Just a self notes to refresh my mind on this, as it keep popping from time to time and I have to google it to answer the question.

• How program executed? - keyword "how main() executed".
• Wikipedia Execution (computing) - not much concrete information but still useful to trigger the long term memory section in your brain ;)
• Understanding the Memory Layout of Linux Executables - keyword - "linux start memory address".
• This post on building OS with Rust also informative on how OS being loaded and execute program.

Injecting into running process

• Attach to the a current running process by gathering its PID.
• Send a SIGSTOP to the program to halt it's execution
• Dump its registers (specifically rip/eip)
• Write your code to the stack where rip is pointing to.
• Send a SIGCONT to the program to return control.
• Profit.

https://www.real0day.com/hacking-tutorials/2017/11/6/injecting-a-running-process-linux

Also look into how strace work.

Summary

• main() was defined in libc - the function in libc that assume the first function in C program to call is named main().
• What address to load that contain the start of the program is defined in ELF headers.

https://stackoverflow.com/questions/7187981/whats-the-memory-before-0x08048000-used-for-in-32-bit-machine

Other similar topics

• What happens when you type google.com into your browser's address box and press enter?
• What happens when you switch on your computer? Unlike above question, I can't find single good write-up, just too many of them:-
  • https://bhavyanshu.me/what-happens-when-you-switch-on-your-computer-facts-that-most-computer-science-engineers-wouldnt-know/10/20/2014/
  • https://medium.com/10xcod3r/what-really-happens-when-you-switch-on-a-computer-defe74ccaba4
  • https://www.quora.com/What-exactly-happens-when-we-switch-on-the-computer