Assembly Language for Beginners

6.2. THREAD LOCAL STORAGE
9 __declspec( thread ) uint32_t rand_state=1234;
10
11 void my_srand (uint32_t init)
12 {
13 rand_state=init;
14 }
15
16 int my_rand ()
17 {
18 rand_state=rand_state*RNG_a;
19 rand_state=rand_state+RNG_c;
20 return rand_state & 0x7fff;
21 }
22
23 int main()
24 {
25 printf ("%d\n", my_rand());
26 };

The code is not different from what we already saw, but in IDA we see:

.tls:00404000 ; Segment type: Pure data

.tls:00404000 ; Segment permissions: Read/Write

.tls:00404000 _tls segment para public 'DATA' use32

.tls:00404000 assume cs:_tls

.tls:00404000 ;org 404000h

.tls:00404000 TlsStart db 0 ; DATA XREF: .rdata:TlsDirectory

.tls:00404001 db 0

.tls:00404002 db 0

.tls:00404003 db 0

.tls:00404004 dd 1234

.tls:00404008 TlsEnd db 0 ; DATA XREF: .rdata:TlsEnd_ptr

...

1234 is there and every time a new thread starts, a newTLSis allocated for it, and all this data, including

1234, will be copied there.

This is a typical scenario:

• Thread A is started. ATLSis created for it, 1234 is copied torand_state.


• Themy_rand()function is called several times in thread A.
  rand_stateis different from 1234.


• Thread B is started. ATLSis created for it, 1234 is copied torand_state, while thread A has a
  different value in the same variable.

TLScallbacks

ButwhatifthevariablesintheTLShavetobefilledwithsomedatathatmustbepreparedinsomeunusual

way?

Let’s say, we’ve got the following task: the programmer can forget to call themy_srand()function to

initialize thePRNG, but the generator has to be initialized at start with something truly random, instead

of 1234. This is a case in whichTLScallbacks can be used.

The following code is not very portable due to the hack, but nevertheless, you get the idea.

What we do here is define a function (tls_callback()) which is to be calledbeforethe process and/or

thread start.

The function initializes thePRNGwith the value returned byGetTickCount()function.

#include <stdint.h>

#include <windows.h>

#include <winnt.h>

// from the Numerical Recipes book:

#define RNG_a 1664525

#define RNG_c 1013904223