File access control relates largely to the secrecy dimension of security. What is the relationship between an access control matrix and the integrity of the objects to which access is being controlled?

questions:
1. Suggest an efficient scheme for maintaining a per-user protection scheme. That is, the system maintains one directory per user, and that directory lists all the objects to which the user is allowed access. Your design should address the needs of a system with 1000 users, of whom no more than 20 are active at any time. Each user has an average of 200 permitted objects; there are 50,000 total objects in the system.
2. Respond to the allegation �An operating system requires no protection for its executable code (in memory) because that code is a duplicate of code maintained on disk.�
3. A directory is also an object to which access should be controlled. Why is it not appropriate to allow users to modify their own directories?
4. Why should the directory of one user not be generally accessible to other users (not even for read-only access)?
5. File access control relates largely to the secrecy dimension of security. What is the relationship between an access control matrix and the integrity of the objects to which access is being controlled?