#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/fs.h>
#include <linux/miscdevice.h>
#include <asm/uaccess.h>


#define MY_MACIG 'G'
#define READ_IOCTL _IOR(MY_MACIG, 0, int)
#define WRITE_IOCTL _IOW(MY_MACIG, 1, int)
 
static int used; 
static char msg[200];

static ssize_t device_read(struct file *filp, char __user *buffer, size_t length, loff_t *offset)
{
  return simple_read_from_buffer(buffer, length, offset, msg, 200);
}

static ssize_t device_write(struct file *filp, const char __user *buff, size_t len, loff_t *off)
{
  if (len > 199)
    return -EINVAL;
  copy_from_user(msg, buff, len);
  msg[len] = '\0';
  return len;
}

static int device_open(struct inode *inode, struct file *file)
{
  used++;
  return 0;
}

static int device_release(struct inode *inode, struct file *file)
{
  used--;
  return 0;
}

char buf[200];
int device_ioctl(struct file *filep, unsigned int cmd, unsigned long arg)
{
  int len = 200;
  switch(cmd) {
  case READ_IOCTL: 
    copy_to_user((char *)arg, buf, 200);
    break;
 
  case WRITE_IOCTL:
    copy_from_user(buf, (char *)arg, len);
    break;

  default:
    return -ENOTTY;
  }
  return len;
}

static struct file_operations fops = {
        .owner = THIS_MODULE,
        .read = device_read,
        .write = device_write,
        .open = device_open,
        .release = device_release,
        .unlocked_ioctl = device_ioctl
};

static struct miscdevice my_miscdev = {
        .name         = "my_device",
        .mode         = S_IRWXUGO,
        .fops         = &fops,
};

static int __init cdevexample_module_init(void)
{
  int ret = misc_register(&my_miscdev);
  if (ret < 0) {
    printk ("Registering the character device failed\n");
    return ret;
  }
  printk("create node with mknod /dev/my_device\n");
  return 0;
}

static void __exit cdevexample_module_exit(void)
{
  misc_deregister(&my_miscdev);
}  

module_init(cdevexample_module_init);
module_exit(cdevexample_module_exit);
MODULE_LICENSE("GPL");

• http://www.all-things-android.com/content/understanding-se-android-policy-files
• https://source.android.com/security/selinux/

0 0

• Herb Sutter @ NWCPP - This is an amazing talk.It shows how your program actually uses your machine's architecture;
• Gerson Robboy Lecture Note - Very good lecture notes with some examples and exercises;
• Great stackoverflow post;
• Table with interesting numbers about latency;
• Latency through a Java perspective;

1 0

1. A "standard" AES CBC implementation in C (the worst performance);
2. The Linux kernel Crypto lib with crypto extensions set to ON (good performance and very easy to use);
3. The AES CBC asm implementation from the OpenSSL lib (good performance and it took me extra time to integrate it into my code);

The Crypto lib

static int pc_aes_encrypt_kernel(const void *key, int key_len,
                                        void *dst, size_t *dst_len,
                                        const void *src, size_t src_len)
{
        struct crypto_blkcipher *blkcipher = NULL;
        char *cipher = "__cbc-aes-ce";
        char iv[AES_BLOCK_SIZE];
        char *charkey = (unsigned char *) key;

        unsigned int ivsize = 0;
        char *scratchpad_in = NULL, *scratchpad_out = NULL; 
        struct scatterlist sg_in, sg_out;
        struct blkcipher_desc desc;
        int ret = -EFAULT;
        int len = ((int)(src_len/16) + 1) * 16;

        memset(iv, 0x00, AES_BLOCK_SIZE); // set the iv to a proper value!!

        blkcipher = crypto_alloc_blkcipher(cipher, 0, 0);
        if (IS_ERR(blkcipher)) {
                PC_LOGV("could not allocate blkcipher handle for %s\n", cipher);
                return -PTR_ERR(blkcipher);
        }

        if (crypto_blkcipher_setkey(blkcipher, charkey, key_len)) {
                PC_LOGV("key could not be set\n");
                ret = -EAGAIN;
                goto out;
        }

        ivsize = crypto_blkcipher_ivsize(blkcipher);
        if (ivsize) {
                if (ivsize != AES_BLOCK_SIZE)
                        PC_LOGV("IV length differs from expected length. It should be : %d\n",ivsize);
                crypto_blkcipher_set_iv(blkcipher, iv, ivsize);
        }

