Quick introduction to reverse engineering for beginners

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loc_804840A: loc_804842E: loc_804843D: loc_8048443: loc_8048444: mov eax, [ebp+var_8] movzx eax, byte ptr [eax] test al, al jz short loc_804842E mov eax, [ebp+var_C] movzx eax, byte ptr [eax] test al, al jz short loc_804842E mov eax, [ebp+var_8] movzx edx, byte ptr [eax] mov eax, [ebp+var_C] movzx eax, byte ptr [eax] cmp dl, al jz short loc_8048402 mov eax, [ebp+var_C] movzx eax, byte ptr [eax] test al, al jnz short loc_804843D mov eax, [ebp+var_4] jmp short locret_8048453 add [ebp+var_4], 1 jmp short loc_8048444 nop locret_8048453: leave retn f endp 4.1.5 Task 1.5 mov eax, [ebp+var_4] movzx eax, byte ptr [eax] test al, al jnz short loc_80483F4 mov eax, 0 This task is rather on knowledge than on reading code. The function is taken from OpenWatcom. Compiled in MSVC 2010 with /Ox flag. _DATA SEGMENT COMM __v:DWORD _DATA ENDS PUBLIC __real@3e45798ee2308c3a 131
PUBLIC __real@4147ffff80000000 PUBLIC __real@4150017ec0000000 PUBLIC _f EXTRN __fltused:DWORD CONST SEGMENT __real@3e45798ee2308c3a DQ 03e45798ee2308c3ar ; 1e-008 __real@4147ffff80000000 DQ 04147ffff80000000r ; 3.14573e+006 __real@4150017ec0000000 DQ 04150017ec0000000r ; 4.19584e+006 CONST ENDS _TEXT SEGMENT _v1$ = -16 ; size = 8 _v2$ = -8 ; size = 8 _f PROC sub esp, 16 ; 00000010H fld QWORD PTR __real@4150017ec0000000 fstp QWORD PTR _v1$[esp+16] fld QWORD PTR __real@4147ffff80000000 fstp QWORD PTR _v2$[esp+16] fld QWORD PTR _v1$[esp+16] fld QWORD PTR _v1$[esp+16] fdiv QWORD PTR _v2$[esp+16] fmul QWORD PTR _v2$[esp+16] fsubp ST(1), ST(0) fcomp QWORD PTR __real@3e45798ee2308c3a fnstsw ax test ah, 65 ; 00000041H jne SHORT $LN1@f or DWORD PTR __v, 1 $LN1@f: add esp, 16 ; 00000010H ret 0 _f ENDP _TEXT ENDS 4.1.6 Task 1.6 Compiled in MSVC 2010 with /Ox option. PUBLIC _f ; Function compile flags: /Ogtpy _TEXT SEGMENT _k0$ = -12 ; size = 4 _k3$ = -8 ; size = 4 _k2$ = -4 ; size = 4 _v$ = 8 ; size = 4 _k1$ = 12 ; size = 4 _k$ = 12 ; size = 4 _f PROC sub esp, 12 ; 0000000cH mov ecx, DWORD PTR _v$[esp+8] mov eax, DWORD PTR [ecx] mov ecx, DWORD PTR [ecx+4] push ebx push esi 132
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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 $SG741 ; ’a
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1.8 switch()/case/default 1.8.1 Few
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1.8.2 A lot of cases If switch() st
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loc_804840C: ; DATA XREF: .rodata:0
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et 0 _main ENDP Nothing very specia
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add esp, 1Ch xor eax, eax ; return
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C/C++ standard defines char type as
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eax=eax+ecx return eax ... and this
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mov eax, ecx imul edx sar edx, 1 mo
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; current stack state: ST(0) = resu
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; in local stack here 8 bytes still
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fld QWORD PTR _a$[esp-4] ; current
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In other words, fnstsw ax / sahf in
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1.13 Arrays Array is just a set of
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mov [esp+70h+var_70], eax call _pri
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jge SHORT $LN1@main mov ecx, DWORD
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ebp 0x15 0x15 esi 0x0 0 edi 0x0 0 e
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1.14 Bit fields A lot of functions
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So, let’s download Linux Kernel 2
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There are some redundant code prese
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y 10 just by dropping last digit (3
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_key$ = 8 ; size = 4 _len$ = 12 ; s
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1.15 Structures It can be defined t
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call DWORD PTR __imp__GetSystemTime
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1.15.4 Fields packing in structure
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}; int b; struct outer_struct { cha
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}; printf ("stepping=%d\n", tmp->st
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call ___printf_chk mov eax, esi shr
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mov edx, DWORD PTR _t$[ebp] or edx,
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1.16 C++ classes I placed a C++ cla
Page 86 and 87: push ecx mov DWORD PTR _this$[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$[esp+36] mov
Page 114 and 115: and r11, r8 and rsi, r8 and r10, r1
Page 116 and 117: pop rdi pop rsi ret 0 s1 ENDP Nothi
Page 118 and 119: 2.1 LEA instruction LEA (Load Effec
Page 120 and 121: 2.3 npad It’s an assembler macro
Page 122 and 123: 2.4 Signed number representations T
Page 124 and 125: call function function: .. do somet
Page 126 and 127: 3.2 String Debugging messages are o
Page 128 and 129: .text:3011E91B DD 1E fstp qword ptr
Page 130 and 131: loc_80483F2: pop ebp retn _f endp 4
Page 132 and 133: loc_8048444: loc_8048448: loc_80484
Page 134 and 135: public f2 f2 proc near push ebp mov
Page 138 and 139: mov esi, DWORD PTR _k$[esp+16] push
Page 140 and 141: or eax, edx retn f endp 4.1.8 Task
Page 142 and 143: 4.2 Middle level 4.2.1 Task 2.1 Wel
Page 144 and 145: mov [edx+eax], bl mov ebx, edi mov
Page 146 and 147: loc_80485AB: ; CODE XREF: f+1E0 ; f
Page 148 and 149: mov [ecx+eax], dl inc eax cmp ebx,
Page 150 and 151: Chapter 5 Tools ∙ IDA as disassem
Page 152 and 153: Chapter 7 More examples 147
Page 154 and 155: .text:00541050 arg_0 = dword ptr 4
Page 156 and 157: .text:005410F3 .text:005410F3 loc_5
Page 158 and 159: .text:005411A9 pop ebx .text:005411
Page 160 and 161: .text:00541249 .text:00541249 next_
Page 162 and 163: .text:005412FC mov esi, offset cube
Page 164 and 165: .text:005413DC call _fwrite ; write
Page 166 and 167: .text:005414A7 push ecx ; Filename
Page 168 and 169: .text:005413A3 mov ecx, [esp+44h+pa
Page 170 and 171: .text:00541408 push offset aRb ; "r
Page 172 and 173: }; }; return; fseek (f, 0, SEEK_END
Page 174 and 175: .text:005411B5 mov ebp, eax Check f
Page 176 and 177: .text:0054124C inc ebp .text:005412
Page 178 and 179: This instruction clears bit, in oth
Page 180 and 181: .text:005410CD add esp, 40h .text:0
Page 182 and 183: }; for (i=0; i
Page 184 and 185: }; 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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