Re: [xacml] Issue #87 CORE ERRATA: resource:xpath needs to be addedin B.6, plus fix needed for 4.2.2 example - updated

This email to follow up on last TC meeting's action item:

  Issue 87:

      Rich: Need xpath feedback from others - i.e. someone who

      "knows" what the xpath constructs are "supposed to be"

       Rich to provide specific proposal for changes. Options of

       required optional/ resource:xpath in attr designator. (will

       be based on deduction of intent of xpath in spec unless

       specific feedback provided)

Based on the above action item, and that afaik there has not been

anyone who has submitted more info on this issue, I will now

explain as best I can my interpretation of the intent of the current

spec and recommend the change necessary to match that intent.

(This will be followed by some supporting comments listing the

main specifics leading to these conclusions.)

I have reviewed the xacml 1.0, xacml 1.1, and xacml 2.0 specs

to set up a context for understanding what changed. My conclusion

is that resource:xpath was accidentally removed from xacml 2.0 and

that the only way to make sense of its use in the example policies

is to assume that when it is specified in a ResourceAttributeDesignator,

that xpath in the AttributeValue of the ResourceMatch must have as

a context node the ResourceContent element of the Request.

Therefore, my proposal is that the proposal in issue 87 for adding back

the resource:xpath identifier with the following text will
substantially 

correct the situation:

    ' This identifier indicates that the resource is specified by an
XPath 

        expression and that the default RequestContextPath for the 

        parent attribute is "//xacml-context:Request/

        xacml-context:Resource/xacml-context:ResourceContent".

    * urn:oasis:names:tc:xacml:1.0:resource:xpath '

The above is the proposal. Now here are some of the specific things

I found which led me to these conclusions:

  1. In xacml 1.1 section B.6 contains several attribute identifiers
including

       resource-id and xpath.

      In xacml 2.0 section B.6 all the above attribute identifiers have
been

       removed, except for resource-id, and a new xacml.2.0 identifier

       has been added: target-namespace.

  2. The description in the example Rule 1 of the ResourceMatch
containing

       the ResourceAttributeDesignator (lines 1166-1174 and lines
1244-1247

       clearly indicate that the resource:xpath identifier is alive and
well and

       required for this Policy to continue to function.

      Given the requirement for the Policy to function, it is clear
that the context

       node for the xpath string specified needs to be the
ResourceContent node

       and not the Resource node.

  3. Given the description of xacml:2.0:resource:target-namespace 

       (lines 5041-5046), it is clear that the place to look for the
namespace that

       is being described is the ResourceContent node.

  4. Given that lines 4907-4908 say the Request element is the context
node for

       every XPath expression, a means to override this is necessary if
we just 

       want to deal with the content under the ResourceContent, which
appears

       to be generally the case.

  5. The xacml:1.0:resource:target-namespace and
xacml:1.0:resource:xpath

       Attributes were removed from xacml 1.1 lines 978-992, and
incorporated

       thru other means in the xacml 2.0. In the former case thru the
xacml 2.0

       defn in section B.6 for xacml:2.0:resource:target-namespace, and
in the

       latter case by what is proposed above as the accidentally left
out defm

       of resource:xpath, which appears to synergistically, as defined,
work

       together with the target-namespace.

Bottom line: the xpath-node-match function is trying to determine if 
the node 

/md:record is under the node ResourceContent.

As indicated in the issue description, there are other ways to address
this

issue or possibly interpret the original intent. However, I believe the
above

proposal is the path of least resistance which is consistent with the
spec

as it currently stands with the examples given.

    Thanks,

    Rich

       

Rich Levinson wrote:
Note:
the previous version of this issue accidentally went to
xacml-demo-tech.

 

Note: this issue should be reviewed carefully to make sure we end up
with a

consistent picture of how xpath is described in the core spec. It
appears that

the changes in section B.6 Resource Attributes from V1.1 to V2.0 were

incomplete. The suggestion in the issue I believe will make the spec
self-consistent,

however as the note at the end of part 1 implies, there may have been
another

intention, which might be a cleaner way to handle it.

  

    Thanks,

    Rich

  

  

  87. CORE
ERRATA:
  resource:xpath needs to be added in B.6,
plus fix needed for 4.2.2
example
  
  
Part 1: Xacml core V2.0 apparently inadvertently
left out resource:xpath in section B.6. (This
identifier is used in a
number of places in the examples: lines 1172, 1337, 1496, 1658) The
following lines (inside the quotes), which are modified from the XACML
V1.1 spec (4555-4556) to take into account the ResourceContextPath, should be
inserted to the XACML V2.0 spec after line 5046: 
  

  
"This identifier indicates that the resource
is specified by an XPath expression and that the default RequestContextPath
for the parent attribute is
"//xacml-context:Request/xacml-context:Resource/xacml-context:ResourceContent".
  

