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Re: [XML Schema 1.1] Can I invent elements if I specify vc:maxVersion="3.2"?

From: C. M. Sperberg-McQueen <cmsmcq@blackmesatech.com>
Date: Thu, 30 Apr 2009 11:11:26 -0600
Cc: "C. M. Sperberg-McQueen" <cmsmcq@blackmesatech.com>, "'xmlschema-dev@w3.org'" <xmlschema-dev@w3.org>
Message-Id: <417510BC-3A14-437B-9A76-70B3F8DB4F59@blackmesatech.com>
To: "Costello, Roger L." <costello@mitre.org>
On 29 Apr 2009, at 11:20 , Costello, Roger L. wrote:

>
> It's my understanding that a 1.1 schema validator will strip out the  
> second Book:
>
> <?xml version="1.0"?>
> <xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
>           xmlns:vc="http://www.w3.org/2007/XMLSchema-versioning"
>           targetNamespace="http://www.books.org"
>           elementFormDefault="qualified">
>
>    <xs:element name="BookStore">
>        <xs:complexType>
>            <xs:sequence>
>                <xs:element name="Book" maxOccurs="unbounded"  
> vc:minVersion="1.1" vc:maxVersion="3.2">
>                    ...
>                </xs:element>
>                <xs:element name="Book" maxOccurs="unbounded"  
> vc:minVersion="3.2">
>                    ...
>                </xs:element>
>            </xs:sequence>
>        </xs:complexType>
>    </xs:element>
> </xs:schema>
>
>
> Correct?

I believe so.

>
> Since the 1.1 schema validator strips out the second Book, I figure  
> that I should be able to put any crazy elements I want in there. For  
> example, here I invent a new element that I call "new":
>
>
> <?xml version="1.0"?>
> <xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
>           xmlns:vc="http://www.w3.org/2007/XMLSchema-versioning"
>           targetNamespace="http://www.books.org"
>           elementFormDefault="qualified">
>    <xs:element name="BookStore">
>        <xs:complexType>
>            <xs:sequence>
>                <xs:element name="Book" maxOccurs="unbounded"  
> vc:minVersion="1.1" vc:maxVersion="3.2">
>                    ...
>                </xs:element>
>                <xs:element name="Book" maxOccurs="unbounded"  
> vc:minVersion="3.2">
>                    <xs:new>
>                           ...
>                    </xs:new>
>                </xs:element>
>            </xs:sequence>
>        </xs:complexType>
>    </xs:element>
> </xs:schema>

Right.  If XSD 3.2 and higher introduce a 'new' element, it will look
to a 1.1 processor pretty much like a random unknown element.  So the
mechanisms introduced to allow a 1.1 processor to tolerate schema  
documents
containing things introduced by later versions of the spec can also
be used to make it tolerate (at least for some sense of the word  
'tolerate'
-- it does strip them out) random unknown elements.

People concerned with preventing the use of covert information  
channels, or
with exploiting them, may see a potential covert channel here, in  
addition
to the existing channels for non-XSD information like appInfo.

> However, when I run this schema using a 1.1 validator (SAXON) I get  
> an error message saying <xs:new> is not allowed within <xs:element>.
>
>
> Why?
>
>
>
> It seems to me, SAXON should never have seen <xs:new>.
>
> Would you clarify this for me please?

As I read the documentation on the Saxon site, at

   http://www.saxonica.com/documentation/schema-processing/schema11.html

Saxon does not claim to be a conforming 1.1 processor, but only to
have implemented some 1.1 features, which can be activated using
the -xsdversion:1.1 flag.  Conditional inclusion and the vc attributes
aren't listed as 1.1 features supported in the current version of
Saxon.  I imagine they will be along eventually; I assume that
the scheduling of work on new features of Saxon is governed by a
subtle interaction among technical interest, technical difficulty,
expressed interest among paying customers, and personal whim, er,
judgement.

I hope this helps.


-- 
****************************************************************
* C. M. Sperberg-McQueen, Black Mesa Technologies LLC
* http://www.blackmesatech.com
* http://cmsmcq.com/mib
* http://balisage.net
****************************************************************
Received on Thursday, 30 April 2009 17:12:06 GMT

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