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I am learning by doing some cracking in files that i made in C. One of this files is this:

file.c:

#include <stdio.h>
#include <stdlib.h>

int main() {
        char s[15] = "disassemba";
        int x;
        x=555;
        printf("%d",x);

        if (x == 2000) {
                printf("You win");
        } else {
                printf("You lose");
        }
}

So i compile with: gcc file.c -o file

open the file with radare: r2 -w file

The commands i use is the following, in radare2, in order:

aaa
s main
V

in visual mode, i need modify the line of code where sets x=555 to x=2000. So i navigate in visual mode to:

enter image description here

And i hit shift+a, but when i try insert

mov dword [var_4h], 0x7D0

the file is not patched (no changes in asm code).

any hint?

note:Preferably in some easy way where i can write the code in asm and not in machine code.

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  • Hi and welcome to RE.SE. This came up recently, I think. There was some flag you needed to activate prior to being able to patch this way. It could be worthwhile checking out recent radare2 questions.
    – 0xC0000022L
    Commented Jan 18, 2022 at 8:01

1 Answer 1

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To patch binaries you need to open the file in read-write mode, by default r2 open the files in read-only. So you can:

  • Use r2 -w
  • Reopen the file in read-write inside the r2 shell with the oo+ command

Actually, if you do what you said, it shows an error message saying exactly what i said above to do (-w or oo+). Hope that helps

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