Disallowing the dynamic loading of agents by default

[Andrei Pangin]

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>:

> 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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