Documentation/filesystems/vfat.txt

   1 USING VFAT
   2 ----------------------------------------------------------------------
   3 To use the vfat filesystem, use the filesystem type 'vfat'.  i.e.
   4   mount -t vfat /dev/fd0 /mnt
   5
   6 No special partition formatter is required.  mkdosfs will work fine
   7 if you want to format from within Linux.
   8
   9 VFAT MOUNT OPTIONS
  10 ----------------------------------------------------------------------
  11 uid=###       -- Set the owner of all files on this filesystem.
  12                  The default is the uid of current process.
  13
  14 gid=###       -- Set the group of all files on this filesystem.
  15                  The default is the gid of current process.
  16
  17 umask=###     -- The permission mask (for files and directories, see umask(1)).
  18                  The default is the umask of current process.
  19
  20 dmask=###     -- The permission mask for the directory.
  21                  The default is the umask of current process.
  22
  23 fmask=###     -- The permission mask for files.
  24                  The default is the umask of current process.
  25
  26 allow_utime=### -- This option controls the permission check of mtime/atime.
  27
  28                   20 - If current process is in group of file's group ID,
  29                        you can change timestamp.
  30                    2 - Other users can change timestamp.
  31
  32                  The default is set from `dmask' option. (If the directory is
  33                  writable, utime(2) is also allowed. I.e. ~dmask & 022)
  34
  35                  Normally utime(2) checks current process is owner of
  36                  the file, or it has CAP_FOWNER capability.  But FAT
  37                  filesystem doesn't have uid/gid on disk, so normal
  38                  check is too unflexible. With this option you can
  39                  relax it.
  40
  41 codepage=###  -- Sets the codepage number for converting to shortname
  42                  characters on FAT filesystem.
  43                  By default, FAT_DEFAULT_CODEPAGE setting is used.
  44
  45 iocharset=<name> -- Character set to use for converting between the
  46                  encoding is used for user visible filename and 16 bit
  47                  Unicode characters. Long filenames are stored on disk
  48                  in Unicode format, but Unix for the most part doesn't
  49                  know how to deal with Unicode.
  50                  By default, FAT_DEFAULT_IOCHARSET setting is used.
  51
  52                  There is also an option of doing UTF-8 translations
  53                  with the utf8 option.
  54
  55                  NOTE: "iocharset=utf8" is not recommended. If unsure,
  56                  you should consider the following option instead.
  57
  58 utf8=<bool>   -- UTF-8 is the filesystem safe version of Unicode that
  59                  is used by the console.  It can be enabled for the
  60                  filesystem with this option. If 'uni_xlate' gets set,
  61                  UTF-8 gets disabled.
  62
  63 uni_xlate=<bool> -- Translate unhandled Unicode characters to special
  64                  escaped sequences.  This would let you backup and
  65                  restore filenames that are created with any Unicode
  66                  characters.  Until Linux supports Unicode for real,
  67                  this gives you an alternative.  Without this option,
  68                  a '?' is used when no translation is possible.  The
  69                  escape character is ':' because it is otherwise
  70                  illegal on the vfat filesystem.  The escape sequence
  71                  that gets used is ':' and the four digits of hexadecimal
  72                  unicode.
  73
  74 nonumtail=<bool> -- When creating 8.3 aliases, normally the alias will
  75                  end in '~1' or tilde followed by some number.  If this
  76                  option is set, then if the filename is
  77                  "longfilename.txt" and "longfile.txt" does not
  78                  currently exist in the directory, 'longfile.txt' will
  79                  be the short alias instead of 'longfi~1.txt'.
  80
  81 usefree       -- Use the "free clusters" value stored on FSINFO. It'll
  82                  be used to determine number of free clusters without
  83                  scanning disk. But it's not used by default, because
  84                  recent Windows don't update it correctly in some
  85                  case. If you are sure the "free clusters" on FSINFO is
  86                  correct, by this option you can avoid scanning disk.
  87
  88 quiet         -- Stops printing certain warning messages.
