Simple message server
(ip:port, binary link)
User can write, read, change messages.
Messages are stored as each file in "/home/msg/db/".
With BOF, attacker can control fd.
If an attacker enters invalid msg, the msg will be written to the file as an error msg.
An attacker can open msg file from phase 2, which triggers BOF.
open, read, write, to read the flag.