最新的linux内核中，写xdp程序时，bpf()函数如何使用，给出一个完整示例，不使用libbpf

在最新的Linux内核中，使用xdp程序时，bpf()函数可以用于加载和更新BPF程序。下面是一个完整的示例，其中使用了bpf()函数来加载和更新BPF程序，而没有使用libbpf。

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/resource.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <linux/if_packet.h>
#include <linux/if_ether.h>
#include <linux/ip.h>
#include <linux/udp.h>
#include <linux/bpf.h>
#include <linux/filter.h>
#include <errno.h>
#include <net/if.h>

#define BPF_PROG_LOAD "/sys/fs/bpf/foo"

static int bpf_prog_load(const char *file, enum bpf_prog_type type, struct bpf_insn **insns, int *len)
{
    int fd, res;
    struct bpf_prog_load_attr attr = {
        .prog_type = type,
        .insns = *insns,
        .insn_cnt = *len,
    };

    fd = open(file, O_RDONLY);
    if (fd < 0) {
        perror("open");
        return -1;
    }

    res = bpf(BPF_PROG_LOAD, &attr, sizeof(attr));
    if (res < 0) {
        perror("bpf");
        close(fd);
        return -1;
    }

    *insns = attr.insns;
    *len = attr.insn_cnt;

    close(fd);
    return res;
}

int main(int argc, char **argv)
{
    int sock, len, res;
    struct sockaddr_ll addr;
    struct packet_mreq mreq;
    struct bpf_insn *insns;
    char buf[4096];

    // Load BPF program
    insns = malloc(sizeof(struct bpf_insn) * 5);
    insns[0] = (struct bpf_insn) {BPF_LD | BPF_W | BPF_ABS, 0, 0, offsetof(struct ethhdr, h_proto)};
    insns[1] = (struct bpf_insn) {BPF_JMP | BPF_JEQ | BPF_K, htons(ETH_P_IP), 0, 1};
    insns[2] = (struct bpf_insn) {BPF_LD | BPF_W | BPF_ABS, 0, 0, offsetof(struct iphdr, protocol)};
    insns[3] = (struct bpf_insn) {BPF_JMP | BPF_JEQ | BPF_K, IPPROTO_UDP, 0, 1};
    insns[4] = (struct bpf_insn) {BPF_RET | BPF_K, 0, 0, 0};
    len = 5;
    res = bpf_prog_load(BPF_PROG_LOAD, BPF_PROG_TYPE_XDP, &insns, &len);
    if (res < 0) {
        fprintf(stderr, "Failed to load BPF program\n");
        return -1;
    }

    // Create socket
    sock = socket(AF_PACKET, SOCK_RAW, htons(ETH_P_ALL));
    if (sock < 0) {
        perror("socket");
        return -1;
    }

    // Attach BPF program to socket
    res = setsockopt(sock, SOL_SOCKET, SO_ATTACH_BPF, &res, sizeof(res));
    if (res < 0) {
        perror("setsockopt");
        return -1;
    }

    // Bind socket to interface
    memset(&addr, 0, sizeof(addr));
    addr.sll_family = AF_PACKET;
    addr.sll_ifindex = if_nametoindex("eth0");
    res = bind(sock, (struct sockaddr *)&addr, sizeof(addr));
    if (res < 0) {
        perror("bind");
        return -1;
    }

    // Join multicast group
    memset(&mreq, 0, sizeof(mreq));
    mreq.mr_ifindex = addr.sll_ifindex;
    mreq.mr_type = PACKET_MR_MULTICAST;
    res = setsockopt(sock, SOL_PACKET, PACKET_ADD_MEMBERSHIP, &mreq, sizeof(mreq));
    if (res < 0) {
        perror("setsockopt");
        return -1;
    }

    // Receive packets
    while (1) {
        len = recv(sock, buf, sizeof(buf), 0);
        if (len < 0) {
            perror("recv");
            return -1;
        }
        printf("Received packet of length %d\n", len);
    }

    return 0;
}

在这个示例中，我们定义了一个BPF程序，它会过滤掉非IP和非UDP数据包。然后，我们使用bpf_prog_load()函数将BPF程序加载到内核中。接下来，我们创建一个原始套接字，将BPF程序附加到套接字上，并将套接字绑定到一个网络接口上。最后，我们加入一个多播组并开始接收数据包。