Re: UDF & dstring

[Pali RohÃr]

On Wednesday 14 June 2017 11:46:14 Jan Kara wrote:
> Hi,
> 
> On Sun 11-06-17 17:10:02, Pali RohÃr wrote:
> > 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?
> 
> Yes, apparently that's what the spec says.
> 
> > 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.
> 
> As far as I'm looking, kernel handles this just fine. Note that
> 'dstring' is actually rather rare in UDF. E.g. filenames are
> recorded as d-characters which is something different. For
> converting dstrings (only used for getting volume and set
> identifiers) we use udf_dstrCS0toUTF8() which uses
> udf_name_from_CS0() and that handles input length of 0 just fine.
> 
> > 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.
> 
> Linux kernel UDF driver never writes any dstring.
> 
> > 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.
> 
> I think you understood the spec correctly. What I think we should do
> is to make udf-tools and blkid accept both variants but create the
> one defined in the spec (to have higher chances for
> interoperability).
> 
> 								Honza

mkudffs creates non-zero dstrings correctly since beginning. Zero 
dstrings have set compression ID (first byte) and length to 1 (last 
byte). This can be fixed, but I'm note sure if it is needed as 
LogicalVolumeId (and others too) according to specification should not 
be empty... But maybe it would make sense to allow user for some 
specific situation to create such disk image (if user knows what is 
doing).

Problem is in blkid parser which includes last byte into buffer for 
decoding. As blkid stops at null byte, problem is only when byte before 
length is non-null. E.g. when LogicalVolumeId (label) has exactly 30 
Latin1 characters (LogicalVolumeId is 32 byte dstring).

I created patch for blkid with test case there: 
https://github.com/karelzak/util-linux/pull/468

Similar patch would be needed also for grub2.

-- 
Pali RohÃr
pali.rohar@xxxxxxxxx

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