Set-Randomuid

When we need to run a program that we do not totally trust, we really do not want to run the program in our own account, because this untrusted program might modify our files. It is desirable if the operating system can create a new user id for us, and allows us to run the program using this new user id. Since the new user id does not own any file, the program cannot read/modify any file unless the file is world-readable/writable. We will design such a mechanism for Minix in this lab.

In this lab, you need to design and implement a Set-RandomUID mechanism. When a Set-RandomUID

program is executed, the operating system randomly generates a non-existing user id, and runs the program with this new user id as the effective uesr id. You can consider Set-RandomUID as an opposite to the Set-UID mechanism: Set-UID allows users to escalate their privileges, while Set-RandomUID allows users to downgrade their privileges. The implementation of Set-RandomUID can be similar to that of Set-UID.

1. To mark a program as a Set-RandomUID program, we can use the unused sticky bit in the permission