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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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Can any one please explain the following disassembly code [on hold]

I was learning x86 assembly and had huge difficulty in understanding many of these lines.
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30 views

Can anyone please explain this disassembly code? [on hold]

Disassembly: 0: 6a 05 push 0x5 2: 58 pop eax 3: 31 c9 xor ecx,ecx 5: 51 push ecx ...
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1answer
79 views

Can anyone please explain the following dissasembly codes?

Disassembly: 0: 31 c9 xor ecx,ecx 2: f7 e1 mul ecx 4: 51 push ecx 5: 68 2f 2f 73 68 push 0x68732f2f a: 68 2f 62 69 ...
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1answer
46 views

How to set a breakpoint in an execve'd program in radare2?

If I have a program A, which uses execve to run program B. How can I set a breakpoint in program B's main routine if I'm debugging program A? After a certain point in the execve routine, program B ...
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1answer
61 views

Understand the CPU cycles of x86 instruction bsr/bsf

I am on the hook to analysis some "timing channels" of some x86 binary code. I am posting one question to comprehend bsf/bsr opcode. So high-levelly, these two opcodes can be modeled as a "loop", ...
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2answers
209 views

XOR encryption - How to find the key?

I'm trying to solve a crackme which basically uses a XOR encryption scheme in which the password is used as the key to an encrypted buffer which is subsequently executed. The following is the ...
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0answers
37 views

How can a segfault happen during the libc exit call on Linux?

I'm relatively new to reverse engineering, and I've decided to practise using the challenges from pwnable.tw. Currently, I am working on the "Silver Bullet" challenge. While testing, I have noticed ...
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1answer
45 views

x64dbg - Conditional breakpoint based on function argument

How can I set a conditional breakpoint based on the argument of a function? I am trying to break on the windows function LoadLibraryExW: https://docs.microsoft.com/en-us/windows/desktop/api/...
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54 views

Reverse Engineer the Galax RGB Lighting Utility

during the last week I tried to find a way to control the lighting on my Galax 1080Ti cause it's software sucks. I come up with this https://github.com/purpl3F0x/GalaxLightingControl . It needs the ...
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1answer
71 views

Radare2 Disassembler How Do You Organize Visual Panels (V!)?

I would like to use Radare2 to learn about how C is assembled into assembly but I am having trouble with the layout. I think the V! mode would be very handy as you can watch the registers update as ...
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1answer
73 views

How do the PSHUFLW and PSHUFD instructions work?

I have been trying to figure out exactly what is happening with these instructions and can't make sense of them. I can see that the PSHUFLW instruction acts upon the first 16 bytes of the XMM register ...
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1answer
32 views

How is the I/O address space on the PC arranged?

I'm disassembling a 16-bit DOS executable, and looking at the following three instructions: mov dx,0x3ce mov ax,0xff08 out dx,ax I know it writes the value 0xff08 to I/O port 0x3ce. I also know that ...
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1answer
77 views

Undocumented MSR Machine Specific register

I'm reversing the BIOS of my laptop, for fun and for learning something new. Inside it, I just stepped into this piece of code: mov ecx, 13Ah rdmsr and eax, 1 jnz SkipCacheAsRAM Looking ...
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1answer
54 views

IDA - large jump table, missing cases in analysis

I have come across the following jump table with a large amount of cases (160+). There seems to be cases missing from IDAs analysis though. For example it skips from cases 22 -> 38 -> 91, and I can ...
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2answers
123 views

Lots of mov instructions into ebp+offset - What is happening?

While learning RE I have come across this large segment of instructions that contains a majority of mov [ebp+offset], value instructions. I believe that ebp + offset are local variables? I think? ...
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1answer
85 views

IDA Interpreting mov instructions in a unusual way

When analysing mach-o binaries IDA seems to interpret const/literal part of x86 mov instruction(0x8B) as an offset. Hex rerpresentation: 8B 40 0A Correct assembly: mov eax,DWORD PTR [eax+0x0A] ; 8B ...
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1answer
280 views

Simplifying assembly

I'm analysing some binary trace, it's pretty much obfuscated. What I need to do is to understand work flow of its alghoritms. However I cannot find any reliable tool which would help me with that. I ...
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1answer
76 views

What purpose of MOVZX EAX,AX after MOVZX EAX, WORD PTR [RBP+ADDR.SA_DATA]?

I thought it was strange when I was looking at the code below. The first instruction clearly has completed the task of data transfer and expansion into 16 bits. Why is there a "mov Eax,ax" in the ...
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1answer
220 views

How to convert assembly to pseudo code? [closed]

I have the following assembly. I don't know how to follow assembly. I was wondering if there is a way to convert assembly to a human readable pseudo code or better yet C code: init: mov eax, [ebp+...
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1answer
91 views

Call instruction changed while running

While debugging and reverse-engineering my exe, I have found a very weird thing. call ds:GetModuleFileNameA This call instruction is formed with below hex bytes in IDA. FF 15 24 C0 41 00 (I think ...
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1answer
113 views

How to locate an specific data structure in an executable?

I need to locate an specific "struct" variable in the data section from an assembly. This structure is used for an specific System Function (Windows) "SetCommState()". I'm wondering how to locate the ...
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1answer
245 views

What does fs and gs registers provide in Linux?

