BPF: `bpf_obj_get_info_by_fd` fails with `Invalid argument`

I try to get the fd of the BPF_MAP_TYPE_XSKMAP in my user-space program.

This is the code:

ret = bpf_get_link_xdp_id(cfg->ifindex, &cfg->prog_id, cfg->xdp_flags);
if (ret) {
    fprintf(stderr, "`bpf_get_link_xdp_id` returned error: %sn", strerror(errno));
    exit(-ret);
}

cfg->prog_fd = bpf_prog_get_fd_by_id(cfg->prog_id);
if(cfg->prog_fd < 0) {
    fprintf(stderr, "Failed to obtain prog_fd: %sn", strerror(errno));
    exit(1);
}

struct bpf_prog_info prog_info;
uint32_t prog_info_len = sizeof(struct bpf_prog_info);
uint32_t map_info_len = sizeof(struct bpf_map_info);
if(bpf_obj_get_info_by_fd(cfg->prog_fd, &prog_info, &prog_info_len) == 0) {

    const uint32_t amnt_of_maps = prog_info.nr_map_ids;
    uint32_t *map_ids = (uint32_t*)malloc(sizeof(uint32_t) * prog_info.nr_map_ids);

    memset(&prog_info, 0, prog_info_len);
    prog_info.nr_map_ids = amnt_of_maps;
    prog_info.map_ids = (uint64_t)(unsigned long)map_ids;

    if(bpf_obj_get_info_by_fd(cfg->prog_fd, &prog_info, &prog_info_len) == 0) {

        for(uint32_t i = 0; i < prog_info.nr_map_ids; i++) {

            const int map_fd = bpf_map_get_fd_by_id(map_ids[i]);

            if(map_fd >= 0) {
                struct bpf_map_info map_info;
                if(bpf_obj_get_info_by_fd(map_fd, &map_info, &map_info_len) == 0) {
                    if(strcmp(map_info.name, "xsks_map") == 0) {
                        cfg->xsks_map_fd = map_fd;
                        printf("Got fd of xsks_map!n");
                        break;
                    }
                } else {
                    close(map_fd);
                }
            } else {
                fprintf(stderr, "Failed to obtain map_fd for id %d: %sn", i, strerror(errno));
            }

        }
    } else {
        fprintf(stderr, "Failed to obtain prog_info 2: %sn", strerror(errno));
    }
} else {
    fprintf(stderr, "Failed to obtain prog_info 1: %sn", strerror(errno));
}

But unfortunately, I get an error for calling bpf_obj_get_info_by_fd(cfg->prog_fd, &prog_info, &prog_info_len): Failed to obtain prog_info 1: Invalid argument.

I don’t know if this is because the XDP-program is loaded from another process? Does the same process have to obtain program information which also loaded the AF-XDP program?

Edit: The only thing I changed in my user-space program was to populate a BPF_MAP_TYPE_HASH via bpf_map_update_elem (the entries are in there, observed with bpftool map dump id <id>) and to use bpf_map_lookup_elem in my XDP-program (same hash map).

And suddenly, the error changes to: Failed to obtain prog_info 1: Bad address

Answer

There is a comment in a function from the kernel code called when you try to retrieve information about your program, stating:

/* If we’re handed a bigger struct than we know of, ensure all the unknown bits are 0 – i.e. new user-space does not rely on any kernel feature extensions we don’t know about yet. […] */

I think this is what you hit in your case. Your libbpf version might use some attributes in struct bpf_prog_info that the kernel is not aware of.

To ensure that the kernel accepts it, simply try to zero-initialise your struct:

struct bpf_prog_info prog_info = {};