Date:   Wed, 6 Feb 2019 09:47:52 +0100
From:   Pali Rohár <pali.rohar@...il.com>
To:     Gabriel Krisman Bertazi <krisman@...labora.com>
Cc:     tytso@....edu, linux-fsdevel@...r.kernel.org,
        linux-ext4@...r.kernel.org, sfrench@...ba.org,
        darrick.wong@...cle.com, samba-technical@...ts.samba.org,
        jlayton@...nel.org, bfields@...ldses.org, paulus@...ba.org
Subject: Re: [PATCH RFC v5 00/11] Ext4 Encoding and Case-insensitive support

On Tuesday 05 February 2019 14:08:00 Gabriel Krisman Bertazi wrote:
> Pali Rohár <pali.rohar@...il.com> writes:
> 
> > On Monday 28 January 2019 16:32:12 Gabriel Krisman Bertazi wrote:
> >> The main change presented here is a proposal to migrate the
> >> normalization method from NFKD to NFD.  After our discussions, and
> >> reviewing other operating systems and languages aspects, I am more
> >> convinced that canonical decomposition is more viable solution than
> >> compatibility decomposition, because it doesn't ignore eliminate any
> >> semantic meaning, like the definitive case of superscript numbers.  NFD
> >> is also the documented method used by HFS+ and APFS, so there is
> >> precedent. Notice however, that as far as my research goes, APFS doesn't
> >> completely follows NFD, and in some cases, like <compat> flags, it
> >> actually does NFKD, but not in others (<fraction>), where it applies the
> >> canonical form.  We take a more consistent approach and always do plain NFD.
> >> 
> >> This RFC, therefore, aims to resume/start conversation with some
> >> stalkeholders that may have something to say regarding the normalization
> >> method used.  I added people from SMB, NFS and FS development who
> >> might be interested on this.
> >
> > Hello! I think that choice of NFD normalization is not right decision.
> > Some reasons:
> >
> > 1) NFD is not widely used. Even Apple does not use it (as you wrote
> >    Apple has own normalization form).
> 
> To be exact, Apple claims to use NFD in their specification [1] .

Interesting...

> What I
> observed is that they don't ignore some types of compatibility
> characters correctly as they should. For instance, the ff ligature is
> decomposed into f + f.

I'm sure that Apple does not do NFD, but their own invented normal form.
Some graphemes are decomposed, and some not.

> > 2) All filesystems which I known either do not use any normalization or
> >    use NFC.
> > 3) Lot of existing Linux application generate file names in NFC.
> >
> 
> Most do use NFC.  But this is an internal representation for ext4 and it
> is name preserving.

Ok. I was in impression that it does not preserve original names, just
like implementation in Apple's system, where char* passed to creat()
does not appear in readdir().

> We only use the normalization when comparing if names
> matches and to calculate dcache and dx hashes.  The unicode standard
> recomends the D forms for internal representation.

Ok, this should be less destructive and less visible to userspace.

> > 4) Linux GUI libraries like Qt and Gtk generate strings from key strokes
> >    in NFC. So if user type file name in Qt/Gtk box it would be in NFC.
> >
> > So why to use NFD in ext4 filesystem if Linux userspace ecosystem
> > already uses NFC?
> 
> NFC is costlier to calculate, usually requiring an intermediate NFD
> step.  Whether it is prohibitively expensive to do in the dcache path, I
> don't know, but since it is a critical path, any gain matters.
> 
> > NFD here just makes another layer of problems, unexpected things and
> > make it somehow different.
> 
> Is there any case where
>    NFC(x) == NFC(y) && NFD(x) != NFD(y)   , or
>    NFC(x) != NFC(y) && NFD(x) == NFD(y)

This is good question. And I think we should get definite answer for it
prior inclusion of normalization into kernel.

> I am having a hard time thinking of an example.  This is the main
> (only?) scenario where choosing C or D form for an internal
> representation would affect userspace.

For decision between normal format, probably yes.

> >
> > Why not rather choose NFS? It would be more compatible with Linux GUI
> > applications and also with Microsoft Windows systems, which uses NFC
> > too.
> >
> > Please, really consider to not use NFD. Most Linux applications really
> > do not do any normalization or do NFC. And usage of decomposition form
> > for application which do not implement full Unicode grapheme algorithms
> > just make for them another problems.
> 
> > Yes, there are still lot of legacy application which expect that one
> > code point = one visible symbol (therefore one Unicode grapheme). And
> > because GUI in most cases generates NFC strings, also existing file
> > names are in NFC, these application works in most cases without problem.
> > Force usage of NFD filenames just break them.
> 
> As I said, this shouldn't be a problem because what the application
> creates and retrieves is the exact name that was used before, we'd
> only use this format for internal metadata on the disk (hashes) and for
> in-kernel comparisons.

There is another problem for userspace applications:

Currently ext4 accepts as file name any sequence of bytes which do not
contain nul byte and '/'. So having Latin1 file name is perfectly
correct.

What would happen if userspace application want to create following two
file names? "\xDF" and "\F0"? First one is sharp S second one is eth (in
Latin1). But file names are invalid UTF-8 sequences. Is it disallowed to
create such file names? Or both file names are internally converted to
"U+FFFD" (replacement character) and because NFD(first U+FFFD) ==
NFD(second U+FFFD) only first file would be created?

And what happen in general with invalid UTF-8 sequences? Because there
are many different types of invalid UTF-8 sequences, like non-shortest
sequence for valid code point, valid sequence for invalid code points
(either surrogate pairs code points, or code points above U+10FFFF,
...), incorrect byte which should start new code point, incorrect byte
when decoding of code point started, ...

Different (userspace) application handles these invalid UTF-8 sequences
differently, some of them accept some kind of "incorrectness" (e.g.
non-shortest form of code point representation), some not. Some
applications replace invalid parts of UTF-8 sequence by sequence of
UTF-8 replacement character, some not. Also it can be observed that some
applications use just one replacement characters and some other replace
invalid UTF-8 sequence by more replacement characters.

So trying to "recover" from invalid UTF-8 sequence to valid one is done
in more ways... And usage of any existing way could cause problems...
E.g. not possible to create two files "\xDF\xF0" and "\xF0\xDF"...

> > (PS: I think that only 2 programming languages implements Unicode
> > grapheme algorithms correctly: Elixir and Perl 6; which is not so
> > much)
> 
> [1] https://developer.apple.com/support/apple-file-system/Apple-File-System-Reference.pdf
> 

-- 
Pali Rohár
pali.rohar@...il.com

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