        scratchpad_in = kmalloc(src_len, GFP_KERNEL);
        if (!scratchpad_in) {
                PC_LOGV("could not allocate scratchpad_in for %s\n", cipher);
                goto out;
        }

        scratchpad_out = kmalloc(len, GFP_KERNEL);
        if (!scratchpad_out) {
                PC_LOGV("could not allocate scratchpad_in for %s\n", cipher);
                goto out;
        }

        memcpy(scratchpad_in,src,src_len);

        desc.flags = 0;
        desc.tfm = blkcipher;
        sg_init_one(&sg_in, scratchpad_in, src_len);
        sg_init_one(&sg_out, scratchpad_out, len);

        crypto_blkcipher_encrypt(&desc, &sg_out, &sg_in, src_len);

        // for decryption, use the following
        // crypto_blkcipher_decrypt(&desc, &sg_out, &sg_in, src_len);

        memcpy(dst,scratchpad_out,sg_out.length);

        *dst_len = sg_out.length;

        ret = 0;
        goto out;

out:
        if (blkcipher)
                crypto_free_blkcipher(blkcipher);
        if (scratchpad_out)
                kzfree(scratchpad_out);
        if (scratchpad_in)
                kzfree(scratchpad_in);

        return ret;

}

Integrating a *.S file as part if your module built process

pc_module-objs := ...
                  src/pc_utility.o src/asm/aesv8-armx-64.o \
                  src/crypto/aes.o src/crypto/crypto.o

static int pc_aes_encrypt_hw(const void *key, int key_len,
                                        void *dst, size_t *dst_len,
                                        const void *src, size_t src_len)
{
        AES_KEY enc_key;

        unsigned char enc_out[src_len];
        unsigned char iv[AES_BLOCK_SIZE];
        unsigned char *aes_input = (unsigned char *) src;
        unsigned char *aes_key = (unsigned char *) key;

        memset(iv, 0x00, AES_BLOCK_SIZE);

        HWAES_set_encrypt_key(aes_key, key_len*8, &enc_key);
        HWAES_cbc_encrypt(aes_input, enc_out, src_len, &enc_key, iv, AES_ENCRYPT);

        *dst_len = src_len;
        memcpy(dst,&enc_out[0],src_len);

        return 0;
}

static int pc_aes_decrypt_hw(const void *key, int key_len,
                                        void *dst, size_t *dst_len,
                                        const void *src, size_t src_len)
{
        AES_KEY dec_key;

        unsigned char iv[AES_BLOCK_SIZE];
        unsigned char dec_out[src_len];
        unsigned char *aes_input = (unsigned char *) src;
        unsigned char *aes_key = (unsigned char *) key;

        memset(iv, 0x00, AES_BLOCK_SIZE);

        HWAES_set_decrypt_key(aes_key, key_len*8, &dec_key);
        HWAES_cbc_encrypt(aes_input, dec_out, src_len, &dec_key, iv, AES_DECRYPT);

        *dst_len = src_len;
        memcpy(dst,&dec_out[0],src_len);

        return 0;
}

0 0

1. Some syscalls have changed, some have been created and some others are not supported anymore. 

2. Much easier than before: now, you only need to change less files. Add the definition of your syscalls in the following files:

3. Don't forget to add asm/unistd.h as part of your head files.

4. Kernel modules are implemented the same way as before. To build it, remember to set cflags properly

5. And of course, the syscall table address table has changed as well. 

0 0

#ifndef FOO_KOBJECT_
#define FOO_KOBJECT_

/* 
 * Partially copied from linux/samples/kobject/kset-example.c
 *
 * Released under the GPL version 2 only.
 */

/*
 * This is our "object" that we will create and register it with sysfs.
 */
struct foo_obj {
 struct kobject kobj;
 int foo;
};
#define to_foo_obj(x) container_of(x, struct foo_obj, kobj)

struct foo_obj *
create_foo_obj(const char *name);

int 
foo_kobj_init(void);

void
foo_kobj_exit(void);

#endif

/*
 * Partially copied from linux/samples/kobject/kset-example.c
 *
 * Released under the GPL version 2 only.
 *
 */

#include <linux/kobject.h>
#include <linux/string.h>
#include <linux/sysfs.h>
#include <linux/slab.h>
#include <linux/module.h>
#include <linux/init.h>

#include "foo_kobject.h"

/*
 * This module shows how to create a kset in sysfs called
 * /sys/kernel/foo
 * Then one kobject is created and assigned to this kset, "foo".  
 * In this kobject, one attribute is also
 * created and if an integer is written to these files, it can be later
 * read out of it.
 */

