Disallowing the dynamic loading of agents by default (revised)

Stephen Felts stephen.felts at oracle.com
Fri Apr 7 12:53:25 UTC 2017

I have a problem with the second point.

Oracle's application server has "FAST SWAP" functionality similar to JRebel built-in so that a developer can speed up the "edit -> build -> deploy -> test" cycle.
It uses an internal agent that attaches to the server so customers don't need to see it, configure it, or even know the name of it.
It only works on application classes in an exploded ear or war file.
This proposal will make it so the JRebel agent will work but our own agent won't.

Ironically, this feature only works in development mode and developers are the ones most likely to run without any scripts having the CLASSPATH set up and just run java weblogic.Server.
So as I've said many times I don't want any JDK9 command line options.

In trying to protect against the guilty, you are killing the innocent.

It seems to me that it's late in Phase 2 of ramp-down of JDK 9 and this feature is not baked or well received.  IMO no changes should be done in this area for JDK 9.

-----Original Message-----
From: jigsaw-dev [mailto:jigsaw-dev-bounces at openjdk.java.net] On Behalf Of mark.reinhold at oracle.com
Sent: Wednesday, April 5, 2017 12:15 PM
To: jigsaw-dev at openjdk.java.net
Subject: Disallowing the dynamic loading of agents by default (revised)

Thanks to everyone for the quick feedback on this topic, and especially to Andrew for the constructive dialogue.

Here's a revised proposal:

  - Define a new VM option, `-XX:+EnableDynamicAgentLoading`, that's
    on by default in JDK 9 but off by default in JDK 10.

    This will allow launch scripts that use this option on JDK 10 to
    work on JDK 9 without change, and will allow early testing of the
    JDK 10 behavior on JDK 9.

  - Revise the `com.sun.tools.attach` API to forbid attachment to the
    current process or to an ancestor of the current process, and
    define a read-only system property that allows such attachment to
    be enabled via the command line.

    This will discourage the inadvertent use of libraries that, for
    better or for worse, intentionally violate strong encapsulation.

  - Enhance the `-jar` launcher option so that if the JAR file being
    launched contains a `Premain-Class` attribute then it's launched
    as both an application and as an agent for that application.

    This will allow `java -jar foo.jar` to be used in place of the
    more verbose `java -javaagent:foo.jar -jar foo.jar` [1].

Taken together, these changes are intended to enable the continued use of legitimate dynamically-loaded agents without change on JDK 9 and with a small change on JDK 10.  That later change will align the treatment of such agents with the other means of breaking encapsulation (`--add-opens`,
etc.) in order to ensure integrity by default for all code.

This proposal does not attempt to lock down platform classes as distinct from user classes.  Many agents have legitimate reasons to transform platform classes, so an additional mechanism to protect those classes does not appear to be worthwhile.


- Mark

[1] http://mail.openjdk.java.net/pipermail/jigsaw-dev/2017-April/012000.html

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