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diff --git a/Documentation/filesystems/vfat.rst b/Documentation/filesystems/vfat.rst new file mode 100644 index 000000000..e85d74e91 --- /dev/null +++ b/Documentation/filesystems/vfat.rst @@ -0,0 +1,387 @@ +==== +VFAT +==== + +USING VFAT +========== + +To use the vfat filesystem, use the filesystem type 'vfat'. i.e.:: + + mount -t vfat /dev/fd0 /mnt + + +No special partition formatter is required, +'mkdosfs' will work fine if you want to format from within Linux. + +VFAT MOUNT OPTIONS +================== + +**uid=###** + Set the owner of all files on this filesystem. + The default is the uid of current process. + +**gid=###** + Set the group of all files on this filesystem. + The default is the gid of current process. + +**umask=###** + The permission mask (for files and directories, see *umask(1)*). + The default is the umask of current process. + +**dmask=###** + The permission mask for the directory. + The default is the umask of current process. + +**fmask=###** + The permission mask for files. + The default is the umask of current process. + +**allow_utime=###** + This option controls the permission check of mtime/atime. + + **-20**: If current process is in group of file's group ID, + you can change timestamp. + + **-2**: Other users can change timestamp. + + The default is set from dmask option. If the directory is + writable, utime(2) is also allowed. i.e. ~dmask & 022. + + Normally utime(2) checks current process is owner of + the file, or it has CAP_FOWNER capability. But FAT + filesystem doesn't have uid/gid on disk, so normal + check is too unflexible. With this option you can + relax it. + +**codepage=###** + Sets the codepage number for converting to shortname + characters on FAT filesystem. + By default, FAT_DEFAULT_CODEPAGE setting is used. + +**iocharset=<name>** + Character set to use for converting between the + encoding is used for user visible filename and 16 bit + Unicode characters. Long filenames are stored on disk + in Unicode format, but Unix for the most part doesn't + know how to deal with Unicode. + By default, FAT_DEFAULT_IOCHARSET setting is used. + + There is also an option of doing UTF-8 translations + with the utf8 option. + +.. note:: ``iocharset=utf8`` is not recommended. If unsure, you should consider + the utf8 option instead. + +**utf8=<bool>** + UTF-8 is the filesystem safe version of Unicode that + is used by the console. It can be enabled or disabled + for the filesystem with this option. + If 'uni_xlate' gets set, UTF-8 gets disabled. + By default, FAT_DEFAULT_UTF8 setting is used. + +**uni_xlate=<bool>** + Translate unhandled Unicode characters to special + escaped sequences. This would let you backup and + restore filenames that are created with any Unicode + characters. Until Linux supports Unicode for real, + this gives you an alternative. Without this option, + a '?' is used when no translation is possible. The + escape character is ':' because it is otherwise + illegal on the vfat filesystem. The escape sequence + that gets used is ':' and the four digits of hexadecimal + unicode. + +**nonumtail=<bool>** + When creating 8.3 aliases, normally the alias will + end in '~1' or tilde followed by some number. If this + option is set, then if the filename is + "longfilename.txt" and "longfile.txt" does not + currently exist in the directory, longfile.txt will + be the short alias instead of longfi~1.txt. + +**usefree** + Use the "free clusters" value stored on FSINFO. It will + be used to determine number of free clusters without + scanning disk. But it's not used by default, because + recent Windows don't update it correctly in some + case. If you are sure the "free clusters" on FSINFO is + correct, by this option you can avoid scanning disk. + +**quiet** + Stops printing certain warning messages. + +**check=s|r|n** + Case sensitivity checking setting. + + **s**: strict, case sensitive + + **r**: relaxed, case insensitive + + **n**: normal, default setting, currently case insensitive + +**nocase** + This was deprecated for vfat. Use ``shortname=win95`` instead. + +**shortname=lower|win95|winnt|mixed** + Shortname display/create setting. + + **lower**: convert to lowercase for display, + emulate the Windows 95 rule for create. + + **win95**: emulate the Windows 95 rule for display/create. + + **winnt**: emulate the Windows NT rule for display/create. + + **mixed**: emulate the Windows NT rule for display, + emulate the Windows 95 rule for create. + + Default setting is `mixed`. + +**tz=UTC** + Interpret timestamps as UTC rather than local time. + This option disables the conversion of timestamps + between local time (as used by Windows on FAT) and UTC + (which Linux uses internally). This is particularly + useful when mounting devices (like digital cameras) + that are set to UTC in order to avoid the pitfalls of + local