/* a custom attribute that works just for a struct foo_obj. */
struct foo_attribute {
 struct attribute attr;
 ssize_t (*show)(struct foo_obj *foo, struct foo_attribute *attr, char *buf);
 ssize_t (*store)(struct foo_obj *foo, struct foo_attribute *attr, const char *buf, size_t count);
};
#define to_foo_attr(x) container_of(x, struct foo_attribute, attr)

/*
 * The default show function that must be passed to sysfs.  This will be
 * called by sysfs for whenever a show function is called by the user on a
 * sysfs file associated with the kobjects we have registered.  We need to
 * transpose back from a "default" kobject to our custom struct foo_obj and
 * then call the show function for that specific object.
 */
static ssize_t foo_attr_show(struct kobject *kobj,
        struct attribute *attr,
        char *buf)
{
 struct foo_attribute *attribute;
 struct foo_obj *foo;

 attribute = to_foo_attr(attr);
 foo = to_foo_obj(kobj);

 if (!attribute->show)
  return -EIO;

 return attribute->show(foo, attribute, buf);
}

/*
 * Just like the default show function above, but this one is for when the
 * sysfs "store" is requested (when a value is written to a file.)
 */
static ssize_t foo_attr_store(struct kobject *kobj,
         struct attribute *attr,
         const char *buf, size_t len)
{
 struct foo_attribute *attribute;
 struct foo_obj *foo;

 attribute = to_foo_attr(attr);
 foo = to_foo_obj(kobj);

 if (!attribute->store)
  return -EIO;

 return attribute->store(foo, attribute, buf, len);
}

/* Our custom sysfs_ops that we will associate with our ktype later on */
static const struct sysfs_ops foo_sysfs_ops = {
 .show = foo_attr_show,
 .store = foo_attr_store,
};

/*
 * The release function for our object.  This is REQUIRED by the kernel to
 * have.  We free the memory held in our object here.
 *
 * NEVER try to get away with just a "blank" release function to try to be
 * smarter than the kernel.  Turns out, no one ever is...
 */
static void foo_release(struct kobject *kobj)
{
 struct foo_obj *foo;

 foo = to_foo_obj(kobj);
 kfree(foo);
}

/*
 * The "foo" file where the .foo variable is read from and written to.
 */
static ssize_t foo_show(struct foo_obj *foo_obj, struct foo_attribute *attr,
   char *buf)
{
 return sprintf(buf, "%d\n", foo_obj->foo);
}

static ssize_t foo_store(struct foo_obj *foo_obj, struct foo_attribute *attr,
    const char *buf, size_t count)
{
 sscanf(buf, "%du", &foo_obj->foo);
 return count;
}

static struct foo_attribute foo_attribute =
 __ATTR(foo, 0666, foo_show, foo_store);

/*
 * Create a group of attributes so that we can create and destroy them all
 * at once.
 */
static struct attribute *foo_default_attrs[] = {
 &foo_attribute.attr,
 NULL, /* need to NULL terminate the list of attributes */
};

/*
 * Our own ktype for our kobjects.  Here we specify our sysfs ops, the
 * release function, and the set of default attributes we want created
 * whenever a kobject of this type is registered with the kernel.
 */
static struct kobj_type foo_ktype = {
 .sysfs_ops = &foo_sysfs_ops,
 .release = foo_release,
 .default_attrs = foo_default_attrs,
};

static struct kset *example_kset;

struct foo_obj *create_foo_obj(const char *name)
{
 struct foo_obj *foo;
 int retval;

 /* allocate the memory for the whole object */
 foo = kzalloc(sizeof(*foo), GFP_KERNEL);
 if (!foo)
  return NULL;

 /*
  * As we have a kset for this kobject, we need to set it before calling
  * the kobject core.
         */
 foo->kobj.kset = example_kset;

 /*
  * Initialize and add the kobject to the kernel.  All the default files
  * will be created here.  As we have already specified a kset for this
  * kobject, we don't have to set a parent for the kobject, the kobject
  * will be placed beneath that kset automatically.
  */
 retval = kobject_init_and_add(&foo->kobj, &foo_ktype, NULL, "%s", name);
 if (retval) {
  kobject_put(&foo->kobj);
  return NULL;
 }

