bug#21410: [TEST-FAIL] 2 tests failed when running `make check' on Debian

[Alex Vong]

Hi Dave,

I have searched the internet according to the information you provided,
I find this bug report <https://github.com/lxc/lxc/issues/250> provides useful 
information.
I have written an example program after going through the clone(2) man page.
It demonstrates the problem and is inlined below.

First, compile the program as `a.out'.

Consider shell session 1:

    root# echo 0 > /proc/sys/kernel/unprivileged_userns_clone
    user$ ./a.out
    I am your parent
    Start cloning...
    Cannot clone!

Consider shell session 2:

    root# echo 1 > /proc/sys/kernel/unprivileged_userns_clone
    user$ ./a.out
    I am your parent
    Start cloning...
    Cloned!
    I am your child

Any idea what's happenning?
I don't know Linux much, for instance I don't know what is container and 
namespace in Linux.

Thanks!

Cheers,
Alex

#define _GNU_SOURCE
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <sched.h>
#include <sys/wait.h>
#include <unistd.h>

#define STACK_SIZE (1024 * 1024)

int
child(void *str)
{
  if (puts(str) < 0)
    return EXIT_FAILURE;
  else
    return EXIT_SUCCESS;
}

int
main(void)
{ void *func_ptr = &child;
  void *stack_base = malloc(STACK_SIZE);
  void *stack_top;
  int flag = CLONE_NEWUSER | SIGCHLD;
  char *msg = "I am your child";
  int errsv = 0;

  puts("I am your parent");
  puts("Start cloning...");

  stack_top = stack_base + STACK_SIZE;
  clone(func_ptr, stack_top, flag, msg);
  errsv = errno;

  if (!errsv)
    puts("Cloned!");
  else
    puts("Cannot clone!");
  return errsv;
}