StackParam

StackParam is a utility that gives method parameters to Java 8 stack traces. It is written in Rust and built to be fairly unobtrusive.

It adds the parameter information to stack trace outputs and can be used to programmatically obtain method parameters (including "this" for non-static methods) up the stack.

Quick Start

StackParam is a shared library loaded as a JVMTI agent (see Installation to get it). For example, say the following Java class is at HelloFailure.java:

public class HelloFailure {
    public static void main(String[] args) {
        throwException(42);
    }

    public static void throwException(int foo) {
        throw new RuntimeException("Hello, Failure!");
    }
}

Compile with javac ./HelloFailure.java which will create the HelloFailure.class file. Now run via Java while specifying the shared library path:

java -agentpath:path/to/shared.ext HelloFailure --some-arg --another-arg 10

The file name shared.ext might be stackparam.dll on Windows, libstackparam.so on Linux, etc. The output is:

Exception in thread "main" java.lang.RuntimeException: Hello, Failure!
        at HelloFailure.throwException(HelloFailure.java:0) [arg0=42]
        at HelloFailure.main(HelloFailure.java:0) [arg0=[--some-arg, --another-arg, 10]]

Compiling with the -g option passed to javac gives debug information to StackParam so the parameter names are accurate. The output for the same code as above compiled with debug info is:

Exception in thread "main" java.lang.RuntimeException: Hello, Failure!
        at HelloFailure.throwException(HelloFailure.java:0) [foo=42]
        at HelloFailure.main(HelloFailure.java:0) [args=[--some-arg, --another-arg, 10]]

Note that StackParam works with all exception stack trace uses and even provides a mechanism to programmatically obtain method parameters up the call stack.

Installation

if you are using 64-bit Windows or Linux, the easiest way is to download the latest stackparam.dll or libstackparam.so from the releases area. For Mac or other architectures, I don't have a precompiled shared lib but it should be easy to build. See Manually Building.

Java Versions

This should work with OpenJDK/Oracle 8. It might also work on OpenJDK/Oracle 7 if manually compiled as the injection points are similar, but this is untested. This will not yet work with OpenJDK/Oracle <= 6 or OpenJDK/Oracle 9. It will also not work on other JREs whose runtimes are not based on the OpenJDK stdlib.

It simply didn't suit my needs to support other JVMs (yet), but it would be fairly trivial to implement if enough people want it.

I doubt Android would be as straightforward. This seems to imply there is no JVMTI interface. If that was in place, the Throwable class there would need the specifically targeted injections to support our needs which is no big deal.

Manually Building

This library is written in Rust and compiles a Java class at build time. Therefore the prerequisites are a recent installation of Rust and a JDK 8 installation with javac on the PATH.

Once the prerequisites are installed, the tests can be run via cargo:

cargo test

This does several things internally including running Gradle (automatically downloaded if not present) to build a Java class and running Gradle again to do some Java-side tests.

If the tests succeed, build can be done via cargo as well:

cargo build --release

Once built, the shared library is in target/release/shared.ext where shared.ext might be stackparam.dll on Windows, libstackparam.so on Linux, etc.

Usage

Once the agent is loaded, it automatically injects strings into stack traces.

Loading the Agent

While JVMTI has a few approaches to deploying an agent, this agent needs to be deployed via command line because it needs to hook into the very earliest part of the JVM load. This is easily done via the -agentpath path parameter of the java command, e.g.:

java -agentpath:path/to/shared.ext HelloWorld

Instead of -agentpath for the exact path, -agentlib could be used to just give the library name. Assuming the stackparam.dll is on the PATH in Windows or libstackparam.so is in the shared library location (e.g. an overridden LD_LIBRARY_PATH) in Linux, you can run:

java -agentlib:stackparam HelloWorld

Note, although untested, this library can likely be placed in the JRE's lib/amd64 folder to get the same effect.

Logging

This library uses Rust's env_logger which lets the logging be controlled by the RUST_LOG environment variable. The binary name is stackparam, so setting RUST_LOG to stackparam=info shows info logs, stackparam=debug shows debug logs, and just stackparam shows all logs. The logs are emitted to the standard error stream.

Programmatic Value Access

In addition to just showing parameters, the stackparam.StackParamNative class is automatically injected. This class provides the following loadStackParams method:

/**
 * Returns the stack params of the given thread for the given depth. It is
 * returned with closest depth first.
 *
 * Each returned sub array (representing a single depth) has params
 * including "this" as the first param for non-static methods. Each param
 * takes 3 values in the array: the string name, the string JVM type
 * signature, and the actual object. All primitives are boxed.
 *
 * In cases where the param cannot be obtained (i.e. non-"this" for native
 * methods), the string "<unknown>" becomes the value regardless of the
 * type's signature.
 *
 * @param thread The thread to get params for
 * @param maxDepth The maximum depth to go to
 * @return Array where each value represents params for a frame. Each param
 *         takes 3 spots in the sub-array for name, type, and value.
 * @throws NullPointerException If thread is null
 * @throws IllegalArgumentException If maxDepth is negative
 * @throws RuntimeException Any internal error we were not prepared for
 */
public static native Object[][] loadStackParams(Thread thread, int maxDepth);

Since this is automatically injected, it can easily be accessed via reflection. The problem with accessing via reflection is there are a few stack frames between the caller and the reflected method that invoke is called on which makes the params less predictably navigable.

