Re: [External] : Re: JDK-8275509: (jlink) SystemModulesPlugin generates a jdk.internal.module.SystemModules$all.class which isn't reproducible
Hello Ioi,
On 02/11/21 12:13 am, Ioi Lam wrote:
Hi Jaikiran,
Thanks for writing the test case to explore the problems in this area.
Please see my comments below:
...
Generally speaking, CDS has two levels of archiving:
[1] archiving class metadata -- classes in the
$JAVA_HOME/lib/classlist are considered to be frequently loaded
classes. They are parsed from classfiles and stored into the CDS
archive. At run time, instead of parsing the classes from classfiles,
the VM directly use the pre-parsed version of these classes (as
InstanceKlass* in C++).
At runtime, all such pre-parsed classes are initially in the "loaded"
state. This means their static constructors will be executed when
these classes are referenced for the first time. So as far as Java
semantic is concerned, there's no difference between a pre-parsed
class vs a class loaded from classfile.
E.g, the examples of loggers in static initializers will be executed
at runtime.
[2] archiving heap objects
As shown in your test, we cannot arbitrarily archive the static fields
that were initialized during -Xshare:dump, because they may have
environment dependency.
The strategy used by CDS is to archive only a few static fields in a
small number of carefully hand-picked system classes. You can see the
list in
https://github.com/openjdk/jdk/blob/977154400be786c500f36ba14188bff79db57075/src/hotspot/share/cds/heapShared.cpp#L97
Thank you for that link. That helped. So essentially even though the
list of classes used for archiving class metadata isn't very tightly
controlled, the list of objects which are archived in the heap is much
more selective.
The reason why my PoC ended up reproducing this issue is because it just
so happened that I selected a class (ModuleDescriptor) which
(indirectly) is hand-picked in that list of classes that can end up in
the archived heap.
These static fields are stored into the CDS archive. At run time,
these fields are essentially copied into the Java heap, and then
picked up by code like this:
https://github.com/openjdk/jdk/blob/977154400be786c500f36ba14188bff79db57075/src/java.base/share/classes/jdk/internal/module/ModuleBootstrap.java#L163
public static ModuleLayer boot() {
Counters.start();
ModuleLayer bootLayer;
ArchivedBootLayer archivedBootLayer = ArchivedBootLayer.get();
if (archivedBootLayer != null) {
assert canUseArchivedBootLayer();
bootLayer = archivedBootLayer.bootLayer();
BootLoader.getUnnamedModule(); // trigger of
BootLoader.
CDS.defineArchivedModules(ClassLoaders.platformClassLoader(),
ClassLoaders.appClassLoader());
// assume boot layer has at least one module providing a
service
// that is mapped to the application class loader.
JLA.bindToLoader(bootLayer, ClassLoaders.appClassLoader());
} else {
bootLayer = boot2();
}
In the case of the module graph, we remove things that depend on the
environment (such as CLASSPATH)
https://github.com/openjdk/jdk/blob/977154400be786c500f36ba14188bff79db57075/src/hotspot/share/cds/heapShared.cpp#L190
The remaining parts of the archived module graph only depend on the
following system properties:
private static boolean canUseArchivedBootLayer() {
return getProperty("jdk.module.upgrade.path") == null &&
getProperty("jdk.module.path") == null &&
getProperty("jdk.module.patch.0") == null && //
--patch-module
getProperty("jdk.module.main") == null && //
--module
getProperty("jdk.module.addmods.0") == null && //
--add-modules
getProperty("jdk.module.limitmods") == null && //
--limit-modules
getProperty("jdk.module.addreads.0") == null && //
--add-reads
getProperty("jdk.module.addexports.0") == null && //
--add-exports
getProperty("jdk.module.addopens.0") == null; //
--add-opens
}
As a result, we will invalidate the archived module graph if these
properties differ between dump time and run time. The Java code above
only asserts that the check has already been done. The actual check is
done in here:
https://github.com/openjdk/jdk/blob/977154400be786c500f36ba14188bff79db57075/src/hotspot/share/runtime/arguments.cpp#L1339
Understood.
Am I misunderstanding the severity of this issue or is this serious
enough that -Xshare:off should be default (or heap archiving disabled
somehow by default till this is fixed) to prevent issues like these
which can at the minimal be hard to debug bugs and on the extreme end
perhaps leak things from the build server where the JDK was built? I
guess it all boils down to which exact classes get
replaced/mapped/copied over from the heap archive? Is there a
definitive list that can be derived in the current JDK?
