/* #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. */