gistfile1.txt

laptix /home/maxux/git/coreX # ls /tmp/new-root/
helloworld  init

laptix /home/maxux/git/coreX # ./poc 
PID: 17504
Parent: waiting...
Executing init
CHILD PID: 1
----
init
.
..
helloworld
----
Hello 0 ! (pid: 1)
Hello 1 ! (pid: 1)
Hello 2 ! (pid: 1)
Hello 3 ! (pid: 1)
...

init.c

#include <stdio.h>
#include <unistd.h>

int main(void) {
    for(int i = 0; i < 64; i++) {
        printf("Hello %d ! (pid: %d)\n", i, getpid());
        sleep(1);
    }

    return 0;
}

poc-chroot.c

#define _GNU_SOURCE
#include <stdio.h>
#include <stdlib.h>
#include <sched.h>
#include <sys/types.h>
#include <unistd.h>
#include <dirent.h>
#include <sys/wait.h>

void diep(char *str) {
    perror(str);
    exit(EXIT_FAILURE);
}

void fileslist(char *path) {
    struct dirent *de;
    DIR *dr;

    if(!(dr = opendir(path))) {
        printf("Could not open current directory");
        return;
    }

    printf("----\n");
    
    while((de = readdir(dr)) != NULL)
        printf("%s\n", de->d_name);
    
    printf("----\n");

    closedir(dr);
}

int main(void) {
    if(unshare(CLONE_FS | CLONE_NEWPID))
        perror("unshare");

    if(chroot("/tmp/new-root") < 0)
        diep("chroot");

    printf("PID: %d\n", getpid());

    pid_t pid = fork();

    if(pid < 0)
        perror("fork");

    if(pid == 0) {
        printf("Executing init\n");
        printf("CHILD PID: %d\n", getpid());

        fileslist("/");

        if(execl("/init", "/init", NULL) < 0)
            perror("system");
    }

    printf("Parent: waiting...\n");
    int value;
    wait(&value);

    return 0;
}