Re: Linux & FAT32 label

["Pali Rohár" <pali.rohar@gmail.com>]

On Wednesday 11 October 2017 23:24:35 Pali Rohár wrote:
> On Wednesday 04 October 2017 17:33:32 Pali Rohár wrote:
> > Hi! There is a big inconsistency in Linux tools which read or write
> > FAT32 label in filesystem images. The most common used are tools:
> > blkid (from util-linux project), fatlabel (previously known as
> > dosfslabel; from dosfstools project) and mlabel (from mtools project).
> > 
> > FAT32 is itself a big mess from Microsoft hell and even FAT32
> > implementation in Microsoft Windows systems is not compliant to the
> > released FAT32 documentation from Microsoft.
> > 
> > In past months I observed that Linux FAT32 tools has its own way how
> > they interpret FAT32 label (known as volume id) and because every GUI
> > application uses one of those low-level command line tool, it is a big
> > mess if one application say that FAT32 label is A and another that it is
> > B. And then Windows XP say, it is C.
> > 
> > I would like to open discussion if it would be possible to change
> > behavior how blkid (from util-linux project) and fatlabel (from
> > dosfstool project) handle FAT32 label. Ideally to report exactly same
> > output.
> > 
> > Basic information about FAT32 label:
> > 
> > 1) It is stored in two locations: boot sector and root directory as
> >    file name.
> > 
> > 2) In both location format is 11 bytes, padded with spaces (not nulls).
> > 
> > 3) Empty label in boot sector is stored as "NO NAME    " and not as
> >    empty string.
> > 
> > 4) Empty label in root directory is stored either as name which starts
> >    with byte 0xE5, or is not stored in root directory at all.
> > 
> > 5) If label contains leading byte 0xE5, then in root directory is stored
> >    as byte 0x05.
> > 
> > 6) Label string is stored according to current DOS code page. Therefore
> >    label string needs to be converted to bytes.
> > 
> > 7) Label string cannot contain control characters and characters from
> >    the set   ? / \ | . , ; : + = [ ] < > "   plus lower case characters
> >    are stored as their upper case variant (not only ASCII).
> > 
> > (Please correct me if I'm wrong in some of those points)
> > 
> > Plus Microsoft Windows systems fully ignores label stored in boot
> > sector. Seems they do not read it nor they do not update it on changes.
> > 
> > Looks like that mlabel (from mtools) applies all above rules and uses
> > DOS code page 850 by default (can be changed in config file).
> > 
> > blkid and fatlabel process special cases from 1) to 5) differently and
> > they operates on raw bytes, not strings (in DOS code page).
> > 
> > mlabel reads label from the root directory (missing entry is interpreted
> > as no label; there is no fallback to boot sector), but "set" operation
> > modify label in both location boot sector + root directory. Basically it
> > is near to Windows implementation. And reason why Gparted GUI
> > application uses mlabel and not fatlabel.
> > 
> > As Linux does not have "current DOS code page" and argv arguments are
> > not (Unicode) strings, but arbitrary bytes, I understand that for point
> > 6) it is easier to operates not on FAT strings (in current code page),
> > but rather on bytes. Which also would be same on all machines with any
> > configuration.
> > 
> > But would it be possible to decide and unify handling of point 2), 3),
> > 4), 5)? Ideally with combination how to handle situation when different
> > label is stored in boot sector and root directory.
> > 
> > As Windows does not use label in boot sector, it is very common
> > situation that label in boot sector differs from the root directory.
> > 
> > The best would be see in all cases same label from blkid, fatlabel and
> > mlabel. Ideally same as Windows machines -- but due to DOS code page,
> > this is possible only for ASCII subset of the 8bit encoding. IIRC most
> > (or all?) DOS code page has same characters in printable ASCII range.
> > 
> > It is really bad situation if I open disk in Gparted which show me label
> > via mlabel and then I open in KDE Partition Manager and I see different
> > label string (as it reads it from fatlabel).
> > 
> > Also note that older version of fatlabel (when it was named dosfslabel)
> > operated only the label stored in boot sector (and label stored in root
> > directory was not read or touched).
> > 
> 
> Hi! I did some testing of FAT32 label with different tools and here are
> results:

Hi! I did more tests with MS-DOS and Windows systems and I'm extending
result tables below:

