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For awhile, I have wanted to write a simple tutorial of in-line patching of binaries and in particular, changing the assembly instructions and having a binary skip to whatever function we desire manually. This involves tweaking the callq instruction call can be altered too, but it refers to a static address vs. I am also assuming that you could find strings within binaries and know how to convert values in hexadecimal.
The example to be used in this test application contains a main function, and two functions function1 and function2 which print different messages.
The goal of this exercise is to modify the application AFTER it has been compiled so that function2 is executed instead of function1. Now compile the code using gcc -Wall -o test test.
Run the command objdump -D test and watch copious amounts of information be displayed on your screen. In particular, you are looking for these lines:. Take note of these lines: Now you are wondering okay so what do these three lines have to do anything; especially the line with the mov instruction.
In short is where we will be making our modifications of the binary in the hex editor, will be used to start our calculation of where function2 is located RELATIVE to where the callq instruction is located.
Next we calculate the relative difference to be used in our modification of the binary: Open hexedit and locate the callq instruction for function1 inside of the main function. Callq can be identified by E8 and 4 additional bytes. Run the binary and you should see function2 being executed. You may ask though, what is this address's sign? In a subsequent tutorial, I will demonstrate how to manipulate the binary to have a new function which was unknown during the original compilation.
And made making the challenge… https: