linux namespace隔离问题

最近正在看《容器与容器云》这本书，根据3.1的代码想自己实现一个隔离所有6种命名空间的简易沙盒，代码如下，基本是照书抄的。

问题是：我做了PID隔离之后子进程的shell pid和外部隔离了，getpid()获取到的值是1，然后子进程就去改/proc/1下的文件了，这当然是改不了的，问题是在子进程中怎么能获取外部进程中所对应的pid呢,用getppid也不行，有没有什么好的办法。

#define _GNU_SOURCE

#include <sys/types.h>
#include <sys/wait.h>
#include <sys/utsname.h>
#include <sys/capability.h>
#include <stdio.h>
#include <sched.h>
#include <signal.h>
#include <unistd.h>

#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", pid);
   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", pid);
   FILE* gid_map = fopen(path, "w");
   fprintf(gid_map, "%d %d %d", inside_id, outside_id, length);
   fclose(gid_map);
}

int child_main(void * args) {
   printf("in child process\n");
   cap_t caps;
   set_uid_map(getpid(), 0, 1000, 1);
   set_gid_map(getpid(), 0, 1000, 1);
   printf("eUID = %ld; eGID = %ld; ", (long) geteuid(), (long) getegid());
   caps = cap_get_proc();
   printf("capabilities: %s\n", cap_to_text(caps, NULL));
   execv(child_args[0], child_args);
   printf("quit child process…\n");
   return 1;
}
int main()
{
    printf("starting process...\n");
    int child_pid = clone(child_main, child_stack + STACK_SIZE,
                          CLONE_NEWUTS|CLONE_NEWIPC|CLONE_NEWPID|CLONE_NEWNET|CLONE_NEWNS|CLONE_NEWUSER|SIGCHLD,
                          NULL);
    if(child_pid == -1)
    {
        printf("error starting child process...\n");
    }
    waitpid(child_pid, NULL, 0);
    printf("quit process...\n");
    return 0;
}