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Re: [RFC v2] Unicode/UTF-8 support for XFS

To: Dave Chinner <david@xxxxxxxxxxxxx>
Subject: Re: [RFC v2] Unicode/UTF-8 support for XFS
From: Olaf Weber <olaf@xxxxxxx>
Date: Fri, 26 Sep 2014 16:50:39 +0200
Cc: Ben Myers <bpm@xxxxxxx>, <linux-fsdevel@xxxxxxxxxxxxxxx>, <tinguely@xxxxxxx>, <xfs@xxxxxxxxxxx>
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <20140924231024.GA4758@dastard>
Organization: SGI
References: <20140918195650.GI19952@xxxxxxx> <20140922222611.GZ4322@dastard> <5422C540.1060007@xxxxxxx> <20140924231024.GA4758@dastard>
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On 25-09-14 01:10, Dave Chinner wrote:
On Wed, Sep 24, 2014 at 03:21:04PM +0200, Olaf Weber wrote:
On 23-09-14 00:26, Dave Chinner wrote:
On Thu, Sep 18, 2014 at 02:56:50PM -0500, Ben Myers wrote:

[...]

TODO: Store the unicode version number of the filesystem on disk in the
super block.

So, if the filesystem has to store the specific unicode version it
was created with so that we know what version to put in trie
lookups, again I'll ask: why are we loading the trie as a generic
kernel module and not as metadata in the filesystem that is demand
paged and cached?

This way the trie can be shared, and the code using it is not
entangled with the XFS code.

The trie parsing code can still be common - just the location and
contents of the data is determined by the end-user.

I'm not sure how common the parsing code can be if needs to be capable of retrieving data from a filesystem.

Note given your and Andi Kleen's feedback on the trie size I've switched to doing algorithmic decomposition for Hangul. This reduces the size of the trie to 89952 bytes.

In addition, if you store the trie in the filesystem, then the only part that needs storing is the version for that particular filesystem, e.g no compatibility info for different unicode versions would be required. This would reduce the trie size to about 50kB for case-sensitive filesystems, and about 55kB on case-folding filesystems.

[...]

[...] Why should we let filesystems say "we fully
understand and support utf8" and then allow them to accept and
propagate invalid utf8 sequences and leave everyone else to have to
clean up the mess?

Because the alternative amounts in my opinion to a demand that every
bit of userspace that may be involved in generating filenames
generate only clean UTF-8. I do not believe that this is a realistic
demand at this point in time.

It's a chicken and egg situation. I'd much prefer we enforce clean
utf8 from the start, because if we don't we'll never be able to do
that. And other filesystems (e.g. ZFS) allow you to do reject
anything that is not clean utf8....

As I understand it, this is optional in ZFS. I wonder what people's experiences are with this.

[...]

Yet normalised strings are only stable and hence comparable
if there are no unassigned code points in them.  What happens when
userspace is not using the same version of unicode as the
filesystem and is using newer code points in it's strings?
Normalisation fails, right?

For the newer code points, yes. This is not treated as a failure to
normalize the string as a whole, as there are clear guidelines in
unicode on how unassigned code points interact with normalization:
they have canonical combining class 0 and no decomposition.

And so effectively are not stable. Which is something we absolutely
have to avoid for information stored on disk. i.e. you're using the
normalised form to build the hash values in the lookup index in the
directory structure, and so having unstable normalisation forms is
just wrong. Hence we'd need to reject anything with unassigned code
points....

On a particular filesystem, the calculated normalization would be stable.

And as an extension of using normalisation for case-folded
comparisons, how do we make case folding work with blobs that can't
be normalised? It seems to me that this just leads to the nasty
situation where some filenames are case sensitive and some aren't
based on what the filesystem thinks is valid utf-8. The worst part
is that userspace has no idea that the filesystem is making such
distinctions and so behaviour is not at all predictable or expected.

Making case-folding work on a blob that cannot be normalized is (in
my opinion) akin to doing an ASCII-based casefold on a Shift-JIS
string: the result is neither pretty nor useful.

Yes, that's exactly my point.

But apparently we draw different conclusions from it.

This is another point in favour of rejecting invalid utf-8 strings
and for keeping the translation tables stable within the
filesystem...

Bear in mind that this means not just rejecting invalid UTF-8
strings, but also rejecting valid UTF-8 strings that encode
unassigned code points.

And that's precisely what I'm suggesting: If we can't normalise the
filename to a stable form then it cannot be used for hashing or case
folding. That means it needs to be rejected, not treated as an
opaque blob.

The moment we start parsing filenames they are no longer opaque
blobs and so all existing "filename are opaque blobs" handling rules
go out the window. They are now either valid so we can use them, or
they are invalid and need to be rejected to avoid unpredictable
and/or undesirable behaviour.

At this point I'd really like other people to weigh in on this and get a sense of how sentiment is spread on the question.

- Forbid non-UTF-8 filenames
- Allow non-UTF-8 filenames
- Make it a mount option
- Make it a mkfs option

[...]

The most contentious part is (should be) ignoring the codepoints with
the Default_Ignorable_Code_Point property. I've included the list
below. My argument, such as it is, is that these code points either
have no visible rendering, or in cases like the soft hyphen, are only
conditionally visible. The problem with these (as I see it) is that on
seeing a filename that might contain them you cannot tell whether they
are present. So I propose to ignore them for the purpose of comparing
filenames for equality.

Which introduces a non-standard "visibility criterial" for
determining what should be or shouldn't be part of the normalised
string for comparison. I don't see any real justification for
stepping outside the standard unicode normalisation here - just
because the user cannot see a character in a specific context does
not mean that it is not significant to the application that created
it.

I agree these characters may be significant to the application. I'm
just not convinced that they should be significant in a file name.

They are significant to the case folding result, right? And
therefore would be significant in a filename...

Case Folding doesn't affect the ignorables, so in that sense at least they're not significant to the case folding result, even if you do not ignore them.

[...]

Hence my comments about NLS integration. The NLS subsystem already
has utf8 support with language dependent case folding tables.  All the
> current filesystems that deal with unicode (including case folding)
> use the NLS subsystem for conversions.

Looking at the NLS subsystem I see support for translating a number of different encodings ("code pages") to unicode and back.

There is support for uppercase/lowercase translation for a number of those encodings. Which is not the same as language dependent case folding.

As for a unicode case fold, I see no support at all. In nls_utf8.c the uppercase/lowercase mappings are set to the identity maps.

I see no support for unicode normalization forms either.

Hmmm - looking at all the NLS code that does different utf format
conversions first: what happens if an application is using UTF16 or
UTF32 for it's filename encoding rather than utf8?

Since UTF-16 and UTF-32 strings contain embedded 0 bytes, those encodings cannot be used to pass a filename across the kernel/userspace interface.


* XFS-specific design notes.
...
If the borgbit (the bit enabling legacy ASCII-based CI in XFS) is set
in the superblock, then case folding is added into the mix. This is
the nfkdicf normalization form mentioned above. It allows for the
creation of case-insensitive filesystems with UTF-8 support.

Please don't overload existing superblock feature bits with multiple
meanings. ASCII-CI is a stand-alone feature and is not in any way
compatible with Unicode: Unicode-CI is a superset of Unicode
support. So it really needs two new feature bits for Unicode and
Unicode-CI, not just one for unicode.

It seemed an obvious extension of the meaning of that bit.

Feature bits refer to a specific on disk format feature. If that bit
is set, then that feature is present. In this case, it means the
filesystem is using ascii-ci. If that bit is passed out to
userspace via the geometry ioctl, then *existing applications*
expect it to mean ascii-ci behaviour from the filesystem. If an
existing utility reads the flag field from disk (e.g. repair,
metadump, db, etc) they all expect it to mean ascii-ci, and will do
stuff based on that specific meaning. We cannot redefine the meaning
of a feature bit after the fact - we have lots of feature bits so
there's no need to overload an existing one for this.

Good point.

Hmmm - another interesting question just popped into my head about
metadump: file name obfuscation.  What does unicode and utf8 mean
for the hash collision calculation algorithm?

Good question.

Olaf

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