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 or disabled
  60                  for the filesystem with this option.
  61                  If 'uni_xlate' gets set, UTF-8 gets disabled.
  62                  By default, FAT_DEFAULT_UTF8 setting is used.
  63
  64 uni_xlate=<bool> -- Translate unhandled Unicode characters to special
  65                  escaped sequences.  This would let you backup and
  66                  restore filenames that are created with any Unicode
  67                  characters.  Until Linux supports Unicode for real,
  68                  this gives you an alternative.  Without this option,
  69                  a '?' is used when no translation is possible.  The
  70                  escape character is ':' because it is otherwise
  71                  illegal on the vfat filesystem.  The escape sequence
  72                  that gets used is ':' and the four digits of hexadecimal
  73                  unicode.
  74
  75 nonumtail=<bool> -- When creating 8.3 aliases, normally the alias will
  76                  end in '~1' or tilde followed by some number.  If this
  77                  option is set, then if the filename is
  78                  "longfilename.txt" and "longfile.txt" does not
  79                  currently exist in the directory, 'longfile.txt' will
  80                  be the short alias instead of 'longfi~1.txt'.
  81
  82 usefree       -- Use the "free clusters" value stored on FSINFO. It'll
  83                  be used to determine number of free clusters without
  84                  scanning disk. But it's not used by default, because
  85                  recent Windows don't update it correctly in some
  86                  case. If you are sure the "free clusters" on FSINFO is
  87                  correct, by this option you can avoid scanning disk.
  88
  89 quiet         -- Stops printing certain warning messages.
  90
  91 check=s|r|n   -- Case sensitivity checking setting.
  92                  s: strict, case sensitive
  93                  r: relaxed, case insensitive
  94                  n: normal, default setting, currently case insensitive
  95
  96 nocase        -- This was deprecated for vfat. Use shortname=win95 instead.
  97
  98 shortname=lower|win95|winnt|mixed
  99               -- Shortname display/create setting.
 100                  lower: convert to lowercase for display,
 101                         emulate the Windows 95 rule for create.
 102                  win95: emulate the Windows 95 rule for display/create.
 103                  winnt: emulate the Windows NT rule for display/create.
 104                  mixed: emulate the Windows NT rule for display,
 105                         emulate the Windows 95 rule for create.
 106                  Default setting is `mixed'.
 107
 108 tz=UTC        -- Interpret timestamps as UTC rather than local time.
 109                  This option disables the conversion of timestamps
 110                  between local time (as used by Windows on FAT) and UTC
 111                  (which Linux uses internally).  This is particularly
 112                  useful when mounting devices (like digital cameras)
 113                  that are set to UTC in order to avoid the pitfalls of
 114                  local time.
 115 time_offset=minutes
 116               -- Set offset for conversion of timestamps from local time
 117                  used by FAT to UTC. I.e. <minutes> minutes will be subtracted
 118                  from each timestamp to convert it to UTC used internally by
 119                  Linux. This is useful when time zone set in sys_tz is
 120                  not the time zone used by the filesystem. Note that this
 121                  option still does not provide correct time stamps in all
 122                  cases in presence of DST - time stamps in a different DST
 123                  setting will be off by one hour.
 124
 125 showexec      -- If set, the execute permission bits of the file will be
 126                  allowed only if the extension part of the name is .EXE,
 127                  .COM, or .BAT. Not set by default.
 128
 129 debug         -- Can be set, but unused by the current implementation.
 130
 131 sys_immutable -- If set, ATTR_SYS attribute on FAT is handled as
 132                  IMMUTABLE flag on Linux. Not set by default.
 133
 134 flush         -- If set, the filesystem will try to flush to disk more
 135                  early than normal. Not set by default.
 136
 137 rodir         -- FAT has the ATTR_RO (read-only) attribute. On Windows,
 138                  the ATTR_RO of the directory will just be ignored,
 139                  and is used only by applications as a flag (e.g. it's set
 140                  for the customized folder).
 141
 142                  If you want to use ATTR_RO as read-only flag even for
 143                  the directory, set this option.
 144
 145 errors=panic|continue|remount-ro
 146               -- specify FAT behavior on critical errors: panic, continue
 147                  without doing anything or remount the partition in
 148                  read-only mode (default behavior).
 149
 150 discard       -- If set, issues discard/TRIM commands to the block
 151                  device when blocks are freed. This is useful for SSD devices
 152                  and sparse/thinly-provisoned LUNs.
 153
 154 nfs=stale_rw|nostale_ro
 155                 Enable this only if you want to export the FAT filesystem
 156                 over NFS.
