In addition to decrypting and loading the embedded executable into memory, which is the userland exec component, the stub may also perform other tasks. It is common for the stub to start anti-debugging and anti-emulation routines that are meant to further protect the binary from being debugged or emulated in order to raise the bar even further so that reverse engineering is even more difficult.
In Chapter 4, ELF Virus Technology – Linux/Unix Viruses, we discussed some anti-debugging techniques used to prevent debugging based on ptrace. This prevents most debuggers, including GDB, from trivially tracing the binary. Later in this chapter, we will summarize the most common anti-debugging techniques used in binary protection for Linux.