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  • Messing with the stack in assembly and c++

    - by user246100
    I want to do the following: I have a function that is not mine (it really doesn't matter here but just to say that I don't have control over it) and that I want to patch so that it calls a function of mine, preserving the arguments list (jumping is not an option). What I'm trying to do is, to put the stack pointer as it was before that function is called and then call mine (like going back and do again the same thing but with a different function). This doesn't work straight because the stack becomes messed up. I believe that when I do the call it replaces the return address. So, I did a step to preserve the return address saving it in a globally variable and it works but this is not ok because I want it to resist to recursitivy and you know what I mean. Anyway, i'm a newbie in assembly so that's why I'm here. Please, don't tell me about already made software to do this because I want to make things my way. Of course, this code has to be compiler and optimization independent. My code (If it is bigger than what is acceptable please tell me how to post it): // A function that is not mine but to which I have access and want to patch so that it calls a function of mine with its original arguments void real(int a,int b,int c,int d) { } // A function that I want to be called, receiving the original arguments void receiver(int a,int b,int c,int d) { printf("Arguments %d %d %d %d\n",a,b,c,d); } long helper; // A patch to apply in the "real" function and on which I will call "receiver" with the same arguments that "real" received. __declspec( naked ) void patch() { _asm { // This first two instructions save the return address in a global variable // If I don't save and restore, the program won't work correctly. // I want to do this without having to use a global variable mov eax, [ebp+4] mov helper,eax push ebp mov ebp, esp // Make that the stack becomes as it were before the real function was called add esp, 8 // Calls our receiver call receiver mov esp, ebp pop ebp // Restores the return address previously saved mov eax, helper mov [ebp+4],eax ret } } int _tmain(int argc, _TCHAR* argv[]) { FlushInstructionCache(GetCurrentProcess(),&real,5); DWORD oldProtection; VirtualProtect(&real,5,PAGE_EXECUTE_READWRITE,&oldProtection); // Patching the real function to go to my patch ((unsigned char*)real)[0] = 0xE9; *((long*)((long)(real) + sizeof(unsigned char))) = (char*)patch - (char*)real - 5; // calling real function (I'm just calling it with inline assembly because otherwise it seems to works as if it were un patched // that is strange but irrelevant for this _asm { push 666 push 1337 push 69 push 100 call real add esp, 16 } return 0; }

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  • Easy way to convert c code to assembly?

    - by Bob
    Is there an easy way (like a free program) that can covert c/c++ code to x86 assembly? I know that any c compiler does something very similar and that I can just compile the c code and then disassemble the complied executable, but that's kind of an overkill, all I want is to convert a few lines of code. Does anyone know of some program that can do that?

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  • interpreting assembly instructions

    - by David Lee
    I am trying to translate the following: Action: pushl %ebp movl %esp, %eax subl $32, %esp movl $0, -8(%eax) movl $0, -4(%eax) movl -4(%eax), %eax cmpl 32(%eax), %ebp movl -4(%ebp), %eax sall $2, %ebp addl 8(%ebp), %ebp movl (%ebp), %ebp addl %ebp, -8(%eax) addl $1, -4(%eax) What is the best way to learn assembly and translating this code?

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  • Error: type or namespace name 'AssemblyKeyFileAttribute' and 'AssemblyKeyFile' could not be found

    To associate an assembly with a strong key file to store it to GAC, we use should include following line after all the imports and before defing namespace. For VB.NET:  <Assembly: AssemblyKeyFile("c:\path\mykey.snk")> For C#:    [assembly: AssemblyKeyFile(@"c:\path\mykey.snk")] but, you might encounter following two errors at the time of creating Assembly for GAC. 1. The type or namespace name 'AssemblyKeyFileAttribute' could not be found (are you missing a using directive or an assembly reference?) 2. The type or namespace name 'AssemblyKeyFile' could not be found (are you missing a using directive or an assembly reference?) How to resolve these errors: Just include "System.Reflection" namespace. It resolve above two errors. span.fullpost {display:none;}

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  • Beginner learning assembly preserving esp after function calls

    - by Daniel
    I'm a beginner learning some assembly, when preserving the ESP register before a function call does it matter if you do it by adding or subtracting? hard to explain, consider the following mov esi, esp sub esp, 12 // on 32bit OS this would mean that there are 3 arguments to the function // push, function call etc cmp esi, esp // should be the same or mov esi, esp // push, function call etc add esp, 12 cmp esi, esp // should be the same Also if for some reason the cmp fails, is it safe to do mov esp, esi to re-align the stack? Thanks EDIT: Also how come i need to do this for a call like sprintf, but MessageBox seems to fix ESP for me? How am i to know what function needs this and what doesn't?

