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 <clinit> 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.<clinit>
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.<clinit> 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 illustration that
this impacts regular static initialization where enums aren't involved,
I decided to add a small piece of code in the
java.lang.module.ModuleDescriptor class. I chose this module system
class more for convenience than any specific reason. Here's what the
updated class looks like (the diff):
diff --git
a/src/java.base/share/classes/java/lang/module/ModuleDescriptor.java
b/src/java.base/share/classes/java/lang/module/ModuleDescriptor.java
index a412dd753cc..d08566afa42 100644
--- a/src/java.base/share/classes/java/lang/module/ModuleDescriptor.java
+++ b/src/java.base/share/classes/java/lang/module/ModuleDescriptor.java
@@ -1214,6 +1214,16 @@ public class ModuleDescriptor
private final Set<String> packages;
private final String mainClass;
+ private static class Hello {
+ private static final String ENV_KEY = "FOO";
+ private static final String envVal = System.getenv(ENV_KEY);
+ }
+
+ /**
+ * javadoc comment
+ */
+ public String fooVal = Hello.envVal;
+
private ModuleDescriptor(String name,
Version version,
String rawVersionString,
@@ -2302,6 +2312,8 @@ public class ModuleDescriptor
if (!provides.isEmpty()) {
sb.append(", provides: ").append(provides);
}
+ sb.append(", Hello.envVal: " + Hello.envVal + " fooVal " + fooVal
+ + " System.getenv() val: " +
System.getenv(Hello.ENV_KEY));
sb.append(" }");
return sb.toString();
}
What this update does is - it uses a class called "Hello" which has a
static initialization logic (the System.getenv("FOO") call). The value
of that static initialization is then used/set as a object instance
value to a new field called "fooVal". The update to the toString()
method is for easy debugging/demo and it prints the value of:
- The System.getenv("FOO") call when toString() is called
- The value set to the object instance field "fooVal"
- The value set to the Hello.envVal field
In a regular lifecycle of a JVM all these 3 values should be the same
value and even same object instance (identity equality).
With this change, I then built the JDK from source. While building the
JDK I set:
export FOO="build server of JDK"
and then
make images
Once the JDK was built, I then came up with a trivial Java program which
looks like this:
import java.lang.module.*;
import java.io.*;
import java.util.*;
public class CDSHeapArchiveIssue {
public static void main(final String[] args) throws Exception {
System.out.println("System.getenv() val is \"" +
System.getenv("FOO") + "\"");
String moduleName = "java.sql";
// load the "java.sql" module from boot layer
Optional<Module> bootModule =
ModuleLayer.boot().findModule(moduleName);
if (bootModule.isEmpty()) {
throw new RuntimeException(moduleName + " module is missing
in boot layer");
}
ModuleDescriptor m1 = bootModule.get().getDescriptor();
System.out.println("Env val in boot MD is \"" + m1.fooVal + "\"");
// now recreate the same module using the ModuleDescriptor.read
on the module's module-info.class
ModuleDescriptor m2;
try (InputStream moduleInfo =
bootModule.get().getResourceAsStream("module-info.class")) {
if (moduleInfo == null) {
throw new RuntimeException("Could not locate
module-info.class in " + moduleName + " module");
}
// this will internally use a ModuleDescriptor.Builder to
construct the ModuleDescriptor
m2 = ModuleDescriptor.read(moduleInfo);
}
System.out.println("Env val in built MD is \"" + m2.fooVal + "\"");
if (m1.fooVal.equals(m2.fooVal) &&
System.getenv("FOO").equals(m1.fooVal)) {
System.out.println("Success");
} else {
System.out.println("Equality check failure");
}
}
}
This code does the following:
- Prints the current System.getenv("FOO") value
- Loads the java.sql boot module using ModuleLayer.boot()... call
(java.sql module isn't special, any other boot module can be used too)
- Prints the ModuleDescriptor.fooVal instance value for that boot
module
- Then builds the same module using the ModuleDescriptor.Builder API.
- Prints the ModuleDescriptor.fooVal instance value of this module
- Expects all 3 values - the System.getenv("FOO"), the m1.fooVal
and m2.fooVal to be "equal()". If this expectation fails, it prints a
failure message, else it prints a success message.
I then run this compile this code and then run it against the JDK I just
built. But before running I set the FOO environment value to something
else like:
export FOO="runtime host"
and then:
java CDSHeapArchiveIssue
When I run this, I get the following output:
java CDSHeapArchiveIssue
System.getenv() val is "runtime host"
Env val in boot MD is "build server of JDK"
Env val in built MD is "runtime host"
Equality check failure
As you can see the value in the boot module descriptor instance is
incorrect and has leaked from the build server where the JDK was built.
Now if I switch off CDS and run the same code without any other changes
as follows:
java -Xshare:off CDSHeapArchiveIssue
System.getenv() val is "runtime host"
Env val in boot MD is "runtime host"
Env val in built MD is "runtime host"
Success
I get the expected result.
So clearly, this issue impacts not just enums but all kinds types of
static initialization. I am not truly familiar with which exact classes
will end up in the boot module layer, but whichever they are, they all
are potentially impacted with this issue, isn't it? I had a look at
which classes are selected in the CDS archive during JDK build as my
limited knowledge around this tells me that there's a
make/jdk/src/classes/build/tools/classlist/HelloClasslist.java[1] which
generates the CDS class list and from what I can say it can pull in any
"random" class (i.e. as far as I can see there isn't a specific static
initialization review done for the classes involved).
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?
Note that I used a call to System.getenv() to make it easier to
visualize and explain the issue. The use of System.getenv() doesn't play
any other role and any other similar static initialization too has the
same issue (I have another piece of code without System.getenv() which
too fails with the same issue).
[1]
https://github.com/openjdk/jdk/blob/master/make/jdk/src/classes/build/tools/classlist/HelloClasslist.java
-Jaikiran
