FreeBSD/Linux Kernel Cross Reference
sys/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.
    */

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