Reverse Engineering for Beginners

CHAPTER 65. THREAD LOCAL STORAGE CHAPTER 65. THREAD LOCAL STORAGE

Listing 65.3: Optimizing MSVC 2013 x64

_TLS SEGMENT

rand_state DD 01H DUP (?)

_TLS ENDS

_DATA SEGMENT

$SG85451 DB '%d', 0aH, 00H

_DATA ENDS

_TEXT SEGMENT

init$ = 8

my_srand PROC

mov edx, DWORD PTR _tls_index

mov rax, QWORD PTR gs:88 ; 58h

mov r8d, OFFSET FLAT:rand_state

mov rax, QWORD PTR [rax+rdx*8]

mov DWORD PTR [r8+rax], ecx

ret 0

my_srand ENDP

my_rand PROC

mov rax, QWORD PTR gs:88 ; 58h

mov ecx, DWORD PTR _tls_index

mov edx, OFFSET FLAT:rand_state

mov rcx, QWORD PTR [rax+rcx*8]

imul eax, DWORD PTR [rcx+rdx], 1664525 ; 0019660dH

add eax, 1013904223 ; 3c6ef35fH

mov DWORD PTR [rcx+rdx], eax

and eax, 32767 ; 00007fffH

ret 0

my_rand ENDP

_TEXT ENDS

InitializedTLSdata

Let’s say, we want to set some fixed value torand_state, so in case the programmer forgets to, therand_statevariable

would be initialized to some constant anyway (line 9):

1 #include <stdint.h>
2 #include <windows.h>
3 #include <winnt.h>
4
5 // from the Numerical Recipes book:
6 #define RNG_a 1664525
7 #define RNG_c 1013904223
8
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 no differ from what we already saw, but in IDA we see: