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

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Message-ID: <20200120224625.GE8904@ZenIV.linux.org.uk>
Date:   Mon, 20 Jan 2020 22:46:25 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
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>,
        Gabriel Krisman Bertazi <krisman@...labora.com>
Subject: Re: vfat: Broken case-insensitive support for UTF-8

On Mon, Jan 20, 2020 at 10:40:46PM +0100, Pali Rohár wrote:

> 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.

Er...  Surely it's OK to just tabulate that function on 65536 values
and see how could that be packed into something more compact?  Whatever
the license of that function might be, this should fall under
interoperability exceptions...

Actually, I wouldn't be surprised if f(x) - x would turn out to be constant
on large enough intervals to provide sufficiently compact representation...

What am I missing here?