1 ====
2 VFAT
3 ====
4
5 USING VFAT
6 ==========
7
8 To use the vfat filesystem, use the filesystem type 'vfat'. i.e.::
9
10 mount -t vfat /dev/fd0 /mnt
11
12
13 No special partition formatter is required,
14 'mkdosfs' will work fine if you want to format from within Linux.
15
16 VFAT MOUNT OPTIONS
17 ==================
18
19 **uid=###**
20 Set the owner of all files on this filesystem.
21 The default is the uid of current process.
22
23 **gid=###**
24 Set the group of all files on this filesystem.
25 The default is the gid of current process.
26
27 **umask=###**
28 The permission mask (for files and directories, see *umask(1)*).
29 The default is the umask of current process.
30
31 **dmask=###**
32 The permission mask for the directory.
33 The default is the umask of current process.
34
35 **fmask=###**
36 The permission mask for files.
37 The default is the umask of current process.
38
39 **allow_utime=###**
40 This option controls the permission check of mtime/atime.
41
42 **-20**: If current process is in group of file's group ID,
43 you can change timestamp.
44
45 **-2**: Other users can change timestamp.
46
47 The default is set from dmask option. If the directory is
48 writable, utime(2) is also allowed. i.e. ~dmask & 022.
49
50 Normally utime(2) checks current process is owner of
51 the file, or it has CAP_FOWNER capability. But FAT
52 filesystem doesn't have uid/gid on disk, so normal
53 check is too inflexible. With this option you can
54 relax it.
55
56 **codepage=###**
57 Sets the codepage number for converting to shortname
58 characters on FAT filesystem.
59 By default, FAT_DEFAULT_CODEPAGE setting is used.
60
61 **iocharset=<name>**
62 Character set to use for converting between the
63 encoding is used for user visible filename and 16 bit
64 Unicode characters. Long filenames are stored on disk
65 in Unicode format, but Unix for the most part doesn't
66 know how to deal with Unicode.
67 By default, FAT_DEFAULT_IOCHARSET setting is used.
68
69 There is also an option of doing UTF-8 translations
70 with the utf8 option.
71
72 .. note:: ``iocharset=utf8`` is not recommended. If unsure, you should consider
73 the utf8 option instead.
74
75 **utf8=<bool>**
76 UTF-8 is the filesystem safe version of Unicode that
77 is used by the console. It can be enabled or disabled
78 for the filesystem with this option.
79 If 'uni_xlate' gets set, UTF-8 gets disabled.
80 By default, FAT_DEFAULT_UTF8 setting is used.
81
82 **uni_xlate=<bool>**
83 Translate unhandled Unicode characters to special
84 escaped sequences. This would let you backup and
85 restore filenames that are created with any Unicode
86 characters. Until Linux supports Unicode for real,
87 this gives you an alternative. Without this option,
88 a '?' is used when no translation is possible. The
89 escape character is ':' because it is otherwise
90 illegal on the vfat filesystem. The escape sequence
91 that gets used is ':' and the four digits of hexadecimal
92 unicode.
93
94 **nonumtail=<bool>**
95 When creating 8.3 aliases, normally the alias will
96 end in '~1' or tilde followed by some number. If this
97 option is set, then if the filename is
98 "longfilename.txt" and "longfile.txt" does not
99 currently exist in the directory, longfile.txt will
100 be the short alias instead of longfi~1.txt.
101
102 **usefree**
103 Use the "free clusters" value stored on FSINFO. It will
104 be used to determine number of free clusters without
105 scanning disk. But it's not used by default, because
106 recent Windows don't update it correctly in some
107 case. If you are sure the "free clusters" on FSINFO is
108 correct, by this option you can avoid scanning disk.
109
110 **quiet**
111 Stops printing certain warning messages.
112
113 **check=s|r|n**
114 Case sensitivity checking setting.
115
116 **s**: strict, case sensitive
117
118 **r**: relaxed, case insensitive
119
120 **n**: normal, default setting, currently case insensitive
121
122 **nocase**
123 This was deprecated for vfat. Use ``shortname=win95`` instead.
124
125 **shortname=lower|win95|winnt|mixed**
126 Shortname display/create setting.
127
128 **lower**: convert to lowercase for display,
129 emulate the Windows 95 rule for create.
130
131 **win95**: emulate the Windows 95 rule for display/create.
132
133 **winnt**: emulate the Windows NT rule for display/create.
134
135 **mixed**: emulate the Windows NT rule for display,
136 emulate the Windows 95 rule for create.
137
138 Default setting is `mixed`.
139
140 **tz=UTC**
141 Interpret timestamps as UTC rather than local time.
142 This option disables the conversion of timestamps
143 between local time (as used by Windows on FAT) and UTC
144 (which Linux uses internally). This is particularly
145 useful when mounting devices (like digital cameras)
146 that are set to UTC in order to avoid the pitfalls of
147 local time.
