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# mount -t overlay overlay -o lowerdir=/home/me,upperdir=/giant_disk/steam/overlay,workdir=/giant_disk/steam/working /giant_disk/steam/main
 
#!/usr/bin/python3
 
import ctypes
import ctypes.util
import os
import unshare
 
 
libc = ctypes.CDLL(ctypes.util.find_library('c'), use_errno=True)
libc.mount.argtypes = (ctypes.c_char_p, ctypes.c_char_p, ctypes.c_char_p, ctypes.c_ulong, ctypes.c_char_p)
 
 
def mount(source: bytes, target: bytes, fs: bytes=b'', flags: int=0, options: bytes=b''):
    ret = libc.mount(source, target, fs, flags, options)
    if ret < 0:
        errno = ctypes.get_errno()
        errstr = os.strerror(errno)
        raise OSError(errno, f"Error mounting {source} ({fs}) on {target} with options '{options}' and flags {flags}: {errstr}")
 
 
uid = os.getuid()
gid = os.getgid()
wd = os.getcwd()
os.chdir('/')
unshare.unshare(unshare.CLONE_NEWNS|unshare.CLONE_NEWUSER)
mount(b'/giant_disk/steam/main', b'/home/me', b'', 4096)
with open('/proc/self/setgroups', 'wb') as f:
    f.write(b'deny')
with open('/proc/self/uid_map', 'wb') as f:
    f.write(f'{uid} {uid} 1\n'.encode('ascii'))
with open('/proc/self/gid_map', 'wb') as f:
    f.write(f'{gid} {gid} 1\n'.encode('ascii'))
os.chdir(wd)
os.execv('/bin/bash', ['/bin/bash'])  # You run steam from this bash shell