Java Runtime (De)compiler

Java Runtime (De)compiler, or JRD for short, allows you to extract bytecode from the running JVM and decompile it with an external decompiler and compile back to running vm.

• talk from devconf 2022: https://www.youtube.com/watch?v=mXcLkaAxNvM&list=PLU1vS0speL2bf9Zfa0pHQLPBhEKbbbsua&index=55
  • quite a good overview
• Main usecases
  • inspect and modify incompelte proprietary blobs
  • watch how instrumetnation affets your code
  • runtime hotpatching
  • verifying what is actually running in your vm
  • for testig, submit missbehaving or reach unreachable code

Configuring decompiler agent

In order to start using Java-Runtime-Decompiler, you will need to select the Decompiler Agent's path in Configure → Agent Path. The Decompiler Agent is a built-in project and can usually be found at ./decompiler_agent/target/decompiler-agent-*.jar. The image should have agent preset.

Configuring external decompilers

Internal javap and javap -v decompiling tools are available by default. In image, we try to keep as many decompilers as possible bundled.

Additionally, external decompilers are supported and can be configured in Configure → Plugins:

• You can download them using the links below and set them up yourself using the New button.
• You can use mvn clean install -PdownloadPlugins from a terminal in the project's directory and import the necessary files using the Import button.

Currently supported decompilers are:

• Fernflower
• Procyon
• CFR
• JD

Assemblers/Disassemblers

• jasm (jdk 17 and up, with lambdas fixed)
• jcoder (jdk 17 and up)
• jasm7 (jdk 11 and down, with known, broken lambdas)
• jcoder7 (jdk 11 and down)
• javap (disassemble only)
• Procyon disassembler(disassemble only)

Assemblers/Disassemblers with additional info

• jasmG (jdk 17 and up, with lambdas fixed, with -g switch)
• jcoderG (jdk 17 and up, with -g switch)
• jasmG7 (jdk 11 and down, with known, broken lambdas, with -g switch)
• jcoderG7 (jdk 11 and down, with -g switch)
• javap-v (disassemble only, javap with -v switch)

From Fedora repositories

The Java-Runtime-Decompiler is packed together with Fernflower and Procyon decompilers in the main Fedora repositories and in EPEL 7 and EPEL 8 and EPEL 9.

dnf install java-runtime-decompiler

The resulting installation is fully preconfigured.

Usage

Local Processes

The list on the top left shows all currently running processes on the local JVM with the name of their main class and their process ID. Selecting any item will display the respective Loaded Classes and Decompiled Bytecode screens. To return to the Welcome screen, simply unselect the item by holding Control while clicking it.

Remote Processes

The list on the bottom left shows any remote processes. Connections to remote processes can be established either by using the + button to the right of the heading, or by going to Connect → New Connection.

Local FS (file system)

Here you can turn JRD to casual, user-friendly jar/folder/file decompiler. You create a FS VM by setting up classpath as you are used for JVM.

Loaded Classes

The middle list contains the classes present in the selected process. This list can be reloaded with the Refresh button at the top and searched through with the Search field.

Decompiled Bytecode

The text area on the right shows decompiled bytecode of the selected class of the selected process. Different results may be achieved with different decompilers; you can select the decompiler from the dropdown menu at the top right.

Overwriting classes

Using the Overwrite button at the top, you can replace the currently selected class' bytecode with your own compiled .class file via a dialog.

CLI

Commandline interface is powerful and allows for bulk processing of VM's, jars and much more...\

Enter ./start.sh --help in a Linux terminal or start.bat --help in a Windows CMD to get started. Decompile:

$ ./start.sh -decompile classpathless-compiler-api-2.1.jar Cfr '.*' -saveas /tmp/cplc -savelike dir
Processing io.github.mkoncek.classpathless.api.ClassIdentifier
Saved: /tmp/cplc/io/github/mkoncek/classpathless/api/ClassIdentifier.java
...
Saved: /tmp/cplc/io/github/mkoncek/classpathless/api/MessagesListener.java

Compile:

$ ./start.sh -compile -cp classpathless-compiler-api-2.1.jar /tmp/cplc/io/github/mkoncek/classpathless/api/ClassIdentifier.java -savelike fqn -saveas .
...
Saved: ./io.github.mkoncek.classpathless.api.ClassIdentifier.class

Upload:

$ ./start.sh -overwrite classpathless-compiler-api-2.1.jar io.github.mkoncek.classpathless.api.ClassIdentifier io.github.mkoncek.classpathless.api.ClassIdentifier.class
WARNING: Class package do not match directories.
Overwrite of class 'io.github.mkoncek.classpathless.api.ClassIdentifier' successful.

