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