On 4/16/18 2:13 PM, David Holmes wrote:
Hi Mandy,

How do you handle dependencies across a set of auxiliary types, i.e if you are defining a new class A, and it depends on B (which you also intend to define), how will the classloader resolve references from A to B when it can't load B itself?

It'll be the java agent's responsibility to arrange that.  The classloader does not find any class for the dynamic generated classes.  For example, if A is a subclass or subinterface of B (direct or indirect), B has to be defined before A or ensure that B is injected in the class path and visible to the class loader. This API is specific for instrumentation similar to the compiler that sometimes needs to define anonymous or bridge classes and they are typically quite straight forward.   The existing java agent has been using sun.misc.Unsafe::defineClass and so this defineClass form should be adequate.

I mention a possible future enhancement to allow the auxiliary classes to be defined together with the transformed classes and allowing them to go through the transformation pipeline.   It could look into relaxing this constraint.

Is the key capability here the ability to inject a class into a package that would otherwise not be open to it?

The public API defining a class to a given runtime package is via protected ClassLoader::defineClass method or Lookup::defineClass but these methods don't satisfy the java agent use cases to aid an instrumentation.  Rafael summarized it quite well in [1] and a brief note: ClassLoader::defineClass can be used from a custom class loader but it won't be able to define a class in the same runtime package as the instrumented class.  Lookup::defineClass requires a Class object as the lookup class while Class object is not available when doing transformation at load time.

What are the security implications here?

The java agent has to be enabled explicitly.  To use this API, the java agent must obtain the Instrumentation object and add the ClassFileTransformer.   The java agent can be started by -javaagent option at startup or be put on the classpath such that the java agent can be dynamically loaded via the attach mechansim.



On 15/04/2018 4:23 PM, mandy chung wrote:

Java agents support both load time and dynamic instrumentation. At load time, the agent's ClassFileTransformer is invoked to transform class bytes.  There is no Class objects at this time.  Dynamic instrumentation is when redefineClasses
or retransformClasses is used to redefine an existing loaded class.  The
ClassFileTransformer is invoked with class bytes where the Class object is present.

Java agent doing instrumentation needs a means to define auxiliary classes that are visible and accessible to the instrumented class. Existing agents have been using sun.misc.Unsafe::defineClass to define aux classes directly or accessing protected ClassLoader::defineClass method with setAccessible to suppress the language access check (see [1] where this issue was brought up).

Instrumentation::appendToBootstrapClassLoaderSearch and appendToSystemClassLoaderSearch
APIs are existing means to supply additional classes.  It's too limited
for example it can't inject a class in the same runtime package as the class
being transformed.


This proposes to add a new ClassFileTransformer.transform method taking additional ClassDefiner parameter.  A transformer can define additional
classes during the transformation process, i.e.
when ClassFileTransformer::transform is invoked. Some details:

1. ClassDefiner::defineClass defines a class in the same runtime package
    as the class being transformed.
2. The class is defined in the same thread as the transformers are being
    invoked.   ClassDefiner::defineClass returns Class object directly
    before the transformed class is defined.
3. No transformation is applied to classes defined by ClassDefiner::defineClass.

The first prototype we did is to collect the auxiliary classes and define them  until all transformers are invoked and have these aux classes to go
through the transformation pipeline.  Several complicated issues would
need to be resolved for example timing whether the auxiliary classes should be defined before the transformed class (otherwise a potential race where
some other thread references the transformed class and cause the code to
execute that in turn reference the auxiliary classes.  The current
implementation has a native reentrancy check that ensure one class is being
transformed to avoid potential circularity issues.  This may need JVM TI
support to be reliable.

This proposal would allow java agents to migrate from internal API and ClassDefiner to be enhanced in the future.


[1] http://mail.openjdk.java.net/pipermail/jdk-dev/2018-January/000405.html

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