Date:   Sun, 11 Jun 2017 17:10:02 +0200
From:   Pali Rohár <pali.rohar@...il.com>
To:     Jan Kara <jack@...e.com>, Steve Kenton <skenton@...edu>,
        Vojtěch Vladyka <xvlady00@...d.feec.vutbr.cz>,
        Karel Zak <kzak@...hat.com>
Cc:     util-linux@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: UDF & dstring

Hi!

I read UDF specification again I found another cryptic part:

=====
2.1.3 Dstrings

The ECMA 167 standard, as well as this document, has normally defined 
byte positions relative to 0. In section 7.2.12 of ECMA 167, dstrings 
are defined in terms of being relative to 1. Since this offers an 
opportunity for confusion, the following shows what the definition would 
be if described relative to 0.

7.2.12 Fixed-length character fields

A dstring of length n is a field of n bytes where d-characters (1/7.2) 
are recorded. The number of bytes used to record the characters shall be 
recorded as a Uint8 (1/7.1.1) in byte n-1, where n is the length of the 
field. The characters shall be recorded starting with the first byte of 
the field, and any remaining byte positions after the characters up 
until byte n-2 inclusive shall be set to #00.

If the number of d-characters to be encoded is zero, the length of the 
dstring shall be zero.

NOTE: The length of a dstring includes the compression code byte (2.1.1) 
except for the case of a zero length string. A zero length string shall 
be recorded by setting the entire dstring field to all zeros.
=====

Next in previous section 2.1.1 Character Sets is Compression Algorithm 
table where IDs 0-7 are reserved.

I'm not sure how to correctly interpret those sections.

Does it mean that every dstring should consist of following buffer?

L - length of encoded characters
N - size of dstring buffer

buffer:
      1   byte: 0x08 (for Latin1) or 0x10 (for UCS-2BE)
  2 - L+2 byte: encoded characters (data either in Latin1 or UCS-2BE)
L+2 - N-2 byte: 0x00
      N-1 byte: number L+1

And in special case when L = 0, then first and last byte is also zero?

Because currently we have different implementation in kernel udf driver, 
util-linux blkid library and in mkudffs from udftools.

None of those implementation accept fully empty buffer as valid dstring.

mkudffs stores at last byte length of encoded characters + 1 (for 
compression id) as written above. On the other hand blkid from util-
linux things that last byte is part of encoded characters and Linux 
kernel driver does not set last byte to some value.

So... how should be understood that UDF specification? Should last byte 
be set to length encoded characters + 1 or not? And should be fully 
empty buffer (also with compression id set to 0x00 which is reserved) 
treated as valid string (empty one)?

And... we should unify implementation of blkid, kernel udf driver and 
mkudffs.

-- 
Pali Rohár
pali.rohar@...il.com

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