Quick introduction to reverse engineering for beginners

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4.2 Middle level 4.2.1 Task 2.1 Well-known algorithm, also included in standard C library. Source code was taken from glibc 2.11.1. Compiled in GCC 4.4.1 with -Os option (code size optimization). Listing was done by IDA 4.9 disassembler from ELF-file generated by GCC and linker. For those who wants use IDA while learning, here you may find .elf and .idb files, .idb can be opened with freeware IDA 4.9: http://conus.info/RE-tasks/middle/1/ f proc near var_150 = dword ptr -150h var_14C = dword ptr -14Ch var_13C = dword ptr -13Ch var_138 = dword ptr -138h var_134 = dword ptr -134h var_130 = dword ptr -130h var_128 = dword ptr -128h var_124 = dword ptr -124h var_120 = dword ptr -120h var_11C = dword ptr -11Ch var_118 = dword ptr -118h var_114 = dword ptr -114h var_110 = dword ptr -110h var_C = dword ptr -0Ch arg_0 = dword ptr 8 arg_4 = dword ptr 0Ch arg_8 = dword ptr 10h arg_C = dword ptr 14h arg_10 = dword ptr 18h push ebp mov ebp, esp push edi push esi push ebx sub esp, 14Ch mov ebx, [ebp+arg_8] cmp [ebp+arg_4], 0 jz loc_804877D cmp [ebp+arg_4], 4 lea eax, ds:0[ebx*4] mov [ebp+var_130], eax jbe loc_804864C mov eax, [ebp+arg_4] mov ecx, ebx mov esi, [ebp+arg_0] lea edx, [ebp+var_110] neg ecx mov [ebp+var_118], 0 mov [ebp+var_114], 0 dec eax imul eax, ebx 137
add eax, [ebp+arg_0] mov [ebp+var_11C], edx mov [ebp+var_134], ecx mov [ebp+var_124], eax lea eax, [ebp+var_118] mov [ebp+var_14C], eax mov [ebp+var_120], ebx loc_8048433: ; CODE XREF: f+28C mov eax, [ebp+var_124] xor edx, edx push edi push [ebp+arg_10] sub eax, esi div [ebp+var_120] push esi shr eax, 1 imul eax, [ebp+var_120] lea edx, [esi+eax] push edx mov [ebp+var_138], edx call [ebp+arg_C] add esp, 10h mov edx, [ebp+var_138] test eax, eax jns short loc_8048482 xor eax, eax loc_804846D: ; CODE XREF: f+CC mov cl, [edx+eax] mov bl, [esi+eax] mov [edx+eax], bl mov [esi+eax], cl inc eax cmp [ebp+var_120], eax jnz short loc_804846D loc_8048482: ; CODE XREF: f+B5 push ebx push [ebp+arg_10] mov [ebp+var_138], edx push edx push [ebp+var_124] call [ebp+arg_C] mov edx, [ebp+var_138] add esp, 10h test eax, eax jns short loc_80484F6 mov ecx, [ebp+var_124] xor eax, eax loc_80484AB: ; CODE XREF: f+10D movzx edi, byte ptr [edx+eax] mov bl, [ecx+eax] 138
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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$[ebp] or edx,
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1.16 C++ classes I placed a C++ cla
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push ecx mov DWORD PTR _this$[ebp],
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call _ZN1c4dumpEv lea eax, [esp+20h
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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$[esp+36] mov
Page 114 and 115: and r11, r8 and rsi, r8 and r10, r1
Page 116 and 117: pop rdi pop rsi ret 0 s1 ENDP Nothi
Page 118 and 119: 2.1 LEA instruction LEA (Load Effec
Page 120 and 121: 2.3 npad It’s an assembler macro
Page 122 and 123: 2.4 Signed number representations T
Page 124 and 125: call function function: .. do somet
Page 126 and 127: 3.2 String Debugging messages are o
Page 128 and 129: .text:3011E91B DD 1E fstp qword ptr
Page 130 and 131: loc_80483F2: pop ebp retn _f endp 4
Page 132 and 133: loc_8048444: loc_8048448: loc_80484
Page 134 and 135: public f2 f2 proc near push ebp mov
Page 136 and 137: loc_804840A: loc_804842E: loc_80484
Page 138 and 139: mov esi, DWORD PTR _k$[esp+16] push
Page 140 and 141: or eax, edx retn f endp 4.1.8 Task
Page 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
Page 186 and 187: }; fread (buf, flen, 1, f); fclose
Page 188 and 189: 7.2 SAP client network traffic comp
Page 190 and 191: .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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