 /*
  * We are always responsible for sending the uevent that the kobject
  * was added to the system.
  */
 kobject_uevent(&foo->kobj, KOBJ_ADD);

 return foo;
}

static void destroy_foo_obj(struct foo_obj *foo)
{
 kobject_put(&foo->kobj);
}

int
foo_kobject_init(void)
{
 /*
  * Create a kset with the name of "kset_foo",
  * located under /sys/kernel/
  */
 example_kset = kset_create_and_add("kset_foo", NULL, kernel_kobj);
 if (!example_kset)
  return -ENOMEM;

}

void 
foo_kobject_exit(void)
{
 destroy_foo_obj(foo_kobj);
 kset_unregister(example_kset);
}

#ifndef FOO_UEVENT_
#define FOO_UEVENT_

enum FOO_event_type {
  FOO_GET = 1,
  FOO_SET
};

struct foo_event {
  enum foo_event_type etype;
};

int foo_init_events(void);

int foo_send_uevent(struct foo_event *fooe);

#endif

#include <linux/kobject.h>
#include "foo_kobject.h"
#include "foo_uevent.h"

static struct foo_obj *foo_kobj;

int foo_init_events(void)
{
  int ret;

  ret = example_init();
  if (ret) 
  { 
    printk("error - could not create ksets\n");  
    goto foo_error;
  } 
 
  foo_kobj = create_foo_obj("foo");
  if (!foo_kobj)
  {
    printk("error - could not create kobj\n"); 
    goto foo_error;
  }
 
  return 0;

foo_error:
  return -EINVAL;
}

int foo_send_uevent(struct foo_event *pce)
{
  char event_string[20];
  char *envp[] = { event_string, NULL };

  if (!foo_kobj)
    return -EINVAL;

  snprintf(event_string, 20, "FOO_EVENT=%d", pce->etype);

  return kobject_uevent_env(&foo_kobj->kobj, KOBJ_CHANGE, envp);
}

...
LOCAL_SRC_FILES:= \
     com_android_server_VibratorService.cpp \
     com_android_server_location_GpsLocationProvider.cpp \
     com_android_server_connectivity_Vpn.cpp \
+    com_android_server_pm_PackageManagerService.cpp \
     onload.cpp
...

class ServerThread extends Thread {
...
         WifiP2pService wifiP2p = null;
         WifiService wifi = null;
         NsdService serviceDiscovery= null;
+        MyServiceBinder myService = null;
         IPackageManager pm = null;
         Context context = null;
         WindowManagerService wm = null;
...
         class ServerThread extends Thread {
...
             }

             try {
+                Slog.i(TAG, "My Service - Example");
+                myService = MyServiceBinder.create(context);
+                ServiceManager.addService(
+                        Context.MY_SERVICE, myService);
+            } catch (Throwable e) {
+                reportWtf("starting Privacy Capsules Service", e);
+            }
+
...

...
public class MyServiceBinder extends Binder {

    private final int FOO_GET = 1;
    private final int FOO_SET = 2;

    private MyServiceWorkerThread mWorker;
    private MyServiceWorkerHandler mHandler;
    private Context mContext;
    private NotificationManager mNotificationManager;

    /*
     * We create an observer to listen to the UEvent we are interested in. 
     * See UEventObserver#startObserving(String). From the UEvent, we extract 
     * an existing integer field and we propagate create a notification to 
     * be seen by the user.
     */
    private final UEventObserver mUEventObserver = new UEventObserver() {
        @Override
        public void onUEvent(UEventObserver.UEvent event) {
            final int eventType = Integer.parseInt(event.get("FOO_EVENT"));
            sendMyServiceNotification(eventType);
        }
    };

    public MyServiceBinder(Context context) {
        super();
        mContext = context;
        mNotificationManager = (NotificationManager)mContext.getSystemService(
                               Context.NOTIFICATION_SERVICE);

         /* We have to specify a filter to receive only certain UEvents from the 
          * kernel. In this case, the UEVENT that we want to listen to contains 
          * the string "FOO_EVENT".
          */
        mUEventObserver.startObserving("FOO_EVENT=");

        mWorker = new MyServiceWorkerThread("MyServiceWorker");
        mWorker.start();
    }

    /*
     * Add potential initialization steps here
     */
    public static MyServiceBinder create(Context context) {
        MyServiceBinder service = new MyServiceBinder(context);
        return service;
    }
    
