From bb10aee9f6a158a203d4afcb4d61234660a5cfda Mon Sep 17 00:00:00 2001
From: Stuart McCulloch <stuart.mcculloch@datadoghq.com>
Date: Fri, 30 Oct 2026 12:04:12 +0000
Subject: [PATCH] Adjust ClassFile super-name check to be more robust

---
 .../instrument/classmatch/ClassFile.java      | 18 +++++----
 .../instrument/classmatch/ClassHeader.java    |  2 +-
 .../instrument/classmatch/ClassFileTest.java  | 40 +++++++++++++++++++
 3 files changed, 51 insertions(+), 9 deletions(-)

diff --git a/class-match/src/main/java/datadog/instrument/classmatch/ClassFile.java b/class-match/src/main/java/datadog/instrument/classmatch/ClassFile.java
index eb915258..717de20d 100644
--- a/class-match/src/main/java/datadog/instrument/classmatch/ClassFile.java
+++ b/class-match/src/main/java/datadog/instrument/classmatch/ClassFile.java
@@ -35,7 +35,6 @@ public final class ClassFile {
   private static final String[] NO_ANNOTATIONS = {};
 
   private static final int ACC_INTERFACE = 0x0200;
-  private static final int ACC_MODULE = 0x8000;
 
   private static final byte[] JAVA_LANG_OBJECT_BYTES = JAVA_LANG_OBJECT.getBytes(US_ASCII);
   private static final byte[] CONSTRUCTOR_BYTES = CONSTRUCTOR.getBytes(US_ASCII);
@@ -213,14 +212,17 @@ private static ClassHeader parse(byte[] bytecode, int offset, boolean onlyHeader
     String superName;
     if ((access & ACC_INTERFACE) != 0) {
       superName = JAVA_LANG_OBJECT;
-    } else if (access != ACC_MODULE) {
-      int superNameOffset = cp[cp[u2(bytecode, cursor)]];
-      superName =
-          utfEquals(bytecode, superNameOffset, JAVA_LANG_OBJECT_BYTES)
-              ? JAVA_LANG_OBJECT
-              : utf(bytecode, superNameOffset);
     } else {
-      superName = null;
+      // java.lang.Object and modules have a zero super_class index (and therefore offset)
+      int superNameOffset = cp[cp[u2(bytecode, cursor)]];
+      if (superNameOffset != 0) {
+        superName =
+            utfEquals(bytecode, superNameOffset, JAVA_LANG_OBJECT_BYTES)
+                ? JAVA_LANG_OBJECT
+                : utf(bytecode, superNameOffset);
+      } else {
+        superName = null;
+      }
     }
     cursor += 2;
 
diff --git a/class-match/src/main/java/datadog/instrument/classmatch/ClassHeader.java b/class-match/src/main/java/datadog/instrument/classmatch/ClassHeader.java
index 38213284..f259f40f 100644
--- a/class-match/src/main/java/datadog/instrument/classmatch/ClassHeader.java
+++ b/class-match/src/main/java/datadog/instrument/classmatch/ClassHeader.java
@@ -22,7 +22,7 @@ public class ClassHeader {
   /** Internal name of this class. */
   public final String className;
 
-  /** Internal name of the super-class declared by this class; {@code null} for modules. */
+  /** Internal name of the super-class; {@code null} for {@code java.lang.Object} and modules. */
   @Nullable public final String superName;
 
   /** Internal names of the interfaces declared by this class. */
diff --git a/class-match/src/test/java/datadog/instrument/classmatch/ClassFileTest.java b/class-match/src/test/java/datadog/instrument/classmatch/ClassFileTest.java
index d51c1153..25a4ec90 100644
--- a/class-match/src/test/java/datadog/instrument/classmatch/ClassFileTest.java
+++ b/class-match/src/test/java/datadog/instrument/classmatch/ClassFileTest.java
@@ -3,8 +3,11 @@
 import static java.util.Arrays.asList;
 import static org.junit.jupiter.api.Assertions.assertArrayEquals;
 import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNotNull;
+import static org.junit.jupiter.api.Assertions.assertNull;
 import static org.junit.jupiter.api.Assertions.fail;
 
+import datadog.instrument.utils.JVM;
 import java.io.ByteArrayOutputStream;
 import java.io.IOException;
 import java.io.InputStream;
@@ -16,6 +19,8 @@
 import java.util.function.Consumer;
 import java.util.jar.JarFile;
 import org.junit.jupiter.api.Test;
+import org.objectweb.asm.ClassReader;
+import org.objectweb.asm.Opcodes;
 
 class ClassFileTest {
 
@@ -48,6 +53,41 @@ void header() {
     testParsing("asm-test.jar", ClassFile::header);
   }
 
+  @Test
+  void objectParsing() throws IOException {
+    try (InputStream in = Object.class.getResourceAsStream("Object.class")) {
+      assertNotNull(in);
+      byte[] bytecode = JVM.readAllBytes(in);
+      for (ClassHeader header :
+          new ClassHeader[] {ClassFile.header(bytecode), ClassFile.outline(bytecode)}) {
+        assertEquals("java/lang/Object", header.className);
+        assertNull(header.superName);
+        assertArrayEquals(new String[0], header.interfaces);
+      }
+    }
+  }
+
+  @Test
+  void moduleParsing() throws IOException {
+    byte[] bytecode;
+    try (JarFile sample = new JarFile("build/sampleBytecode/asm-test.jar");
+        InputStream in = sample.getInputStream(sample.getEntry("jdk9/module-info.class"))) {
+      bytecode = JVM.readAllBytes(in);
+    }
+    // also check a module with extra access flags, such as ACC_SYNTHETIC
+    byte[] syntheticBytecode = bytecode.clone();
+    int accessOffset = new ClassReader(syntheticBytecode).header;
+    syntheticBytecode[accessOffset] |= (byte) (Opcodes.ACC_SYNTHETIC >> 8);
+    for (byte[] moduleBytecode : new byte[][] {bytecode, syntheticBytecode}) {
+      for (ClassHeader header :
+          new ClassHeader[] {ClassFile.header(moduleBytecode), ClassFile.outline(moduleBytecode)}) {
+        assertEquals("module-info", header.className);
+        assertNull(header.superName);
+        assertArrayEquals(new String[0], header.interfaces);
+      }
+    }
+  }
+
   @Test
   void unicodeHeader() {
     ClassHeader header = ClassFile.header(sampleUnicodeClass);