148
149 **time_offset=minutes**
150 Set offset for conversion of timestamps from local time
151 used by FAT to UTC. I.e. <minutes> minutes will be subtracted
152 from each timestamp to convert it to UTC used internally by
153 Linux. This is useful when time zone set in ``sys_tz`` is
154 not the time zone used by the filesystem. Note that this
155 option still does not provide correct time stamps in all
156 cases in presence of DST - time stamps in a different DST
157 setting will be off by one hour.
158
159 **showexec**
160 If set, the execute permission bits of the file will be
161 allowed only if the extension part of the name is .EXE,
162 .COM, or .BAT. Not set by default.
163
164 **debug**
165 Can be set, but unused by the current implementation.
166
167 **sys_immutable**
168 If set, ATTR_SYS attribute on FAT is handled as
169 IMMUTABLE flag on Linux. Not set by default.
170
171 **flush**
172 If set, the filesystem will try to flush to disk more
173 early than normal. Not set by default.
174
175 **rodir**
176 FAT has the ATTR_RO (read-only) attribute. On Windows,
177 the ATTR_RO of the directory will just be ignored,
178 and is used only by applications as a flag (e.g. it's set
179 for the customized folder).
180
181 If you want to use ATTR_RO as read-only flag even for
182 the directory, set this option.
183
184 **errors=panic|continue|remount-ro**
185 specify FAT behavior on critical errors: panic, continue
186 without doing anything or remount the partition in
187 read-only mode (default behavior).
188
189 **discard**
190 If set, issues discard/TRIM commands to the block
191 device when blocks are freed. This is useful for SSD devices
192 and sparse/thinly-provisioned LUNs.
193
194 **nfs=stale_rw|nostale_ro**
195 Enable this only if you want to export the FAT filesystem
196 over NFS.
197
198 **stale_rw**: This option maintains an index (cache) of directory
199 *inodes* by *i_logstart* which is used by the nfs-related code to
200 improve look-ups. Full file operations (read/write) over NFS is
201 supported but with cache eviction at NFS server, this could
202 result in ESTALE issues.
203
204 **nostale_ro**: This option bases the *inode* number and filehandle
205 on the on-disk location of a file in the MS-DOS directory entry.
206 This ensures that ESTALE will not be returned after a file is
207 evicted from the inode cache. However, it means that operations
208 such as rename, create and unlink could cause filehandles that
209 previously pointed at one file to point at a different file,
210 potentially causing data corruption. For this reason, this
211 option also mounts the filesystem readonly.
212
213 To maintain backward compatibility, ``'-o nfs'`` is also accepted,
214 defaulting to "stale_rw".
215
216 **dos1xfloppy <bool>: 0,1,yes,no,true,false**
217 If set, use a fallback default BIOS Parameter Block
218 configuration, determined by backing device size. These static
219 parameters match defaults assumed by DOS 1.x for 160 kiB,
220 180 kiB, 320 kiB, and 360 kiB floppies and floppy images.
221
222
223
224 LIMITATION
225 ==========
226
227 The fallocated region of file is discarded at umount/evict time
228 when using fallocate with FALLOC_FL_KEEP_SIZE.
229 So, User should assume that fallocated region can be discarded at
230 last close if there is memory pressure resulting in eviction of
231 the inode from the memory. As a result, for any dependency on
232 the fallocated region, user should make sure to recheck fallocate
233 after reopening the file.
234
235 TODO
236 ====
237 Need to get rid of the raw scanning stuff. Instead, always use
238 a get next directory entry approach. The only thing left that uses
239 raw scanning is the directory renaming code.
240
241
242 POSSIBLE PROBLEMS
243 =================
244
245 - vfat_valid_longname does not properly checked reserved names.
246 - When a volume name is the same as a directory name in the root
247 directory of the filesystem, the directory name sometimes shows
248 up as an empty file.
249 - autoconv option does not work correctly.
250
251
252 TEST SUITE
253 ==========
254 If you plan to make any modifications to the vfat filesystem, please
255 get the test suite that comes with the vfat distribution at
256
257 `<http://web.archive.org/web/*/http://bmrc.berkeley.edu/people/chaffee/vfat.html>`_
258
259 This tests quite a few parts of the vfat filesystem and additional
260 tests for new features or untested features would be appreciated.
261
262 NOTES ON THE STRUCTURE OF THE VFAT FILESYSTEM
263 =============================================
264 This documentation was provided by Galen C. Hunt gchunt@cs.rochester.edu and
265 lightly annotated by Gordon Chaffee.
266
267 This document presents a very rough, technical overview of my
268 knowledge of the extended FAT file system used in Windows NT 3.5 and
269 Windows 95. I don't guarantee that any of the following is correct,
270 but it appears to be so.
271
272 The extended FAT file system is almost identical to the FAT
273 file system used in DOS versions up to and including *6.223410239847*
274 :-). The significant change has been the addition of long file names.