    /*
     * Sends notification to the user. In this case, we are create notifications. 
     * If the user clicks on the notification, we send an Intent so that an system 
     * application is launched. For example, if we are listening to notifications 
     * for USB UEvents, the USB Settings application can be launched when the 
     * notification is clicked.
     */
    private final void sendNotificationToUser(int type) {
        String notificationMessage = "";
        Resources r = mContext.getResources();
        switch(type) {
                case FOO_GET:
                // do something if the event type is A
                notificationMessage = "This is event type FOO_GET";
                break;
                case FOO_SET:
                // do something if the event type is B
                notificationMessage = "This is event type FOO_SET";      
                break;
                default:
                break;
        }  

        String notificationTitle = r.getString(R.string.my_service_notification_title);
        long notificationWhen = System.currentTimeMillis();
        int requestID = (int) notificationWhen;
 
        Intent intent = new Intent(Intent.ACTION_FOO_EVENT);
        intent.addFlags(Intent.FLAG_ACTIVITY_SINGLE_TOP
                        | Intent.FLAG_ACTIVITY_CLEAR_TOP);

        intent.putExtra(FooManager.EXTRA_ETYPE, type);
        intent.putExtra(FooManager.EXTRA_MESSAGE, notificationMessage);

        PendingIntent pi = PendingIntent.getActivityAsUser(mContext, requestID,
                                intent, 0, null, UserHandle.CURRENT);

        Notification notification = new Notification.Builder(mContext)
                        .setSmallIcon(R.drawable.indicator_input_error)
                        .setContentTitle(notificationTitle)
                        .setContentText(notificationMessage)
                        .setPriority(Notification.PRIORITY_HIGH)
                        .setDefaults(0)
                        .setWhen(notificationWhen)
                        .setOngoing(true)
                        .setContentIntent(pi)
                        .setAutoCancel(true)
                        .build();

        mNotificationManager.notifyAsUser(null,
                        R.string.my_service_notification_title,
                        notification, UserHandle.ALL);
    }


    /*
     * Runner for the 
     *
     */
    private void sendMyServiceNotification(final int type) {
        
        mHandler.post(new Runnable() {
            @Override
            public void run() {
                sendNotificationToUser(type);
            }
        });

    }

    private class MyServiceWorkerThread extends Thread {

        public MyServiceWorkerThread(String name) {
            super(name);
        }

        public void run() {
            Looper.prepare();
            mHandler = new MyServiceWorkerHandler();
            Looper.loop();
        }

    }
 
    private class MyServiceWorkerHandler extends Handler {

        private static final int MESSAGE_SET = 0;

        @Override
        public void handleMessage(Message msg) {
            try {
                if (msg.what == MESSAGE_SET) {
                    Log.i(TAG, "set message received: " + msg.arg1);
                }
            } catch (Exception e) {
                // Log, don't crash!
                Log.e(TAG, "Exception in MyServiceWorkerHandler.handleMessage:", e);
            }
        }

    }
}

0 0

• The option CONFIG_MODULES is NOT enabled by default. If you want to insert modules into the kernel, make sure to set CONFIG_MODULES;
• Android emulator does NOT work with Kernels > 2.6.X So, make sure to use the proper Kernel. For devices, any support Kernel will work.

1 0

1. Building Android ICS
2. Building/Running Android Goldfish kernel 
3. Creating a Goldfish kernel module
4. Adding new syscalls into the Goldfish kernel

1 0

• Follow the instructions described on http://source.android.com/source/building.html
• It also provides information about Android Kernel building.
• Check which compiler best fits into the version of Android platform you are using.
• Usually, due to new features (for example, C++11 support on GCC/++ 4.7), it is a bit more complicated to build it with new compilers... you will see several compiling errors. For example, for the ICS, GCC/++ 4.6 works better than GCC/++ 4.7.
• You may need some changes in order to compile it properly (mostly on Makefile). Again, depending on your compiler, you may apply more or less changes on your code.
• For ICS, GCC/++ 4.6 works better.

1 0

• Checkout LLVM:
  $ cd where-you-want-llvm-to-live
  $ svn co http://llvm.org/svn/llvm-project/llvm/trunk llvm
  
• Checkout Clang:
  $ cd where-you-want-llvm-to-live
  $ cd llvm/tools
  $ svn co http://llvm.org/svn/llvm-project/cfe/trunk clang
• Checkout Compiler-RT:
  $ cd where-you-want-llvm-to-live
  $ cd llvm/projects
  $ svn co http://llvm.org/svn/llvm-project/compiler-rt/trunk compiler-rt
• Get the Test Suite Source Code [Optional]
  $ cd where-you-want-llvm-to-live
  $ cd llvm/projects
  $ svn co http://llvm.org/svn/llvm-project/test-suite/trunk test-suite