time. + +**time_offset=minutes** + Set offset for conversion of timestamps from local time + used by FAT to UTC. I.e. <minutes> minutes will be subtracted + from each timestamp to convert it to UTC used internally by + Linux. This is useful when time zone set in ``sys_tz`` is + not the time zone used by the filesystem. Note that this + option still does not provide correct time stamps in all + cases in presence of DST - time stamps in a different DST + setting will be off by one hour. + +**showexec** + If set, the execute permission bits of the file will be + allowed only if the extension part of the name is .EXE, + .COM, or .BAT. Not set by default. + +**debug** + Can be set, but unused by the current implementation. + +**sys_immutable** + If set, ATTR_SYS attribute on FAT is handled as + IMMUTABLE flag on Linux. Not set by default. + +**flush** + If set, the filesystem will try to flush to disk more + early than normal. Not set by default. + +**rodir** + FAT has the ATTR_RO (read-only) attribute. On Windows, + the ATTR_RO of the directory will just be ignored, + and is used only by applications as a flag (e.g. it's set + for the customized folder). + + If you want to use ATTR_RO as read-only flag even for + the directory, set this option. + +**errors=panic|continue|remount-ro** + specify FAT behavior on critical errors: panic, continue + without doing anything or remount the partition in + read-only mode (default behavior). + +**discard** + If set, issues discard/TRIM commands to the block + device when blocks are freed. This is useful for SSD devices + and sparse/thinly-provisoned LUNs. + +**nfs=stale_rw|nostale_ro** + Enable this only if you want to export the FAT filesystem + over NFS. + + **stale_rw**: This option maintains an index (cache) of directory + *inodes* by *i_logstart* which is used by the nfs-related code to + improve look-ups. Full file operations (read/write) over NFS is + supported but with cache eviction at NFS server, this could + result in ESTALE issues. + + **nostale_ro**: This option bases the *inode* number and filehandle + on the on-disk location of a file in the MS-DOS directory entry. + This ensures that ESTALE will not be returned after a file is + evicted from the inode cache. However, it means that operations + such as rename, create and unlink could cause filehandles that + previously pointed at one file to point at a different file, + potentially causing data corruption. For this reason, this + option also mounts the filesystem readonly. + + To maintain backward compatibility, ``'-o nfs'`` is also accepted, + defaulting to "stale_rw". + +**dos1xfloppy <bool>: 0,1,yes,no,true,false** + If set, use a fallback default BIOS Parameter Block + configuration, determined by backing device size. These static + parameters match defaults assumed by DOS 1.x for 160 kiB, + 180 kiB, 320 kiB, and 360 kiB floppies and floppy images. + + + +LIMITATION +========== + +The fallocated region of file is discarded at umount/evict time +when using fallocate with FALLOC_FL_KEEP_SIZE. +So, User should assume that fallocated region can be discarded at +last close if there is memory pressure resulting in eviction of +the inode from the memory. As a result, for any dependency on +the fallocated region, user should make sure to recheck fallocate +after reopening the file. + +TODO +==== +Need to get rid of the raw scanning stuff. Instead, always use +a get next directory entry approach. The only thing left that uses +raw scanning is the directory renaming code. + + +POSSIBLE PROBLEMS +================= + +- vfat_valid_longname does not properly checked reserved names. +- When a volume name is the same as a directory name in the root + directory of the filesystem, the directory name sometimes shows + up as an empty file. +- autoconv option does not work correctly. + + +TEST SUITE +========== +If you plan to make any modifications to the vfat filesystem, please +get the test suite that comes with the vfat distribution at + +`<http://web.archive.org/web/*/http://bmrc.berkeley.edu/people/chaffee/vfat.html>`_ + +This tests quite a few parts of the vfat filesystem and additional +tests for new features or untested features would be appreciated. + +NOTES ON THE STRUCTURE OF THE VFAT FILESYSTEM +============================================= +This documentation was provided by Galen C. Hunt gchunt@cs.rochester.edu and +lightly annotated by Gordon Chaffee. + +This document presents a very rough, technical overview of my +knowledge of the extended FAT file system used in Windows NT 3.5 and +Windows 95. I don't guarantee that any of the following is correct, +but it appears to be so. + +The extended FAT file system is almost identical to the FAT +file system used in DOS versions up to and including *6.223410239847* +:-). The significant change has been the addition of long file names. +These names support up to 255 characters