Questions tagged [x86]

A family of instruction set architectures originally based on the Intel 8086 CPU that functions with a very wide array of operating systems, including Windows, MacOS X, Linux, BSD, and Solaris.

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3
votes
1answer
648 views

What is the proper way to deal with an x86/64 instruction containing multiple prefixes from the same group?

Olly, specifically, will only accept the last prefix it finds if there are more than one from the same group. For example lock rep add dword ptr[eax],eax ; f0 f3 01 00 becomes ??? ...
2
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1answer
388 views

What C construct could generate this assembly sequence?

I have the following disassembly: mov BL, [EAX] inc EAX mov [EDX], BL inc EDX I could see this being the result of: uint8_t foo = bar; ++bar; uint8_t tmp = foo; ++foo; But this assumes C99. Is ...
30
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5answers
10k views

Why there are not any disassemblers that can generate re-assemblable asm code?

I am struggling on this problem for around three months: How to use disassemblers (IDA Pro and others...) to generate re-assemblable asm code and assemble it back My experience is that: There is NO ...
3
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1answer
490 views

What do the first 16 bytes on the stack represent?

Whenever I attach a process in OllyDbg v1.10 on my Windows 7 64-bit machine, I notice that the first saved EBP on the stack doesn't point to the very base of the stack. Instead it points 16 bytes ...
6
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1answer
999 views

understanding the stack

I have de-assembled a x86 application use ida, it generates the following code .text:1084FF10 push ebp .text:1084FF11 mov ebp, esp .text:1084FF13 ...
6
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1answer
1k views

I need quick correction on this x86 opcode conundrum

I am trying to figure out how to encode the instruction MOV DS, AX, but I am confused from the opcode table as follows: Is MOV DS all one byte, or is MOV one byte, DS the second, and the AX the third?...
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2answers
3k views

Replacing common x86 instructions with less known ones

Is there any kind of software or research or paper which discusses replacement of frequent x86 instructions with ones which are less common and thus less understandable to the attacker (floating point/...
6
votes
1answer
944 views

How to get IDA Pro Free to automatically change segment on retf?

I have been unable to get IDA pro to do a simple disassembly of a flat 16-bit binary (bootloader) file. I'm just getting started, and it seems to be harder than it should be. There's a simple set of ...
15
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4answers
4k views

Open-Source library for Complete Binary Disassembly

One of the major hurdles of x86 disassembly is separating code from data. All available open-source disassembly library only perform a straight line disassembly (starts from the top and skips errors ...
21
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1answer
8k views

How to use sysenter under Linux?

I would like to know what are the different ways to perform a system call in x86 assembler under Linux. But, with no cheating, only assembler must be used (i.e. compilation with gcc must be done with -...
6
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1answer
782 views

Unusual x86 switch statement?

I have come across the following x86 (Built with some version of Visual Studio AFAIK) switch statement: 0x1009E476 cmp edx, 0x3B 0x1009E479 jnz switch_statement switch_statement: 0x1009E591 movzx ...
7
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1answer
834 views

Can I set entry point at code in PE headers?

If I set something like 0x00000040 (my code is located at this address), then the program crashes with this error: The application was unable to start correctly (0xc000007b) But if I jmp from the ...
7
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1answer
869 views

GAS/x86 disassembled a bare gs register as an instruction, is it a bug?

I encountered a strange x86-32 instruction (opcode 0x65) decoded by objdump as gs (not %gs but gs). I found it while a full linear sweep of a binary (objdump -D), so the decoding was surely incorrect. ...
8
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1answer
3k views

What x86 calling convention passes first parameter via ESI?

I am looking at some x86 code, which I believe was built using a Microsoft tool chain, and am trying to figure out the calling convention used during this call: push esi ; save ESI (it gets ...
3
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2answers
5k views

What is an effective way to debug a 32 bits application within a 64 bits OS?

Currently I am on an x64-based system and have been doing some reverse engineering for a while. However I'm still a newbie, and the biggest issue I keep running into is the memory layout differences. ...
31
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4answers
35k views

What is the meaning of movabs in gas/x86 AT&T syntax?

I just found a strange instruction by assembling (with gas) and disassembling (with objdump) on a amd64 architecture. The original amd64 assembly code is: mov 0x89abcdef, %al And, after gas ...
2
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1answer
1k views

What is this assembly code doing with the command line arguments [duplicate]

I know something is happening with the command line arguments but I don't know what. Can someone walk me through this code? 0x401050 <main>: push %ebp 0x401051 <main+1>: ...
8
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3answers
90k views

How to obtain x86-64 pseudocode in IDA Pro?

