samples: bpf: trivial eBPF program in C

this example does the same task as previous socket example
in assembler, but this one does it in C.

eBPF program in kernel does:
    /* assume that packet is IPv4, load one byte of IP->proto */
    int index = load_byte(skb, ETH_HLEN + offsetof(struct iphdr, protocol));
    long *value;

    value = bpf_map_lookup_elem(&my_map, &index);
    if (value)
        __sync_fetch_and_add(value, 1);

Corresponding user space reads map[tcp], map[udp], map[icmp]
and prints protocol stats every second

Signed-off-by: Alexei Starovoitov <ast@plumgrid.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

samples/bpf/sockex1_user.c

@@ -0,0 +1,49 @@
+#include <stdio.h>
+#include <assert.h>
+#include <linux/bpf.h>
+#include "libbpf.h"
+#include "bpf_load.h"
+#include <unistd.h>
+#include <arpa/inet.h>
+
+int main(int ac, char **argv)
+{
+	char filename[256];
+	FILE *f;
+	int i, sock;
+
+	snprintf(filename, sizeof(filename), "%s_kern.o", argv[0]);
+
+	if (load_bpf_file(filename)) {
+		printf("%s", bpf_log_buf);
+		return 1;
+	}
+
+	sock = open_raw_sock("lo");
+
+	assert(setsockopt(sock, SOL_SOCKET, SO_ATTACH_BPF, prog_fd,
+			  sizeof(prog_fd[0])) == 0);
+
+	f = popen("ping -c5 localhost", "r");
+	(void) f;
+
+	for (i = 0; i < 5; i++) {
+		long long tcp_cnt, udp_cnt, icmp_cnt;
+		int key;
+
+		key = IPPROTO_TCP;
+		assert(bpf_lookup_elem(map_fd[0], &key, &tcp_cnt) == 0);
+
+		key = IPPROTO_UDP;
+		assert(bpf_lookup_elem(map_fd[0], &key, &udp_cnt) == 0);
+
+		key = IPPROTO_ICMP;
+		assert(bpf_lookup_elem(map_fd[0], &key, &icmp_cnt) == 0);
+
+		printf("TCP %lld UDP %lld ICMP %lld packets\n",
+		       tcp_cnt, udp_cnt, icmp_cnt);
+		sleep(1);
+	}
+
+	return 0;
+}