Linux kernel module - memory allocation

["q.kontinuum" <achsis_of_elvis@yahoo.co.uk>]

I'm new to this group, new to Ada programming, and a bit out of touch with
Linux driver development for some time. Since I want to learn Ada and want
to get back in touch with Linux I'm trying to implement a driver for my 
raspberry-pi, to access a DS18B20 temperature sensor via onewire protocol.

The steps I already took was to implement a small wrapper in C to link 
with a first Hello world module in Ada and to access the hr_timer of the 
Linux kernel (here I again used to wrapper and called it from Ada to avoid 
cloning the whole hrtimer struct from Linux kernel header files)

Where I'm getting stuck now is a function returning a String. I simplified 
the function to this:

   function UInt2String(I : Integer) return String is
   begin      
      return "";
   end;

As soon as this function is used somewhere, I can't load the module. 
Here is the kernel log output when I try to load the module:

May 18 10:19:39 localhost kernel: [87978.938801] wrapper: 
   Unknown symbol system__secondary_stack__ss_allocate (err 0)
May 18 10:19:39 localhost kernel: [87978.938812] wrapper: 
   Unknown symbol system__secondary_stack__ss_release (err 0)
May 18 10:19:39 localhost kernel: [87978.938819] wrapper: 
   Unknown symbol system__secondary_stack__ss_mark (err 0)
May 18 10:19:39 localhost kernel: wrapper: 
   Unknown symbol system__secondary_stack__ss_allocate (err 0)
May 18 10:19:39 localhost kernel: wrapper: 
   Unknown symbol system__secondary_stack__ss_release (err 0)
May 18 10:19:39 localhost kernel: wrapper: 
   Unknown symbol system__secondary_stack__ss_mark (err 0)

I will need to store a binary tree later on to store the addresses of the 
devices found on the onewire bus. In C, I'd probably use kmalloc and 
casts to allocate memory for a node of the tree. How should I do that in 
Ada?

Sorry if this is a stupid question. I'm entirely happy with a RTFM + link
to a good M on this topic.