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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112
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2answers
95k 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)...
79
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3answers
117k 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 ...
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, ...
36
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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 ...
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 ...
30
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4answers
34k 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 ...
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 ...
22
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1answer
36k views

What does mov qword ptr ds:[rax+18], r8 mean?

mov qword ptr ds:[rax+18], r8 In the above code, where are we copying the value of r8 register into? I know that ds means data segment which is a segment in memory, but what does exactly [rax+18] ...
21
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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,...
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 -...
18
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3answers
12k views

Is it possible to access the higher part of the 32-bit and 64-bit registers? If so, which ones?

I don't know whether this is a silly question, but I couldn't find any answer. With the evolution of CPU architecture, register size was extended, from 8, to 16, 32, and eventually 64-bit. I was ...
18
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1answer
2k views

What is the protocol for x87 floating point emulation in MS-DOS?

I'm trying to get useful results using the Reko decompiler on a dusty old MS-DOS binary compiled with Borland C++ that appears to be performing a lot of floating point arithmetic. I'm seeing code ...
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? ...
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 ...
11
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2answers
2k 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/...
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++ ...
11
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4answers
7k views

How can I generate a call graph from an unstripped x86 Linux ELF?

There's a piece of software, for which I only have the binary, not the source code. It's: unobfuscated (so, not polymorphic or anything. It is optimized a bit, though) unstripped x86 32 bit ...
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 ...
11
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1answer
829 views

Randomly picking up a x86 register for an instruction

I came around a piece of malware which i am analyzing and have found that it uses some kind of math to randomly selecting a register for a specific instruction Which i don't understand how this ...
10
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2answers
4k views

What is the purpose of these instructions before the main preamble?

I wrote a small C program below: #include <stdlib.h> int sub(int x, int y){ return 2*x+y; } int main(int argc, char ** argv){ int a; a = atoi(argv[1]); return sub(argc,a); } Compiled ...
10
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2answers
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Why does IDA add "db" statements between disassembled code

I'm trying to analyze a MS-DOS COM file that I wrote a few years ago with IDA Free 5.0, I've since renamed the segment to code_and_data and named constants and set data types correctly. However, when ...
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 ...
10
votes
1answer
39k views

What does the `TEST` instruction do

I'm having trouble understanding the TEST instruction and its use. I'm looking at the following code at the end of a loop 0040A3D1 A9 00010181 TEST EAX,81010100 0040A3D6 74 E8 ...
10
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2answers
560 views

How did this 80286 detection code work?

I'm poking through a disassembled 16-bit DOS game circa 1992. The original system requirements state that the game needs an IBM AT-compatible machine or later, with the 286 processor, to run. And ...
10
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2answers
2k views

How to do static analysis to identify pointer from concrete value in assembly?

So basically I am writing some code to do analysis work on disassembled assembly code. I am trapped in this issue for a while, here is an simple example of a disassembled asm code by objdump, ...
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 ...
9
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2answers
10k views

nop with argument in x86_64

A program I'm investigating has the following code (disassembled by IDA with some light censorship by me): __text:....B9 call <redacted> __text:....BE nop ...
9
votes
3answers
902 views

What methodology can be used to change code flow atomically during program execution?

I have been reading a lot about the different techniques for Windows API hooking (a technique I'm particularly fascinated by and fond of), and it seems a major problem in implementing a realiable ...
8
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3answers
6k views

Is there any tool to visually see program stack in linux while debugging?

By stack I mean the location where local variable, return address etc are stored. Which is pointed to by the esp and ebp.
8
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1answer
4k views

How to investigate Windows 32/64bit (WOW64) transition

There is surprisingly little information about Windows WOW64 mechanism. I'm trying to investigate it. So when we have system call in 32-land, it calls an address that is stored in FS, which leads us ...
8
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2answers
2k views

basic reversing question about local variable

I do not understand the following: in a very simple dummy C function: void vulnerable_function(char* string) { char buffer[100]; } When I use gdb to disassemble it, I obtain: 0x08048464 <+...
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 ...
7
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3answers
87k 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 ...
7
votes
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 ...
7
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2answers
1k views

What's this assembly doing?

I have been trying to figure out the assembly for part of a DOS game and there is an operation that keeps getting called that uses all 4 registers. I can see what each line does but I can't for the ...
7
votes
2answers
3k views

How to recover information stored in .ctors section?

Test is on x86, 32-bit Linux. I am using g++ 4.6.3 and objdump 2.22 Here is a simple C++ code I am working on: #include <iostream> using namespace std; main() { cout << "Hello ...
7
votes
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?
7
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1answer
4k views

How to recover the exception info from .gcc_except_table and .eh_handle sections?

For C++ program with try catch defined, when using g++ to compile it into assembly code (test is on x86 32bit Linux, g++ 4.6.3) g++ -S cppexcept.cc A specified section called .gcc_except_table is ...
7
votes
1answer
849 views

WHY and WHEN do we need SEH for buffer overflowing

In our course in university, we recently moved on from standard buffer-overflows to SEH based ones. My exploit is already ready and working thanks to some nice tutorials, although I am still not sure ...
7
votes
1answer
855 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. ...
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?...
6
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2answers
6k views

Question about LEA instruction

i'm learning to reverse engineer. So i'm coding some programs and try to understand their assembly. I stumbled upon a curious case and i think i can't solve it alone. Here's the c code: #include &...
6
votes
1answer
992 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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2answers
3k views

What are the difference syscall and int 0x2E instructions?

From this question: How does the Windows Native API communicate with the kernel? Here is an example of ZwClose(HANDLE Handle); system call in NTDLL.DLL in Windows 10 X86_64: NtClose proc near ...
6
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2answers
6k views

Disassembly shows `LEA` with RIP?

When I disassemble this instruction using pad on Radare I get an LEA with rip [0x00000000]> pad 8d 15 c8 90 04 08 lea edx, [rip + 0x80490c8] I got this instruction from this post here I'm ...
6
votes
1answer
821 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 ...
6
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2answers
8k views

When was the MOVABS instruction introduced?

When trying to write an instruction set analysis tool for disassembled code (https://superuser.com/a/832440/384221) I have found opcode MOVABS which was not included in my opcode source database (...
6
votes
1answer
419 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 ...
6
votes
1answer
11k views

Difference between INT 0X80 and SYSCALL

The SYSCALL instruction is said to be the 64-bit version of INT 0X80, however it's still possible to use the latter in 64-bit code (although strace decodes it wrong because of the 64-bit ABI I guess) ...

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