path: root/fs/umsdos/specs

/* #Specification: umsdos / readdir
 * umsdos_readdir() should fill a struct dirent with
 * an inode number. The cheap way to get it is to
 * do a lookup in the MSDOS directory for each
 * entry processed by the readdir() function.
 * This is not very efficient, but very simple. The
 * other way around is to maintain a copy of the inode
 * number in the EMD file. This is a problem because
 * this has to be maintained in sync using tricks.
 * Remember that MSDOS (the OS) does not update the
 * modification time (mtime) of a directory. There is
 * no easy way to tell that a directory was modified
 * during a DOS session and synchronise the EMD file.
 */
		/* #Specification: readdir / . and ..
		 * The msdos filesystem manages the . and .. entry properly
		 * so the EMD file won't hold any info about it.
		 * 
		 * In readdir, we assume that for the root directory
		 * the read position will be 0 for ".", 1 for "..". For
		 * a non root directory, the read position will be 0 for "."
		 * and 32 for "..".
		 */
		/*
		 * This is a trick used by the msdos file system (fs/msdos/dir.c)
		 * to manage . and .. for the root directory of a file system.
		 * Since there is no such entry in the root, fs/msdos/dir.c
		 * use the following:
		 * 
		 * if f_pos == 0, return ".".
		 * if f_pos == 1, return "..".
		 * 
		 * So let msdos handle it
		 * 
		 * Since umsdos entries are much larger, we share the same f_pos.
		 * if f_pos is 0 or 1 or 32, we are clearly looking at . and
		 * ..
		 * 
		 * As soon as we get f_pos == 2 or f_pos == 64, then back to
		 * 0, but this time we are reading the EMD file.
		 * 
		 * Well, not so true. The problem, is that UMSDOS_REC_SIZE is
		 * also 64, so as soon as we read the first record in the
		 * EMD, we are back at offset 64. So we set the offset
		 * to UMSDOS_SPECIAL_DIRFPOS(3) as soon as we have read the
		 * .. entry from msdos.
		 * 
		 * Now (linux 1.3), umsdos_readdir can read more than one
		 * entry even if we limit (umsdos_dir_once) to only one:
		 * It skips over hidden file. So we switch to
		 * UMSDOS_SPECIAL_DIRFPOS as soon as we have read successfully
		 * the .. entry.
		 */
			/* #Specification: umsdos / lookup / inode info
			 * After successfully reading an inode from the MSDOS
			 * filesystem, we use the EMD file to complete it.
			 * We update the following field.
			 * 
			 * uid, gid, atime, ctime, mtime, mode.
			 * 
			 * We rely on MSDOS for mtime. If the file
			 * was modified during an MSDOS session, at least
			 * mtime will be meaningful. We do this only for regular
			 * file.
			 * 
			 * We don't rely on MS-DOS for mtime for directories
			 * because the MS-DOS date on a directory is its
			 * creation time (strange MSDOS behavior) which
			 * corresponds to none of the three Unix time stamps.
			 */
	/* #Specification: umsdos / conversion mode
	 * The msdos filesystem can do some inline conversion
	 * of the data of a file.  It can translate silently
	 * from the MS-DOS text file format to the Unix one
	 * (CRLF -> LF) while reading, and the reverse
	 * while writing. This is activated using the mount
	 * option conv=....
	 * 
	 * This is not useful for Linux files in a promoted
	 * directory.  It can even be harmful.  For this
	 * reason, the binary (no conversion) mode is
	 * always activated.
	 */
	/* #Specification: umsdos / conversion mode / todo
	 * A flag could be added to file and directories
	 * forcing an automatic conversion mode (as
	 * done with the msdos filesystem).
	 * 
	 * This flag could be setup on a directory basis
	 * (instead of file) and all files in it would
	 * logically inherit it.  If the conversion mode
	 * is active (conv=) then the i_binary flag would
	 * be left untouched in those directories.
	 * 
	 * It was proposed that the sticky bit be used to set
	 * this.  A problem with that is that new files would
	 * be written incorrectly.  The other problem is that
	 * the sticky bit has a meaning for directories. So
	 * another bit should be used (there is some space
	 * in the EMD file for it) and a special utility
	 * would be used to assign the flag to a directory).
	 * I don't think it is useful to assign this flag
	 * on a single file.
	 */
 * #Specification: weakness / rename
 * There is a case where UMSDOS rename has a different behavior
 * than a normal Unix file system.  Renaming an open file across
 * directory boundary does not work.  Renaming an open file within
 * a directory does work, however.
 * 
 * The problem may is in Linux VFS driver for msdos.
 * I believe this is not a bug but a design feature, because
 * an inode number represents some sort of directory address
 * in the MSDOS directory structure, so moving the file into
 * another directory does not preserve the inode number.
 */
/* #Specification: rename / new name exist
 * If the destination name already exists, it will
 * silently be removed.  EXT2 does it this way
 * and this is the spec of SunOS.  So does UMSDOS.
 * 
 * If the destination is an empty directory it will
 * also be removed.
 */
/* #Specification: rename / new name exist / possible flaw
 * The code to handle the deletion of the target (file
 * and directory) use to be in umsdos_rename_f, surrounded
 * by proper directory locking.  This was ensuring that only
 * one process could achieve a rename (modification) operation
 * in the source and destination directory.  This was also
 * ensuring the operation was "atomic".
 * 
 * This has been changed because this was creating a
 * stack overflow (the stack is only 4 kB) in the kernel.  To avoid
 * the code doing the deletion of the target (if exist) has
 * been moved to a upper layer. umsdos_rename_f is tried
 * once and if it fails with EEXIST, the target is removed
 * and umsdos_rename_f is done again.
 * 
 * This makes the code cleaner and may solve a
 * deadlock problem one tester was experiencing.
 * 
 * The point is to mention that possibly, the semantic of
 * "rename" may be wrong. Anyone dare to check that :-)
 * Be aware that IF it is wrong, to produce the problem you
 * will need two process trying to rename a file to the
 * same target at the same time. Again, I am not sure it
 * is a problem at all.
 */

