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From "David W. Van Couvering" <David.Vancouver...@Sun.COM>
Subject Re: context classloader versus default classloader
Date Wed, 23 Nov 2005 16:33:44 GMT
Yes, great background info.  I am a little concerned about the 
classloading work I am doing clashing with your security work.  I am 
thinking of changing getApplicationClass() to load org.apache.derby 
classes with the default classloader as you suggest -- is that going to 
step on your toes?


Daniel John Debrunner wrote:
> David W. Van Couvering wrote:
>>The concern I have is that DatabaseClasses.loadApplicationClass() uses
>>the thread context classloader, and if the class is not found there (or
>>there is no context classloader), only then does it use the default
>>Let's say I'm able to create a classloader whose search path only
>>includes specific derby jar files (to avoid the issues of mixed versions
>>of Derby).
>>Then let's say I try to make use of this when getting connections:
>>URLClassLoader cl = new MyClassLoader()
>>EmbeddedDataSource ds =
>>Connection conn = ds.getConnection(...);
> Of course the variable ds here would have to be declared as
> javax.sql.DataSource and not EmbeddedDataSource. :-)
>>This effectively sets the default classloader for EmbeddedDataSource
>>(and all its dependent classes) to be MyClassLoader.
>>What I am concerned about is that there is all this code in the Derby
>>engine that calls DatabaseClasses.loadApplicationClass().  It looks like
>>this is used to load classes for VTIs and Java stored procedures (it's
>>hard to tell) as well as a number of other things I'm not clear on.  If
>>any of these classes are Derby classes (for instance internal VTIs) then
>>it would seem that my attempt to correctly load Derby classes from the
>>right jar files will be thwarted.
> So the use of the class context class loader was added for J2EE
> applications. In those the application's classes for procedures,
> functions, vti's etc. can be stored in the WAR or EAR file, and those
> classes are available to the system through the context class loader.
> The context class loader is set up by the container manager, ie. the
> application server, WAS or Geronimo etc.
>>Probably the right thing for me to do is test this, but if you have any
>>observations about this that might guide me it would be much appreciated.
> One of the changes I made to the Cloudscape code before it was open
> sourced as Derby, was to separate the packages at a high level.
> Previously all the code, product and tests, had a common package prefix,
> COM.cloudscape or com.ibm.db2j. This meant there was no way to tell from
> a class name if the class was a Derby class or a test class (e.g.
> function). With Derby I separated the code so that tests were all under
> org.apache.derbyTesting and product under org.apache.derby (and some
> other splits.) This then allows (though it's not done yet) class loading
> code to be more precise in loading classes, since it's obvious that any
> org.apache.derby. class is  part of the system and not a test procedure etc.
> As an example, related to the security changes I'm working on, I'm
> thinking about making the class loaders for the application installed
> jars (sqlj.install_jar) refuse to load any class starting with the
> prefix 'org.apache.derby.'. This would then remove one potential hole
> where an application could try to subvert an engine class.
> There may be similar possible changes for loadApplicationClass(), don't
> load org.apache.derby. classes through the context class loader.
> It may be that ensuring the classes are loaded from the same jar/class
> loader is good for security and for the multi-version issue.
> In addition most of the packages in derby.jar are now sealed, which
> means that classes in a sealed package must be loaded from the same jar.
>  This is not sufficient because if a class is loaded from another jar an
> error is thrown rather than ignoring the load. Thus, without changes, an
> application can try to subvert and inject invalid versions of Derby
> classes, which will not be accepted by the vm, but could cause a denial
> of service.
> Hope this helps as background info.
> Dan.

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