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

[Thomas Stüfe]

Thank you for the clarification.

Oddly enough, -XX:-EnableDynamicAgentLoading seems to be broken. Tried head
(fastdebug, release) and JDK17, even with this switch my sample library
loads just fine:

```
thomas at starfish$ ./images/jdk/bin/java -XX:-EnableDynamicAgentLoading
-XX:+PrintFlagsFinal  -cp $REPROS_JAR de.stuefe.repros.Simple


[Global flags]

...
     bool EnableDynamicAgentLoading                = false
                    {product} {command line}
...
<press key>
OnAttach! Loading JVMTI sample agent
```

Investigation shows that there seems to be a bug in attachListener.cpp
where we compare AttachOperation::name for "load", but it contains "jcmd":

```
Thread 22 "Attach Listener" hit Breakpoint 1, attach_listener_thread_entry
(thread=0x7fff94000fd0, __the_thread__=0x7fff94000fd0) at
/shared/projects/openjdk/jdk-jdk/source/src/hotspot/share/services/attachListener.cpp:404
404         } else if (!EnableDynamicAgentLoading && strcmp(op->name(),
"load") == 0) {
(gdb) p op
$1 = (AttachOperation *) 0x7fff7401b640
(gdb) p *op
$2 = {<CHeapObj<(MEMFLAGS)9>> = {<No data fields>}, _vptr.AttachOperation =
0x7ffff7b61210 <vtable for LinuxAttachOperation+16>, _name = "jcmd\000",
'\361' <repeats 11 times>, <incomplete sequence \361>, _arg = {
    "JVMTI.agent_load /shared/projects/jvmti-sample/sample.so\000", '\361'
<repeats 967 times>..., "\000", '\361' <repeats 1023 times>..., "\000",
'\361' <repeats 1023 times>...}}
(gdb) p op->name()
$3 = 0x7fff7401b648 "jcmd"
```

This was on Linux x64.

So if people have been using -XX:-EnableDynamicAgentLoading to check their
code, this may not have worked as intended.

Cheers, Thomas



On Fri, Mar 17, 2023 at 2:42 PM Ron Pressler <ron.pressler at oracle.com>
wrote:

>
>
> On 17 Mar 2023, at 13:33, Thomas Stüfe <thomas.stuefe at gmail.com> wrote:
>
> Hi Ron,
>
> Will this affect attaching via jcmd?
>
>
> The Attach mechanism will not be disabled by default, just the ability to
> load agents via the Attach mechanism.
> So the only jcmd command that will be affected is JVMTI.agent_load.
>
> To see the effect of the change today, launch java with
> -XX:-EnableDynamicAgentLoading, which is
> to become the new default.
>
> — Ron
>
>
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