  89
  90 check=s|r|n   -- Case sensitivity checking setting.
  91                  s: strict, case sensitive
  92                  r: relaxed, case insensitive
  93                  n: normal, default setting, currently case insensitive
  94
  95 nocase        -- This was deprecated for vfat. Use shortname=win95 instead.
  96
  97 shortname=lower|win95|winnt|mixed
  98               -- Shortname display/create setting.
  99                  lower: convert to lowercase for display,
 100                         emulate the Windows 95 rule for create.
 101                  win95: emulate the Windows 95 rule for display/create.
 102                  winnt: emulate the Windows NT rule for display/create.
 103                  mixed: emulate the Windows NT rule for display,
 104                         emulate the Windows 95 rule for create.
 105                  Default setting is `mixed'.
 106
 107 tz=UTC        -- Interpret timestamps as UTC rather than local time.
 108                  This option disables the conversion of timestamps
 109                  between local time (as used by Windows on FAT) and UTC
 110                  (which Linux uses internally).  This is particularly
 111                  useful when mounting devices (like digital cameras)
 112                  that are set to UTC in order to avoid the pitfalls of
 113                  local time.
 114 time_offset=minutes
 115               -- Set offset for conversion of timestamps from local time
 116                  used by FAT to UTC. I.e. <minutes> minutes will be subtracted
 117                  from each timestamp to convert it to UTC used internally by
 118                  Linux. This is useful when time zone set in sys_tz is
 119                  not the time zone used by the filesystem. Note that this
 120                  option still does not provide correct time stamps in all
 121                  cases in presence of DST - time stamps in a different DST
 122                  setting will be off by one hour.
 123
 124 showexec      -- If set, the execute permission bits of the file will be
 125                  allowed only if the extension part of the name is .EXE,
 126                  .COM, or .BAT. Not set by default.
 127
 128 debug         -- Can be set, but unused by the current implementation.
 129
 130 sys_immutable -- If set, ATTR_SYS attribute on FAT is handled as
 131                  IMMUTABLE flag on Linux. Not set by default.
 132
 133 flush         -- If set, the filesystem will try to flush to disk more
 134                  early than normal. Not set by default.
 135
 136 rodir         -- FAT has the ATTR_RO (read-only) attribute. On Windows,
 137                  the ATTR_RO of the directory will just be ignored,
 138                  and is used only by applications as a flag (e.g. it's set
 139                  for the customized folder).
 140
 141                  If you want to use ATTR_RO as read-only flag even for
 142                  the directory, set this option.
 143
 144 errors=panic|continue|remount-ro
 145               -- specify FAT behavior on critical errors: panic, continue
 146                  without doing anything or remount the partition in
 147                  read-only mode (default behavior).
 148
 149 discard       -- If set, issues discard/TRIM commands to the block
 150                  device when blocks are freed. This is useful for SSD devices
 151                  and sparse/thinly-provisoned LUNs.
 152
 153 nfs=stale_rw|nostale_ro
 154                 Enable this only if you want to export the FAT filesystem
 155                 over NFS.
 156
 157                 stale_rw: This option maintains an index (cache) of directory
 158                 inodes by i_logstart which is used by the nfs-related code to
 159                 improve look-ups. Full file operations (read/write) over NFS is
 160                 supported but with cache eviction at NFS server, this could
 161                 result in ESTALE issues.
 162
 163                 nostale_ro: This option bases the inode number and filehandle
 164                 on the on-disk location of a file in the MS-DOS directory entry.
 165                 This ensures that ESTALE will not be returned after a file is
 166                 evicted from the inode cache. However, it means that operations
 167                 such as rename, create and unlink could cause filehandles that
 168                 previously pointed at one file to point at a different file,
 169                 potentially causing data corruption. For this reason, this
 170                 option also mounts the filesystem readonly.
