DBUS as IPC medium for communicating signals/events

[Thiago Macieira]

On quarta-feira, 1 de agosto de 2012 17.26.09, rajnikant jachak wrote:
> Hi All,
> 
> I'm considering DBUS (version-1.6.2/ linux platform) as option to signal
> different modules within event driven system. Eg. Module-A sends signal to
> Module-B, Module-B process signal and replies module-A with response
> signal.
> 
> So I wanted to send some extra information with signal across different
> module. I tried sending extra information in Byte array. Following is code
> to do so:
> 
> struct MyExample {
>     char cchar[30];
>     int iint;
>     double ddouble;
>     char c;
> };
> 
> /--------------- Send Signal Code -------------------/
>     // Extra Information with signal is in MyExample Object
>     struct MyExample obj;
>     strcpy(obj.cchar,"String");
>     obj.iint = 9209;
>     obj.ddouble = 10.12;
>     obj.c = 'A';
>     char tBuff[100];
>     memcpy(tBuff, &obj,sizeof(obj));  // Serialising obj into buffer

If you send this buffer, please note that you might be leaking security-
sensitive data to the world. The structure you defined above, with 30 
characters in a string, did not get overwritten by that strcpy, so it still 
contain information from the stack which could be important.

You wanted strncpy -- and really, that's the only place where strncpy's weird 
length behaviour comes in handy.

>     int len = sizeof(obj);
>     unsigned char *val = (unsigned char *)tBuff;
>     if (!dbus_message_append_args (signal,
>                 DBUS_TYPE_ARRAY, DBUS_TYPE_BYTE, &val, len,
>                 DBUS_TYPE_INVALID))
>         die ("No memory");
>     if (!dbus_connection_send (connection, signal, NULL))
>         die ("No memory\n");
> /------------------ End Code ----------------/
> 
> On receiver side, ideally I should get correct 'extra information' after
> type casting the byte array I would be getting by
> dbus_message_iter_get_fixed_array API. But when I type cast the buffer, I'm
> seeing some weird data. Following is the code to read the byte array:
> /---------------- Processing Signal --------------/
>             DBusMessageIter iter;
>             dbus_message_iter_init (message,& iter);
>             int argType, eleType;
>             argType = dbus_message_iter_get_arg_type(&iter);
>             if (argType == DBUS_TYPE_ARRAY) {
>                 if (dbus_message_iter_get_element_type(&iter) ==
> DBUS_TYPE_BYTE) {
>                     DBusMessageIter arrayIter;
>                     dbus_message_iter_recurse(&iter,&arrayIter);
>                     unsigned char buff[50];
>                     int length;
> 
> dbus_message_iter_get_fixed_array(&arrayIter,&buff,&length);
>                     struct MyExample *obj = (struct MyExample*) buff;
>                      printf ("\nLength: %d Name: %s, Int: %d, Double:%f,
> Char: %c\n",length, obj->cchar, obj->iint,obj->ddouble,obj->c);
>                  }
>              }
> /---------------- End Processing -----------------/
> 
> I got the correct information when I used 'dbus_message_iter_get_basic' API
> iterating over byte array. But I dont want to use this method as its
> execution is expensive.

You passed the wrong parameters to dbus_message_iter_get_fixed_array.

The correct code would have been:
    int len;
    char* data;
    dbus_message_iter_get_fixed_array(&arrayIter,&data,&len);

You get a pointer to the message's internal buffers. You can use it as long as 
you don't unref that message.


> 1. Basic question is, is Byte Array is good choice to use for information
> forwarding in DBUS?

No. D-Bus has a proper complex type system. You can pass your struct in a 
nicer manner and avoid the security leak I pointed out.

Note that D-Bus strings are *always* UTF-8 encoded. If your char* strings are 
arbitrary data, use byte arrays, not strings.

> 2. What am I missing cause of which I'm not getting correct data on
> receiver side?
-- 
Thiago Macieira - thiago (AT) macieira.info - thiago (AT) kde.org
   Software Architect - Intel Open Source Technology Center
      PGP/GPG: 0x6EF45358; fingerprint:
      E067 918B B660 DBD1 105C  966C 33F5 F005 6EF4 5358
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