Simple XDP example

Makefile

LLVM_PREFIX ?= /usr/bin
CLANG ?= $(LLVM_PREFIX)/clang
LLC ?= $(LLVM_PREFIX)/llc
INCLUDEFLAGS = -I/usr/include

all: xdp_prog.elf

clean:
        -$(RM) *.elf

%.elf : %.o
        $(LLC) -march=bpf -filetype=obj -o $@ $<

%.o : %.c
        $(CLANG) $(INCLUDEFLAGS) -target bpf -O2 -emit-llvm -g \
                -Wall -Werror \
                -Wno-unused-variable \
                -Wno-compare-distinct-pointer-types \
                -Wno-implicit-function-declaration \
                -c $< -o $@

xdp_prog.c

#include <linux/bpf.h>
#include <linux/in.h>
#include <linux/if_ether.h>
#include <linux/ip.h>
#include <linux/ipv6.h>

typedef unsigned short u16;
typedef unsigned int u32;
typedef unsigned long u64;

#define BPF_FUNC_map_lookup_elem 1

static void *(*bpf_map_lookup_elem)(void *map, void *key) =
	(void *) BPF_FUNC_map_lookup_elem;

#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
# define __bpf_htons(x)			__builtin_bswap16(x)
# define __bpf_constant_htons(x)	___constant_swab16(x)
#elif __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
# define __bpf_htons(x)			(x)
# define __bpf_constant_htons(x)	(x)
#else
# error "Fix your compiler's __BYTE_ORDER__?!"
#endif

# define bpf_htons(x)				\
	(__builtin_constant_p(x) ?		\
	 __bpf_constant_htons(x) : __bpf_htons(x))

struct bpf_map_def {
	u32 type;
	u32 key_size;
	u32 value_size;
	u32 max_entries;
	u32 flags;
	u32 inner_map_idx;
};

#define SEC(NAME) __attribute__((section(NAME), used))
struct bpf_map_def SEC("maps") rxcnt = {
	.type = BPF_MAP_TYPE_ARRAY,
	.key_size = sizeof(u32),
	.value_size = sizeof(long),
	.max_entries = 256,
};

static int parse_ipv4(void *data, u64 nh_off, void *data_end)
{
	struct iphdr *iph = data + nh_off;

	if (iph + 1 > data_end)
		return 0;
	return iph->protocol;
}

static int parse_ipv6(void *data, u64 nh_off, void *data_end)
{
	struct ipv6hdr *ip6h = data + nh_off;

	if (ip6h + 1 > data_end)
		return 0;
	return ip6h->nexthdr;
}

SEC("xdp")
int xdp_prog(struct xdp_md *ctx)
{
	void *data_end = (void *)(long)ctx->data_end;
	void *data = (void *)(long)ctx->data;
	struct ethhdr *eth = data;
	long *value;
	u16 h_proto;
	u64 nh_off;
	u32 ipproto;

	nh_off = sizeof(*eth);
	if (data + nh_off > data_end)
		return XDP_DROP;

	h_proto = eth->h_proto;

	if (h_proto == bpf_htons(ETH_P_IP))
		ipproto = parse_ipv4(data, nh_off, data_end);
	else if (h_proto == bpf_htons(ETH_P_IPV6))
		ipproto = parse_ipv6(data, nh_off, data_end);
	else
		ipproto = 0;

	value = bpf_map_lookup_elem(&rxcnt, &ipproto);
	if (value)
		*value += 1;

	return XDP_PASS;
}

char _license[] SEC("license") = "MIT";