CS208 Introduction to Computer Systems
Monday, 27 Feb 2023

+ Are you buffers overflowing? Any questions?

+ How might we thwart buffer overflow attacks?
- [OS developers, compiler developers, instruction set designers] Suppose the OS places the program's instructions in a different place every time it executes. Then ctarget.phase1's contents need to change to...a random number?

- [Application programmers] Check the bounds of the input buffer and don't allow overflow. (e.g., use strncpy instead of strcpy)

- [OS, chip, instruction set designers] Make all return addresses read-only.
  Or..
  Don't allow instructions stored in the stack to be executed

- [Programming language designers] Don't allow out-of-range indexing of arrays.
    Java, Python, Rust,...
  Slower

===
+ Next
- One more programming assignment (threads)
- Takehome final
    - handed out Mon, Mar 6
    - due Wed, Mar 15
    - predictable topics: floating point representation; threads;
        network programming

====

+ Scheduler reminder
- CPU interrupts
- Atomic operations
- Disabling/enabling interrupts

    "pre-emptive multi-tasking"

+ What does "lock shared memory" look like?

    process A wants to check and manipulate SharedThing

    A:
        if lock on SharedThing is locked   <--- needs to be
            go to sleep                    <--- atomic
        else:
            lock the lock                  <--- this one too!
            use SharedThing
            unlock the lock                <--- me too!
                                                also, this needs to wake up
                                                a sleeper
        continue about business
    

+ How does a pipe work?
- What does a pipe consist of?
- Reading, writing, races
- How to fix? 

+ Threads