Re: Linux & FAT32 label

From: "Pali Rohár" <pali.rohar@gmail.com>
To: Andreas Bombe <aeb@debian.org>,
	Andy Shevchenko <andy.shevchenko@gmail.com>
Cc: util-linux@vger.kernel.org, linux-kernel@vger.kernel.org,
	"Andrius Štikonas" <andrius@stikonas.eu>,
	"Curtis Gedak" <gedakc@gmail.com>, "Pavel Machek" <pavel@ucw.cz>,
	"Karel Zak" <kzak@redhat.com>
Subject: Re: Linux & FAT32 label
Date: Thu, 30 Nov 2017 00:21:26 +0100	[thread overview]
Message-ID: <20171129232126.3hymurdf7uedl6c2@pali> (raw)
In-Reply-To: <20171120111256.zqvmpzdcf6ncyi5m@ws.net.home>

On Monday 20 November 2017 12:12:56 Karel Zak wrote:
> On Sun, Nov 19, 2017 at 01:44:40PM +0100, Pali Rohár wrote:
> > On Thursday 09 November 2017 22:21:31 Pali Rohár wrote:
> > > So from all tests and discussion I would propose new unification:
> > > 
> > > 1. Read label only from the root directory. If label in root directory
> > >    is missing then disk would be treated as without label. Label from
> > >    boot sector would not be read.
> > > 
> > >    --> 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.
> > > 
> > >        This logic is used by all tested MS-DOS and Windows versions,
> > >        plus also by mtools on Linux.
> > > 
> > > 2. Write label to to both location, boot sector and root directory.
> > > 
> > >    --> Reason: MS-DOS 6.22, MS-DOS 7.10, Windows 98 and also mtools on
> > >        Linux do this. This is also what is written in FAT specification.
> > > 
> > >        It also provides backward compatibility with old dosfslabel
> > >        versions which read label only from boot sector.
> > > 
> > > 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 sector.
> > > 
> > >        Also in FAT specification is written that empty label is stored
> > >        as 'NO NAME    '.
> > > 
> > > With this change we would get compatibility with MS-DOS, Windows (both
> > > DOS-based and NT-based) and also with Linux mtools, modulo problems DOS
> > > code page.
> > > 
> > > There are just two negatives:
> > > 
> > > 1) Labels set by old dosfslabel versions (which stored them only to boot
> > >    sector) would not be visible. But they are already not visible on
> > >    MS-DOS or Windows machines, and also via mlabel (from mtools).
> > > 
> > > 2) Behavior of blkid and fatlabel would be changed as both tools have
> > >    different as proposed above, and based on tests they also differ each
> > >    from other.
> > > 
> > > Andreas, Karel, what do you think about it?
> > 
> > Also for other people, do any have comments on my proposed solution?
> 
> Go ahead and send patch :-) (also with LABEL_FATBOOT=)

Now I implemented changes for dosfstools project, pull request is there:
https://github.com/dosfstools/dosfstools/pull/73

Just waiting for the Andreas response...

Andy, you wanted some manpage update. I did it in this commit:
https://github.com/dosfstools/dosfstools/pull/73/commits/3f4f122b7ec8eeb4a0ae0db8e94b8829f51d1163
Can you check if changes in manpage are OK?

-- 
Pali Rohár
pali.rohar@gmail.com