275 These names support up to 255 characters including spaces and lower
276 case characters as opposed to the traditional 8.3 short names.
277
278 Here is the description of the traditional FAT entry in the current
279 Windows 95 filesystem::
280
281 struct directory { // Short 8.3 names
282 unsigned char name[8]; // file name
283 unsigned char ext[3]; // file extension
284 unsigned char attr; // attribute byte
285 unsigned char lcase; // Case for base and extension
286 unsigned char ctime_ms; // Creation time, milliseconds
287 unsigned char ctime[2]; // Creation time
288 unsigned char cdate[2]; // Creation date
289 unsigned char adate[2]; // Last access date
290 unsigned char reserved[2]; // reserved values (ignored)
291 unsigned char time[2]; // time stamp
292 unsigned char date[2]; // date stamp
293 unsigned char start[2]; // starting cluster number
294 unsigned char size[4]; // size of the file
295 };
296
297
298 The lcase field specifies if the base and/or the extension of an 8.3
299 name should be capitalized. This field does not seem to be used by
300 Windows 95 but it is used by Windows NT. The case of filenames is not
301 completely compatible from Windows NT to Windows 95. It is not completely
302 compatible in the reverse direction, however. Filenames that fit in
303 the 8.3 namespace and are written on Windows NT to be lowercase will
304 show up as uppercase on Windows 95.
305
306 .. note:: Note that the ``start`` and ``size`` values are actually little
307 endian integer values. The descriptions of the fields in this
308 structure are public knowledge and can be found elsewhere.
309
310 With the extended FAT system, Microsoft has inserted extra
311 directory entries for any files with extended names. (Any name which
312 legally fits within the old 8.3 encoding scheme does not have extra
313 entries.) I call these extra entries slots. Basically, a slot is a
314 specially formatted directory entry which holds up to 13 characters of
315 a file's extended name. Think of slots as additional labeling for the
316 directory entry of the file to which they correspond. Microsoft
317 prefers to refer to the 8.3 entry for a file as its alias and the
318 extended slot directory entries as the file name.
319
320 The C structure for a slot directory entry follows::
321
322 struct slot { // Up to 13 characters of a long name
323 unsigned char id; // sequence number for slot
324 unsigned char name0_4[10]; // first 5 characters in name
325 unsigned char attr; // attribute byte
326 unsigned char reserved; // always 0
327 unsigned char alias_checksum; // checksum for 8.3 alias
328 unsigned char name5_10[12]; // 6 more characters in name
329 unsigned char start[2]; // starting cluster number
330 unsigned char name11_12[4]; // last 2 characters in name
331 };
332
333
334 If the layout of the slots looks a little odd, it's only
335 because of Microsoft's efforts to maintain compatibility with old
336 software. The slots must be disguised to prevent old software from
337 panicking. To this end, a number of measures are taken:
338
339 1) The attribute byte for a slot directory entry is always set
340 to 0x0f. This corresponds to an old directory entry with
341 attributes of "hidden", "system", "read-only", and "volume
342 label". Most old software will ignore any directory
343 entries with the "volume label" bit set. Real volume label
344 entries don't have the other three bits set.
345
346 2) The starting cluster is always set to 0, an impossible
347 value for a DOS file.
348
349 Because the extended FAT system is backward compatible, it is
350 possible for old software to modify directory entries. Measures must
351 be taken to ensure the validity of slots. An extended FAT system can
352 verify that a slot does in fact belong to an 8.3 directory entry by
353 the following:
354
355 1) Positioning. Slots for a file always immediately proceed
356 their corresponding 8.3 directory entry. In addition, each
357 slot has an id which marks its order in the extended file
358 name. Here is a very abbreviated view of an 8.3 directory
359 entry and its corresponding long name slots for the file
360 "My Big File.Extension which is long"::
361
362 <proceeding files...>
363 <slot #3, id = 0x43, characters = "h is long">
364 <slot #2, id = 0x02, characters = "xtension whic">
365 <slot #1, id = 0x01, characters = "My Big File.E">
366 <directory entry, name = "MYBIGFIL.EXT">
367
368
369 .. note:: Note that the slots are stored from last to first. Slots
370 are numbered from 1 to N. The Nth slot is ``or'ed`` with
371 0x40 to mark it as the last one.
372
373 2) Checksum. Each slot has an alias_checksum value. The
374 checksum is calculated from the 8.3 name using the
375 following algorithm::
376
377 for (sum = i = 0; i < 11; i++) {
378 sum = (((sum&1)<<7)|((sum&0xfe)>>1)) + name[i]
379 }
380
381
382 3) If there is free space in the final slot, a Unicode ``NULL (0x0000)``
383 is stored after the final character. After that, all unused
384 characters in the final slot are set to Unicode 0xFFFF.
385
386 Finally, note that the extended name is stored in Unicode. Each Unicode
387 character takes either two or four bytes, UTF-16LE encoded.
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