[External] : Re: Disallowing the dynamic loading of agents by default

[Ron Pressler]

Hi.

The majority of serviceability tools don’t require dynamically loading an agent, and the majority of applications never load an agent dynamically.

True, there are some tools that will be affected, which is why the decision was to introduce the flag in JDK 9 and to announce this change, but change the default in a later version to give tools ample time to prepare their users. The rationale for this change then hasn’t changed, but will be reiterated in a JEP (we just wanted to announce this ahead of the JEP to give tool authors another reminder more than six months ahead of JDK 21). The only change between then and now is that even fewer use cases require dynamically loaded agents, and so the impact is even smaller.

It is also true that, when starting an application you don’t know that you *will* need to load an agent, but in most situations you know that you might. E.g. processes that are too critical to bring down even for deep maintenance (although not many of these are written in modern version of Java anyone) or canary services that are under trial. The relatively few sophisticated users who know how to write ad-hoc agents can even opt to enable dynamic agent loading on all their servers; these users are better equipped to can weigh the risks and tradeoffs involved.

Finally, some tools that require a dynamically loaded JVM TI agents, such as profilers that profile native code, are so tied to the VM's internals that the best place for them is in the JDK. If anything, the bigger problem is not that profilers are used too much in production, but too little, including less advanced ones that don’t require an agent. There is plenty of time to enhance the JDK’s built-in profiling capabilities ahead of demand.

— Ron

On 20 Mar 2023, at 01:21, Andrei Pangin <andrei.pangin at gmail.com<mailto:andrei.pangin at gmail.com>> wrote:

Hi all,

Serviceability has been one of the biggest Java strengths, but the proposed change is going to have a large negative impact on it.

Disallowing dynamic agents by default means it will no longer be possible to attach a profiler to a running app in runtime. JFR cannot close this gap due to lack of capabilities modern Java profilers have (that's a separate topic though).

When an issue happens with a live app, it's already too late to add a command line argument. Furthermore, it may not be even feasible to add an agent at startup in containerized applications. Starting profiler on demand from the host OS or from a sidecar is the only viable solution in these cases.

Next, it's hard to predict beforehand what tools exactly might be useful for troubleshooting: e.g., one tool may be better for finding memory leaks, a different one for analyzing CPU performance. Adding all possible tools at startup does not seem a reasonable approach, especially when tools may conflict with each other.

The most important aspect of dynamic agents is the possibility to make a special tool just in time for solving a particular problem. A typical example is to get a value of some field in a live app without dumping the entire 60 GB heap. Another common use case is hot patching for fixing trivial bugs or for adding debug logs dynamically. The prominent example is when the dynamic agent has proved irreplaceable aid in addressing the notorious log4j vulnerabilities CVE-2021-44228 and CVE-2021-45046.

I would be grateful to know more about the reasons why we should give up all the above advantages of dynamic agents in their good and legitimate use cases.

Thank you,
Andrei

чт, 16 мар. 2023 г. в 18:48, Ron Pressler <ron.pressler at oracle.com<mailto:ron.pressler at oracle.com>>:
Hi.

In JDK 21 we intend to disallow the dynamic loading of agents by default. This
will affect tools that use the Attach API to load an agent into a JVM some time
after the JVM has started [1]. There is no change to any of the mechanisms that
load an agent at JVM startup (-javaagent/-agentlib on the command line or the
Launcher-Agent-Class attribute in the main JAR's manifest).

This change in default behavior was proposed in 2017 as part of JEP 261 [2][3].
At that time the consensus was to switch to this default not in JDK 9 but in a
later release to give tool maintainers sufficient time to inform their users.
To allow the dynamic loading of agents, users will need to specify
-XX:+EnableDynamicAgentLoading on the command line.

I'll post a draft JEP for review shortly.

-- Ron

[1]: https://docs.oracle.com/en/java/javase/19/docs/api/jdk.attach/com/sun/tools/attach/package-summary.html
[2]: https://openjdk.org/jeps/261
[3]: https://mail.openjdk.org/pipermail/jigsaw-dev/2017-April/012040.html

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