130 lines
4.6 KiB
Plaintext
130 lines
4.6 KiB
Plaintext
$NetBSD: README,v 1.5 2011/07/18 08:58:38 uch Exp $
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makefs - build a file system image from a directory tree
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NOTES:
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* This tool uses modified local copies of source found in other
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parts of the tree. This is intentional.
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* makefs is a work in progress, and subject to change.
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user overview:
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--------------
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makefs creates a file system image from a given directory tree.
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the following file system types can be built:
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ffs BSD fast file system
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cd9660 ISO 9660 file system
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v7fs 7th edition(V7) file system
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Support for the following file systems maybe be added in the future
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ext2fs Linux EXT2 file system
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fat MS-DOS `FAT' file system (FAT12, FAT16, FAT32)
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Various file system independent parameters and contraints can be
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specified, such as:
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- minimum file system size (in KB)
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- maximum file system size (in KB)
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- free inodes
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- free blocks (in KB)
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- mtree(8) specification file containing permissions and ownership
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to use in image, overridding the settings in the directory tree
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- file containing list of files to specifically exclude or include
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- fnmatch(3) pattern of filenames to exclude or include
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- endianness of target file system
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File system specific parameters can be given as well, with a command
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line option such as "-o fsspeccific-options,comma-separated".
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For example, ffs would allow tuning of:
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- block & fragment size
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- cylinder groups
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- number of blocks per inode
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- minimum free space
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Other file systems might have controls on how to "munge" file names to
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fit within the constraints of the target file system.
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Exit codes:
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0 all ok
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1 fatal error
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2 some files couldn't be added during image creation
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(bad perms, missing file, etc). image will continue
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to be made
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Implementation overview:
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------------------------
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The implementation must allow for easy addition of extra file systems
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with minimal changes to the file system independent sections.
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The main program will:
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- parse the options, including calling fs-specific routines to
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validate fs-specific options
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- walk the tree, building up a data structure which represents
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the tree to stuff into the image. The structure will
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probably be a similar tree to what mtree(8) uses internally;
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a linked list of entries per directory with a child pointer
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to children of directories. ".." won't be stored in the list;
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the fs-specific tree walker should add this if required by the fs.
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this builder have the smarts to handle hard links correctly.
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- (optionally) Change the permissions in the tree according to
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the mtree(8) specfile
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- Call an fs-specific routine to build the image based on the
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data structures.
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Each fs-specific module should have the following external interfaces:
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prepare_options optional file system specific defaults that need to be
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setup before parsing fs-specific options.
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parse_options parse the string for fs-specific options, feeding
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errors back to the user as appropriate
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cleanup_options optional file system specific data that need to be
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cleaned up when done with this filesystem.
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make_fs take the data structures representing the
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directory tree and fs parameters,
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validate that the parameters are valid
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(e.g, the requested image will be large enough),
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create the image, and
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populate the image
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prepare_options and cleanup_options are optional and can be NULL.
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NOTE: All file system specific options are referenced via the fs_specific
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pointer from the fsinfo_t strucutre. It is up to the filesystem to allocate
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and free any data needed for this via the prepare and cleanup callbacks.
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Each fs-specific module will need to add it's routines to the dispatch array
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in makefs.c and add prototypes for these to makefs.h
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All other implementation details should not need to change any of the
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generic code.
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ffs implementation
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------------------
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In the ffs case, we can leverage off sbin/newfs/mkfs.c to actually build
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the image. When building and populating the image, the implementation
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can be greatly simplified if some assumptions are made:
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- the total required size (in blocks and inodes) is determined
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as part of the validation phase
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- a "file" (including a directory) has a known size, so
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support for growing a file is not necessary
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Two underlying primitives are provided:
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make_inode create an inode, returning the inode number
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write_file write file (from memory if DIR, file descriptor
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if FILE or SYMLINK), referencing given inode.
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it is smart enough to know if a short symlink
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can be stuffed into the inode, etc.
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When creating a directory, the directory entries in the previously
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built tree data structure is scanned and built in memory so it can
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be written entirely as a single write_file() operation.
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