Quick introduction to reverse engineering for beginners

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_f ENDP Nothing specially new to us, with the exception that compiler moving input variable a to temporary local variable tv64. If to compile the same in GCC 4.4.1, we’ll get alsmost the same, even with maximal optimization turned on (-O3 option). Now let’s turn on optimization in MSVC (/Ox): cl 1.c /Fa1.asm /Ox _a$ = 8 ; size = 4 _f PROC mov eax, DWORD PTR _a$[esp-4] sub eax, 0 je SHORT $LN4@f sub eax, 1 je SHORT $LN3@f sub eax, 1 je SHORT $LN2@f mov DWORD PTR _a$[esp-4], OFFSET $SG791 ; ’something unknown’, 0aH, 00H jmp _printf $LN2@f: mov DWORD PTR _a$[esp-4], OFFSET $SG789 ; ’two’, 0aH, 00H jmp _printf $LN3@f: mov DWORD PTR _a$[esp-4], OFFSET $SG787 ; ’one’, 0aH, 00H jmp _printf $LN4@f: mov DWORD PTR _a$[esp-4], OFFSET $SG785 ; ’zero’, 0aH, 00H jmp _printf _f ENDP Here we can see even dirty hacks. First: a is placed into EAX and 0 subtracted from it. Sounds absurdly, but it may need to check if 0 was in EAX before? If yes, flag ZF will be set (this also mean that subtracting from zero is zero) and first conditional jump JE (Jump if Equal or synonym JZ — Jump if Zero) will be triggered and control flow passed to $LN4@f label, where ’zero’ message is begin printed. If first jump was not triggered, 1 subtracted from input value and if at some stage 0 will be resulted, corresponding jump will be triggered. And if no jump triggered at all, control flow passed to printf() with argument ’something unknown’. Second: we see unusual thing for us: string pointer is placed into a variable, and then printf() is called not via CALL, but via JMP. This could be explained simply. Caller pushing to stack some value and via CALL calling our function. CALL itself pushing returning address to stack and do unconditional jump to our function address. Our function at any place of its execution (since it do not contain any instruction moving stack pointer) has the following stack layout: ∙ ESP — pointing to return address ∙ ESP+4 — pointing to a variable On the other side, when we need to call printf() here, we need exactly the same stack layout, except of first printf() argument pointing to string. And that is what our code does. It replaces function’s first argument to different and jumping to printf(), as if not our function f() was called firstly, but immediately printf(). printf() printing some string to stdout and then execute RET instruction, which POPping return address from stack and control flow is returned not to f(), but to f()’s callee, escaping f(). All it’s possible because printf() is called right at the end of f() in any case. In some way, it’s all similar to longjmp() 18 . And of course, it’s all done for the sake of speed. 18 http://en.wikipedia.org/wiki/Setjmp.h 23
1.8.2 A lot of cases If switch() statement contain a lot of case’s, it is not very handy for compiler to emit too large code with a lot JE/JNE instructions. void f (int a) { switch (a) { case 0: printf ("zero\n"); break; case 1: printf ("one\n"); break; case 2: printf ("two\n"); break; case 3: printf ("three\n"); break; case 4: printf ("four\n"); break; default: printf ("something unknown\n"); break; }; }; We got (MSVC 2010): tv64 = -4 ; size = 4 _a$ = 8 ; size = 4 _f PROC push ebp mov ebp, esp push ecx mov eax, DWORD PTR _a$[ebp] mov DWORD PTR tv64[ebp], eax cmp DWORD PTR tv64[ebp], 4 ja SHORT $LN1@f mov ecx, DWORD PTR tv64[ebp] jmp DWORD PTR $LN11@f[ecx*4] $LN6@f: push OFFSET $SG739 ; ’zero’, 0aH, 00H call _printf add esp, 4 jmp SHORT $LN9@f $LN5@f: push OFFSET $SG741 ; ’one’, 0aH, 00H call _printf add esp, 4 jmp SHORT $LN9@f $LN4@f: push OFFSET $SG743 ; ’two’, 0aH, 00H call _printf add esp, 4 jmp SHORT $LN9@f $LN3@f: push OFFSET $SG745 ; ’three’, 0aH, 00H call _printf add esp, 4 jmp SHORT $LN9@f $LN2@f: push OFFSET $SG747 ; ’four’, 0aH, 00H call _printf add esp, 4 24
Page 1 and 2: Quick introduction to reverse engin
Page 3 and 4: 1.16 C++ classes . . . . . . . . .
Page 5 and 6: Preface Here (will be) some of my n
Page 7 and 8: 1.1 Hello, world! Let’s start wit
Page 9 and 10: 1.2 Stack Stack — is one of the m
Page 11 and 12: (_snprintf() function works just li
Page 13 and 14: 1.3 printf() with several arguments
Page 15 and 16: 1.4 scanf() Now let’s use scanf()
Page 17 and 18: GCC replaced first printf() call to
Page 19 and 20: }; return 0; printf ("What you ente
Page 21 and 22: 1.5 Passing arguments via stack Now
Page 23 and 24: 1.6 One more word about results ret
Page 25 and 26: push OFFSET $SG741 ; ’a
Page 27: 1.8 switch()/case/default 1.8.1 Few
Page 31 and 32: loc_804840C: ; DATA XREF: .rodata:0
Page 33 and 34: et 0 _main ENDP Nothing very specia
Page 35 and 36: add esp, 1Ch xor eax, eax ; return
Page 37 and 38: C/C++ standard defines char type as
Page 39 and 40: eax=eax+ecx return eax ... and this
Page 41 and 42: mov eax, ecx imul edx sar edx, 1 mo
Page 43 and 44: ; current stack state: ST(0) = resu
Page 45 and 46: ; in local stack here 8 bytes still
Page 47 and 48: fld QWORD PTR _a$[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
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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
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xor eax, eax pop esi mov esp, ebp p
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Let’s compile it in MSVC 2010 (I
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comp() function: public comp comp p
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Some compilers can do vectorization
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mov ecx, [esp+10h+var_10] mov edx,
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GCC In all other cases, non-SSE2 co
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add ebx, edx lea esi, [esi+0] loc_8
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movdqa xmm1, XMMWORD PTR [ecx+16] a
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1.20 x86-64 It’s a 64-bit extensi
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} x40 = a3 | x31; x41 = x24 & ~x37;
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or ebx, DWORD PTR _x6$[esp+36] mov
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and r11, r8 and rsi, r8 and r10, r1
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pop rdi pop rsi ret 0 s1 ENDP Nothi
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2.1 LEA instruction LEA (Load Effec
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2.3 npad It’s an assembler macro
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2.4 Signed number representations T
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call function function: .. do somet
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3.2 String Debugging messages are o
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.text:3011E91B DD 1E fstp qword ptr
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loc_80483F2: pop ebp retn _f endp 4
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loc_8048444: loc_8048448: loc_80484
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public f2 f2 proc near push ebp mov
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loc_804840A: loc_804842E: loc_80484
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mov esi, DWORD PTR _k$[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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