Replacement of sun.reflect.Reflection#getCallerClass

David M. Lloyd david.lloyd at
Tue Sep 3 17:41:14 UTC 2013

On 09/03/2013 12:16 PM, Peter Levart wrote:
> On 09/03/2013 04:39 PM, Nick Williams wrote:
>>> >Do you mean sun.reflect.CallerSensitive can go away?  This is very
>>> important part of the design that we need to detect which methods are
>>> caller-sensitive.   I keep seeing you suggest this and it is unclear
>>> to me if you only mean to remove java.lang. at CallerSensitive in your
>>> proposal.
>> Yes, that's what I mean. If you carefully examine the (existing)
>> native code that backs getCallerClass, you see that @CallerSensitive
>> is/only/  used as an enforcement mechanism. When I first read about
>> @CallerSensitive, I/thought/  you could take a call stack like this one:
>> @CallerSensitive getCallerClass()
>> @CallerSensitive someMethod1()
>> @CallerSensitive someMethod2()
>> @CallerSensitive someMethod3()
>> @CallerSensitive someMethod4()
>> actualCallerMethod()
>> And calling getCallerClass would return the class for
>> actualCallerMethod(). However, I was wrong. getCallerClass/always/
>> returns someMethod1(). @CallerSensitive is/not/  used to determine
>> when to stop looking for the caller. It's just an enforcement
>> mechanism to ensure that only built-in JVM classes can call
>> getCallerClass.
> *AND* that Reflection.getCallerClass() can only be called from within
> methods annotated with @CallerSensitive.
> Now for that part, the public API equivalent
> (StackTraceFrame.getCallerClass() or whatever it is called) need not be
> restricted to methods annotated with any annotation, but that means that
> this public API should not be used to implement security decisions since
> MethodHandles API allows caller to be spoofed unless looking-up a method
> annotated with @CallerSensitive...

Using an annotation for security decisions is pretty far off from the 
standard security model in any case.  Why not use a regular permission 
check?  If performance is a concern, the check need only be made one 
time when an instance is acquired that has these methods on it.

>>   This is/not/  how I did it, this is how it already was. Because of
>> this, you could delete the @CallerSensitive annotation completely and
>> getCallerClass still be fully functional the way it is. It just
>> wouldn't be restricted to annotated methods anymore.
> For security unrelated things (like logging and similar) public API need
> not include any enabling annotation, but it needs to be documented that
> it should not be used for security decisions.

s/need not/should not/, IMO

> ***
> Regarding ability to obtain j.l.Class instances for classes that client
> code would otherwise have no access to:
> What about a simple restriction on methods returning such instances that
> Class objects are only returned when they are resolvable from the
> ClassLoader of client code. If they are not resolvable, null is
> returned. For example, the equivalent of:
> public class StackTraceFrame {
>      private final Class<?> declaringClass;
>      @CallerSensitive
>      public Class<?> getDeclaringClass() {
>          try {
>              return Class.forName(declaringClass.getName(),
>                                   false,
> Reflection.getCallerClass().getClassLoader())
>                     == declaringClass ? declaringClass : null;
>              }
>          } catch (ClassNotFoundException ignore) {}
>          return null;
>      }
>      // the name can be exposed without fear...
>      public String getDeclaringClassName() {
>          return declaringClass.getName();
>      }
> This example could be implemented more efficiently then above code
> (using private Class/ClassLoader API).

I don't think this will hold up.  Why would (for example) a logging API 
have access to every class loader that might need to log something?  In 
any system of appreciable size, there is typically at least *some* class 
loader isolation, often a lot of it.  Utilities like logging or security 
code that need to do this kind of check do not typically have direct 
access to classes which are "higher on the stack", so to speak.


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