Instead, you can either add the above contents in a stackparam.StackParamNative class in your source code or you can add a dependency to the native library JAR to your build file via JitPack. Even though it is included, the file/JAR will never be directly used because the actual class is injected early at VM startup.

Once in place, you can do neat things like grab the caller, e.g.:

public class CallerTest {
    /** Check that this is only called via instance method of foo.Bar */
    public boolean isFooBarInstanceMethod() {
        // We have to give a max depth of 3 and access the third one because on
        // the stack, "loadStackParams" is at 0, this method ("checkCaller") is
        // at 1, and then the caller is at 2.
        Object[] callerParams = stackparam.StackParamNative.loadStackParams(Thread.currentThread(), 3)[2];
        return callerParams.length != 0 &&
            "this".equals(callerParams[0]) &&
            callerParams[2] instanceof foo.Bar;
    }
}

While this kind of programming/validation in general is very bad and not portable, it demonstrates usage of the library. Also, it is very high performing.

There is also a public static String appendParamsToFrameString(String frameString, Object[] params) method on the class which takes the given set of params triplets and appends it (after a space) to the given frameString and returns it. It is mostly a helper for the library, but can be used by others.

Production Usage?

I wouldn't, but I took care to silently fail and fall back to original JVM functionality in most cases. There are a few possible burdensome performance issues:

• Stack walking at exception creation (CPU) - This is not extremely heavy and note that the JVM does its own native stack walking at this time to build the stack trace in the first place. Usually this is not an issue except for those using exceptions for control flow, and in those cases the developers should be overriding fillInStackTrace() to do nothing which will also make this library do nothing.
• Holding object references (memory) - For every exception stack trace created, a multi-dimensional array is created to hold references to all local params, their names, and their signatures. Usually once an exception is thrown, many of the local variables on the stack go out of scope and are eligible for garbage collection. With StackParam, the references (and the strings) live at least as long as the exception does. Besides just the multi-dimensional array on Throwable, a reference is also placed on StackTraceElement to the individual array item.
• According to this StackOverflow question, since we tell the JVM we want local variables, a couple of optimizations might be disabled. I have not independently verified this.

There are also a couple of security concerns:

• Low-level library - Care should always be taken when adding native code behind the JVM in production. There are no guarantees of the safety of the software. Granted, the danger surface area is not much higher than untrusted Java code.
• Sensitive data - StackParam does not know what is considered confidential data and what is not. Data/method filtering is not yet implemented. So your parameters to BCrypt.checkpw for example would be visible to all if an exception occurred inside it.

How Does it Work?

StackParam takes a series of steps to do what it does. In no particular order, they are:

• During Rust build time, compile stackparam.StackParamNative to a class file and include the bytes into the shared library.
• On agent start, ask the JVM to access local vars and class file load events. Also register callbacks for VM init and class file load hook.
• On VM init, take the stackparam.StackParamNative bytes and inject the class via DefineClass.
• Just before Throwable class load, transform the class bytes to:
• Add a private transient Object[][] stackParams field to the class.
• Add a private native Throwable stackParamFillInStackTrace(Thread) method to the class.
• Change the existing fillInStackTrace method to find the internal native fillStackTrace overload call. Then inject instructions to call our stackParamFillInStackTrace(Thread) method afterwards.
• Rename the existing getOurStackTrace method to $$stack_param$$getOurStackTrace.
• Add a private synchronized native StackTraceElement[] getOurStackTrace method to the class.
• Just before StackTraceElement class load, transform the class bytes to:
• Add a transient Object[] paramInfo field to the class.
• Rename the existing toString method to $$stack_param$$toString.
• Add a public native String toString method to the class.
• On native invoke of stackparam.StackParamNative.loadStackParams, walk up the stack grabbing params and return them.
• On the native invoke of Throwable.stackParamFillInStackTrace:
• Call getStackTraceDepth to fetch the depth of the current stack trace.
• Walk up the stack grabbing params for only the stack trace depth (plus a little for ourselves).
• Take the last depth amount of params, and store in Throwable.stackParams.
• On the native invoke of Throwable.getOurStackTrace:
• Record state of Throwable.stackTrace.
• Call Throwable.$$stack_param$$getOurStackTrace for the array of StackTraceElement values.
• Bail if the result isn't different than the recorded field value.
• Take the resulting array of StackTraceElement values and set each one's paramInfo field to its set of params from the Throwable.stackParams field.
• On the native invoke of StackTraceElement.toString:
• Call StackTraceElement.$$stack_param$$toString.
• Run the result through stackparam.StackParamNative.appendParamsToFrameString method along with the StackTraceElement.paramInfo value to return a string with parameters.

While it looks like a good bit of reverse engineering and a bit brittle, it's not that bad. Failures are handled gracefully for the most part. And it is not that difficult to add conditionals and do different bytecode manipulation based on what is seen (e.g. for different Java versions or different JVMs).

Why?

Primary goals: * Learn Rust (only kinda, I live in unsafe code which isn't cool) * Learn Rust + JNI/JVMTI * Learn intricacies of JVMTI and more about JVM internals * Provide nice example project for others wanting to hook into the JVM

Secondary goals: * Actually get method params on the stack trace

TODO

• Other JVM versions
• Especially Java 9, where we can just have params on StackFrame and the callers can choose how they walk it.
• Proper ignoring of certain OOM exceptions, see