Quick introduction to reverse engineering for beginners

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1.16 C++ classes I placed a C++ classes description here intentionally after structures description, because internally, C++ classes representation is almost the same as structures representation. Let’s try an example with two variables, couple of constructors and one method: #include class c { private: int v1; int v2; public: c() // ctor { v1=667; v2=999; }; }; c(int a, int b) // ctor { v1=a; v2=b; }; void dump() { printf ("%d; %d\n", v1, v2); }; int main() { class c c1; class c c2(5,6); }; c1.dump(); c2.dump(); return 0; Here is how main() function looks like translated into assembler: _c2$ = -16 ; size = 8 _c1$ = -8 ; size = 8 _main PROC push ebp mov ebp, esp sub esp, 16 ; 00000010H lea ecx, DWORD PTR _c1$[ebp] call ??0c@@QAE@XZ ; c::c push 6 push 5 79
lea ecx, DWORD PTR _c2$[ebp] call ??0c@@QAE@HH@Z ; c::c lea ecx, DWORD PTR _c1$[ebp] call ?dump@c@@QAEXXZ ; c::dump lea ecx, DWORD PTR _c2$[ebp] call ?dump@c@@QAEXXZ ; c::dump xor eax, eax mov esp, ebp pop ebp ret 0 _main ENDP So what’s going on. For each object (instance of class c) 8 bytes allocated, that’s exactly size of 2 variables storage. For c1 a default argumentless constructor ??0c@@QAE@XZ is called. For c2 another constructor ??0c@@QAE@HH@Z is called and two numbers are passed as arguments. A pointer to object (this in C++ terminology) is passed in ECX register. This is called thiscall 2.5.4 — a pointer to object passing method. MSVC doing it using ECX register. Needless to say, it’s not a standardized method, other compilers could do it differently, for example, via first function argument (like GCC). Why these functions has so odd names? That’s name mangling 59 . C++ class may contain several methods sharing the same name but having different arguments — that’s polymorphism. And of course, different classes may own methods sharing the same name. Name mangling allows to encode class name + method name + all method argument types in one ASCIIstring, which will be used as internal function name. That’s all because neither linker, nor DLL operation system loader (mangled names may be among DLL exports as well) knows nothing about C++ or OOP. dump() function called two times after. Now let’s see constructors’ code: _this$ = -4 ; size = 4 ??0c@@QAE@XZ PROC ; c::c, COMDAT ; _this$ = ecx push ebp mov ebp, esp push ecx mov DWORD PTR _this$[ebp], ecx mov eax, DWORD PTR _this$[ebp] mov DWORD PTR [eax], 667 ; 0000029bH mov ecx, DWORD PTR _this$[ebp] mov DWORD PTR [ecx+4], 999 ; 000003e7H mov eax, DWORD PTR _this$[ebp] mov esp, ebp pop ebp ret 0 ??0c@@QAE@XZ ENDP ; c::c _this$ = -4 ; size = 4 _a$ = 8 ; size = 4 _b$ = 12 ; size = 4 ??0c@@QAE@HH@Z PROC ; c::c, COMDAT ; _this$ = ecx push ebp mov ebp, esp 59 Wikipedia: Name mangling 80
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Quick introduction to reverse engin
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1.16 C++ classes . . . . . . . . .
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Preface Here (will be) some of my n
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1.1 Hello, world! Let’s start wit
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1.2 Stack Stack — is one of the m
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(_snprintf() function works just li
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1.3 printf() with several arguments
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1.4 scanf() Now let’s use scanf()
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GCC replaced first printf() call to
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}; return 0; printf ("What you ente
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1.5 Passing arguments via stack Now
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1.6 One more word about results ret
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push OFFSET $SG741 ; ’a
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1.8 switch()/case/default 1.8.1 Few
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1.8.2 A lot of cases If switch() st
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loc_804840C: ; DATA XREF: .rodata:0
Page 33 and 34: et 0 _main ENDP Nothing very specia
Page 35 and 36: add esp, 1Ch xor eax, eax ; return
Page 37 and 38: C/C++ standard defines char type as
Page 39 and 40: eax=eax+ecx return eax ... and this
Page 41 and 42: mov eax, ecx imul edx sar edx, 1 mo
Page 43 and 44: ; current stack state: ST(0) = resu
Page 45 and 46: ; in local stack here 8 bytes still
Page 47 and 48: fld QWORD PTR _a$[esp-4] ; current
Page 49 and 50: In other words, fnstsw ax / sahf in
Page 51 and 52: 1.13 Arrays Array is just a set of
Page 53 and 54: mov [esp+70h+var_70], eax call _pri
Page 55 and 56: jge SHORT $LN1@main mov ecx, DWORD
Page 57 and 58: ebp 0x15 0x15 esi 0x0 0 edi 0x0 0 e
Page 59 and 60: 1.14 Bit fields A lot of functions
Page 61 and 62: So, let’s download Linux Kernel 2
Page 63: There are some redundant code prese
Page 66 and 67: y 10 just by dropping last digit (3
Page 68 and 69: _key$ = 8 ; size = 4 _len$ = 12 ; s
Page 70 and 71: 1.15 Structures It can be defined t
