linux-kernel - Re: Linux & FAT32 label

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Message-ID: <20171105140745.ze4ttkazeczrqsy7@pali>
Date:   Sun, 5 Nov 2017 15:07:45 +0100
From:   Pali Rohár <pali.rohar@...il.com>
To:     Andy Shevchenko <andy.shevchenko@...il.com>
Cc:     Andreas Bombe <aeb@...ian.org>, Karel Zak <kzak@...hat.com>,
        util-linux@...r.kernel.org,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Andrius Štikonas <andrius@...konas.eu>,
        Curtis Gedak <gedakc@...il.com>, pavel@....cz
Subject: Re: Linux & FAT32 label

On Sunday 05 November 2017 15:56:53 Andy Shevchenko wrote:
> On Sun, Nov 5, 2017 at 3:39 PM, Pali Rohár <pali.rohar@...il.com> wrote:
> > On Tuesday 31 October 2017 10:35:48 Andy Shevchenko wrote:
> >> On Mon, Oct 16, 2017 at 4:12 AM, Andreas Bombe <aeb@...ian.org> wrote:
> >> > On Thu, Oct 12, 2017 at 10:49:31PM +0200, Pali Rohár wrote:
> >> >> On Thursday 12 October 2017 12:13:11 Karel Zak wrote:
> 
> >> > I was worried that there might be some scripts or programs that expect
> >>
> >> If we really care about such scripts another approach might be to
> >> introduce a CLI switch to "spec compatible mode" to each tool and
> >> suggest in documentation to use it.
> >>
> >> There are also variants:
> >> - spec compatible
> >> - WinXX compatible
> >> - DOS compatible
> >> - etc
> >
> > I did tests with MS-DOS and Windows versions (results in previous
> > email), and they seems to be compatible how they read label.
> >
> > Based on results I would suggest to ignore label from the boot sector
> > when reading label.
> 
> So, for tools which are not doing that to add
> 
>  --ignore-boot-sector-label (or alike) [recommended]
> 
> right?
> 
> We don't actually know how many users (scripts) are relying on current
> behaviour.
> If there are only few, we may introduce backward compatibility switch
> 
> --read-boot-sector-label

Current behavior of the last blkid and fatlabel tools is: Try to read
label from the root directory. If it does not exist, then fallback to
label stored in boot sector. And when fatlabel is changing label it
updates both locations.

So tools which already uses fatlabel for get & set operations should not
be affected as setting new label makes boot and root in sync.

New proposed behavior is: Try to read label from the root directory. If
not exist, then treat disk as without label.

As in current behavior there is no way via fatlabel to read "just only
label from boot sector", I think that problems for current scripts is
minimal.

What about making this new behavior in fatlabel as default with new
switch --fallback-to-boot-sector (switch to current behavior) or
--read-only-boot-sector (ignores label in root directory) for people who
are interested in label stored in boot sector?

And what to do with blkid? That cannot have any switch :-( and can have
only one behavior.

> > This makes behavior consistent with older MS-DOS
> > systems and also all Windows systems. This change would be a problem
> > only for users who have label stored only in boot sector. After change
> > they would not see label anymore -- exactly same what MS-DOS or Windows
> > show them. Seems that mkdosfs stores label to both location, since
> > support for label was introduced. So different label would be visible
> > only for users who used dosfslabel prior to version 3.0.16.
> >
> > What do you think?
> 
> So, in summary it looks like a documentation needs update (to mark
> your research).

Which documentation do you mean?

-- 
Pali Rohár
pali.rohar@...il.com