Wcast-qual was useuful to discover those errors in extra(): the returned ByteBuffer is fully writable, but the backing memory isn't really intended to be written to (and you never should modify the descriptors in memory).

So fixed by getting a read-only ByteBuffer and passing that back to Java instead.

src/main/c/src/ConfigDescriptor.c

@@ -162,8 +162,7 @@ JNIEXPORT jobject JNICALL METHOD_NAME(ConfigDescriptor, extra)
     struct libusb_config_descriptor *config = unwrapConfigDescriptor(env, this);
     if (!config) return NULL;
 
-    return (*env)->NewDirectByteBuffer(env, (void *) config->extra,
-        config->extra_length);
+    return NewDirectReadOnlyByteBuffer(env, config->extra, config->extra_length);
 }
 
 /**

src/main/c/src/EndpointDescriptor.c

@@ -156,8 +156,7 @@ JNIEXPORT jobject JNICALL METHOD_NAME(EndpointDescriptor, extra)
     struct libusb_endpoint_descriptor *ep = unwrapEndpointDescriptor(env, this);
     if (!ep) return NULL;
 
-    return (*env)->NewDirectByteBuffer(env, (void *) ep->extra,
-        ep->extra_length);
+    return NewDirectReadOnlyByteBuffer(env, ep->extra, ep->extra_length);
 }
 
 /**

src/main/c/src/InterfaceDescriptor.c

@@ -198,7 +198,7 @@ JNIEXPORT jobject JNICALL METHOD_NAME(InterfaceDescriptor, extra)
         unwrapInterfaceDescriptor(env, this);
     if (!interface) return NULL;
 
-    return (*env)->NewDirectByteBuffer(env, (void *) interface->extra,
+    return NewDirectReadOnlyByteBuffer(env, interface->extra,
         interface->extra_length);
 }
 

src/main/c/src/usb4java.c

@@ -19,7 +19,23 @@ jint illegalState(JNIEnv *env, const char *message)
     return (*env)->ThrowNew(env, cls, message);
 }
 
-jint JNI_OnLoad(JavaVM *vm, void *reserved) {
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wcast-qual"
+jobject NewDirectReadOnlyByteBuffer(JNIEnv *env, const void *mem,
+    int mem_length)
+{
+    jobject buffer = (*env)->NewDirectByteBuffer(env, (void *) mem, mem_length);
+
+    // Get a read-only buffer from this buffer.
+    jclass cls = (*env)->GetObjectClass(env, buffer);
+    jmethodID method = (*env)->GetMethodID(env, cls, "asReadOnlyBuffer",
+        "()Ljava/nio/ByteBuffer;");
+    return (*env)->CallObjectMethod(env, buffer, method);
+}
+#pragma GCC diagnostic pop
+
+jint JNI_OnLoad(JavaVM *vm, void *reserved)
+{
     jvm = vm;
     return JNI_VERSION_1_4;
 }

src/main/c/src/usb4java.h

@@ -88,5 +88,7 @@ extern JavaVM *jvm;
 
 jint illegalArgument(JNIEnv *env, const char *message);
 jint illegalState(JNIEnv *env, const char *message);
+jobject NewDirectReadOnlyByteBuffer(JNIEnv *env, const void *mem,
+    int mem_length);
 
 #endif