Does libbpf provide an API that allows user space to pass a value to kernel space?

Question

I am using libbpf and I am familiar with passing data using ebpf maps from kernel space to user space. However, how do I do it the other way around?

The closest API I find is below, but I think this is to pass value from kernel to user and it wasn't working for me when I tried it in user space.

int bpf_map_update_elem(int fd, const void *key, const void *value,
      __u64 flags)

Answer 1

bpf_map_update_elem is indeed the way to update the contents of a map from userspace. I have changed the sockex1_user.c program to demonstrate how you would use this function:

  
// SPDX-License-Identifier: GPL-2.0
#include <stdio.h>
#include <assert.h>
#include <linux/bpf.h>
#include <bpf/bpf.h>
#include <bpf/libbpf.h>
#include "sock_example.h"
#include <unistd.h>
#include <arpa/inet.h>

int main(int ac, char **argv)
{
    struct bpf_object *obj;
    int map_fd, prog_fd;
    char filename[256];
    int i, sock, key;
    long value;
    FILE *f;

    snprintf(filename, sizeof(filename), "%s_kern.o", argv[0]);

    if (bpf_prog_load(filename, BPF_PROG_TYPE_SOCKET_FILTER,
                &obj, &prog_fd))
        return 1;

    map_fd = bpf_object__find_map_fd_by_name(obj, "my_map");

    sock = open_raw_sock("lo");

    assert(setsockopt(sock, SOL_SOCKET, SO_ATTACH_BPF, &prog_fd,
                sizeof(prog_fd)) == 0);

    key = 1;
    value = 123;
    bpf_map_update_elem(map_fd, &key, &value, BPF_ANY);

    return 0;
}

The BPF_ANY flag is the one you will use the most. It is defined here along with its meaning: https://elixir.bootlin.com/linux/v5.15.11/source/include/uapi/linux/bpf.h#L1169