Date: Fri, 2 May 2014 12:22:25 +0200 From: Michael Tuexen <Michael.Tuexen@lurchi.franken.de> To: FreeBSD Net <freebsd-net@freebsd.org> Subject: RX checksum offloading problem Message-ID: <0EB8F4F6-65C2-4B90-8101-FCC53A15C6F9@lurchi.franken.de>
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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. 2. Map CSUM_DATA_VALID to some other generic value in mbuf.h. However, = CSUM_L4_CALC, which would make perfect sense in my view, is already used for = CSUM_PSEUDO_HDR. Therefore we would have to define a new generic value. So what is the best way to fix this? Best regards Michael=
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