 157
 158                 stale_rw: This option maintains an index (cache) of directory
 159                 inodes by i_logstart which is used by the nfs-related code to
 160                 improve look-ups. Full file operations (read/write) over NFS is
 161                 supported but with cache eviction at NFS server, this could
 162                 result in ESTALE issues.
 163
 164                 nostale_ro: This option bases the inode number and filehandle
 165                 on the on-disk location of a file in the MS-DOS directory entry.
 166                 This ensures that ESTALE will not be returned after a file is
 167                 evicted from the inode cache. However, it means that operations
 168                 such as rename, create and unlink could cause filehandles that
 169                 previously pointed at one file to point at a different file,
 170                 potentially causing data corruption. For this reason, this
 171                 option also mounts the filesystem readonly.
 172
 173                 To maintain backward compatibility, '-o nfs' is also accepted,
 174                 defaulting to stale_rw
 175
 176 dos1xfloppy  -- If set, use a fallback default BIOS Parameter Block
 177                 configuration, determined by backing device size. These static
 178                 parameters match defaults assumed by DOS 1.x for 160 kiB,
 179                 180 kiB, 320 kiB, and 360 kiB floppies and floppy images.
 180
 181
 182 <bool>: 0,1,yes,no,true,false
 183
 184 LIMITATION
 185 ---------------------------------------------------------------------
 186 * The fallocated region of file is discarded at umount/evict time
 187   when using fallocate with FALLOC_FL_KEEP_SIZE.
 188   So, User should assume that fallocated region can be discarded at
 189   last close if there is memory pressure resulting in eviction of
 190   the inode from the memory. As a result, for any dependency on
 191   the fallocated region, user should make sure to recheck fallocate
 192   after reopening the file.
 193
 194 TODO
 195 ----------------------------------------------------------------------
 196 * Need to get rid of the raw scanning stuff.  Instead, always use
 197   a get next directory entry approach.  The only thing left that uses
 198   raw scanning is the directory renaming code.
 199
 200
 201 POSSIBLE PROBLEMS
 202 ----------------------------------------------------------------------
 203 * vfat_valid_longname does not properly checked reserved names.
 204 * When a volume name is the same as a directory name in the root
 205   directory of the filesystem, the directory name sometimes shows
 206   up as an empty file.
 207 * autoconv option does not work correctly.
 208
 209 BUG REPORTS
 210 ----------------------------------------------------------------------
 211 If you have trouble with the VFAT filesystem, mail bug reports to
 212 chaffee@bmrc.cs.berkeley.edu.  Please specify the filename
 213 and the operation that gave you trouble.
 214
 215 TEST SUITE
 216 ----------------------------------------------------------------------
 217 If you plan to make any modifications to the vfat filesystem, please
 218 get the test suite that comes with the vfat distribution at
 219
 220   http://web.archive.org/web/*/http://bmrc.berkeley.edu/
 221   people/chaffee/vfat.html
 222
 223 This tests quite a few parts of the vfat filesystem and additional
 224 tests for new features or untested features would be appreciated.
 225
 226 NOTES ON THE STRUCTURE OF THE VFAT FILESYSTEM
 227 ----------------------------------------------------------------------
 228 (This documentation was provided by Galen C. Hunt <gchunt@cs.rochester.edu>
 229  and lightly annotated by Gordon Chaffee).
 230
 231 This document presents a very rough, technical overview of my
 232 knowledge of the extended FAT file system used in Windows NT 3.5 and
 233 Windows 95.  I don't guarantee that any of the following is correct,
 234 but it appears to be so.
 235
 236 The extended FAT file system is almost identical to the FAT
 237 file system used in DOS versions up to and including 6.223410239847
 238 :-).  The significant change has been the addition of long file names.
 239 These names support up to 255 characters including spaces and lower
 240 case characters as opposed to the traditional 8.3 short names.
