example on how to use pivot_root, and demonstration of fd's still being valid after pivot

pivot_root_example.c

#define _GNU_SOURCE
#include <sys/syscall.h>
#include <sys/mount.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <sched.h>
#include <unistd.h>
#include <fcntl.h>
#include <stdio.h>

int main(int argc, char* const argv[], char* const envp[]) {
        int fd1 = open("/etc/passwd", O_RDONLY);

        unshare(CLONE_NEWNS);
        mount(NULL, "/", NULL, MS_PRIVATE | MS_REC, NULL);

        char cwd[1024];
        getcwd(cwd, sizeof(cwd));

        char put_old[1024];
        snprintf(put_old, sizeof(put_old) - 1, "%s/oldroot", cwd);
        mkdir(put_old, 0777);

        mount(cwd, cwd, NULL, MS_BIND, NULL);

        syscall(SYS_pivot_root, cwd, put_old);
        chdir("/");
        umount2("/oldroot", MNT_DETACH);
        rmdir("/oldroot");

        /*
        mkdir("/sys", 0777);
        mount(NULL, "/sys", "sysfs", 0, NULL);
        mkdir("/proc", 0777);
        mount(NULL, "/proc", "proc", 0, NULL);
        */

        char buf[1024];
        while (1) {
                ssize_t rd = read(fd1, buf, sizeof(buf));
                if (rd <= 0) {
                        break;
                }
                write(1, buf, rd);
        }
        close(fd1);

        execve("/busybox", &argv[1], envp);
}