Quick introduction to reverse engineering for beginners

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jmp SHORT $LN9@f $LN1@f: push OFFSET $SG749 ; ’something unknown’, 0aH, 00H call _printf add esp, 4 $LN9@f: mov esp, ebp pop ebp ret 0 npad 2 $LN11@f: DD $LN6@f ; 0 DD $LN5@f ; 1 DD $LN4@f ; 2 DD $LN3@f ; 3 DD $LN2@f ; 4 _f ENDP OK, what we see here is: there are a set of printf() calls with various arguments. All them has not only addresses in process memory, but also internal symbolic labels generated by compiler. All these labels are also places into $LN11@f internal table. At the function beginning, if a is greater than 4, control flow is passed to label $LN1@f, where printf() with argument ’something unknown’ is called. And if a value is less or equals to 4, let’s multiply it by 4 and add $LN1@f table address. That is how address inside of table is constructed, pointing exactly to the element we need. For example, let’s say a is equal to 2. 2*4 = 8 (all table elements are addresses within 32-bit process, that is why all elements contain 4 bytes). Address of $LN11@f table + 8 = it will be table element where $LN4@f label is stored. JMP fetch $LN4@f address from the table and jump to it. Then corresponding printf() is called with argument ’two’. Literally, jmp DWORD PTR $LN11@f[ecx*4] instruction mean jump to DWORD, which is stored at address $LN11@f + ecx * 4. npad 2.3 is assembler macro, aligning next label so that it will be stored at address aligned by 4 bytes (or 16). This is very suitable for processor, because it can fetch 32-bit values from memory through memory bus, cache memory, etc, in much effective way if it is aligned. Let’s see what GCC 4.4.1 generate: public f f proc near ; CODE XREF: main+10 var_18 = dword ptr -18h arg_0 = dword ptr 8 push ebp mov ebp, esp sub esp, 18h ; char * cmp [ebp+arg_0], 4 ja short loc_8048444 mov eax, [ebp+arg_0] shl eax, 2 mov eax, ds:off_804855C[eax] jmp eax loc_80483FE: ; DATA XREF: .rodata:off_804855C mov [esp+18h+var_18], offset aZero ; "zero" call _puts jmp short locret_8048450 25
loc_804840C: ; DATA XREF: .rodata:08048560 mov [esp+18h+var_18], offset aOne ; "one" call _puts jmp short locret_8048450 loc_804841A: ; DATA XREF: .rodata:08048564 mov [esp+18h+var_18], offset aTwo ; "two" call _puts jmp short locret_8048450 loc_8048428: ; DATA XREF: .rodata:08048568 mov [esp+18h+var_18], offset aThree ; "three" call _puts jmp short locret_8048450 loc_8048436: ; DATA XREF: .rodata:0804856C mov [esp+18h+var_18], offset aFour ; "four" call _puts jmp short locret_8048450 loc_8048444: ; CODE XREF: f+A mov [esp+18h+var_18], offset aSomethingUnkno ; "something unknown" call _puts locret_8048450: ; CODE XREF: f+26 ; f+34... leave retn f endp off_804855C dd offset loc_80483FE ; DATA XREF: f+12 dd offset loc_804840C dd offset loc_804841A dd offset loc_8048428 dd offset loc_8048436 It is almost the same, except little nuance: argument arg_0 is multiplied by 4 with by shifting it to left by 2 bits (it is almost the same as multiplication by 4) 1.14.3. Then label address is taken from off_804855C array, address calculated and stored into EAX, then JMP EAX do actual jump. 26
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 $SG741 ; ’a
Page 27 and 28: 1.8 switch()/case/default 1.8.1 Few
Page 29: 1.8.2 A lot of cases If switch() st
Page 33 and 34: et 0 _main ENDP Nothing very specia
Page 35 and 36: add esp, 1Ch xor eax, eax ; return
Page 37 and 38: C/C++ standard defines char type as
Page 39 and 40: eax=eax+ecx return eax ... and this
Page 41 and 42: mov eax, ecx imul edx sar edx, 1 mo
Page 43 and 44: ; current stack state: ST(0) = resu
Page 45 and 46: ; in local stack here 8 bytes still
Page 47 and 48: fld QWORD PTR _a$[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
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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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