Page 72 and 73: call DWORD PTR __imp__GetSystemTime
Page 74 and 75: 1.15.4 Fields packing in structure
Page 76 and 77: }; int b; struct outer_struct { cha
Page 78 and 79: }; printf ("stepping=%d\n", tmp->st
Page 80 and 81: call ___printf_chk mov eax, esi shr
Page 82 and 83: mov edx, DWORD PTR _t$[ebp] or edx,
Page 86 and 87: push ecx mov DWORD PTR _this$[ebp],
Page 88 and 89: call _ZN1c4dumpEv lea eax, [esp+20h
Page 90 and 91: 1.17 Unions 1.17.1 Pseudo-random nu
Page 92 and 93: xor eax, eax pop esi mov esp, ebp p
Page 94 and 95: Let’s compile it in MSVC 2010 (I
Page 96 and 97: comp() function: public comp comp p
Page 98 and 99: Some compilers can do vectorization
Page 100 and 101: mov ecx, [esp+10h+var_10] mov edx,
Page 102 and 103: GCC In all other cases, non-SSE2 co
Page 104 and 105: add ebx, edx lea esi, [esi+0] loc_8
Page 106 and 107: movdqa xmm1, XMMWORD PTR [ecx+16] a
Page 108 and 109: 1.20 x86-64 It’s a 64-bit extensi
Page 110 and 111: } x40 = a3 | x31; x41 = x24 & ~x37;
Page 112 and 113: or ebx, DWORD PTR _x6$[esp+36] mov
Page 114 and 115: and r11, r8 and rsi, r8 and r10, r1
Page 116 and 117: pop rdi pop rsi ret 0 s1 ENDP Nothi
Page 118 and 119: 2.1 LEA instruction LEA (Load Effec
Page 120 and 121: 2.3 npad It’s an assembler macro
Page 122 and 123: 2.4 Signed number representations T
Page 124 and 125: call function function: .. do somet
Page 126 and 127: 3.2 String Debugging messages are o
Page 128 and 129: .text:3011E91B DD 1E fstp qword ptr
Page 130 and 131: loc_80483F2: pop ebp retn _f endp 4
Page 132 and 133: loc_8048444: loc_8048448: loc_80484
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public f2 f2 proc near push ebp mov
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loc_804840A: loc_804842E: loc_80484
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mov esi, DWORD PTR _k$[esp+16] push
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or eax, edx retn f endp 4.1.8 Task
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4.2 Middle level 4.2.1 Task 2.1 Wel
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mov [edx+eax], bl mov ebx, edi mov
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loc_80485AB: ; CODE XREF: f+1E0 ; f
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mov [ecx+eax], dl inc eax cmp ebx,
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Chapter 5 Tools ∙ IDA as disassem
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Chapter 7 More examples 147
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.text:00541050 arg_0 = dword ptr 4
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.text:005410F3 .text:005410F3 loc_5
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.text:005411A9 pop ebx .text:005411
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.text:00541249 .text:00541249 next_
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.text:005412FC mov esi, offset cube
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.text:005413DC call _fwrite ; write
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.text:005414A7 push ecx ; Filename
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.text:005413A3 mov ecx, [esp+44h+pa
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.text:00541408 push offset aRb ; "r
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}; }; return; fseek (f, 0, SEEK_END
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.text:005411B5 mov ebp, eax Check f
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.text:0054124C inc ebp .text:005412
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This instruction clears bit, in oth
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.text:005410CD add esp, 40h .text:0
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}; for (i=0; i
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}; tmp[y][z]=get_bit (row, y, z); f
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}; fread (buf, flen, 1, f); fclose
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7.2 SAP client network traffic comp
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.text:64413F68 .text:64413F68 loc_6
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.text:64405171 call dbg .text:64405
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.text:64404FF7 push offset aLogging
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.text:64405113 loc_64405113: .text:
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Now let’s dig deeper and find con
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.text:64411E0B .text:64411E0B loc_6
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Chapter 8 Other things 8.1 Compiler
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Chapter 9 Tasks solutions 9.1 Easy
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9.1.5 Task 1.5 Hint #1: Keep in min
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} 9.1.8 Task 1.8 Solution: two 100*
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#define PUSH(low, high) ((void) ((t
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} } ignore one or both. Otherwise,
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Quick introduction to reverse engineering for beginners

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