I believe the currently implementation is still safe
Re: [External] : Re: JDK-8275509: (jlink) SystemModulesPlugin generates a jdk.internal.module.SystemModules$all.class which isn't reproducible
Hi Jaikiran,
Thanks for writing the test case to explore the problems in this area.
Please see my comments below:
On 10/29/21 5:55 AM, Jaikiran Pai wrote:
Hello Ioi (and others),
On 22/10/21 1:39 pm, Jaikiran Pai wrote:
Hello Ioi,
On 22/10/21 12:29 pm, Ioi Lam wrote:
On 10/21/21 9:09 PM, Jaikiran Pai wrote:
Hello Ioi,
This is my initial analysis of the problem.
>>> Can anyone think of similar problems that may happen
elsewhere?
The static constructors of enum classes are executed at both CDS
dump time and run time. E.g.,
public enum Modifier {
OPEN
}
The method essentially does this:
public static final Modifier OPEN = new Modifier("OPEN");
If a reference of Modifier.OPEN is stored inside the CDS archived
heap during dump time, it will be different than the value of
Modifier.OPEN that is re-created at runtime by the execution of
Modifier.
I have almost next to nothing knowledge about CDS internals. My
only understanding of it is based on some documentation that I have
read. One of them being this one
https://docs.oracle.com/en/java/javase/17/vm/class-data-sharing.html#GUID-7EAA3411-8CF0-4D19-BD05-DF5E1780AA91.
Based on that documentation (and other similar ones), it was my
understanding that CDS was meant to store/share class "metadata"
like it states in that doc:
"When the JVM starts, the shared archive is memory-mapped to allow
sharing of read-only JVM metadata for these classes among multiple
JVM processes."
But from what you explain in that enum example, it looks like it
also stores class instance data that is computed at build time on
the host where the JDK image was generated? Did I understand it
correctly? Is this only for enums or does it also store the static
initialization data of "class" types too? If it does store the
static init data of class types too, then wouldn't such data be
host/build time specific and as such the classes that need to be
enrolled into the default CDS archive of the JDK should be very
selective (by reviewing what they do in their static init)? Like I
said, I haven't looked into this in detail so perhaps it already is
selective in the JDK build?
Hi Jaikiran,
Thank you very much for the detailed response.
CDS also has the ability to archive Java heap object. Since
https://bugs.openjdk.java.net/browse/JDK-8244778 , we have archived
the entire module graph to improve start-up time. At run time, the
module graph (as well as other archived heap objects) are loaded
from the CDS archive and put into the Java heap (either through
memory mapping or copying).
That is interesting and something that I hadn't known.
You can see the related code in
jdk.internal.module.ModuleBootstrap::boot()
I just had a look at it and it's quite elaborate and it'll take a me
while to fully grasp it (if at all) given its understandable complexity.
When the module system has started up, the module graph will
reference a copy of the OPEN enum object that was created as part of
the archive. However, the Modifier. will also be executed at
VM start-up, causing a second copy of the OPEN enum object to be
stored into the static field Modified::OPEN.
Thank you for that detail. That helps me understand this a bit more
(and opens a few questions). To be clear - the VM startup code which
creates that other copy, ends up creating that copy because that
piece of initialization happens even before the module system has
fully started up and created those references from the archived
state? Otherwise, the classloaders I believe would be smart enough to
not run that static init again, had the module system with that graph
from the archived state been fully "up"?
So would this mean that this not just impacts enums but essentially
every class referenced within the module system (of just boot layer?)
that has state which is initialized during static init? To be more
precise, consider the very common example of loggers which are
typically static initialized and stored in a static (final) field:
private static final java.util.logger.Logger logger =
Logger.getLogger(SomeClass.class);
If the boot layer module graph has any classes which has state like
this, it would then mean that if such classes do get initialized very
early on during VM startup, then they too are impacted and the module
graph holding instances of such classes will end up using a different
instance for such fields as compared to the rest of the application
code?
In essence, such classes which get accessed early (before module
system with the archived graph is "up") during VM startup can end up
_virtually_ having their static initialization run twice (I
understand it won't be run twice, but that's the end result, isn't it)?
I was really curious why this was only applicable to enums and why
other static initialization (like the one I explained above) wasn't
impacted. So I decided to do a small PoC. To come up with an
