RX checksum offloading problem

Michael Tuexen Michael.Tuexen at lurchi.franken.de
Thu May 8 18:40:35 UTC 2014

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.
>>> 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?
>>> 3. How the controller can compute TCP checksum of fragmented
>>>  packets?
At least it does it right for UDP...

Best regards
>>> Since you have the controller I guess it's easy to verify all
>>> cases.  For case 3, I believe the controller can't handle
>>> fragmented frames so driver should have to explicitly check ip_off
>>> field of IPv4 header.  See how gem(4)/sk(4)/hme(4) and fxp(4)
>>> handle it.
>> Let me check this. Is there a tool to send UDP/TCP with IP level options
>> or do I need to write a small test program myself?
> I recall I used buggy ipsend of ipfilter package in the past but it
> would be more easy to write a simple test program or patch driver
> to generate those frames.

More information about the freebsd-net mailing list