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fsck

Overview

A file system checker for the xv6 operating system.

This file system checker will check/validate an operating system to make sure itsatisfies the following conditions:

  • Each inode is either unallocated or one of the valid types (T_FILE, T_DIR, T_DEV). ERROR: bad inode
  • For in-use inodes, each address that is used by inode is valid (points to a valid datablock address within the image). Note: must check indirect blocks too, when they are in use.
    ERROR: bad address in inode
  • Root directory exists, and it is inode number 1.
    ERROR: root directory does not exist
  • Each directory contains . and .. entries.
    ERROR: directory not properly formatted
  • Each .. entry in directory refers to the proper parent inode, and parent inode points back to it.
    ERROR: parent directory mismatch
  • For in-use inodes, each address in use is also marked in use in the bitmap.
    ERROR: address used by inode but marked free in bitmap
  • For blocks marked in-use in bitmap, actually is in-use in an inode or indirect block somewhere.
    ERROR: bitmap marks block in use but it is not in use
  • For in-use inodes, any address in use is only used once.
    ERROR: address used more than once
  • For inodes marked used in inode table, must be referred to in at least one directory.
    ERROR: inode marked use but not found in a directory
  • For inode numbers referred to in a valid directory, actually marked in use in inode table.
    ERROR: inode referred to in directory but marked free
  • Reference counts (number of links) for regular files match the number of times file is referred to in directories (i.e., hard links work correctly).
    ERROR: bad reference count for file

Testing/Usage

First, run this on an uncorrupted file system:

make qemu-nox //run xv6  
mkdir xyz //create some files and/or directories  
exit xv6 (ctrl+a followed by x)  
run chkfs on fs.img  

Then, try it out on a corrupted filesystem:

cp fs.img corrupted.img  
./corruptfs corrupted.img TYPE  
chkfs corrupted.img  

TYPE is respective to the errors lsited above e.g. bad inode would be type 1

The corruptfs program randomly chooses which directory, inode, etc. to corrupt. To make the random selection deterministic, include a random seed (i.e., a positive integer) as an additional command line argument to corruptfs. Also, because of the random nature of corruptfs, it will occasionally make a random choice such that it is impossible to introduce the specified type of corruption. If this occurs, corruptfs will indicate that it failed to corrupt the image; simply run corruptfs again (with a different random seed) so it makes a different random choice.

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A xv6-based file system checker

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