Sharing maps between same eBPF programs loaded onto different interfaces with libbpf

Question

I want to load the same eBPF program for XDP hook onto different interfaces of a switch and all the programs should share the same map. I have gone through the post (exactly my target) post 1 and post 2. However, I could not get things up and running.

I know that I need to use bpf_map__reuse_fd(), but how exactly do I use it without creating the maps first? Also, individual programs in the interfaces will create their own maps, isn't it so?

In comments of post 2, following the steps we first create structs for both prog1 and prog2, then load prog1, get the map fds to use in prog2. But, in this case, how will the maps be created without loading the BPF program and then shared?

I have only two programs - one kernel space BPF program and one user space program that loads the code. Here are my code snippets. In file xxx_kern.c

struct {
    __uint(type, BPF_MAP_TYPE_HASH);
    __type(key, __u32);
    __type(value, __u32);
    __uint(max_entries, 10);
} xdp_map_t SEC(".maps");

In file xxx_user.c

int main()
{
    struct bpf_object *obj = NULL;
    obj = bpf_object__open("xxx_kern.o");
    struct bpf_map *map1 = bpf_object__find_map_by_name(obj, "xdp_map_t ");
    int map1_fd = bpf_object__find_map_fd_by_name(obj, "xdp_map_t ");
    bpf_map__reuse_fd(map1, map1_fd);
    
    // load and attach the program with xdp_program__attach()

    // verifythe fd value of map ```xdp_map_t``` from different interfaces
    printf("\nMap1_fd value: %d\n", map1_fd);
}

Can someone tell the proper steps in this scenario? Note that I am using libbpf.

Answer 1

My suggest is to follow these tutorial enter link description here. It pins the map in a bpffs, a virtual file system that permit to access the map and the bpf structures every time you want. Keep your definition of the map and do not use the bpf_map_def cause it's deprecated.