From 266ab299cc7c780f62d2b8cd237200194c8ac465 Mon Sep 17 00:00:00 2001
From: Luca Longinotti <l@longi.li>
Date: Thu, 13 Jun 2013 13:43:27 +0200
Subject: [PATCH] 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    |  3 +--
 src/main/c/src/EndpointDescriptor.c  |  3 +--
 src/main/c/src/InterfaceDescriptor.c |  2 +-
 src/main/c/src/usb4java.c            | 18 +++++++++++++++++-
 src/main/c/src/usb4java.h            |  2 ++
 5 files changed, 22 insertions(+), 6 deletions(-)

diff --git a/src/main/c/src/ConfigDescriptor.c b/src/main/c/src/ConfigDescriptor.c
index c95059f..f26c13c 100644
--- a/src/main/c/src/ConfigDescriptor.c
+++ b/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);
 }
 
 /**
diff --git a/src/main/c/src/EndpointDescriptor.c b/src/main/c/src/EndpointDescriptor.c
index 7e40a56..9d5973a 100644
--- a/src/main/c/src/EndpointDescriptor.c
+++ b/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);
 }
 
 /**
diff --git a/src/main/c/src/InterfaceDescriptor.c b/src/main/c/src/InterfaceDescriptor.c
index d2834d5..b3b722e 100644
--- a/src/main/c/src/InterfaceDescriptor.c
+++ b/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);
 }
 
diff --git a/src/main/c/src/usb4java.c b/src/main/c/src/usb4java.c
index ebcd1cc..7a05959 100644
--- a/src/main/c/src/usb4java.c
+++ b/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;
 }
diff --git a/src/main/c/src/usb4java.h b/src/main/c/src/usb4java.h
index 0ca325f..cee6e39 100644
--- a/src/main/c/src/usb4java.h
+++ b/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