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Operating Systems Lecture No.
Operating Systems
Lecture No. 44
Reading Material
Chapters 11 and 12 of the textbook Lecture 44 on Virtual TV
Summary
File Protection In-Memory Data Structures Space Allocation Techniques Contiguous, Linked, Index
Protection
The need to protect files is a direct result of the ability to access files. Systems that do not permit access to the files of other users do not need protection. Thus we could provide complete protection by prohibiting access. Alternatively we could provide free access with no protection. Both approaches are too extreme for general use. What is needed is controlled access. File owner/creator should be able to control What can be done By whom Several types of operations may be controlled: Read: read from the file Write: write or rewrite to the file Execute: Load the file into memory and execute it Append: Write new information at the end of the file Delete: Delete the file and free its space for possible reuse List: List the name and attributes of the file
UNIX Protection UNIX recognizes three modes of access: rreeaadd , wwrriittee , and eexxeeccuuttee (r, w, x). The execute permission on a directory specifies permission to seseaarrcchh the directory. The three classes of users are: ^ OOwwnneerr::^ ususeerr iiss tthhee oowwnneerr ooff tthhee ffiillee GGrroouupp: someone who belongs to the same group as the owner OOtthheerrss:: eveveerryyoonnee eellssee wwhhoo hhaass aann aaccccoouunntt oonn tthhee ssyysstteemm A user’s access to a file can be specified by an octal digit. The first bit of the octal digit specifies the read permission, the second bit specifies the write permission, and the third bit specifies the execute permission. A bit value 1 indicates permission for access and 0 indicates no permission. Here is an example:
rr ww xx a) Owner access: 7 1 1 11 1 1 b) Group access: 6 1 1 11 0 0 c) Public access: 1 0 0 00 1 1
Each user in a UNIX system belongs to a group of users as assigned by the system administrator when a user is allocated an account on the system. A user can belong to multiple groups, but a typical UNIX user belongs to a single group. For a particular file or subdirectory, we need to set appropriate (^) aacccceessss peperrmmiissssiioonnss for proper protection.
DDeeffaauulltt PPeerrmmiissssiioonnss The default permissions on a UNIX/Linux system are 777 for executable files and directories and 666 for text files. You can use the uummaasskk command to set permission bits on newly created files and directories to 1, except for those bits that are set to 1 in the ‘mask’. You can use the chmod command to set permissions on existing files and directories. We give some examples of the chmod and umask commands below.
ow ownneerr (^) ggrroouupp (^) ootthheerrss
chmod 761 game
Read, Write, Execute
Read, Write Execute
File Control Block
A file control block is a memory data structure that contains most of the attributes of a file. In UNIX, this data structure is called inode (for index node). Here are possible values in this data structure.
UNIX inode
In-Memory Data Structures
The following upper-level data structures needed for file system support. An in-memory partition table containing information about each mounted partition An in-memory directory structure that holds the directory information of recently accessed directories The system-wide open file table contains pointer to the FCB (UNIX inode) of each open file as well as read/write pointer The FCB for each open file The per process file descriptor table contains a pointer to the appropriate entry in the system wide open file table as well as other information Here are the connections between various in-memory data structures. UNIX specific mappings follow this diagram.
Connections between various in-memory data structures
From File Descriptor to File Contents—The UNIX/Linux In-Memory Data Structures
The open call passes a file name to the file system. When a file is opened, the directory structure is searched for the given file name and file’s inode. An entry is made in the per process open-file table (aka the file descriptor table), with a pointer to the entry in the system wide open file table. The system wide open file table contains the pointer to the current location in the file and a pointer to file’s inode. The open call returns an index for the appropriate entry in the per-process file system table. All file operations are performed via this index, which is called the file descriptor in UNIX/Linux jargon.
Space Allocation Methods
We now turn to some file system implementation issues, in particular space allocation techniques and free space management methods. Here are the three commonly used methods for file space allocation. ^ CCoonnttiigguuoouuss aallllooccaattiioonn ^ LLiinnkkeedd aallllooccaattiioonn ^ IInnddeexxeedd aallllooccaattiioonn
Contiguous Allocation The contiguous allocation method requires each file to occupy a set of contiguous blocks on the disk. The directory entry for each file contains starting block number and file size (in blocks). Disk addresses define a linear ordering on the disk. With this ordering, assuming only one job is accessing the disk, accessing b+1 block after block b normally requires no head movement. When head movement is needed it is only one track. Both sequential and direct access can be supported by contiguous allocation. For direct access to block I of a file that starts at block b we can immediately access block b+i.
Per Process File Descriptor Table
OPEN_MAX — 1
File’s contents
File Descriptor
…^ …
File Table
Inode Table
Linked allocation
Index Allocation Indexed allocation brings all the pointers to the block together into a disk block, known as the (^) iinnddeexx bblloocckk .. HHeerree iiss tthhee llooggiiccaall vviieeww ooff tthhee rreellaattiioonnsshhiipp bbeettwweeeenn aann iinnddeexx bblloocckk aanndd aa ffiillee’’ss ddaattaa bblloocckkss..
Logical view of index allocation
Each file has its own index block, which is an array of disk block addresses. The i th entry in the index block points to the i th block of the file. The directory contains the address of the index block. To read the i th block, we use the pointer in the i th index-block entry to find and read the desired block Here is an example of index allocation.
Index allocation
Index table
Data blocks
