ebpf bpf_get_socket_uid错误: insn[0].code 0x85的relo无效

Question

我在内核源代码中构建了一个ebpf演示。演示代码如下：

cgroup_kern.c

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

#include <bpf_helpers.h>
#include <linux/bpf.h>


struct bpf_map_def SEC("maps") my_map = {
        .type = BPF_MAP_TYPE_ARRAY,
        .key_size = sizeof(u32),
        .value_size = sizeof(long),
        .max_entries = 256,
};

SEC("cgroupskb/ingress/stats")
int bpf_cgroup_ingress(struct __sk_buff* skb) {
        uint32_t sock_uid = bpf_get_socket_uid(skb);
        return 0;
}

cgroup_user.c

#include <stdio.h>
#include <assert.h>
#include <linux/bpf.h>
#include "libbpf.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;
        }
        return 0;
}

然后，我为我的cgroup ebpf演示添加了一些Makefile。当我做的时候，得到了一个警告：

/usr/src/linux-source-4.15.0/linux-source-4.15.0/samples/bpf/cgroup_kern.c:20:22: warning: implicit declaration of function 'bpf_get_socket_uid' is invalid in C99 [-Wimplicit-function-declaration]
        uint32_t sock_uid = bpf_get_socket_uid(skb);

当我想要附加我的ebpf演示:无效的insn.code 0x85的relo

任何建议都将不胜感激。

Answer 1

我猜你是从相同的内核源代码(4.15)中检索到bpf_helpers.h的。不幸的是，在4.15中，该文件缺少几个助手声明。此问题已在5.1版本的commit f2bb538中修复

commit f2bb53887eb3e8f859ac7cfc09d1a3801492c009
Author: Willem de Bruijn <willemb@google.com>
Date:   Wed Feb 27 11:08:06 2019 -0500

    bpf: add missing entries to bpf_helpers.h
    
    This header defines the BPF functions enumerated in uapi/linux.bpf.h
    in a callable format. Expand to include all registered functions.
    
    Signed-off-by: Willem de Bruijn <willemb@google.com>
    Acked-by: Song Liu <songliubraving@fb.com>
    Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

您应该能够简单地通过向bpf_helpers.h添加适当的定义来修复错误

static unsigned int (*bpf_get_socket_uid)(void *ctx) =
    (void *) BPF_FUNC_get_socket_uid;