  
    
urn:oasi:names:tc:xacml:1.0:resource:xpath" 

  
  
It has been deduced from analysis of section B.6
(lines 5041-5045) description of "resource:target-namespace",
plus the
fact that the "resource:xpath" identifier apparently was
inadvertently
left out of XACML 2.0, plus examination of the XPaths in the examples
that begin with "/md:record" require that the statement about the about
the <xacml-context:Request> element being the context node for
every XPath expression (Sec A.3.15 lines 4907-4908) be overridden in
order to set the ResourceContextPath to the <ResourceContent>
element. See analysis in this email: http://lists.oasis-open.org/archives/xacml-dev/200710/msg00005.html
  

  
In addition, to understand this situation please
refer to lines 1155-1176 of Rule 1: 
  

  
    

      
<Resources>  

      
        

          
<Resource>  

          
            

              
<ResourceMatch MatchId="urn:oasis:names:tc:xacml:1.0:function:string-equal">
              

              
                

                  
<AttributeValue DataType="http://www.w3.org/2001/XMLSchema#string">
                   

                  
                    

                      
http://www.med.example.com/schemas/record.xsd
                      

                    

                  
                  
</AttributeValue>
                  <ResourceAttributeDesignator
                   

                  
                    

                      
AttributeId="urn:oasis:names:tc:xacml:2.0:resource:target-namespace" DataType="http://www.w3.org/2001/XMLSchema#string"/>
                      

                    

                  
                

              
              
</ResourceMatch>
              <ResourceMatch
              MatchId="urn:oasis:names:tc:xacml:1.0:function:xpath-node-match">
              

              
                

                  
<AttributeValue DataType="http://www.w3.org/2001/XMLSchema#string">
                   

                  
                    
/md:record 

                  
                  
</AttributeValue>
                  <ResourceAttributeDesignator
                   

                  
                    

                      
AttributeId="urn:oasis:names:tc:xacml:1.0:resource:xpath" DataType="http://www.w3.org/2001/XMLSchema#string"/>
                      

                    

                  
                

              
              
</ResourceMatch>
              

            

          
          
</Resource> 

        

      
      
</Resources> 

    

  
  
Note: that the 2nd ResourceMatch is a
somewhat "odd"
construct in that it uses an AttributeDesignator
to specify
an XPath. Be that as it may, it is clear that the intent of the
  resource:xpath identifier is implicitly make
the <ResourceContent>
node the xpath context node, which is consistent with intention of the
  resource:target-namespace
identifier as well, which is used in the
first ResourceMatch.
  

  
Note: an alternative solution might be to not use
the resource:xpath identifier at all and simply
modify the description
of the resource:target-namespace to say
that it implies that the ResourceContent
element is the xpath context node. However, it might be too late for
that, although we could add both and mark the resource:xpath
as
deprecated. 
  

  
Part 2: In addition, on line 1064 there is a typo
4.2.2 Example request that should be fixed: 
  

  
    

      
"xmlns:(md=:Resource/ResourceContent/xpointer"
      

    

  
  
should be changed to read: 
  

  
    

      
"xmlns:(md=http:www.med.example.com/schemas/record.xsd)
xpointer" 

    

  
  
Note: it had been decided before to fix this: http://lists.oasis-open.org/archives/xacml/200702/msg00001.html
and it has been brought up again independently: http://lists.oasis-open.org/archives/xacml-dev/200710/msg00000.html,
  http://lists.oasis-open.org/archives/xacml-dev/200710/msg00002.html
with a preliminary response: http://lists.oasis-open.org/archives/xacml/200710/msg00009.html
  

  
Part 3: Finally, some clarifying text about
requirements for whether XPointer needs to be supported should be added
in section 3.3.1.1 "Rule Target" lines 657-658 where it is explicitly
referenced. It shows up in the example on line 1064. i.e. apparently
providing XPath capabilities does not implicitly include XPointer, and
we should say something about how to include it if desired. Note: in
the example it comes in as part of the request. What behavior would be
expected if the PDP did not support it? 
  

  
Status: OPEN 
  

  
Champions: Rich