Skip to content

fix: properly instrument nested records #1009

New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Draft
wants to merge 1 commit into
base: main
Choose a base branch
from
+59 −1
Draft

fix: properly instrument nested records #1009

Show file tree
Hide file tree
Changes from all commits
Commits
Show all changes
1 commit
Select commit
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
2 changes: 1 addition & 1 deletion src/main/java/com/code_intelligence/jazzer/instrumentor/TraceDataFlowInstrumentor.kt
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -52,7 +52,7 @@ internal class TraceDataFlowInstrumentor(
}
}

val writer = ClassWriter(ClassWriter.COMPUTE_MAXS)
val writer = ClassWriter(reader, ClassWriter.COMPUTE_MAXS)
Copy link
Contributor

@fmeum fmeum Nov 5, 2025

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

How did you figure out that this was causing the bug? 😅

Copy link
Contributor Author

@oetr oetr Nov 5, 2025

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

I am still unsure if this is the 1) right solution, or 2) if we should do this only for records; or 3) if it's a bug in asm.

My debugging approach was to get as far as possible with manual work and then ask AI (claude free vs) before I read any docs myself! Now that the segfault issue seems to be solved, I am actually reading the docs and the source code for ClassWriter to figure out which point above is true (1-3).

After initial analysis, I could minimize the reproducer to the code below. Removing the @NotNull annotation, or moving the record to the top level, or using a local annotation, or some other java annotation (e.g. we tried @Deprecated) works fine. It must be a Jazzer annotation:

package reproducer;

import com.code_intelligence.jazzer.mutation.annotation.NotNull;
import com.code_intelligence.jazzer.junit.FuzzTest;

public class ReproduceCrash {
  record NestedRecordWithExternalAnnotation(@NotNull String fileName) {}

  @FuzzTest
  public void fuzz_test (boolean ignored) {
    var components = NestedRecordWithExternalAnnotation.class.getRecordComponents();
  }
}

Here is the stack trace after a segfault that happens in fuzzing mode only, and not in regression mode:

V  [libjvm.so+0x6b09c0]  ConstantPool::symbol_at(int) const+0x5c  (constantPool.hpp:442)
V  [libjvm.so+0xd0ba3f]  java_lang_reflect_RecordComponent::create(InstanceKlass*, RecordComponent*, JavaThread*)+0x245  (javaClasses.cpp:3398)
V  [libjvm.so+0xe22b58]  JVM_GetRecordComponents+0x2d4  (jvm.cpp:1828)

I narrowed down the cause to TraceDataFlowInstrumentor.instrument() by tracing the class with the nested record. Then I saw that simply reading and then writing back the bytecode already triggers the segfault (I removed the for-loop for methods):

val node = ClassNode()
val reader = ClassReader(bytecode)
reader.accept(node, 0)
random = DeterministicRandom("trace", node.name)
//for (method in node.methods) {
//    if (shouldInstrument(method)) {
//        addDataFlowInstrumentation(method)
//    }
//}
val writer = ClassWriter(ClassWriter.COMPUTE_MAXS)
node.accept(writer)
return writer.toByteArray()

At this point I asked AI, and its first suggestion was to add COMPUTE_FRAMES reader/writer options, which didn't solve the segfault.
The second try was to add reader to the writer, and here we are!

Copy link
Contributor

@fmeum fmeum Nov 5, 2025

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

If you save the transformed class file to a file and load it in a separate JVM process, does that also trigger the crash? This looks like a JVM bug first and foremost, but reporting one would probably require a reproducer that doesn't use as many unsafe tricks as Jazzer in fuzzing mode. 😉

node.accept(writer)
return writer.toByteArray()
}
Expand Down
31 changes: 31 additions & 0 deletions tests/BUILD.bazel
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -1077,6 +1077,37 @@ java_fuzz_target_test(
],
)

java_fuzz_target_test(
name = "NestedRecordFuzzer",
timeout = "short",
srcs = [
"src/test/java/com/example/NestedRecordFuzzer.java",
],
# This regression test checks if we instrument fuzz test classes with nested records correctly,
# and can start a fuzzing run at all without getting segfaults.
fuzzer_args = [
"-print_final_stats=1",
"-runs=1",
],
javacopts = [
"--release",
"17",
],
tags = [
"no-jdk8",
],
target_class = "com.example.NestedRecordFuzzer",
runtime_deps = [
"@maven//:org_junit_jupiter_junit_jupiter_engine",
],
deps = [
"//deploy:jazzer-junit",
"//deploy:jazzer-project",
"@maven//:org_junit_jupiter_junit_jupiter_api",
"@maven//:org_junit_jupiter_junit_jupiter_params",
],
)

java_fuzz_target_test(
name = "BigDecimalFuzzer",
srcs = [
Expand Down
27 changes: 27 additions & 0 deletions tests/src/test/java/com/example/NestedRecordFuzzer.java
View file
Open in desktop
Original file line number Diff line number Diff line change
@@ -0,0 +1,27 @@
/*
* Copyright 2024 Code Intelligence GmbH
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/

package com.example;

import com.code_intelligence.jazzer.junit.FuzzTest;
import com.code_intelligence.jazzer.mutation.annotation.NotNull;

public class NestedRecordFuzzer {
record Address(@NotNull String street, String city, String zipCode) {}

@FuzzTest
public void test(Address ignored) {}
}