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Author SHA1 Message Date
Jesper Dangaard Brouer e2e3224122 samples/bpf: xdp2skb_meta comment explain why pkt-data pointers are invalidated
Improve the 'unknown reason' comment, with an actual explaination of why
the ctx pkt-data pointers need to be loaded after the helper function
bpf_xdp_adjust_meta().  Based on the explaination Daniel gave.

Fixes: 36e04a2d78 ("samples/bpf: xdp2skb_meta shows transferring info from XDP to SKB")
Reported-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
2018-01-18 01:49:09 +01:00
Jesper Dangaard Brouer 36e04a2d78 samples/bpf: xdp2skb_meta shows transferring info from XDP to SKB
Creating a bpf sample that shows howto use the XDP 'data_meta'
infrastructure, created by Daniel Borkmann.  Very few drivers support
this feature, but I wanted a functional sample to begin with, when
working on adding driver support.

XDP data_meta is about creating a communication channel between BPF
programs.  This can be XDP tail-progs, but also other SKB based BPF
hooks, like in this case the TC clsact hook. In this sample I show
that XDP can store info named "mark", and TC/clsact chooses to use
this info and store it into the skb->mark.

It is a bit annoying that XDP and TC samples uses different tools/libs
when attaching their BPF hooks.  As the XDP and TC programs need to
cooperate and agree on a struct-layout, it is best/easiest if the two
programs can be contained within the same BPF restricted-C file.

As the bpf-loader, I choose to not use bpf_load.c (or libbpf), but
instead wrote a bash shell scripted named xdp2skb_meta.sh, which
demonstrate howto use the iproute cmdline tools 'tc' and 'ip' for
loading BPF programs.  To make it easy for first time users, the shell
script have command line parsing, and support --verbose and --dry-run
mode, if you just want to see/learn the tc+ip command syntax:

 # ./xdp2skb_meta.sh --dev ixgbe2 --dry-run
 # Dry-run mode: enable VERBOSE and don't call TC+IP
 tc qdisc del dev ixgbe2 clsact
 tc qdisc add dev ixgbe2 clsact
 tc filter add dev ixgbe2 ingress prio 1 handle 1 bpf da obj ./xdp2skb_meta_kern.o sec tc_mark
 # Flush XDP on device: ixgbe2
 ip link set dev ixgbe2 xdp off
 ip link set dev ixgbe2 xdp obj ./xdp2skb_meta_kern.o sec xdp_mark

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
2018-01-11 01:02:25 +01:00