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

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Message-ID: <20200119233348.es5m63kapdvyesal@pali>
Date:   Mon, 20 Jan 2020 00:33:48 +0100
From:   Pali Rohár <pali.rohar@...il.com>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        "Theodore Y. Ts'o" <tytso@....edu>,
        OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>,
        Namjae Jeon <linkinjeon@...il.com>,
        Gabriel Krisman Bertazi <krisman@...labora.com>
Subject: Re: vfat: Broken case-insensitive support for UTF-8

On Sunday 19 January 2020 23:08:09 Al Viro wrote:
> On Sun, Jan 19, 2020 at 11:14:55PM +0100, Pali Rohár wrote:
> 
> > So when UTF-8 on VFS for VFAT is enabled, then for VFS <--> VFAT
> > conversion are used utf16s_to_utf8s() and utf8s_to_utf16s() functions.
> > But in fat_name_match(), vfat_hashi() and vfat_cmpi() functions is used
> > NLS table (default iso8859-1) with nls_strnicmp() and nls_tolower().
> > 
> > Which means that fat_name_match(), vfat_hashi() and vfat_cmpi() are
> > broken for vfat in UTF-8 mode.
> > 
> > I was thinking how to fix it, and the only possible way is to write a
> > uni_tolower() function which takes one Unicode code point and returns
> > lowercase of input's Unicode code point. We cannot do any Unicode
> > normalization as VFAT specification does not say anything about it and
> > MS reference fastfat.sys implementation does not do it neither.
> 
> Then how can that possibly be broken?  If it matches the native behaviour,
> that's it.

VFAT is case insensitive.

> > As you can see lowercase 'd' and uppercase 'D' are same, but lowercase
> > 'č' and uppercase 'Č' are not same. This is because 'č' is two bytes
> > 0xc4 0x8d sequence and comparing is done by Latin1 table. 0xc4 is in
> > Latin 'Ä' which is already in uppercase. 0x8d is control char so is not
> > changed by tolower/toupper function.
> 
> Again, who the hell cares?

All users who use also non-Linux fat implementations.

> Does the behaviour match how Windows handles that thing?

Linux behavior does not match Windows behavior.

On Windows is FAT32 (fastfat.sys) case insensitive and file names "č"
and "Č" are treated as same file. Windows does not allow you to create
both files. It says that file already exists.

> "Case" is not something well-defined; the only definition
> is "whatever weird crap does the native implementation choose to do".

You are right that case sensitiveness is not well-defined, but in
Unicode we have also language-independent and basically well-defined
conversion.

And because VFAT is Unicode fs (internally UTF-16) it make sense that
well-defined Unicode folding should be used.

> That's the only reason to support that garbage at all...

What do you mean by garbage? Where? All filenames which I specified are
valid UTF-8 sequences, valid Unicode code points and therefore have
valid UTF-16 representation stored in VFAT fs.

Sorry, but I did not understand your comment.

-- 
Pali Rohár
pali.rohar@...il.com

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