Support for parsing GSO_PARTIAL packets

[williamtu]

Hi,
We are running Intel i40e driver with Geneve TSO offload and collecting TX traffic using tcpdump. When enabling the GSO_PARTIAL, the Linux network stack sends a 64k byte packet to the driver, but the tcpdump reports packet length of 1500, even though the caplen is 64k.

The reason is due to tunnel encapsulation with GSO_PARTIAL. The outer header could be vxlan or geneve, and GSO_PARTIAL carries multiple inner frame, with each inner frame an 1500 MTU size. So this confuses the tcpdump, which parses only the first inner frame and report length of 1500, instead of the sum of all inner frames.

An pcap file is attached:
geneve_gso_partial-2.pcap.txt

$ tcpdump -r geneve_gso_partial-2.pcap
IP 10.12.14.146.24321 > 10.12.14.147.6081: Geneve, Flags [none], 
vni 0x0: IP 192.168.0.146.51260 > 192.168.0.147.33997: Flags [.], 
seq 1:1449, ack 0, win 29, options [nop,nop,TS val 24588549 ecr 431281855], length 1448

It shows only the first frame of length 1448, but the actual caplen is 64k because there are more frame behind the first.

I looked at the source code and don't have a good solution. One way is to pass the caplen all the way to ip_print, tcp_print, so we know that there are still bytes not processed yet. Another quick solution is to enable printing a warning when tcpdump processes bytes less than caplen, or in command line adds an option to print out caplen so when people see the output "length" != caplen, they have to check by themselves.

About GSO partial
https://netdevconf.org/1.2/slides/oct5/09_LCO_GSO_Partial.pdf

System Info
tcpdump --version
tcpdump version 4.10.0-PRE-GIT_2017_03_13
libpcap version 1.7.4
OpenSSL 1.0.2g 1 Mar 2016
OS: ubuntu 16.04, Linux kernel 4.8
Steps to reproduce: download the pcap file and tcpdump -r

[mcr]

Do you think libpcap should decapsulate the GSO and return multiple packets upwards, or should tcpdump know how to decode GSO?

[infrastation]

What would be the steps to reproduce this in a test lab? What would be the most common use case that would deliver such packets out of the box?