RX checksum offloading problem

Michael Tuexen Michael.Tuexen at lurchi.franken.de
Mon May 12 09:54:52 UTC 2014

On 12 May 2014, at 03:45, Yonghyeon PYUN <pyunyh at gmail.com> wrote:

> On Fri, May 09, 2014 at 04:22:36PM +0200, Michael Tuexen wrote:
>> On 09 May 2014, at 12:46, Michael Tuexen <Michael.Tuexen at lurchi.franken.de> wrote:
>>> On 09 May 2014, at 03:35, Yonghyeon PYUN <pyunyh at gmail.com> wrote:
>>>> On Thu, May 08, 2014 at 08:40:22PM +0200, Michael Tuexen wrote:
>>>>> On 07 May 2014, at 10:37, Yonghyeon PYUN <pyunyh at gmail.com> wrote:
>>>>>> On Wed, May 07, 2014 at 10:07:09AM +0200, Michael Tuexen wrote:
>>>>>>> On 07 May 2014, at 09:56, Yonghyeon PYUN <pyunyh at gmail.com> wrote:
>>>>>>>> On Sat, May 03, 2014 at 11:52:47AM +0200, Michael Tuexen wrote:
>>>>>>>>> On 02 May 2014, at 16:02, Bjoern A. Zeeb <bz at FreeBSD.org> wrote:
>>>>>>>>>> On 02 May 2014, at 10:22 , Michael Tuexen <Michael.Tuexen at lurchi.franken.de> wrote:
>>>>>>>>>>> Dear all,
>>>>>>>>>>> during testing I found that FreeBSD head (on a raspberry pi) accepts SCTP packet
>>>>>>>>>>> with bad checksums. After debugging this I figured out that this is a problem with
>>>>>>>>>>> the csum_flags defined in mbuf.h.
>>>>>>>>>>> The SCTP code on its input path checks for CSUM_SCTP_VALID, which is defined in mbuf.h:
>>>>>>>>>>> #define CSUM_SCTP_VALID         CSUM_L4_VALID
>>>>>>>>>>> This makes sense: If CSUM_SCTP_VALID is set in csum_flags, the packet is considered
>>>>>>>>>>> to have a correct checksum.
>>>>>>>>>>> For UDP and TCP some drivers calculate the UDP/TCP checksum and set CSUM_DATA_VALID in
>>>>>>>>>>> csum_flags to indicate that the UDP/TCP should consider csum_data to figure out if
>>>>>>>>>>> the packet has a correct checksum. The problem is that CSUM_DATA_VALID is defined as
>>>>>>>>>>> #define CSUM_DATA_VALID         CSUM_L4_VALID
>>>>>>>>>>> In this case the semantic is not that the packet has a valid checksum, but the csum_data
>>>>>>>>>>> field contains information.
>>>>>>>>>>> Now the following happens (on the raspberry pi the driver used is
>>>>>>>>>>> dev/usb/net/if_smsc.c
>>>>>>>>>>> 1. A packet is received and if it is not too short, the checksum computed
>>>>>>>>>>> is stored in csum_data and the flag CSUM_DATA_VALID is set. This happens
>>>>>>>>>>> for all IP packets, not only for UDP and TCP packets.
>>>>>>>>>>> 2. In case of SCTP packets, the SCTP interprets CSUM_DATA_VALID as CSUM_SCTP_VALID
>>>>>>>>>>> and accepts the packet. So no SCTP checksum check ever happened.
>>>>>>>>>>> Alternatives to fix this:
>>>>>>>>>>> 1. Change all drivers to set CSUM_DATA_VALID only in case of UDP or TCP packets, since
>>>>>>>>>>> it only makes sense in these cases.
>>>>>>>>>> Wait, or for SCTP in cad the crc32 (I think it was)  was actually checked but not otherwise.   This is how it should be imho.  It seems like a driver bug.
>>>>>>>>> I went through the list of drivers and you are right, it seems to be a bug
>>>>>>>>> in if_smsc.c. Most of the other drivers check for UDP/TCP, a small set I can't tell.
>>>>>>>> I'm not sure how the controller computes TCP/UDP checksum values.
>>>>>>>> It seems the publicly available data sheet was highly sanitized so
>>>>>>>> it was useless to me.  The comment in the driver says that the
>>>>>>> Same for me...
>>>>>>>> controller computes RX checksum after the IPv4 header to the end of
>>>>>>>> ethernet frame. After seeing that comment, three questions popped
>>>>>>>> up:
>>>>> OK, I did some testing. It looks like the card is just computing the
>>>>> checksum over the IP payload taking the correct IP header length into account.
>>>>>>>> 1. Is the controller smart enough to skip IP options header in
>>>>>>>> TCP/UDP checksum offloading?
>>>>> Yes, I can send fragmented and un-fragmented UDP packets with IP options
>>>>> and they are handled correctly. Even if the last fragment is too short.
>>>> I'm assuming you're taking about receiving fragmented UDP packets
>>>> with RX checksum offloading, right?
>>> Correct.
>>>>>>>> 2. How controller handles UDP checksum value 0x0000(i.e. sender
>>>>>>>> didn't compute UDP checksum)?
>>>>> This case isn't handled. However, udp_input() looks first for zero checksums
>>>>> and only after that in the csum_flags. So it doesn't result in any problems.
>>>>> Would you prefer not to set CSUM_DATA_VALID in this case?
>>>> At least, it correctly updates UDP stats of netstat(1).
>>> Let me double check that...
>> I double checked it. The statistic counters are incremented.
>> Please note that we had a bug in the sending code of head, which
>> made it impossible to send UDP packets with 0 checksum. That is
>> fixed in
>> http://svnweb.freebsd.org/base?view=revision&revision=265776
> Thanks.
>> So any preference whether to report CSUM_DATA_VALID if a UDP packet
>> with checksum 0 is received or not? I'm pretty open, since it does
>> not have any effect right now...
> Because upper stack correctly counts for these packets, your change
> (report CSUM_DATA_VALID for UDP packet with checksum value 0) looks
> fine.  I don't remember how pf/ipf handles that case though but we
> can easily fix pf/ipf once we see breakage.

Best regards
>> Best regards
>> Michael

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