How to create an "empty" space in an executable at a definite address (gcc,linux)?
executable, gcc, linux, portable-executable
Solution
At least for PEs and ELFs, you can append data to the end of the executable without affecting the program at all.
A standard approach is to append your data to the executable, and then append a number indicating how many bytes have been appended. The executable then opens itself for reading, looks at the last N bytes indicating the data length, and then `seek`s backwards by that value, to the beginning of the appended data.
This article goes into pretty good detail on how to use the above method to make a self-extracting executable. That's a little different from what you want, but the principle of reading data contained in the executable remains the same.
Problem
What I essentially want to do is have another program write data into this "empty space" for the executable to "work" on I thought of appending a signature to the application and then writing the data, searching for it later, but that doesn't quite sound right... Now, other important thing ... I know it should be possible to create a code cave by using code like : ``` void function(void) { __asm { nop nop nop nop }; } ``` then, even this is practically the same (apart from the fact that it will be in the .data section, so not executable): ``` const char data[3]; ``` The problem then is that the other application will not have a definite address to write to.