tjh.dev
Linux kernel mirror (for testing)
git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
kernel
os
linux
Documentation: filesystems: convert vfat.txt to RST
Converts vfat.txt to the reStructuredText format, improving presentation
without changing the underlying content.
Signed-off-by: Daniel W. S. Almeida <dwlsalmeida@gmail.com>
-----------------------------------------------------------
Changes in v3:
Removed unnecessary markup.
Removed section "BUG REPORTS" as recommended by the maintainer.
Changes in v2:
Refactored long lines as pointed out by Jonathan
Copied the maintainer
Updated the reference in the MAINTAINERS file for vfat
I did not move this into admin-guide, waiting on what the
maintainer has to say about this and also about old sections
in the text, if any.
Link: https://lore.kernel.org/r/20191223010030.434902-1-dwlsalmeida@gmail.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
authored by
Daniel W. S. Almeida
and committed by
Jonathan Corbet
a1986433
e43630ed
+389
-348
+387
Documentation/filesystems/vfat.rst
+387
Documentation/filesystems/vfat.rst
···
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 unflexible. 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-provisoned 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.
-347
Documentation/filesystems/vfat.txt
-347
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 either two or four bytes, UTF-16LE encoded.