RX checksum offloading problem

Michael Tuexen Michael.Tuexen at lurchi.franken.de
Wed May 7 08:07:14 UTC 2014

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:
> 1. Is the controller smart enough to skip IP options header in
>   TCP/UDP checksum offloading?
> 2. How controller handles UDP checksum value 0x0000(i.e. sender
>   didn't compute UDP checksum)?
> 3. How the controller can compute TCP checksum of fragmented
>   packets?
> 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?

Best regards
>> Best regards
>> Michael 
>>> ? 
>>> Bjoern A. Zeeb             "Come on. Learn, goddamn it.", WarGames, 1983

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