all the bytes, regardless of how smart header parsing can be on the NIC)

'Buggy switch' would add random bytes after IP frames, but as I mentioned, any AF_PACKET user
can cook arbitrary padding after a valid IP (or IPv6) frame.

> 
> I will try it.
> 
> Thanks.
>