A quick review on how native executables are loaded by the OS

For better understanding on how packers modify files, let us have a quick review of how executable files are loaded by the operating system. Native executables are better known as PE files for Windows and ELF files for Linux. These files are compiled down to their low-level format; that is, using assembly language like x86 instructions. Every executable is structured with a header, code section, data section, and other pertinent sections. The code section contains the actual low-level instruction codes, while the data section contains actual data used by the code. The header contains information about the file, the sections, and how the file should be mapped as a process in the memory. This is shown in the following diagram:

 

 

The header information can be classified as raw and virtual. Raw information consists of appropriate information about the physical file, such as file offsets and size. The offsets are relative to file offset...