在Ubuntu上，实现容器隔离（用户名称空间）、more/proc/[pid]/gid_映射和nothing输出

当我学习docker时，[用户名称空间]一章中的代码：
演示的意义是使用[用户名称空间]实现：
容器外是普通用户权限，容器内是根权限

#define _GNU_SOURCE
#include <sys/types.h>
#include <sys/wait.h>
#include <stdio.h>
#include <sched.h>
#include <signal.h>
#include <unistd.h>
#include <sys/capability.h>    // sudo apt install libcap-dev

#define STACK_SIZE (1024 * 1024)

static char child_stack[STACK_SIZE];
char* const child_args[] = {
    "/bin/bash",
    NULL
};

void set_uid_map(pid_t pid, int inside_id, int outside_id, int length){
    char path[256];
    sprintf(path, "/proc/%d/uid_map", getpid());
    FILE* uid_map = fopen(path, "w");
    fprintf(uid_map, "%d %d %d", inside_id, outside_id, length);
    fclose(uid_map);
}

void set_gid_map(pid_t pid, int inside_id, int outside_id, int length){
    char path[256];
    sprintf(path, "/proc/%d/gid_map", getpid());
    FILE* gid_map = fopen(path, "w");
    //printf("*****gid---Pid: %d *****\n", getpid());
    fprintf(gid_map, "%d %d %d", inside_id, outside_id, length);
    fclose(gid_map);
}

int child_main(void* args){
    cap_t caps;
    printf("In the Child process !\n");
    //sethostname("NewNameSpace", 12);
    //cap_t caps;
    printf("*****Pid: %d *****\n", getpid());
    //set_uid_map(getpid(), 0, 1000, 1);
    set_gid_map(getpid(), 0, 1000, 1);
    set_uid_map(getpid(), 0, 1000, 1);
    printf("eUID = %ld; eGID = %ld; \n\n", (long)geteuid(), (long)getegid());
    caps = cap_get_proc();
    printf("capabilities: %s\n", cap_to_text(caps, NULL));
    execv(child_args[0], child_args);
    return 1;
}

int main(){
    printf("Start: \n");
    //int child_pid = clone(child_main, child_stack + STACK_SIZE, CLONE_NEWUSER | CLONE_NEWNET | CLONE_NEWNS | CLONE_NEWPID | CLONE_NEWIPC | CLONE_NEWUTS | SIGCHLD, NULL);
    int child_pid = clone(child_main, child_stack + STACK_SIZE, CLONE_NEWUSER | SIGCHLD, NULL);
    waitpid(child_pid, NULL, 0);
    printf("Already Exit \n");
    return 0;
}

并展示：

Start: 
In the Child process !
*****Pid: 24072 *****
eUID = 0; eGID = 65534; 

capabilities: = cap_chown,cap_dac_override,cap_dac_read_search,cap_fowner,cap_fsetid,cap_kill,cap_setgid,cap_setuid,cap_setpcap,cap_linux_immutable,cap_net_bind_service,cap_net_broadcast,cap_net_admin,cap_net_raw,cap_ipc_lock,cap_ipc_owner,cap_sys_module,cap_sys_rawio,cap_sys_chroot,cap_sys_ptrace,cap_sys_pacct,cap_sys_admin,cap_sys_boot,cap_sys_nice,cap_sys_resource,cap_sys_time,cap_sys_tty_config,cap_mknod,cap_lease,cap_audit_write,cap_audit_control,cap_setfcap,cap_mac_override,cap_mac_admin,cap_syslog,cap_wake_alarm,cap_block_suspend,37+ep

当我在子shell中输入命令时

# more /proc/24590/uid_map 

显示：0 1000 1

# more /proc/24590/gid_map

不露声色

键eUID和eGID应为0，但我的eGID应为65534。
我不知道为什么我的eGID=65534，而不是=0

gcc版本5.4.0 20160609（Ubuntu 5.4.0-6 ubuntu1~16.04.4）

英语不是我的母语；请原谅打字错误

#include need sudo apt install libcap dev#include need sudo apt install libcap dev

# more /proc/24590/gid_map