Date: Fri, 2 May 2014 14:02:49 +0000 From: "Bjoern A. Zeeb" <bz@FreeBSD.org> To: Michael Tuexen <Michael.Tuexen@lurchi.franken.de> Cc: FreeBSD Net <freebsd-net@freebsd.org> Subject: Re: RX checksum offloading problem Message-ID: <A345E6A0-D6FF-4E69-AFBD-9BB67B82F02E@FreeBSD.org> In-Reply-To: <0EB8F4F6-65C2-4B90-8101-FCC53A15C6F9@lurchi.franken.de> References: <0EB8F4F6-65C2-4B90-8101-FCC53A15C6F9@lurchi.franken.de>
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On 02 May 2014, at 10:22 , Michael Tuexen = <Michael.Tuexen@lurchi.franken.de> wrote: > 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. 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. =97=20 Bjoern A. Zeeb "Come on. Learn, goddamn it.", WarGames, 1983
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