CODEWITHSUNDEEP
ccompilationhexexepatch
LLL
LLL

Reputation: 658

Define an address to be modified after the compilation of a C program

I've this little sample C program:

#include <stdio.h>

#define CIAO 0x15151515;
#define PROVA 0x14141414;

int main(){
    unsigned char *ptr = (unsigned char *)CIAO;
    long int nbytes = PROVA;

    long int i;

    int cpt = 0;

    char key[] = { 'a','b','a','b','a','b','a','b','\0' };
    int keyLength = 8;

    for (i = 0; i < nbytes; i++){
        ptr[i] = ptr[i] ^ key[cpt];
        cpt = cpt + 1;
        if (cpt == keyLength)
            cpt = 0;
    }
    return 0;
}

My goal is to compile it and then using a HexEditor, find the specific values 0x15151515 and 0x14141414 and replace them with another value.

I've tried with other smaller sample program i managed to do this really easily. Using this instead i'm having some problem. 0x15151515 got written as 0x16151515 and i searched for 0x14141414 but I didn't manage to find anything similar.

Here is the relevant hex view of the exe: https://pastebin.com/dLi4tfU5

Do you have any clue on why sometimes the value written seems to change? Is there any way to avoid this and be sure that whenever i write "CIAO" then in the exe viewed as hex I will always found 0x15151515?

p.s. I know it's not something that you should normally do and of course i know that editing the exe could lead to unexpected behaviour of the program. I'm just doing it for fun

Upvotes: 1

Views: 89

Answers (1)

Govind Parmar
Govind Parmar

Reputation: 21542

An optimizing compiler may make changes to constants in your code.

You can use the volatile keyword as a hint to the compiler that the value of a variable may be modified or accessed by something external to the program itself, and thus to avoid optimizing it too heavily:

#include <stdint.h>

volatile const uint32_t CIAO = 0x15151515;
volatile const uint32_t PROVA = 0x14141414;

Try replacing your #define statements with this code and then open the binary and look for these values.

Upvotes: 1

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