Date: Thu, 8 May 2014 20:40:22 +0200 From: Michael Tuexen <Michael.Tuexen@lurchi.franken.de> To: pyunyh@gmail.com Cc: FreeBSD Net <freebsd-net@freebsd.org>, "Bjoern A. Zeeb" <bz@FreeBSD.org> Subject: Re: RX checksum offloading problem Message-ID: <415C1CB5-3AF9-44E4-943A-74116037980E@lurchi.franken.de> In-Reply-To: <20140507083751.GB1376@michelle.cdnetworks.com> References: <0EB8F4F6-65C2-4B90-8101-FCC53A15C6F9@lurchi.franken.de> <A345E6A0-D6FF-4E69-AFBD-9BB67B82F02E@FreeBSD.org> <B149FC4B-4F15-4619-A04F-F1A08DDC1741@lurchi.franken.de> <20140507075612.GA1376@michelle.cdnetworks.com> <36469814-FAC8-4172-A792-487E2AB8ECB9@lurchi.franken.de> <20140507083751.GB1376@michelle.cdnetworks.com>
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On 07 May 2014, at 10:37, Yonghyeon PYUN <pyunyh@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@gmail.com> wrote: >>=20 >>> On Sat, May 03, 2014 at 11:52:47AM +0200, Michael Tuexen wrote: >>>> On 02 May 2014, at 16:02, Bjoern A. Zeeb <bz@FreeBSD.org> wrote: >>>>=20 >>>>>=20 >>>>> On 02 May 2014, at 10:22 , Michael Tuexen = <Michael.Tuexen@lurchi.franken.de> wrote: >>>>>=20 >>>>>> Dear all, >>>>>>=20 >>>>>> 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. >>>>>>=20 >>>>>> 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. >>>>>>=20 >>>>>> 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. >>>>>>=20 >>>>>> Now the following happens (on the raspberry pi the driver used is >>>>>> dev/usb/net/if_smsc.c >>>>>>=20 >>>>>> 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. >>>>>>=20 >>>>>> Alternatives to fix this: >>>>>>=20 >>>>>> 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. >>>>>=20 >>>>> 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. >>>>=20 >>>=20 >>> 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: >>>=20 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 Michael >>>=20 >>> 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? >>=20 >=20 > 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. >=20
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