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

From: "Pali Rohár" <pali.rohar@gmail.com>
To: David Laight <David.Laight@ACULAB.COM>
Cc: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
	"Theodore Y. Ts'o" <tytso@mit.edu>,
	Namjae Jeon <linkinjeon@gmail.com>,
	Gabriel Krisman Bertazi <krisman@collabora.com>
Subject: Re: vfat: Broken case-insensitive support for UTF-8
Date: Mon, 20 Jan 2020 16:20:09 +0100	[thread overview]
Message-ID: <20200120152009.5vbemgmvhke4qupq@pali> (raw)
In-Reply-To: <89eba9906011446f8441090f496278d2@AcuMS.aculab.com>

On Monday 20 January 2020 15:07:20 David Laight wrote:
> From: Pali Rohár
> > Sent: 20 January 2020 11:05
> > On Monday 20 January 2020 13:04:42 OGAWA Hirofumi wrote:
> > > Pali Rohár <pali.rohar@gmail.com> writes:
> > >
> > > > Which means that fat_name_match(), vfat_hashi() and vfat_cmpi() are
> > > > broken for vfat in UTF-8 mode.
> > >
> > > Right. It is a known issue.
> > 
> > Could be this issue better documented? E.g. in mount(8) manpage where
> > are written mount options for vfat? I think that people should be aware
> > of this issue when they use "utf8=1" mount option.
> 
> What happens if the filesystem has filenames that invalid UTF8 sequences

Could you please describe what you mean by this question?

VFAT filesystem stores file names in UTF-16. Therefore you cannot have
UTF-8 on FS (and therefore also you cannot have invalid UTF-8).

Ehm... UTF-16 is not fully truth, MS FAT32 implementations allows half
of UTF-16 surrogate pair stored in FS.

Therefore practically, on VFAT you can store any uint16_t[] sequence as
filename, there is no invalid sequence (except those characters like
:<>?... which are invalid in MS-DOS).

If by "the filesystem has filenames" you do not mean filesystem file
names, but rather Linux VFS file names (e.g. you call creat() call with
invalid UTF-8 sequence) then function utf8s_to_utf16s() (called in
namei_vfat.c) fails and returns error. Which should be propagated to
open() / creat() call that it is not possible to create filename with
such UTF-8 sequence.

> or multiple filenames that decode from UTF8 to the same 'wchar' value.

This is not possible. There is 1:1 mapping between UTF-8 sequence and
Unicode code point. wchar_t in kernel represent either one Unicode code
point (limited up to U+FFFF in NLS framework functions) or 2bytes in
UTF-16 sequence (only in utf8s_to_utf16s() and utf16s_to_utf8s()
functions).

> Never mind ones that are just case-differences for the same filename.
> 
> UTF8 is just so broken it should never have been allowed to become
> a standard.

Well, UTF-16 is worse then UTF-8... incompatible with ASCII, variable
length and space consuming.

-- 
Pali Rohár
pali.rohar@gmail.com