Overview of the OS CS 450 : Operating Systems Michael Saelee - - PowerPoint PPT Presentation

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Overview of the OS CS 450 : Operating Systems Michael Saelee - - PowerPoint PPT Presentation

Mar 11, 2024 105 likes •496 views

Overview of the OS CS 450 : Operating Systems Michael Saelee <lee@iit.edu> 1 Computer Science Science Agenda - what is an operating system? - what are its main responsibilities? - how does it achieve them? - how is an operating

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Overview of the OS

CS 450 : Operating Systems Michael Saelee <lee@iit.edu>

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Agenda

  • what is an operating system?
  • what are its main responsibilities?
  • how does it achieve them?
  • how is an operating system organized?
  • what is an operating system kernel?

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§What is an OS?

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  • perating system

noun the software that supports a computer's basic functions, such as scheduling tasks, executing applications, and controlling peripherals.

New Oxford American Dictionary

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tasks & applications = running programs = Processes peripherals = I/O devices

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OS duties revolve around aiding and abetting user processes

  • setting up a consistent view of system

(e.g., virtual memory)

  • simplifying access to disparate devices

(e.g., open/close/read/write API)

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Problem: there’s never enough hardware to go around

  • OS multiplexes hardware (time/space)
  • must also isolate processes from each
  • ther (and the OS itself)

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primary OS services: isolation, h.w. abstraction and concurrency (and another, arising from first: interaction)

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How to enforce isolation? Two routes: software / hardware

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Is isolation possible solely via software? I.e., can you write a program (the OS) to execute other (user) programs, and guarantee separation & robustness without hardware support?

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Some software attack vectors:

  • address fabrication (e.g., integer-to-

address cast for cross-space pointers)

  • buffer overruns (e.g., on syscalls)
  • run-time errors (e.g., intentional/

accidental stack overflows)

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Software prevention mechanisms:

  • static verification (e.g., type-checking)

— programs must “pass” to be run

  • run-time tools (e.g., garbage collection,

exception handling)

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Is isolation possible solely via software?

  • maybe — but difficult/impractical
  • the popular approach (all commercial

OSes) is to rely on hardware support

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e.g., Intel x86 architecture provides a 2-bit current privilege level (CPL) flag

  • implements 4 protection ring levels

1 2 3 most to least privileged

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CPL=3 ➞ “user” mode CPL=0 ➞ “supervisor/kernel” mode

  • access to special instructions

& hardware

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How to modify CPL? Q: Ok to allow user to directly modify CPL before invoking OS? A: No! User can set CPL=0 and run arbitrary code before calling OS

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Q: What about combining CPL “set” instruction with “jump” instruction to force instruction pointer (eip) change? A: Bad! User can set CPL=0 and jump to user code to masquerade as OS.

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Q: What about combining CPL “set” instruction with “jump” instruction that must target OS codespace? A: Not good enough. User code may jump to delicate location in OS.

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Solution: x86 provides int instruction:

  • sets CPL=0
  • loads a pre-defined OS entry point

from interrupt descriptor table (IDT)

  • IDT base address can only be set when

CPL=0 (by privileged lidt instr)

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Privileged instruction & hardware access prevented, but how is memory protected?

  • Each segment/page of memory in x86

is associated with a minimum CPL

  • Only permit current process to access

its own segments/pages

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Finally, how can OS regain control from unruly user process? (E.g., running in tight loop, never executing int)

  • hardware sends periodic clock interrupt
  • preempts user; summons OS

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Isolation accomplished. How to achieve h.w. abstraction & concurrency?

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h.w. abstraction = user traps to OS (via int) with service request; OS carries

  • ut task and returns result — “syscall”

i.e., hardware (e.g., NIC) is exposed as a software stack (e.g., TCP/IP)

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concurrency = clock interrupt drives context switches and hardware multiplexing, carried

  • ut by OS scheduler (and others)

enables multitasking on limited hardware (compare to parallelism)

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Different approaches to multitasking:

  • cooperative: processes voluntarily control
  • preemptive: OS periodically interrupts
  • real-time: more stringent requirements

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§How is an OS organized?

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i.e., what are the top-level modules of an OS, and which must run in privileged mode (e.g., CPL=0)?

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some modules:

  • virtual memory
  • scheduler
  • device drivers
  • file system
  • IPC

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privileged modules constitute the “core”

  • f the operating system; i.e. the OS kernel

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traditional approach: all are privileged

  • i.e., entire “OS” runs in kernel mode
  • known as monolithic kernel
  • pros/cons?

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alternative approach: minimum privileged

  • i.e., have a “microkernel” with minimal

set of privileged services

  • everything else runs in user mode
  • microkernel relays requests
  • pros/cons?

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courtesy of Wikimedia Commons

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… suffice it to say that among the people who actually design operating systems, the debate is essentially over. Microkernels have won

  • Andrew Tanenbaum

(noted OS researcher)

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The whole “microkernels are simpler” argument is just bull, and it is clearly shown to be bull by the fact that whenever you compare the speed of development of a microkernel and a traditional kernel, the traditional kernel wins. By a huge amount, too.

  • Linus Torvalds

(chief architect, Linux)

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your opinion? ➞ assignment 1 (paper)

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Yet another route: why not just implement OS as a low-level library?

  • loss of isolation, but big efficiency gain

(and flexibility in using h.w. directly)

  • used by many embedded systems

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And finally, what about hosting multiple OSes on a single machine? (Useful/ feasible on large, multi-core machines)

  • hypervisors provide low-level virtual

machines to guest OSes

  • yet another layer of isolation!

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