netdev - Re: veth regression with "don’t modify ip_summed; doing so treats packets with bad checksums as good."

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Message-ID: <CAKUBDd8ojaAYHZ6L_8HBetqL1P6v9iy9W9ThZwnnzf3-npt-Vg@mail.gmail.com>
Date:	Fri, 25 Mar 2016 00:41:57 -0400
From:	Vijay Pandurangan <vijayp@...ayp.ca>
To:	Ben Greear <greearb@...delatech.com>
Cc:	Cong Wang <xiyou.wangcong@...il.com>,
	netdev <netdev@...r.kernel.org>, Evan Jones <ej@...njones.ca>,
	Cong Wang <cwang@...pensource.com>
Subject: Re: veth regression with "don’t modify ip_summed; doing so treats packets with bad checksums as good."

agreed. It should maybe be set to CHECKSUM_UNNECESSARY. The comment
seems to imply that it's treated the same as CHECKSUM_NONE but that's
evidently not true. I think that would fix the checksumming issue but
I'm fearful it may break something else:
http://lxr.free-electrons.com/source/include/linux/skbuff.h#L177

I'm really worried there are other equally subtle bugs hidden in this
code. Do we have any kind of regression test, or any automated way to
test all possible values on an skb to determine side effects to any
change here? (I'm new to the kernel so sorry if there's an answer in
an FAQ somewhere).

If not,
1. how should we ensure that our change doesn't break something else?
2. Should we audit / simplify the checksum code or come up with a list
of test cases that covers all uses?

On Fri, Mar 25, 2016 at 12:34 AM, Ben Greear <greearb@...delatech.com> wrote:
>
>
> On 03/24/2016 06:44 PM, Vijay Pandurangan wrote:
>>
>> Oops, I think my last email didn't go through due to an inadvertent
>> html attachment from my phone mail client.
>>
>> Can you send us a copy of a packet you're sending and/or confirm that
>> the IP and UDP4 checksums are set correctly in the packet?
>>
>> If those are set right, I think we need to read through the networking
>> code again to see why this is broken...
>
>
> Wireshark decodes the packet as having no checksum errors.
>
> I think the contents of the packet is correct, but the 'ip_summed'
> field is set incorrectly to 'NONE' when transmitting on a raw packet
> socket.
>
>
> Thanks,
> Ben
>
> --
> Ben Greear <greearb@...delatech.com>
> Candela Technologies Inc  http://www.candelatech.com