In a 64 bit ELF binary I found it mainly uses fs register to get some values. How can I know which value it wants to access? I'm familiar with that NT kernel uses fs and gs register for TEB structure ...
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1answer
68 views

While loop function disassembly x86

I am trying to convert this binary to C but I am having trouble with the while loop. What does sub esp, 8 mean? as well as push [ebp+_a] ? I am able to get the esp 8 sometimes while changing the ...
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1answer
68 views

Where to find address of registers populated in a binary

This question might sound very naive, however, I got stuck when one of my friends asked me where can I find the address of an initialized register in the stack. For e.g., info registers in gdb gives ...
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0answers
70 views

IDA 7 handling strings in debugger pseudocode

When running a remote debugger to a linux server, is there a way to get IDA (7.0) to properly handle strings? When I go to the pseudocode view, I get things like this; v7 = sub_804AD60((std::string *)...
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1answer
122 views

Finding the main function without symbols

for a C/C++ program compiled on windows using the x86 architecture, what is the best way to find the main function within disassembled code? I've found various answers to questions similar to this, ...
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1answer
82 views

Changing local variable in caller from callee in C?

I'm implementing a function which is going to perform some time-consuming task, and I'd like to hack the benchmarking part a bit by changing the variable indicating the CPU time my function has ...
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1answer
365 views

XCHG RAX, RAX: 0x09, what does this code do and how does it work?

Given the following short assembly snippet: shr rax, 3 adc rax, 0 I worked this out a bit: We know SHR sets the CF with the last bit shifted. We know ADC dest, 0 is just adding the CF. So ...
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1answer
95 views

XCHG RAX, RAX: 0x03, what does this code do and how does it work?

"XCHG RAX, RAX" is a kind of riddle book that provides assembly code for you to reverse and undercover the meaning. Some of the examples calculate the Fibonacci sequence others bit-twiddle to toggle ...
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0answers
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getting %localappdata% in asm

I have an old windows x86 application that no longer works properly since windows 8. This app creates files within its own folders for things like app settings. Unfortunately, Microsoft no longer ...
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2answers
128 views

Why there are two different shellcodes for bin/sh with much different syntax and which one is better to use? (x86)

I have found two shellcodes for spawning /bin/sh which are much different from each other. The second one is hardly ever found on the net, and it mostly comes up in reverse engineering websites: 1. "\...
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1answer
98 views

Is the value of EBP before the main function important?

So before we come to the main function, it looks we are in a function with a name __libc_start_main and before we come to our main function we obviously push EBP value in the stack ( the value of EBP ...
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2answers
141 views

Getting opcode Length , How to

I am apparently not very good with assembly as I am novice to somethings! Hence i wanted to make a question again. Now i got this from here to make something like a LDE (Length Disassembly Engine) ...
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4answers
867 views

Calculation of jmp address through subtraction

I don't get why the two addresses of the functions are subtracted in order to get the jump destination. mov eax, [ebp+func1] sub eax, [ebp+func2] sub eax, 5 mov [ebp+var_4], eax Which ...
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1answer
167 views

What is the password to this file? I can't figure it out with radare2 or gdb

Here is the binary file. https://drive.google.com/file/d/1ywN60yZYIhPPRyZMGbQudZRYoLYBhkeJ/view?usp=sharing I am able to figure out some of the assembly, but can't get the correct password. I know ...
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2answers
155 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 ...
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1answer
159 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 ...
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0answers
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Recognizing instructions containing low-level control flows

There are some x86 instructions which containing a low-level** control flow in its semantics, for example div because its semantics is roughly as: if (source == 0) throw exception... else ...
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2answers
345 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 ...
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1answer
219 views

XMM register instructions and their c equivalents

I'm trying to convert some x86 assembly back into C++, and I cannot figure out how this set of instructions was originally written. movd xmm0,eax ; byte read from device '0x04' cvtdq2pd xmm0,xmm0 ; ...
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2answers
179 views

How does the XADD instruction modify the Zero Flag?

I've come across a set of instructions using lock xadd which I'm guessing came from one of the C++ smart pointer templates. (It's been a very long time since I coded C++ so I'm not sure which). lea ...
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1answer
129 views

Using ECX after being modified by a function call

I have a peculiar snippet of code which I cannot wrap my head around: push 0xC ; arg1 for call mov ecx,edi ; set the this pointer for call call sdk.100039F0 ; make the call (...
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1answer
77 views

GS Cookie and exception handlers

I'm trying to understand how this GS cookie implementation is working. From what I've read on the topic, a cookie is set during the prologue then checked again in the epilogue. Well I can see the ...
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5answers
3k views

Older versions of IDA free?

I'm looking for an IDA free version previous to 7.0 so I can use it in x86 systems, the current one being only x64, but can't seem to find one and it doesn't look like I saved one neither. Does ...
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1answer
148 views

What does this magic number do? [duplicate]

I've opened a 64 bit DLL file in IDA. One function has this pseudocode: unsigned __int64 output; ULONG input; output = (unsigned __int64)(input * (unsigned __int128)0xE38E38E38E38E38Fui64 >> 64)...
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1answer
91 views

How to debug a crash in wow64.dll in 32bit process

I have a 32bit application and a few wrapper tools that I use one after another to inject code, hook system dll functions (using the patch-first-bytes-of-the-function-with-jump approach). Also I have ...
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1answer
64 views

Passing a (string) argument via the return address?

I'm debugging an old PC BIOS and it has most of its constant strings interspersed directly with the code, like this: As the comment notes, the function putsc will take the string as its argument, ...
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1answer
83 views

Why are these 0-byte jumps used as a delay?

I'm debugging an old PC BIOS and see a lot of what I assume are delay sequences, using 0-byte jump instructions (EB 00 = jmp short $+2). Why this particular instruction? I'm guessing it must have ...
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1answer
252 views

Trap Flag - Anti-debugging trick

I am trying to reverse a program which is employing some kind of anti-debugging trick based on the use of SEHs and the TF. The code stars with: So apparently it's generating an exception with XOR ...
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1answer
88 views

Data moved after program rebase

Consider the following piece of code: .text:00F74B42 call sub_12D1130 .text:00F74B47 mov eax, dword_15A0C80 Now, I want to add a sanity check but as I don't have enough space to do that I ...