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SubjectRe: Linux & FAT32 label
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

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

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

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