为什么样本/bpf中的ebpf程序不起作用? [英] Why ebpf program inside samples/bpf doesn't work?

问题描述

目标:在内核源代码树4.18.0的samples/bpf目录中编写一个新的ebpf示例，对其进行编译和执行.

GOAL: write a new ebpf example within samples/bpf directory in the kernel source tree of 4.18.0, compile and execute it.

问题::在我运行sudo ./mine时将其编译后，它将终止.

PROBLEM: after compiling it when I run sudo ./mine it just terminates.

mine_kern.c

#include <uapi/linux/bpf.h>
#include <uapi/linux/if_ether.h>
#include <uapi/linux/ip.h>
#include <linux/in.h>
#include <linux/if_packet.h>
#include "bpf_helpers.h" 

int icmp_filter(struct __sk_buff *skb){

        int proto = load_byte(skb, ETH_HLEN + offsetof(struct iphdr, protocol));
        if(proto == IPPROTO_ICMP && skb->pkt_type == PACKET_OUTGOING){
           return -1;
        } else {
           return 0;
        }
}

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

mine_user.c

#include <stdio.h>
#include <assert.h>
#include <linux/bpf.h>
#include <bpf/bpf.h>
#include "bpf_load.h"
#include "sock_example.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;

    char buf[65535];

    for(i=0; i<20; i++){
           int res = recvfrom(sock, buf, sizeof(buf), 0, NULL, 0);
           printf("res=%d\n", res);
     } 

     return 0;
}

我还修改了samples/bpf内的Makefile，并在需要时添加了mine_user.c和mine_kern.c. 问题:此代码有什么问题?

I've also modified the Makefile inside samples/bpf adding mine_user.c and mine_kern.c where required. QUESTIONS: what is wrong with this code?

推荐答案

由于load_bpf_file()加载函数的方式，您需要将BPF程序函数放在单独的ELF节中.例如，我可以用以下命令加载程序:

Because of the way load_bpf_file() loads the function, you need to put your BPF program function in a separate ELF section. For example, I could load the program with:

SEC("socket")
int icmp_filter(struct __sk_buff *skb){
        ...
}

在那之后，我在运行程序时看到连续的res=-1.这是因为open_raw_sock()从sock_example.h将您的套接字设置为非阻塞:

After that, I see a succession of res=-1 when running the program. This is because your socket is set as non-blocking by open_raw_sock(), from sock_example.h:

sock = socket(PF_PACKET, SOCK_RAW|SOCK_NONBLOCK|SOCK_CLOEXEC, htons(ETH_P_ALL));

因此，当没有要接收的数据包时，recvfrom()只需返回-1(并将errno设置为-EGAIN-您应该考虑打印strerror(errno))，而不是等待数据包.因此，您可能也想更改它.

So when there is no packet to receive, recvfrom() simply returns with -1 (and sets errno to -EGAIN -- you should consider printing strerror(errno) by the way) instead of waiting for a packet. So you may want to change that too.

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