Date: Wed, 7 May 2014 17:37:51 +0900 From: Yonghyeon PYUN <pyunyh@gmail.com> To: Michael Tuexen <Michael.Tuexen@lurchi.franken.de> Cc: FreeBSD Net <freebsd-net@freebsd.org>, "Bjoern A. Zeeb" <bz@FreeBSD.org> Subject: Re: RX checksum offloading problem Message-ID: <20140507083751.GB1376@michelle.cdnetworks.com> In-Reply-To: <36469814-FAC8-4172-A792-487E2AB8ECB9@lurchi.franken.de> 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>
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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: > > > 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: > >> > >>> > >>> On 02 May 2014, at 10:22 , Michael Tuexen <Michael.Tuexen@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? > 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.
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