linux-kernel - Re: vfat: Broken case-insensitive support for UTF-8

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Message-ID: <85wo9knxqp.fsf@collabora.com>
Date:   Tue, 21 Jan 2020 19:25:18 -0500
From:   Gabriel Krisman Bertazi <krisman@...labora.com>
To:     Pali Rohár <pali.rohar@...il.com>
Cc:     OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        "Theodore Y. Ts'o" <tytso@....edu>,
        Namjae Jeon <linkinjeon@...il.com>
Subject: Re: vfat: Broken case-insensitive support for UTF-8

Pali Rohár <pali.rohar@...il.com> writes:

> On Monday 20 January 2020 21:07:12 OGAWA Hirofumi wrote:
>> Pali Rohár <pali.rohar@...il.com> writes:
>> 
>> >> To be perfect, the table would have to emulate what Windows use. It can
>> >> be unicode standard, or something other.
>> >
>> > Windows FAT32 implementation (fastfat.sys) is opensource. So it should
>> > be possible to inspect code and figure out how it is working.
>> >
>> > I will try to look at it.
>> 
>> I don't think the conversion library is not in fs driver though,
>> checking implement itself would be good.
>
> Ok, I did some research. It took me it longer as I thought as lot of
> stuff is undocumented and hard to find all relevant information.
>
> So... fastfat.sys is using ntos function RtlUpcaseUnicodeString() which
> takes UTF-16 string and returns upper case UTF-16 string. There is no
> mapping table in fastfat.sys driver itself.
>
> RtlUpcaseUnicodeString() is a ntos kernel function and after my research
> it seems that this function is using only conversion table stored in
> file l_intl.nls (from c:\windows\system32).
>
> Project wine describe this file as "unicode casing tables" and seems
> that it can parse this file format. Even more it distributes its own
> version of this file which looks like to be generated from official
> Unicode UnicodeData.txt via Perl script make_unicode (part of wine).
>
> So question is... how much is MS changing l_intl.nls file in their
> released Windows versions?
>
> I would try to decode what is format of that file l_intl.nls and try to
> compare data in it from some Windows versions.
>
> Can we reuse upper case mapping table from that file?

Regarding fs/unicode, we have some infrastructure to parse UCD files,
handle unicode versioning, and store the data in a more compact
structure.  See the mkutf8data script.

Right now, we only store the mapping of the code-point to the NFD + full
casefold, but it would be possible to extend the parsing script to store
the un-normalized uppercase version in the data structure.  So, if
l_intl.nls is generated from UnicodeData.txt, you might consider to
extend fs/unicode to store it.  We store the code-points in an optimized
format to decode utf-8, but the infrastructure is half way there
already.

-- 
Gabriel Krisman Bertazi