Check Can Open

script.sh

adduser brian --system --disabled-password --uid=1000
adduser john --system --disabled-password --uid=1001
echo "00900111512262233733448445595566066"> brian.txt
chown brian brian.txt
chmod 700 brian.txt

pip install cffi

#########

python
import threading
from time import sleep
from cffi import FFI
import os
ffi = FFI()

ffi.cdef("""
int setfsuid(int fsuid);
int getgroups(int fsuid, int[]);
int setgroups(int size, const int *list);
""")

sys = ffi.dlopen(None)

def setfsuid(uid, message):
    old = sys.setfsuid(ffi.cast("int", uid))
    actual = sys.setfsuid(ffi.cast("int", uid))
    print(f"{message} old fsuid: {old} new: {actual}")

def getgroups():
    ptr = ffi.new("int[]", [])
    size = sys.getgroups(ffi.cast("int", 0), ptr)
    ret_val = ffi.new("int[]", size)
    sys.getgroups(size, ret_val)
    return ret_val


def setgroups(groups):
    new_groups = ffi.new("const int[]", groups)
    size = sys.setgroups(len(new_groups), new_groups)
    return size

def check_open():
    try:
        open("brian.txt").close()
        return True	
    except Exception as e:
        return False

def worker(brians_file):
    """thread worker function"""
    setfsuid(1000, "  Thread 1 - Making sure I'm Brian")
    print(f"  Thread 1 - Brian reads more - {brians_file.read(5)}")
    print(f"  Thread 1 - Check Open - {check_open()}")
    print(brians_file.read(5))
    sleep(3)
    print(f"  Thread 1 - Still Brian reads more - {brians_file.read(5)}")
    print(f"  Thread 1 - Check Open - {check_open()}")
    setfsuid(1001, "  Thread 1 - ")
    sleep(3)
    print(f"  Thread 1 - Now John reads: {brians_file.read(5)}")
    print(f"  Thread 1 - Check Open - {check_open()}")


def doit():
    setfsuid(1000, "Main Process - ")
    brians_file = open("brian.txt", "r")
    print(f"Main Process - Brian reads {brians_file.read(5)}")
    print(f"Main Process - Can open - {check_open()}")
    threads = []
    t = threading.Thread(target=worker, args=(brians_file,))
    t.start()
    sleep(2)
    setfsuid(1001, "Main Process - ")
    print(f"Main Process - John reads {brians_file.read(5)}")
    print(f"Main Process - Can Open - {check_open()}")
    sleep(1)
    t.join()

doit()