Disassemble:

$ ./start.sh  -decompile classpathless-compiler-api-2.1.jar jasm '.*' -saveas /tmp/cplc -savelike dir
Saved: /tmp/cplc/io/github/mkoncek/classpathless/api/ClassIdentifier.java
...
Saved: /tmp/cplc/io/github/mkoncek/classpathless/api/IdentifiedSource.java
Saved: /tmp/cplc/io/github/mkoncek/classpathless/api/MessagesListener.java

Assemble:

$ ./start.sh -compile -p jasm /tmp/cplc -r -saveas /tmp/bin
...
jasm compiler caled with input of: 8
entering into /tmp/jrd-jasm14590159245433727064tmp.dir
writing tmp file into /tmp/jrd-jasm14590159245433727064tmp.dir/src/io.github.mkoncek.classpathless.api.IdentifiedBytecode.java
...
Saved: /tmp/bin/io/github/mkoncek/classpathless/api/ClassIdentifier.class
Saved: /tmp/bin/io/github/mkoncek/classpathless/api/MessagesListener.class

Don't forget that all operations are same over classpath, remote vm, or process of VM - runtime compiler/decompiler!

For developers

Relase

• Should be done in -Plegacy to enable injecting to jdk8 vms.
  • note, that newest asmtools, are jdk16 and up

Installation

• JRD 4 and lower: Note that Git, Maven & JDK 8 with its tools.jar or JDK11 and higher are required to run or help develop this app.*
• JRD 5 and up: Note that Git, Maven & JDK11 and higher are required to run or help develop this app.*

From GIT

Initial setup

$ git clone https://github.com/pmikova/java-runtime-decompiler.git
$ cd java-runtime-decompiler
$ mvn clean install  # builds the runtime decpompiler and agent, downloads also plugins
$ mvn clean install -PdownloadPlugins # builds the decompiler and downloads the decompiler plugins for future use, if not already downloaded
$ mvn clean install -Pimages # on Linux, bundles the plugins and JRD into a standalone portable image. Make some basic verifications
$ mvn clean install -Plegacy # take care to build agent in oldest resonable way

# $PLUGINS and $VERIFY_CP variables may help to solve some weird image building issues.

usually the development command is:

$ mvn clean install -Dcheckstyle.skip -Dspotbugs.skip=true 

Note, that -Dcheckstyle.skip -Dspotbugs.skip=true is not cowardly avoiding the valid issues those two finds, but to format sources. It alo downloads all the plugins. usually the release command is:

$ mvn clean install -DskipTests -Pimages -Plegacy

if the development command was never run, then plugins profile must be included

$ mvn clean install -DskipTests -Pimages -Plegacy -PdownloadPlugins

Note, that tests requires valid DISPLAY, to have some top level components (like HexEditor) pass

Then, in images/target/runtime-decompiler... ./start.sh in a Linux terminal or start.bat in a Windows CMD to start the application. The usage of top level start.sh/bat is only for development purposes.

Known issues

• mvn clean install results in BUILD FAILURE with the error java.lang.RuntimeException: Unexpected error: java.security.InvalidAlgorithmParameterException: the trustAnchors parameter must be non-empty (spotted on Windows)

  Temporary solution: Use mvn clean install -Dmaven.wagon.http.ssl.insecure=true -Dmaven.wagon.http.ssl.allowall=true -Dmaven.wagon.http.ssl.ignore.validity.dates=true to disregard SSL certificates when building.

• mvn clean install results in TEST FAILURE

  Temporary solution: Use mvn clean install -DskipTests to not run test classes when building.