/* #Specification: hard link / strategy
 * Hard links are difficult to implement on top of an MS-DOS FAT file
 * system. Unlike Unix file systems, there are no inodes. A directory
 * entry holds the functionality of the inode and the entry.
 * 
 * We will used the same strategy as a normal Unix file system
 * (with inodes) except we will do it symbolically (using paths).
 * 
 * Because anything can happen during a DOS session (defragment,
 * directory sorting, etc.), we can't rely on an MS-DOS pseudo
 * inode number to record the link. For this reason, the link
 * will be done using hidden symbolic links. The following
 * scenario illustrates how it works.
 * 
 * Given a file /foo/file
 * 
 * #
 * ln /foo/file /tmp/file2
 * 
 * become internally
 * 
 * mv /foo/file /foo/-LINK1
 * ln -s /foo/-LINK1 /foo/file
 * ln -s /foo/-LINK1 /tmp/file2
 * #
 * 
 * Using this strategy, we can operate on /foo/file or /foo/file2.
 * We can remove one and keep the other, like a normal Unix hard link.
 * We can rename /foo/file or /tmp/file2 independently.
 * 
 * The entry -LINK1 will be hidden. It will hold a link count.
 * When all link are erased, the hidden file is erased too.
 */

/* #Specification: weakness / hard link
 * The strategy for hard link introduces a side effect that
 * may or may not be acceptable. Here is the sequence
 * 
 * #
 * mkdir subdir1
 * touch subdir1/file
 * mkdir subdir2
 * ln    subdir1/file subdir2/file
 * rm    subdir1/file
 * rmdir subdir1
 * rmdir: subdir1: Directory not empty
 * #
 * 
 * This happen because there is an invisible file (--link) in
 * subdir1 which is referenced by subdir2/file.
 * 
 * Any idea ?
 */
/* #Specification: weakness / hard link / rename directory
 * Another weakness of hard link come from the fact that
 * it is based on hidden symbolic links. Here is an example.
 * 
 * #
 * mkdir /subdir1
 * touch /subdir1/file
 * mkdir /subdir2
 * ln    /subdir1/file subdir2/file
 * mv    /subdir1 subdir3
 * ls -l /subdir2/file
 * #
 * 
 * Since /subdir2/file is a hidden symbolic link
 * to /subdir1/..hlinkNNN, accessing it will fail since
 * /subdir1 does not exist anymore (has been renamed).
 */
/* #Specification: hard link / directory
 * A hard link can't be made on a directory. EPERM is returned
 * in this case.
 */
/* #Specification: hard link / first hard link
 * The first time a hard link is done on a file, this
 * file must be renamed and hidden. Then an internal
 * symbolic link must be done on the hidden file.
 * 
 * The second link is done after on this hidden file.
 * 
 * It is expected that the Linux MSDOS file system
 * keeps the same pseudo inode when a rename operation
 * is done on a file in the same directory.
 */
/* #Specification: function name / convention
 * A simple convention for function names has been used in
 * the UMSDOS filesystem. First, all functions use the prefix
 * umsdos_ to avoid name clashes with other parts of the kernel.
 * 
 * Standard VFS entry points use the prefix UMSDOS (upper case)
 * so it's easier to tell them apart.
 * N.B. (FIXME) PTW, the order and contents of this struct changed.
 */

/* #Specification: mount / options
 * Umsdos run on top of msdos. Currently, it supports no
 * mount option, but happily pass all option received to
 * the msdos driver. I am not sure if all msdos mount option
 * make sense with Umsdos. Here are at least those who
 * are useful.
 * uid=
 * gid=
 * 
 * These options affect the operation of umsdos in directories
 * which do not have an EMD file. They behave like normal
 * msdos directory, with all limitation of msdos.
 */

/* #Specification: pseudo root / mount
 * When a umsdos fs is mounted, a special handling is done
 * if it is the root partition. We check for the presence
 * of the file /linux/etc/init or /linux/etc/rc or
 * /linux/sbin/init. If one is there, we do a chroot("/linux").
 * 
 * We check both because (see init/main.c) the kernel
 * try to exec init at different place and if it fails
 * it tries /bin/sh /etc/rc. To be consistent with
 * init/main.c, many more test would have to be done
 * to locate init. Any complain ?
 * 
 * The chroot is done manually in init/main.c but the
 * info (the inode) is located at mount time and store
 * in a global variable (pseudo_root) which is used at
 * different place in the umsdos driver. There is no
 * need to store this variable elsewhere because it
 * will always be one, not one per mount.
 * 
 * This feature allows the installation
 * of a linux system within a DOS system in a subdirectory.
 * 
 * A user may install its linux stuff in c:\linux
 * avoiding any clash with existing DOS file and subdirectory.
 * When linux boots, it hides this fact, showing a normal
 * root directory with /etc /bin /tmp ...
 * 
 * The word "linux" is hardcoded in /usr/include/linux/umsdos_fs.h
 * in the macro UMSDOS_PSDROOT_NAME.
 */