 171
 172                 To maintain backward compatibility, '-o nfs' is also accepted,
 173                 defaulting to stale_rw
 174
 175 dos1xfloppy  -- If set, use a fallback default BIOS Parameter Block
 176                 configuration, determined by backing device size. These static
 177                 parameters match defaults assumed by DOS 1.x for 160 kiB,
 178                 180 kiB, 320 kiB, and 360 kiB floppies and floppy images.
 179
 180
 181 <bool>: 0,1,yes,no,true,false
 182
 183 LIMITATION
 184 ---------------------------------------------------------------------
 185 * The fallocated region of file is discarded at umount/evict time
 186   when using fallocate with FALLOC_FL_KEEP_SIZE.
 187   So, User should assume that fallocated region can be discarded at
 188   last close if there is memory pressure resulting in eviction of
 189   the inode from the memory. As a result, for any dependency on
 190   the fallocated region, user should make sure to recheck fallocate
 191   after reopening the file.
 192
 193 TODO
 194 ----------------------------------------------------------------------
 195 * Need to get rid of the raw scanning stuff.  Instead, always use
 196   a get next directory entry approach.  The only thing left that uses
 197   raw scanning is the directory renaming code.
 198
 199
 200 POSSIBLE PROBLEMS
 201 ----------------------------------------------------------------------
 202 * vfat_valid_longname does not properly checked reserved names.
 203 * When a volume name is the same as a directory name in the root
 204   directory of the filesystem, the directory name sometimes shows
 205   up as an empty file.
 206 * autoconv option does not work correctly.
 207
 208 BUG REPORTS
 209 ----------------------------------------------------------------------
 210 If you have trouble with the VFAT filesystem, mail bug reports to
 211 chaffee@bmrc.cs.berkeley.edu.  Please specify the filename
 212 and the operation that gave you trouble.
 213
 214 TEST SUITE
 215 ----------------------------------------------------------------------
 216 If you plan to make any modifications to the vfat filesystem, please
 217 get the test suite that comes with the vfat distribution at
 218
 219   http://web.archive.org/web/*/http://bmrc.berkeley.edu/
 220   people/chaffee/vfat.html
 221
 222 This tests quite a few parts of the vfat filesystem and additional
 223 tests for new features or untested features would be appreciated.
 224
 225 NOTES ON THE STRUCTURE OF THE VFAT FILESYSTEM
 226 ----------------------------------------------------------------------
 227 (This documentation was provided by Galen C. Hunt <gchunt@cs.rochester.edu>
 228  and lightly annotated by Gordon Chaffee).
 229
 230 This document presents a very rough, technical overview of my
 231 knowledge of the extended FAT file system used in Windows NT 3.5 and
 232 Windows 95.  I don't guarantee that any of the following is correct,
 233 but it appears to be so.
 234
 235 The extended FAT file system is almost identical to the FAT
 236 file system used in DOS versions up to and including 6.223410239847
 237 :-).  The significant change has been the addition of long file names.
 238 These names support up to 255 characters including spaces and lower
 239 case characters as opposed to the traditional 8.3 short names.
 240
 241 Here is the description of the traditional FAT entry in the current
 242 Windows 95 filesystem:
 243
 244         struct directory { // Short 8.3 names
 245                 unsigned char name[8];          // file name
 246                 unsigned char ext[3];           // file extension
 247                 unsigned char attr;             // attribute byte
 248                 unsigned char lcase;            // Case for base and extension
 249                 unsigned char ctime_ms;         // Creation time, milliseconds
 250                 unsigned char ctime[2];         // Creation time
 251                 unsigned char cdate[2];         // Creation date
 252                 unsigned char adate[2];         // Last access date
 253                 unsigned char reserved[2];      // reserved values (ignored)
 254                 unsigned char time[2];          // time stamp
 255                 unsigned char date[2];          // date stamp
 256                 unsigned char start[2];         // starting cluster number
 257                 unsigned char size[4];          // size of the file
 258         };
 259
 260 The lcase field specifies if the base and/or the extension of an 8.3
 261 name should be capitalized.  This field does not seem to be used by
 262 Windows 95 but it is used by Windows NT.  The case of filenames is not
 263 completely compatible from Windows NT to Windows 95.  It is not completely
 264 compatible in the reverse direction, however.  Filenames that fit in
 265 the 8.3 namespace and are written on Windows NT to be lowercase will
 266 show up as uppercase on Windows 95.