Does anyone know about obtaining pseudocode for x86-64 in IDA Pro? I already tried the Hex-Rays plugin to obtain pseudocode, but when I try it, the following error pops up: "only 32bit code is ...
3
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1answer
2k views

Modifying step value that affects xmm# float value

I don't have much experience with assembly and I am trying to change a step value that changes a final value, these are the opcodes that I am working with and this is how they are set: movss xmm0,[...
7
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3answers
4k views

Reverse engineering a VC++ video game

So I know x86 machine language and C++/C language. In the last weeks I've dedicated to learn Windows internals and especially PE format. My target is retrieving the Tomb Raider The Angel of Darkness ...
10
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2answers
7k views

Mixed 16/32-bit code reversing using IDA

I am trying to reverse engineer a binary blob I expect to transition from 16-bit real mode into 32-bit protected mode (it is boot time code), so I expect the code to contain code of both sorts. When ...
5
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2answers
1k views

OllyDBG's disassembled syntax and c-equivalent

This is probably a pretty simple question as I'm not too used to how the syntax looks for OllyDBG's disassembler. Does this following assembler statement: MOV EAX, DWORD PTR [ESI + 14] Be roughly ...
7
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2answers
4k views

NOP instruction

I have recently learned that nop instruction is actually xchg eax, eax... what it does is basically exchanges eax with itself. As far as CPU goes, does the exchange actually happen?
9
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3answers
4k views

Why does the function pointer get overwritten even though is declared before the vulnerable buffer?

I am working on io-wargames for fun right now, level3: I do understand why there is a stack-overflow in this code (strlen(argv[1])), but what I don't understand is why it overflows the function ...
3
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1answer
354 views

Take A Snippet of Assembly and Make All Side Effects Explicit?

I came across a small function whose only side effects were to modify the state of some processor flags. When I decompiled this function using the Hex Rays decompiler, I simply got an empty function, ...
5
votes
1answer
688 views

What could this sequence of assembly instructions possibly mean?

I have recently came across the following sequence of assembly instructions. call ds:WSAStartup push ecx push edi mov ecx, 69E65AC4h mov edi, 2776452Ah pop edi pop ecx jmp ...
42
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4answers
22k views

convert this x86 ASM to C?

How could this 32-bit x86 assembly be written in C? loc_536FB0: mov cl, [eax] cmp cl, ' ' jb short loc_536FBC cmp cl, ',' jnz short loc_536FBF loc_536FBC: mov byte ptr [eax], ' ' loc_536FBF mov cl, ...
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3answers
119k views

How are the segment registers (fs, gs, cs, ss, ds, es) used in Linux?

I try to understand the process of memory segmentation for the i386 and amd64 architectures on Linux. It seems that this is heavily related to the segment registers %fs, %gs, %cs, %ss, %ds, %es. Can ...
11
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2answers
13k views

Better explanation of different x86 CALL variations

Hoping to get a better explanation of x86 call instruction. I sort of understand the call near and call far. But I don't fully understand the segment part. A little insight into my main problem, I am ...
116
votes
2answers
99k views

What is PLT/GOT?

From time to time, when disassembling x86 binaries, I stumble on reference to PLT and GOT, especially when calling procedures from a dynamic library. For example, when running a program in gdb: (gdb)...
6
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1answer
3k views

What does %reg:value mean in ATT assembly?

When I disassemble a function, I encounter from time to time an expression of the form %reg:value. Typically, I encounter this syntax when I activate the canaries in GCC (-fstack-protector), as in the ...
5
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1answer
239 views

Strange GDB behavior in OSX

I'm reversing some malware on an OSX VM when I noticed something peculiar. While stepping through the instructions, the instruction just after a int 0x80 gets skipped i.e. gets executed without me ...
21
votes
1answer
11k views

What's 'fs:[0]' doing and how can I execute it step by step?

In a 32 bits Windows binary, I see this code: push next push fs:[0] mov fs:[0], esp int3 ... next: I see that something happens on the int3 (an error), but I don't understand why,...
36
votes
5answers
7k views

What is the reason for this method to call itself?

While reversing a 32bit Mach-O binary with Hopper, I noticed this peculiar method. The instruction on 0x0000e506 seems to be calling an address right below the instruction. What would be the reason ...
6
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1answer
422 views

How can I check I've moved outside the stack without triggering a protection fault?

I'm adding a feature to my Linux debugger (I'm using Ptrace to manipulate the traced process as well as libbfd/libopcodes) to unwind the stack and determine if discrepancies exist between each CALL's ...
11
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3answers
8k views

Reverse engineer an old DOS QBasic executable

I recently decided to try my hand at reverse engineer an old DOS text based game that was coded in QBasic. I found some info online about old DOS games disassembly but mostly for Wacom compiled C/C++ ...
17
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3answers
10k views

Are there any tools or scripts for identifying compression algorithms in executables?

I know there are tools for identifying common ciphers and hash algorithms in code, but are there any similar scripts / tools / plugins for common compression algorithms such as gzip, deflate, etc? ...
29
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2answers
10k views

Why are machine code decompilers less capable than for example those for the CLR and JVM?

Java and .NET decompilers can (usually) produce an almost perfect source code, often very close to the original. Why can't the same be done for the native code? I tried a few but they either don't ...
24
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8answers
4k views

How are x86 CPU instructions encoded?

I'm writing a small utility library for hooking functions at run time. I need to find out the length of the first few instructions because I don't want to assume anything or require the developer to ...

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