examples: add example for XDP

[wedaly]

Add an example for attaching an eBPF program to a network
interface with XDP. The example program parses the IPv4
source address from packets (if available) and records
packet counts by IP address in an LRU hash map.

Issue: #645

[wedaly]

Working on fixing this error in the build:

In file included from /ebpf/examples/xdp/xdp.c:5:
In file included from /usr/include/linux/if_ether.h:25:
/usr/include/linux/types.h:5:10: fatal error: 'asm/types.h' file not found
#include <asm/types.h>
         ^~~~~~~~~~~~~

[markpash]

Working on fixing this error in the build:

In file included from /ebpf/examples/xdp/xdp.c:5:
In file included from /usr/include/linux/if_ether.h:25:
/usr/include/linux/types.h:5:10: fatal error: 'asm/types.h' file not found
#include <asm/types.h>
         ^~~~~~~~~~~~~

@wedaly you can solve that by copying the types you need from the linux headers into your C code, or throw them in the common headers, and then not including header files from linux.

[wedaly]

Copied the types I needed into examples/common.h. Maybe I should pull in the header changes from #711 (or wait until after that PR merges?) to avoid conflicts?

[markpash]

Nice example! Just a few non-critical suggestions.

[markpash]

Suggested change

	var sb strings.Builder
	var key, val uint32
	iter := m.Iterate()
	for iter.Next(&key, &val) {
	sourceIP := net.IPv4(byte(key>>24), byte(key>>16), byte(key>>8), byte(key))
	packetCount := val
	var (
	sb strings.Builder
	key []byte
	val uint32
	iter = m.Iterate()
	)
	for iter.Next(&key, &val) {
	sourceIP := net.IP(key)
	packetCount := val

[markpash]

Since you do the byte swap of the IP in BPF, we can just get key in the form of []byte and coerce it to a Go net.IP. (This should be even easier with go1.18 and netip.Addr since it implements encoding.BinaryUnmarshaler!)

It would also make it super easy if someone wanted to adapt this to support IPv6!

[wedaly]

good call, updated! When I tested this, I noticed after switching to net.IP(key), the IP address octets were being printed in reverse order, so I also updated xdp.c to store them in network byte order (removed the call to bpf_ntohl)

[dadav]

I'm having a hard time getting the ipv6 network byte order thingy working, would be awesome if someone could share an example someday :)

[kolinfluence]

anyone care to share the ipv6 example?

[lmb]

Thanks!