Disallowing the dynamic loading of agents by default

Stephen Felts stephen.felts at oracle.com
Mon Apr 3 17:23:30 UTC 2017

One of the key problems with the current proposal is the required use of the command line to enabled this feature.
Alternative proposals (besides suggesting moving this to JDK10, which I agree with) have suggested other ways to configure the option.
Since JDK9 has set the precedent of configuring options for a JAVA_HOME by using files under the conf directory, an alternative way to configure this would be to have a file under conf to do it (not tied to the security manager).
Developers could then configure it once for all of their subsequent work.  I know that I have functional QA tests that automatically set/unset conf options when running specific tests.

-----Original Message-----
From: Alan Bateman 
Sent: Monday, April 3, 2017 11:50 AM
To: Andrew Dinn
Cc: jigsaw-dev at openjdk.java.net
Subject: Re: Disallowing the dynamic loading of agents by default

On 03/04/2017 15:19, Andrew Dinn wrote:

> :
> Well, Byteman is definitely in the cross hairs by virtue of both 
> counts if those are the criteria (but I still don't see Byteman as a 
> problem that requires disruption :-)
> That said, I don't quite follow how your last statement relates to the 
> proposed change. It reads to me as if EnableDynamicAgentLoading=true 
> is intended to stop out of process loading of instrumentation agents 
> like Byteman as well as foil code that hoists an agent into the 
> current JVM process. Have I misread the proposed behaviour? Or have I read it right?
> (with the corollary, I assume, that here you are you merely setting 
> out what you would prefer to implement in contrast to what you can 
> actually achieve).
> Also, can you provide a reason why you are so agin agent-hoisting from 
> within a JVM? Is there a reason why it is such a cardinal sin?
As Mark said, this discussion got off to a bad start. We might have to explain the issue again.

Java SE 9 / JDK 9 brings strong encapsulation. The access control for the Java Language and VM has been extended to modules so that modules that don't want their internals to be accessed from code outside the module can do so. None of the core modules want their internals to be accessed so none of the core modules are open or open any packages. A consequence of this is that code on the class path or module path doesn't get to break in these modules. This is really nice but it exposes a lot of technical debt in existing code (as we've seen in mails here over the last 18 months).

If libraries and applications can't break in then what about tools? 
Tools are special, they get the capability to instrument almost every class. As you know, their capabilities have been expanded in Java SE 9 / JDK 9 as all the tool APIs have been updated to support modules. This means they can instrument code in modules as well as modify modules to export or open packages to other modules.

Now bring the attach API and late binding agents into the picture. This is where things blur and where the problem arises. A library can use the attach mechanism to load an agent into the current VM and break into any module. It's much easier in JDK 9 compared to previous releases because the jdk.attach module is resolved by default. All it takes is someone to post a solution on stackoverflow that spins a sneaky agent to leak the Instrumentation object to the library. It's just too easy to "migrate" 
existing reflection hacks.

The attach mechanism was of course never intended to be used this way. 
It was meant for troubleshooting tools and profilers/similar to load agents into running VMs. Back in the JDK 6 then we did consider disallowing attaching to the current VM but didn't enforce it - one reason is that it's not hard to just fork a VM with tools.jar on the class path and connect back to the parent.

So that is the context for the discussion. We need to find a good way to put the Genie back in its bottle. It may be that we have to disable attaching to the current or ancestor VMs. We may have to prohibit the instrumentation of core modules by late binding agents. We may have to do some disabling of agent loading. Maybe a combination. Suggestions and proposals are of course welcome.


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