 267
 268 Note that the "start" and "size" values are actually little
 269 endian integer values.  The descriptions of the fields in this
 270 structure are public knowledge and can be found elsewhere.
 271
 272 With the extended FAT system, Microsoft has inserted extra
 273 directory entries for any files with extended names.  (Any name which
 274 legally fits within the old 8.3 encoding scheme does not have extra
 275 entries.)  I call these extra entries slots.  Basically, a slot is a
 276 specially formatted directory entry which holds up to 13 characters of
 277 a file's extended name.  Think of slots as additional labeling for the
 278 directory entry of the file to which they correspond.  Microsoft
 279 prefers to refer to the 8.3 entry for a file as its alias and the
 280 extended slot directory entries as the file name.
 281
 282 The C structure for a slot directory entry follows:
 283
 284         struct slot { // Up to 13 characters of a long name
 285                 unsigned char id;               // sequence number for slot
 286                 unsigned char name0_4[10];      // first 5 characters in name
 287                 unsigned char attr;             // attribute byte
 288                 unsigned char reserved;         // always 0
 289                 unsigned char alias_checksum;   // checksum for 8.3 alias
 290                 unsigned char name5_10[12];     // 6 more characters in name
 291                 unsigned char start[2];         // starting cluster number
 292                 unsigned char name11_12[4];     // last 2 characters in name
 293         };
 294
 295 If the layout of the slots looks a little odd, it's only
 296 because of Microsoft's efforts to maintain compatibility with old
 297 software.  The slots must be disguised to prevent old software from
 298 panicking.  To this end, a number of measures are taken:
 299
 300         1) The attribute byte for a slot directory entry is always set
 301            to 0x0f.  This corresponds to an old directory entry with
 302            attributes of "hidden", "system", "read-only", and "volume
 303            label".  Most old software will ignore any directory
 304            entries with the "volume label" bit set.  Real volume label
 305            entries don't have the other three bits set.
 306
 307         2) The starting cluster is always set to 0, an impossible
 308            value for a DOS file.
 309
 310 Because the extended FAT system is backward compatible, it is
 311 possible for old software to modify directory entries.  Measures must
 312 be taken to ensure the validity of slots.  An extended FAT system can
 313 verify that a slot does in fact belong to an 8.3 directory entry by
 314 the following:
 315
 316         1) Positioning.  Slots for a file always immediately proceed
 317            their corresponding 8.3 directory entry.  In addition, each
 318            slot has an id which marks its order in the extended file
 319            name.  Here is a very abbreviated view of an 8.3 directory
 320            entry and its corresponding long name slots for the file
 321            "My Big File.Extension which is long":
 322
 323                 <proceeding files...>
 324                 <slot #3, id = 0x43, characters = "h is long">
 325                 <slot #2, id = 0x02, characters = "xtension whic">
 326                 <slot #1, id = 0x01, characters = "My Big File.E">
 327                 <directory entry, name = "MYBIGFIL.EXT">
 328
 329            Note that the slots are stored from last to first.  Slots
 330            are numbered from 1 to N.  The Nth slot is or'ed with 0x40
 331            to mark it as the last one.
 332
 333         2) Checksum.  Each slot has an "alias_checksum" value.  The
 334            checksum is calculated from the 8.3 name using the
 335            following algorithm:
 336
 337                 for (sum = i = 0; i < 11; i++) {
 338                         sum = (((sum&1)<<7)|((sum&0xfe)>>1)) + name[i]
 339                 }
 340
 341         3) If there is free space in the final slot, a Unicode NULL (0x0000)
 342            is stored after the final character.  After that, all unused
 343            characters in the final slot are set to Unicode 0xFFFF.
 344
 345 Finally, note that the extended name is stored in Unicode.  Each Unicode
 346 character takes two bytes.