Quick introduction to reverse engineering for beginners

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Now let’s dig deeper and find connection between 0x15 offset in load_command_line() (I gave that name to that function) and this+0x3D variable in CDwsGui::PrepareInfoWindow. Are we sure that the value is the same? I’m starting to search for all occurences of 0x15 value in code. For some small programs like SAPGUI, it sometimes works. Here is the first occurence I got: .text:64404C19 sub_64404C19 proc near .text:64404C19 .text:64404C19 arg_0 = dword ptr 4 .text:64404C19 .text:64404C19 push ebx .text:64404C1A push ebp .text:64404C1B push esi .text:64404C1C push edi .text:64404C1D mov edi, [esp+10h+arg_0] .text:64404C21 mov eax, [edi] .text:64404C23 mov esi, ecx ; ESI/ECX are pointers to some unknown object. .text:64404C25 mov [esi], eax .text:64404C27 mov eax, [edi+4] .text:64404C2A mov [esi+4], eax .text:64404C2D mov eax, [edi+8] .text:64404C30 mov [esi+8], eax .text:64404C33 lea eax, [edi+0Ch] .text:64404C36 push eax .text:64404C37 lea ecx, [esi+0Ch] ; demangled name: ATL::CStringT::operator=(class ATL::CStringT ... &) .text:64404C3A call ds:mfc90_817 .text:64404C40 mov eax, [edi+10h] .text:64404C43 mov [esi+10h], eax .text:64404C46 mov al, [edi+14h] .text:64404C49 mov [esi+14h], al .text:64404C4C mov al, [edi+15h] ; copy byte from 0x15 offset .text:64404C4F mov [esi+15h], al ; to 0x15 offset in CDwsGui object That function was called from the function named CDwsGui::CopyOptions! And thanks again for debugging information. But the real answer in the function CDwsGui::Init(): .text:6440B0BF loc_6440B0BF: .text:6440B0BF mov eax, [ebp+arg_0] .text:6440B0C2 push [ebp+arg_4] .text:6440B0C5 mov [esi+2844h], eax .text:6440B0CB lea eax, [esi+28h] ; ESI is pointer to CDwsGui object .text:6440B0CE push eax .text:6440B0CF call CDwsGui__CopyOptions Finally, we understand: array filled in load_command_line() is actually placed in CDwsGui class but on this+0x28 address. 0x15 + 0x28 is exactly 0x3D. OK, we found the place where the value is copied to. Let’s also find other places where 0x3D offset is used. Here is one of them in CDwsGui::SapguiRun function (again, thanks to debugging calls): .text:64409D58 cmp [esi+3Dh], bl ; ESI is pointer to CDwsGui object .text:64409D5B lea ecx, [esi+2B8h] .text:64409D61 setz al 193
.text:64409D64 push eax ; arg_10 of CConnectionContext:: CreateNetwork .text:64409D65 push dword ptr [esi+64h] ; demangled name: const char* ATL::CSimpleStringT::operator PCXSTR .text:64409D68 call ds:mfc90_910 .text:64409D68 ; no arguments .text:64409D6E push eax .text:64409D6F lea ecx, [esi+2BCh] ; demangled name: const char* ATL::CSimpleStringT::operator PCXSTR .text:64409D75 call ds:mfc90_910 .text:64409D75 ; no arguments .text:64409D7B push eax .text:64409D7C push esi .text:64409D7D lea ecx, [esi+8] .text:64409D80 call CConnectionContext__CreateNetwork Let’s check our findings. Replace setz al here to xor eax, eax / nop, clear TDW_NOCOMPRESS environment variable and run SAPGUI. Wow! There is no nagging window (just as expected, because variable is not set), but in Wireshark we can see that the network packets are not compressed anymore! Obviously, this is the place where compression flag is to be set in CConnectionContext object. So, compression flag is passed in the 5th argument of function CConnectionContext::CreateNetwork. Inside that function, another one is called: ... .text:64403476 push [ebp+compression] .text:64403479 push [ebp+arg_C] .text:6440347C push [ebp+arg_8] .text:6440347F push [ebp+arg_4] .text:64403482 push [ebp+arg_0] .text:64403485 call CNetwork__CNetwork Compression flag is passing here in the 5th argument to CNetwork::CNetwork constructor. And here is how CNetwork constructor sets some flag in CNetwork object according to the 5th argument and some another variable which probably could affect network packets compression too. .text:64411DF1 cmp [ebp+compression], esi .text:64411DF7 jz short set_EAX_to_0 .text:64411DF9 mov al, [ebx+78h] ; another value may affect compression? .text:64411DFC cmp al, ’3’ .text:64411DFE jz short set_EAX_to_1 .text:64411E00 cmp al, ’4’ .text:64411E02 jnz short set_EAX_to_0 .text:64411E04 .text:64411E04 set_EAX_to_1: .text:64411E04 xor eax, eax .text:64411E06 inc eax ; EAX -> 1 .text:64411E07 jmp short loc_64411E0B .text:64411E09 ; --------------------------------------------------------------------------- .text:64411E09 .text:64411E09 set_EAX_to_0: .text:64411E09 .text:64411E09 xor eax, eax ; EAX -> 0 194
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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
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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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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
Page 192 and 193: .text:64405171 call dbg .text:64405
Page 194 and 195: .text:64404FF7 push offset aLogging
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Page 200 and 201: .text:64411E0B .text:64411E0B loc_6
Page 202 and 203: Chapter 8 Other things 8.1 Compiler
Page 204 and 205: Chapter 9 Tasks solutions 9.1 Easy
Page 206 and 207: 9.1.5 Task 1.5 Hint #1: Keep in min
Page 208 and 209: } 9.1.8 Task 1.8 Solution: two 100*
Page 210 and 211: #define PUSH(low, high) ((void) ((t
Page 212 and 213: } } ignore one or both. Otherwise,
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