$ export $PATH=$PATH:/home/user/CodeSourcery/Sourcery_G++_Lite/bin

$ cd where-you-want-to-build-llvm
$ mkdir build (for building without polluting the source dir)
$ cd build
$ ../configure --target=arm-none-linux-gnueabi --host=arm-none-linux-gnueabi --enable-optimized (the target/host value may change depending on the toolchain that you have chosen.)
$ make

• Install NFS packages
  @desktop$ sudo apt-get install portmap nfs-kernel-server nfs-common
• Edit /etc/exports and add the following line
  /home/user/llvm 192.168.1.0/24(rw,sync,no_subtree_check)
• Restart NFS system
  @desktop$ sudo service nfs-kernel-server restart
• Change access permission of your shared folder (in this case, I use the same folder)
  @desktop$ chmod -R 777 /home/user/llvm

• Install NFS packages (most of Raspberry distributions contain NFS installed, so maybe this step is not necessary)
  @remote$ sudo apt-get install portmap nfs-kernel-server nfs-common
• Create the host folder
  @remote$ mkdir llvm
  @remote$ chmod 777 llvm
• Mount the NFS-shared folder
  @remote$ sudo mount 192.168.1.4:/home/user/llvm /home/user/llvm

2 1

static char *start_health_session(DBusConnection *conn)
{

  DBusMessage *msg, *reply;
  DBusMessageIter args;
  DBusError err;
  const char *reply_path;
  char *path;

  msg = dbus_message_new_method_call("org.bluez", 
                                     "/org/bluez", 
                                     "org.bluez.HealthManager",
                                     "CreateApplication");

  if (!msg) {
      printf(" network:dbus Can't allocate new method call\n");
      return NULL;
  }

  // append arguments

  dbus_message_iter_init_append(msg, &args);

  if ( !iter_append_dictionary(&args, DATA_TYPE_VALUE, 
                                          ROLE_VALUE,
                                          DESCRIPTION_VALUE, 
                                          CHANNEL_TYPE_VALUE) ) {
      printf(" network:dbus Can't append parameters\n");
      dbus_message_unref(msg);
      return NULL;
  }

  dbus_error_init(&err);

....
}

static int iter_append_dictionary(DBusMessageIter *iter, 
                                  dbus_uint16_t dataType,
                                  const char *role,
                                  const char *description,
                                  const char *channelType)
{
  DBusMessageIter dict;

  dbus_message_iter_open_container(iter, DBUS_TYPE_ARRAY,
            DBUS_DICT_ENTRY_BEGIN_CHAR_AS_STRING
            DBUS_TYPE_STRING_AS_STRING DBUS_TYPE_VARIANT_AS_STRING
            DBUS_DICT_ENTRY_END_CHAR_AS_STRING, &dict);

  dict_append_entry(&dict, "DataType", DBUS_TYPE_UINT16, &dataType);

  dict_append_entry(&dict, "Role", DBUS_TYPE_STRING, &role);

  dict_append_entry(&dict, "Description", DBUS_TYPE_STRING, &description);

  dict_append_entry(&dict, "ChannelType", DBUS_TYPE_STRING, &channelType);

  dbus_message_iter_close_container(iter, &dict);
}

DBUS_DICT_ENTRY_BEGIN_CHAR_AS_STRING
DBUS_TYPE_STRING_AS_STRING DBUS_TYPE_VARIANT_AS_STRING
DBUS_DICT_ENTRY_END_CHAR_AS_STRING

static void append_variant(DBusMessageIter *iter, int type, void *val)
{
  DBusMessageIter value;
  char sig[2] = { type, '\0' };

  dbus_message_iter_open_container(iter, DBUS_TYPE_VARIANT, sig, &value);

  dbus_message_iter_append_basic(&value, type, val);

  dbus_message_iter_close_container(iter, &value);
}

static void dict_append_entry(DBusMessageIter *dict,
   const char *key, int type, void *val)
{
  DBusMessageIter entry;

  if (type == DBUS_TYPE_STRING) {
    const char *str = *((const char **) val);
    if (str == NULL)
      return;
  }

  dbus_message_iter_open_container(dict, DBUS_TYPE_DICT_ENTRY,
                                   NULL, &entry);

  dbus_message_iter_append_basic(&entry, DBUS_TYPE_STRING, &key);

  append_variant(&entry, type, val);

  dbus_message_iter_close_container(dict, &entry);
}

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