Tiny init system to use with Android kernel

README.md

This is a tiny init system, which in combination with the original kernel can be used to build a "functional GNU/Linux system". Can be used with pretty much any android system

Usage

Get aarch64-none-linux-gnu cross-compiler toolchain (for example here). Compile init using aarch64-none-linux-gnu-gcc init.c -o init -static Repack boot.img (or a TWRP recovery) using Android boot image editor Profit!

init.c

#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>

#include <fcntl.h>
#include <sys/mount.h>
#include <sys/stat.h>
#include <sys/sysmacros.h>
#include <linux/kd.h>
#include <unistd.h>

void fpwrite(const char *f, const char *s) {
  FILE *fi = fopen(f, "w");
  if (!fi) {
    return;
  }
  fprintf(fi, "%s", s);
  fclose(fi);
}

int main() {
  mkdir("/dev", 0755);
  mkdir("/proc", 0755);
  mkdir("/sys", 0755);

  mount("devtmpfs", "/dev", "devtmpfs", MS_NOSUID, "mode=0755");
  mount("proc", "/proc", "proc", 0, NULL);
  mount("sysfs", "/sys", "sysfs", 0, NULL);

  fpwrite("/sys/class/android_usb/android0/enable", "0");
  fpwrite("/sys/class/android_usb/android0/idVendor", "18d1");
  fpwrite("/sys/class/android_usb/android0/idProduct", "4ee2");
  fpwrite("/sys/class/android_usb/android0/functions", "acm");
  fpwrite("/sys/class/android_usb/android0/f_acm/instances", "1");
  fpwrite("/sys/class/android_usb/android0/enable", "1");
  fpwrite("/sys/class/android_usb/android0/iManufacturer", "Xiaomi");
  fpwrite("/sys/class/android_usb/android0/iProduct", "Redmi");
  fpwrite("/sys/class/android_usb/android0/iSerial", "0123456789ABCDEF");

  {
    mknod("/dev/ttyGS0", 0777 | S_IFCHR, makedev(233, 0));
    int fd;
    if ((fd = open("/dev/ttyGS0", O_RDWR)) < 0) {
      fd = open("/dev/null", O_RDWR);
    }
    ioctl(fd, TIOCSCTTY, 0);
    dup2(fd, 0);
    dup2(fd, 1);
    dup2(fd, 2);
  }

  symlink("/dev/ttyGS0", "/dev/console");

  setenv("LD_LIBRARY_PATH", "/lib:/usr/lib:/sbin", 1);
  setenv("PATH", "/usr/sbin:/usr/bin:/sbin:/bin", 1);

  execl("/sbin/sh", "/sbin/sh", NULL);
}