namespace.c

#define _GNU_SOURCE                                                                                                                                                                                          
#include <sched.h>                                                                                                                                                                                           

#include <stdio.h>                                                                                                                                                                                           
#include <stdlib.h>                                                                                                                                                                                          
#include <signal.h>                                                                                                                                                                                          
#include <linux/capability.h>                                                                                                                                                                                
#include <sys/mount.h>                                                                                                                                                                                       
#include <sys/types.h>                                                                                                                                                                                       
#include <sys/wait.h>                                                                                                                                                                                        
#include <unistd.h>                                                                                                                                                                                          

int child(void *args)                                                                                                                                                                                        
{                                                                                                                                                                                                            
    printf("pid as seen in the child: %lu\n", (unsigned long)getpid());                                                                                                                                      

    pid_t p = mount("overlay", "./hatch/mnt", "overlay", 0, "lowerdir=./hatch/lower,upperdir=./hatch/upper,workdir=./hatch/work");                                                                           
    if (p == -1){                                                                                                                                                                                            
        perror("mount");                                                                                                                                                                                     
        exit(1);                                                                                                                                                                                             
    }                                                                                                                                                                                                        

    // Expose the mount to the parent namespace                                                                                                                                                              
    p = mount("none", "./hatch/mnt", NULL, MS_SHARED, NULL);                                                                                                                                                 
    if (p == -1){                                                                                                                                                                                            
        perror("mount");                                                                                                                                                                                     
        exit(1);                                                                                                                                                                                             
    }                                                                                                                                                                                                        

    char *newargv[] = { "/bin/bash", NULL };                                                                                                                                                                 

    execv("/bin/bash", newargv);                                                                                                                                                                             
    perror("exec");                                                                                                                                                                                          
    exit(EXIT_FAILURE);                                                                                                                                                                                      

    return 0;                                                                                                                                                                                                
}                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          
int main()                                                                                                                                                                                                   
{                                                                                                                                                                                                            
    pid_t p = clone(child, malloc(4096) + 4096, CLONE_NEWNS | CLONE_NEWUSER | SIGCHLD, NULL);                                                                                                                
    if (p == -1) {                                                                                                                                                                                           
        perror("clone");                                                                                                                                                                                     
        exit(1);                                                                                                                                                                                             
    }                                                                                                                                                                                                        

    printf("child pid: %lu\n", (unsigned long)p);                                                                                                                                                            
    waitpid(p, NULL, 0);                                                                                                                                                                                     
    return 0;                          
}