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  • conversion of assembly

    - by lego69
    hello, can somebody please explain is it possible to convert this snippet of the code to assembly of pdp11? movq %rdi, -8(%rbp) movl %esi, -12(%rbp) movl %edx, -16(%rbp) movl -16(%rbp), %eax cltq leaq 0(,%rax,4), %rdi movq -8(%rbp), %r8 movl -12(%rbp), %eax cltq leaq 0(,%rax,4), %rcx movq -8(%rbp), %rsi movl -16(%rbp), %eax cltq leaq 0(,%rax,4), %rdx movq -8(%rbp), %rax movl (%rdx,%rax), %eax addl (%rcx,%rsi), %eax movl %eax, (%rdi,%r8) movl -12(%rbp), %eax cltq leaq 0(,%rax,4), %rdi movq -8(%rbp), %r8 movl -16(%rbp), %eax cltq leaq 0(,%rax,4), %rcx movq -8(%rbp), %rsi movl -12(%rbp), %eax cltq leaq 0(,%rax,4), %rdx movq -8(%rbp), %rax movl (%rdx,%rax), %edx movl (%rcx,%rsi), %eax subl %edx, %eax movl %eax, (%rdi,%r8) movl -16(%rbp), %eax cltq leaq 0(,%rax,4), %rdi movq -8(%rbp), %r8 movl -16(%rbp), %eax cltq leaq 0(,%rax,4), %rcx movq -8(%rbp), %rsi movl -12(%rbp), %eax cltq leaq 0(,%rax,4), %rdx movq -8(%rbp), %rax movl (%rdx,%rax), %edx movl (%rcx,%rsi), %eax subl %edx, %eax movl %eax, (%rdi,%r8) leave ret it is only small part of all code that I have...

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  • Test assembly code on a mac

    - by happyCoding25
    Hello, A while back I was following some tutorials an assembly. I was running it all on a windows machine, compiling with NASM and then writing the compiled code to a floppy disk, then reboot and try the code. This process was long and time consuming and sadly was not on a mac. When I found out that Xcode for mac installed NASM I immediately tried to compile some code. The code compiled fine. The issue is testing it. On a mac I have no floppy (not like I want to use one) so Im not sure how to test this. I looked in to Q (kju) and found it would only emulate things on an ISO file. So I guess what Im asking is is it possible to install the compiled code on an ISO file for testing? (Note: the code when compiled forms a .bin file) Thanks for any help

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  • Stack / base pointers in assembly

    - by flyingcrab
    I know this topic has been covered ad-naseum here, and other places on the internet - but hopefully the question is a simple one as I try to get my head around assembly... So if i understand correctly the ebp (base pointer) will point to the top of the stack, and the esp (stack pointer) will point to the bottom -- since the stack grows downward. esp therefore points to the 'current location'. So on a function call, once you've saved the ebp on the stack you insert a new stack frame - for the function. So in the case of the image below, if you started from N-3 you would go to N-2 with a function call. But when you are at N-2 - is your ebp == 25 and the esp == 24 (at least initially, before any data is placed on the stack)? Is this correct or am I of on a tangent here? Thanks!

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  • Operand size conflict in x86 Assembly??

    - by Mark V.
    I'm a novice programmer who is attempting assembly for the first time. Sorry in advance if this is an incredibly lame question. I have a character stored in the EAX register, but I need to move it to my DL register. When I try: mov dl, eax I get an error C2443: operand size conflict. I know that the eax register is 32 bit while the dl is 8 bit... am I on to something?? How do I go about solving this.

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  • "Could not load file or assembly" error on trying to access auxillary assemblies from COM

    - by Codex
    We have the project structure as follows: COMExposedCCW.dll -refers- BusinessLayer.dll -refers- Utils.dll COMExposedCCW.dll -refers- Utils.dll The COMExposedCCW.dll has been registered for COM interop. From the COM application{Excel} we can successfully create the CCW object and access its properties, but when we try to invoke methods that refer to the Business/Utils layer, it throws an exception that the file or assembly{Business/Utils} could not be found. The Business/Utils dlls are present in the same folder as the CCW. On copying the Business/Utils to the COM application directory it works fine. I've tried the following: Set "HintPath/" in the CCW project I dont want to dynamically load the assemblies using Reflection {there should be a more elegant solution} Thanks in advance.

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  • Using an array in embedded x86 assembly??

