BPF: `bpf_obj_get_info_by_fd` 失败并出现 `Invalid argument`

Question

我尝试在我的 user-space 程序中获取 BPF_MAP_TYPE_XSKMAP 的 fd。

这是代码：

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: %s\n", 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: %s\n", 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: %s\n", i, strerror(errno));
            }

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

但不幸的是，调用 bpf_obj_get_info_by_fd(cfg->prog_fd, &prog_info, &prog_info_len) 时出现错误：Failed to obtain prog_info 1: Invalid argument。

不知道是不是因为XDP程序是从别的进程加载的？同一个进程是否必须获取同样加载了AF-XDP程序的程序信息？

编辑：我在我的 user-space 程序中唯一改变的是通过 bpf_map_update_elem 填充 BPF_MAP_TYPE_HASH（条目在那里，用 [=17= 观察） ]) 并在我的 XDP 程序中使用 bpf_map_lookup_elem（相同的哈希映射）。

突然，错误变成了： Failed to obtain prog_info 1: Bad address

Answer 1

a function from the kernel code 中有一条评论在您尝试检索有关您的程序的信息时调用，指出：

/* 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. [...] */

我认为这就是您遇到的情况。您的 libbpf 版本可能在 struct bpf_prog_info 中使用了一些内核不知道的属性。

为确保内核接受它，只需尝试零初始化您的结构:

struct bpf_prog_info prog_info = {};

BPF: `bpf_obj_get_info_by_fd` 失败并出现 `Invalid argument`

BPF: `bpf_obj_get_info_by_fd` fails with `Invalid argument`

c

linux

sockets

xdp-bpf