File Sharing

In multiuser system, allow files to be shared among users
Two issues
- Access rights. Allowing users to share files raises a major issue: protection. A general approach is to provide controlled access to files through a set of operations such as read, write, delete, list, and append. Then permit users to perform one or more operations. One popular protection mechanism is a condensed version of access list, where the system recognizes three classifications of users with each file and directory: user, group, other
- Management of simultaneous access
Access Rights
- None
  - User may not know of the existence of the file
  - User is not allowed to read the user directory that includes the file
- Knowledge; User can only determine that the file exists and who its owner is
- Execution; The user can load and execute a program but cannot copy it
- Reading; The user can read the file for any purpose, including copying and execution
- Appending; The user can add data to the file but cannot modify or delete any of the file's contents
- Updating; The user can modify, deleted, and add to the file's data. This includes creating the file, rewriting it, and removing all or part of the data
- Changing protection; User can change access rights granted to other users
- Deletion; User can delete the file
- Owners
  - Has all rights previously listed
  - May grant rights to others using the following classes of users; Specific user, User groups, All for public files
```
total 1704
drwxr-x--- 3 user group 4096 Oct 14 08:13 .
drwxr-x--- 3 user group 4096 Oct 14 08:14 ..
drwxr-x--- 2 user group 4096 Oct 14 08:12 backup
-rw-r----- 1 user group 141133 Oct 14 08:13 eniac3.jpg
-rw-r----- 1 user group 1580544 Oct 14 08:13 wk11.ppt
```
- First letter: file type
  - d for directories
  - - for regular files
- Three user categories; user, group, and other
- Three access rights per category; read, write, and execute
- drwxrwxrwx; user group other
- Execute permission for directory? Permission to access files in the directory
- To list a directory requires read permissions
- What about drwxr-x-x ?
- Problematic example
  - A owns file foo.bar
  - A wishes to keep his file private
  - Inaccessible to the general public
  - A wishes to give B read and write access
  - A wishes to give C readonly access
  - ???????
Management of Simultaneous Access
- Most Oses provide mechanisms for users to manage concurrent access to files; Example: lockf(), flock() system calls
- Typically
  - User may lock entire file when it is to be updated
  - User may lock the individual records during the update
- Mutual exclusion and deadlock are issues for shared access

2004-05-25