including spaces and lower +case characters as opposed to the traditional 8.3 short names. + +Here is the description of the traditional FAT entry in the current +Windows 95 filesystem:: + + struct directory { // Short 8.3 names + unsigned char name[8]; // file name + unsigned char ext[3]; // file extension + unsigned char attr; // attribute byte + unsigned char lcase; // Case for base and extension + unsigned char ctime_ms; // Creation time, milliseconds + unsigned char ctime[2]; // Creation time + unsigned char cdate[2]; // Creation date + unsigned char adate[2]; // Last access date + unsigned char reserved[2]; // reserved values (ignored) + unsigned char time[2]; // time stamp + unsigned char date[2]; // date stamp + unsigned char start[2]; // starting cluster number + unsigned char size[4]; // size of the file + }; + + +The lcase field specifies if the base and/or the extension of an 8.3 +name should be capitalized. This field does not seem to be used by +Windows 95 but it is used by Windows NT. The case of filenames is not +completely compatible from Windows NT to Windows 95. It is not completely +compatible in the reverse direction, however. Filenames that fit in +the 8.3 namespace and are written on Windows NT to be lowercase will +show up as uppercase on Windows 95. + +.. note:: Note that the ``start`` and ``size`` values are actually little + endian integer values. The descriptions of the fields in this + structure are public knowledge and can be found elsewhere. + +With the extended FAT system, Microsoft has inserted extra +directory entries for any files with extended names. (Any name which +legally fits within the old 8.3 encoding scheme does not have extra +entries.) I call these extra entries slots. Basically, a slot is a +specially formatted directory entry which holds up to 13 characters of +a file's extended name. Think of slots as additional labeling for the +directory entry of the file to which they correspond. Microsoft +prefers to refer to the 8.3 entry for a file as its alias and the +extended slot directory entries as the file name. + +The C structure for a slot directory entry follows:: + + struct slot { // Up to 13 characters of a long name + unsigned char id; // sequence number for slot + unsigned char name0_4[10]; // first 5 characters in name + unsigned char attr; // attribute byte + unsigned char reserved; // always 0 + unsigned char alias_checksum; // checksum for 8.3 alias + unsigned char name5_10[12]; // 6 more characters in name + unsigned char start[2]; // starting cluster number + unsigned char name11_12[4]; // last 2 characters in name + }; + + +If the layout of the slots looks a little odd, it's only +because of Microsoft's efforts to maintain compatibility with old +software. The slots must be disguised to prevent old software from +panicking. To this end, a number of measures are taken: + + 1) The attribute byte for a slot directory entry is always set + to 0x0f. This corresponds to an old directory entry with + attributes of "hidden", "system", "read-only", and "volume + label". Most old software will ignore any directory + entries with the "volume label" bit set. Real volume label + entries don't have the other three bits set. + + 2) The starting cluster is always set to 0, an impossible + value for a DOS file. + +Because the extended FAT system is backward compatible, it is +possible for old software to modify directory entries. Measures must +be taken to ensure the validity of slots. An extended FAT system can +verify that a slot does in fact belong to an 8.3 directory entry by +the following: + + 1) Positioning. Slots for a file always immediately proceed + their corresponding 8.3 directory entry. In addition, each + slot has an id which marks its order in the extended file + name. Here is a very abbreviated view of an 8.3 directory + entry and its corresponding long name slots for the file + "My Big File.Extension which is long":: + + <proceeding files...> + <slot #3, id = 0x43, characters = "h is long"> + <slot #2, id = 0x02, characters = "xtension whic"> + <slot #1, id = 0x01, characters = "My Big File.E"> + <directory entry, name = "MYBIGFIL.EXT"> + + + .. note:: Note that the slots are stored from last to first. Slots + are numbered from 1 to N. The Nth slot is ``or'ed`` with + 0x40 to mark it as the last one. + + 2) Checksum. Each slot has an alias_checksum value. The + checksum is calculated from the 8.3 name using the + following algorithm:: + + for (sum = i = 0; i < 11; i++) { + sum = (((sum&1)<<7)|((sum&0xfe)>>1)) + name[i] + } + + + 3) If there is free space in the final slot, a Unicode ``NULL (0x0000)`` + is stored after the final character. After that, all unused + characters in the final slot are set to Unicode 0xFFFF. + +Finally, note that the extended name is stored in Unicode. Each Unicode +character takes either two or four bytes, UTF-16LE encoded. |