 241
 242 Here is the description of the traditional FAT entry in the current
 243 Windows 95 filesystem:
 244
 245         struct directory { // Short 8.3 names
 246                 unsigned char name[8];          // file name
 247                 unsigned char ext[3];           // file extension
 248                 unsigned char attr;             // attribute byte
 249                 unsigned char lcase;            // Case for base and extension
 250                 unsigned char ctime_ms;         // Creation time, milliseconds
 251                 unsigned char ctime[2];         // Creation time
 252                 unsigned char cdate[2];         // Creation date
 253                 unsigned char adate[2];         // Last access date
 254                 unsigned char reserved[2];      // reserved values (ignored)
 255                 unsigned char time[2];          // time stamp
 256                 unsigned char date[2];          // date stamp
 257                 unsigned char start[2];         // starting cluster number
 258                 unsigned char size[4];          // size of the file
 259         };
 260
 261 The lcase field specifies if the base and/or the extension of an 8.3
 262 name should be capitalized.  This field does not seem to be used by
 263 Windows 95 but it is used by Windows NT.  The case of filenames is not
 264 completely compatible from Windows NT to Windows 95.  It is not completely
 265 compatible in the reverse direction, however.  Filenames that fit in
 266 the 8.3 namespace and are written on Windows NT to be lowercase will
 267 show up as uppercase on Windows 95.
 268
 269 Note that the "start" and "size" values are actually little
 270 endian integer values.  The descriptions of the fields in this
 271 structure are public knowledge and can be found elsewhere.
 272
 273 With the extended FAT system, Microsoft has inserted extra
 274 directory entries for any files with extended names.  (Any name which
 275 legally fits within the old 8.3 encoding scheme does not have extra
 276 entries.)  I call these extra entries slots.  Basically, a slot is a
 277 specially formatted directory entry which holds up to 13 characters of
 278 a file's extended name.  Think of slots as additional labeling for the
 279 directory entry of the file to which they correspond.  Microsoft
 280 prefers to refer to the 8.3 entry for a file as its alias and the
 281 extended slot directory entries as the file name.
 282
 283 The C structure for a slot directory entry follows:
 284
 285         struct slot { // Up to 13 characters of a long name
 286                 unsigned char id;               // sequence number for slot
 287                 unsigned char name0_4[10];      // first 5 characters in name
 288                 unsigned char attr;             // attribute byte
 289                 unsigned char reserved;         // always 0
 290                 unsigned char alias_checksum;   // checksum for 8.3 alias
 291                 unsigned char name5_10[12];     // 6 more characters in name
 292                 unsigned char start[2];         // starting cluster number
 293                 unsigned char name11_12[4];     // last 2 characters in name
 294         };
 295
 296 If the layout of the slots looks a little odd, it's only
 297 because of Microsoft's efforts to maintain compatibility with old
 298 software.  The slots must be disguised to prevent old software from
 299 panicking.  To this end, a number of measures are taken:
 300
 301         1) The attribute byte for a slot directory entry is always set
 302            to 0x0f.  This corresponds to an old directory entry with
 303            attributes of "hidden", "system", "read-only", and "volume
 304            label".  Most old software will ignore any directory
 305            entries with the "volume label" bit set.  Real volume label
 306            entries don't have the other three bits set.
 307
 308         2) The starting cluster is always set to 0, an impossible
 309            value for a DOS file.
 310
 311 Because the extended FAT system is backward compatible, it is
 312 possible for old software to modify directory entries.  Measures must
 313 be taken to ensure the validity of slots.  An extended FAT system can
 314 verify that a slot does in fact belong to an 8.3 directory entry by
 315 the following:
 316
 317         1) Positioning.  Slots for a file always immediately proceed
 318            their corresponding 8.3 directory entry.  In addition, each
 319            slot has an id which marks its order in the extended file
 320            name.  Here is a very abbreviated view of an 8.3 directory
 321            entry and its corresponding long name slots for the file
 322            "My Big File.Extension which is long":
 323
 324                 <proceeding files...>
 325                 <slot #3, id = 0x43, characters = "h is long">
 326                 <slot #2, id = 0x02, characters = "xtension whic">
 327                 <slot #1, id = 0x01, characters = "My Big File.E">
 328                 <directory entry, name = "MYBIGFIL.EXT">
 329
 330            Note that the slots are stored from last to first.  Slots
 331            are numbered from 1 to N.  The Nth slot is or'ed with 0x40
 332            to mark it as the last one.
 333
 334         2) Checksum.  Each slot has an "alias_checksum" value.  The
 335            checksum is calculated from the 8.3 name using the
 336            following algorithm:
 337
 338                 for (sum = i = 0; i < 11; i++) {
 339                         sum = (((sum&1)<<7)|((sum&0xfe)>>1)) + name[i]
 340                 }
 341
 342         3) If there is free space in the final slot, a Unicode NULL (0x0000)
 343            is stored after the final character.  After that, all unused
 344            characters in the final slot are set to Unicode 0xFFFF.
 345
 346 Finally, note that the extended name is stored in Unicode.  Each Unicode
 347 character takes two bytes.