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:
> 
>                                                  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'
> 
> 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
> 
> 
> 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.
> 

Based on results I would propose following unification:

1. If there is entry for label in root directory, including erased entry
   which starts with 0xE5, then do *not* process boot entry.

   --> Reason: Setting label to empty on Windows XP just marks label in
       root directory as erased (set first byte to 0xE5) and it does not
       touch boot sector when label can be always old. Test case is in
       file fat32_mkdosfs_label1_xp_erase.

       Same behavior can be seen in mlabel.

2. Process 'NO NAME    ' label in root directory as 'NO NAME' name. Not
   as empty label.

   --> Reason: 'NO NAME    ' is regular entry in root directory and both
       Windows XP and mlabel handle it in this way.

3. Process 'NO NAME    ' label in boot directory as empty label. Not as
   label with name 'NO NAME'.

   --> Reason: On Windows XP when formatting empty disk and label is not
       specified then 'NO NAME    ' is stored to boot directory.

4. Prefer label from the root directory. If there is none entry (means
   there is also no erased entry), then read label from root sector.

   --> Reason: Windows XP and mlabel ignores what is written in boot
       sector. Windows XP even do not update boot sector, so label
       stored in boot sector is incorrect after any change done by
       Windows XP.

       But due to compatibility with older dosfslabel, which stores
       label only to boot sector, there is need for some fallback. Due
       to point 1. the best seems to be to process also erased label in
       root directory (marked with leading 0xE5) and fallback to boot
       sector only in case label in root directory is missing.

What do you think about it?

-- 
Pali Rohár
pali.rohar@gmail.com