>                                                  dosfslabel 3.0.12   fatlabel 4.1       blkid 2.20.1   mlabel 4.0.12   label.exe Windows XP
> fat32_mkdosfs_label1                             'label1     '       'label1     '      'label1'       'label1     '   'label1'
> fat32_mkdosfs_label1_dosfslabel_empty            '           '       '           '      none           '           '   none
> fat32_mkdosfs_label1_dosfslabel_label2           'label2     '       'label2     '      'label2'       'label2     '   'label2'
> fat32_mkdosfs_label1_dosfslabel_NO_NAME          'NO NAME    '       'NO NAME    '      none           'NO NAME    '   'NO NAME'
> fat32_mkdosfs_label1_mlabel_erase                'NO NAME    '       'NO NAME    '      none           none            none
> fat32_mkdosfs_label1_mlabel_NO_NAME              'NO NAME    '       'NO NAME    '      none           'NO NAME    '   'NO NAME'
> fat32_mkdosfs_label1_xp_erase                    'label1'    '       0xE5'abel1     '   'label1'       none            none
> fat32_mkdosfs_label1_xp_label2                   'label1'    '       'LABEL2     '      'LABEL2'       'LABEL2     '   'LABEL2'
> fat32_mkdosfs_none                               '           '       '           '      none           none            none
> fat32_mkdosfs_none_dosfslabel_label1             'label1     '       'label1     '      'label1'       none            none
> fat32_mkdosfs_none_dosfslabel_label1_xp_label2   'label1'    '       'LABEL2     '      'LABEL2'       'LABEL2     '   'LABEL2'
> fat32_mkdosfs_none_dosfslabel_NO_NAME            'NO NAME    '       'NO NAME    '      none           none            none
> fat32_mkdosfs_none_xp_label1                     '           '       'LABEL1     '      'LABEL1'       'LABEL1     '   'LABEL1'
> fat32_mkdosfs_none_xp_label1_dosfslabel_label2   'label2     '       'label2     '      'label2'       'label2     '   'label2'
> fat32_xp_label1                                  'NO NAME    '       'LABEL1     '      'LABEL1'       'LABEL1     '   'LABEL1'
> fat32_xp_none                                    'NO NAME    '       'NO NAME    '      none           none            none
> fat32_xp_none_dosfslabel_label1                  'label1     '       'label1     '      'label1'       none            none
> fat32_xp_none_mlabel_label1                      'LABEL1     '       'LABEL1     '      'LABEL1'       'LABEL1     '   'LABEL1'

                                                   label.exe MS-DOS 7.10   label.exe Windows 98   label.exe Windows 10
fat32_mkdosfs_label1                               'label1'                'label1'               'label1'
fat32_mkdosfs_label1_dosfslabel_empty              ''                      ''                     none
fat32_mkdosfs_label1_dosfslabel_label2             'label2'                'label2'               'label2'
fat32_mkdosfs_label1_dosfslabel_NO_NAME            'NO NAME'               'NO NAME'              'NO NAME'
fat32_mkdosfs_label1_mlabel_erase                  none                    none                   none
fat32_mkdosfs_label1_mlabel_NO_NAME                'NO NAME'               'NO NAME'              'NO NAME'
fat32_mkdosfs_label1_xp_erase                      none                    none                   none
fat32_mkdosfs_label1_xp_label2                     'LABEL2'                'LABEL2'               'LABEL2'
fat32_mkdosfs_none                                 none                    none                   none
fat32_mkdosfs_none_dosfslabel_label1               none                    none                   none
fat32_mkdosfs_none_dosfslabel_label1_xp_label2     'LABEL2'                'LABEL2'               'LABEL2'
fat32_mkdosfs_none_dosfslabel_NO_NAME              none                    none                   none
fat32_mkdosfs_none_xp_label1                       'LABEL1'                'LABEL1'               'LABEL1'
fat32_mkdosfs_none_xp_label1_dosfslabel_label2     'label2'                'label2'               'label2'
fat32_xp_label1                                    'LABEL1'                'LABEL1'               'LABEL1'
fat32_xp_none                                      none                    none                   none
fat32_xp_none_dosfslabel_label1                    none                    none                   none
fat32_xp_none_mlabel_label1                        'LABEL1'                'LABEL1'               'LABEL1'

Seems that behavior of reading label from FAT32 volume is consistent
between MS-DOS and different Windows versions. The only exception is
when in label in the root directory is stored as empty string (11
spaces). MS-DOS and Windows 98 treat it as label with empty string, but
Windows XP and Windows 10 as disk without label.

> In the first column is image name (all images are compressed and
> attached) which contains steps of operations, e.g. file name
> fat32_mkdosfs_none_dosfslabel_label1_xp_label2 means:
> 
> 1. create filesystem with mkdosfs without specifying label
> 2. change label with dosfslabel (3.0.12) to 'label1'
> 3. change label under Windows XP to 'label2'
> 
> From testing it looks like that different tools and different version of
> them have different behavior how they read or write FAT32 label, see
> following table:
> 
>                              read boot   write boot   read root   write root
> dosfslabel 3.0.0  - 3.0.6    YES         YES          NO          NO
> dosfslabel 3.0.7  - 3.0.15   YES         YES          NO          BUGGY (YES - if already exists; NO - otherwise)
> dosfslabel 3.0.16 - 4.1      YES         YES          YES         YES
> label.exe Windows XP         NO          NO           YES         YES
> blkid                        YES         NO           YES         NO
> mlabel                       NO          YES          YES         YES

label.exe MS-DOS 6.22          NO          YES          YES         YES
label.exe MS-DOS 7.10          NO          YES          YES         YES
label.exe Windows 98 SE        NO          YES          YES         YES
label.exe Windows 10           NO          NO           YES         YES

Older MS-DOS 6.22 does not support FAT32 disks, only FAT16. MS-DOS 7.10
has support for FAT32 and also for LFN. But both tested MS-DOS versions
and Windows 98 updates label in both locations: boot sector and root
directory. Also in case when label is changed in Windows 98 via
"My Computer" GUI.

>From above tests it can be seen that both MS-DOS and all Windows
versions ignores label which is stored in boot sector and show to user
only label from root directory.

Also it can be seen that both MS-DOS versions do not have problems when
label contains lower case letters.

> Attached images in compressed form has only 600 kB and I think they can
> be useful for testing either blkid or dosfstools project, so I'm sending
> them here.

-- 
Pali Rohár
pali.rohar@gmail.com