/*
 * fj-stub.c
 *
 * PUBLIC DOMAIN
 *
 * Create folders first:
    sudo mkdir /tmp/firejail
    sudo mkdir /tmp/firejail/mnt/oroot
    sudo mkdir /tmp/firejail/mnt/odiff
    sudo mkdir /tmp/firejail/mnt/owork

 * Compile:
    gcc fj-stub.c -o fj-stub
    sudo chown root fj-stub
    sudo chgrp root fj-stub
    sudo chmod u+s fj-stub

 * Confirm issue:
    ./fj-stub
    ls -la /home

 * NOTE: if /home is nosuid then copy the binary before chmod/confirm
*/

#include <stdio.h>
#include <stdlib.h>
#include <sys/mount.h>
#include <errno.h>
#include <unistd.h>

int main(int argc, char **argv) {

    if (setreuid(-1,0)<0){
        fprintf(stderr, "Error: Unable to setuid root %i\n",errno);
        return -1;
    }

    if (mount("overlay", "/tmp/firejail/mnt/oroot/", "overlay", MS_MGC_VAL, "lowerdir=/,upperdir=/tmp/firejail/mnt/odiff/,workdir=/tmp/firejail/mnt/owork/") < 0){
        printf("Failed overlay: %i\n",errno);
        return -1;
    }

    if (chroot("/tmp/firejail/mnt/oroot/") == -1){
        printf("Failed chroot: %i\n",errno);
        return -1;
    }
    system("/usr/bin/bash");

    return 0;
}