    - by Mark V.
    Hey all I have a method (C++) that returns a character and takes an array of characters as its parameters. I'm messing with assembly for the first time and just trying to return the first character of the array in the dl register. Here's what I have so far: char returnFirstChar(char arrayOfLetters[]) { char max; __asm { push eax push ebx push ecx push edx mov dl, 0 mov eax, arrayOfLetters[0] xor edx, edx mov dl, al mov max, dl pop edx pop ecx pop ebx pop eax } return max; } For some reason this method returns a ? Any idea whats going on? Thanks

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  • from C to assembly

    - by lego69
    how can I get assembly code from C program I used this recommendation and I use something like this -c -fmessage-length=0 -O2 -S in Eclipse, but I've got an error, thanks in advance for any help I have this error **** Internal Builder is used for build **** gcc -O0 -g3 -Wall -c -fmessage-length=0 -O2 -S -oatam.o ..\atam.c gcc -oatam.exe atam.o D:\technion\2sem\matam\eclipse\eclipse\mingw\bin\..\lib\gcc\mingw32\3.4.5\..\..\..\..\mingw32\bin\ld.exe:atam.o: file format not recognized; treating as linker script D:\technion\2sem\matam\eclipse\eclipse\mingw\bin\..\lib\gcc\mingw32\3.4.5\..\..\..\..\mingw32\bin\ld.exe:atam.o:1: syntax error collect2: ld returned 1 exit status Build error occurred, build is stopped Time consumed: 281 ms.

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  • Assembly Jump conditionals -- jae vs. jbe

    - by Raven Dreamer
    Hi, all! I'm working on an assembly program (intel 8086). I'm trying to determine whether an input character (stored in dl) is within a certain range of hex values. cmp dl, 2Eh ;checks for periods je print ;jumps to print a "." input cmp dl, 7Ah ;checks for outside of wanted range jae input ; returns to top Please confirm that this is a correct interpretation of my code: step 1: if dl = 2E, goto print Step 2: if dl = 7A is false, goto input [if dl < 7A, goto input]

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  • Declaring local variables in assembly

    - by dcmoebius
    Is it possible to allocate locally-scoped memory in assembly? For example, consider the following (completely contrived) situation: I have two macros, one of which is dependent on the other. The first is: minimum MACRO dest, num1, num2 ; Finds the minimum of two unsigned numbers, stores the result in dest And the second is: tripMin MACRO dest, num1, num2, num3 ; Finds the minimum of three unsigned numbers, stores the result in dest minimum firstMin, num1, num2 minimum secondMin, num2, num3 minimum dest, firstMin, secondMin (I know that this isn't a realistic example for a variety of reasons, but bear with me.) Assuming that all the registers are otherwise occupied, is there any way to declare firstMin and secondMin locally within the macro? Or am I just better off freeing a register by pushing its value onto the stack and popping it back when I'm done?

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  • Segmentation Fault when using "mov" in Assembly

    - by quithakay207
    I am working on a simple assembly program for a class, and am encountering an odd segmentation fault. It's a pretty simple program to convert bytes into kilobytes. However, within the function that does the conversion, I get a segmentation fault when I try to move the value 1024 into the ebx register. I've never had this kind of problem before when working with registers. Does someone know what could be causing this? I imagine it is something simple that I'm overlooking. Thank you! asm_main: enter 0,0 pusha mov eax, 0 mov ebx, 0 call read_int push eax call functionA popa mov leave ret functionA: mov eax, [esp + 4] call print_int call print_nl mov ebx, 1024 ;segmentation fault occurs here div ebx call print_int ret UPDATE: One interesting discovery is that if I delete the lines interacting with the stack, push eax and mov eax, [esp + 4], there is no longer a segmentation fault. However, I get a crazy result in eax after performing div ebx.

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  • Getting sign of an integer in Assembly

    - by Sagekilla
    Hi all, I'm writing some assembly for a project using MASM (32-bit), and I was wondering what would be the easiest way to do this: int delta = A - B; int value = floor((delta + sign(delta)) / 2); Which is basically the following mapping: For 1 < A < 9, and B = 5: A = [1, 2] -> -2 A = [3, 4] -> -1 A = [5] -> 0 A = [6, 7] -> +1 A = [8, 9] -> +1 Any help is much appreciated!

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  • Windows API calls from assembly while minimizing program size

    - by takteek
    I'm trying to write a program in assembly and make the resulting executable as small as possible. Some of what I'm doing requires windows API calls to functions such as WriteProcessMemory. I've had some success with calling these functions, but after compiling and linking, my program comes out in the range of 14-15 KB. (From a source of less than 1 KB) I was hoping for much, much less than that. I'm very new to doing low level things like this so I don't really know what would need to be done to make the program smaller. I understand that the exe format itself takes up quite a bit of space. Can anything be done to minimize that? I should mention that I'm using NASM and GCC but I can easily change if that would help.

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