CSE 220: Systems Programming PDF

The Kernel and User Mode CSE 220: Systems Programming Ethan Blanton, Carl Alphonce, & Eric Mikida Department of Computer Science and Engineering University at Buffalo Introduction Protection Domains Exceptions System Calls Summary References The Oper...

The Kernel and User Mode CSE 220: Systems Programming Ethan Blanton, Carl Alphonce, & Eric Mikida Department of Computer Science and Engineering University at Buffalo Introduction Protection Domains Exceptions System Calls Summary References The Operating System Kernel We have talked about the operating system or the kernel. The operating system manages the hardware. On our systems, it also: Supports the dedicated computer model Provides protection against misbehaving programs The kernel is the code of the inner core of the OS. In some sense the OS and Kernel are just programs. © 2024 Ethan Blanton, Carl Alphonce, & Eric Mikida / CSE 220: Systems Programming 2 Introduction Protection Domains Exceptions System Calls Summary References User Mode Programs Our programs run in user mode. User mode programs appear to run on a dedicated computer. This means that shared resources must be managed for them. User mode programs ask the kernel for access to shared resources. If the user mode program has a dedicated computer … where is the kernel? © 2024 Ethan Blanton, Carl Alphonce, & Eric Mikida / CSE 220: Systems Programming 3 Introduction Protection Domains Exceptions System Calls Summary References Exceptions Exceptions are another type of control flow. Unlike if, for, etc., they: Allow non-local (to another function or even program) transfer of control Can be asynchronous (triggered by an external event) Exceptions may be caused by hardware or software. The handling of exceptions requires both. © 2024 Ethan Blanton, Carl Alphonce, & Eric Mikida / CSE 220: Systems Programming 4 Introduction Protection Domains Exceptions System Calls Summary References System Calls A system call is a special kind of exception. It allows a program to: “break out” of its dedicated computer, and contact the kernel System calls are synchronous but non-local transfer of control. © 2024 Ethan Blanton, Carl Alphonce, & Eric Mikida / CSE 220: Systems Programming 5 Introduction Protection Domains Exceptions System Calls Summary References Lecture Question Ask a review question! © 2024 Ethan Blanton, Carl Alphonce, & Eric Mikida / CSE 220: Systems Programming 6 Introduction Protection Domains Exceptions System Calls Summary References The Kernel and Supervisor Mode The kernel does not have a dedicated computer.¶ The kernel has the real computer! It runs in a special mode (often called supervisor mode). It can: Access hardware directly Manipulate virtual memory mappings Modify process memory … © 2024 Ethan Blanton, Carl Alphonce, & Eric Mikida / CSE 220: Systems Programming 7 Introduction Protection Domains Exceptions System Calls Summary References Protection Domains Protection domains represent the amount of privilege to access the “real computer” allowed to a process. Supervisor mode is a special protection domain. User mode is a less-privileged protection domain. Protection domains are a hardware capability. User programs run in user mode, the kernel in supervisor mode. The hardware enforces access restrictions on user mode. Some hardware has more than two protection domains. © 2024 Ethan Blanton, Carl Alphonce, & Eric Mikida / CSE 220: Systems Programming 8 Introduction Protection Domains Exceptions System Calls Summary References Changing Protection Domains Changing protection domains is a supervisor mode operation. This prevents programs from breaking out of user mode. It also means there must be a safe way to switch domains! We will see how exceptions provide a controlled mode change. Changing protection domains can be slow and expensive. © 2024 Ethan Blanton, Carl Alphonce, & Eric Mikida / CSE 220: Systems Programming 9 Introduction Protection Domains Exceptions System Calls Summary References Exception Flow When an exception occurs, control passes to the kernel. If control is already in the kernel, it changes location. If control is in a user mode program, it switches contexts. User mode Kernel Exception at In Kernel takes over Program resumes at In+1 © 2024 Ethan Blanton, Carl Alphonce, & Eric Mikida / CSE 220: Systems Programming 10 Introduction Protection Domains Exceptions System Calls Summary References Types of Exceptions There are four major types of exceptions: Interrupts are asynchronous notifications from hardware Traps are synchronous exceptions caused by software intentionally Faults are synchronous exceptions caused by software due to potentially recoverable errors Aborts are synchronous exceptions caused by unrecoverable errors outside of software control We have only seen faults thus far (remember page faults?). We are currently most interested in traps. © 2024 Ethan Blanton, Carl Alphonce, & Eric Mikida / CSE 220: Systems Programming 11 Introduction Protection Domains Exceptions System Calls Summary References Interrupts Interrupts are a way for hardware to signal the OS. Examples: A network packet has arrived A clock has “ticked” A disk has completed a read Interrupts are handled by the kernel. We will not discuss them more, although they are in the text. © 2024 Ethan Blanton, Carl Alphonce, & Eric Mikida / CSE 220: Systems Programming 12 Introduction Protection Domains Exceptions System Calls Summary References Faults We have already seen faults! Segmentation fault (core dumped) Faults are program errors that may be recoverable. When a fault occurs, the kernel may: try to fix it notify the program It may also terminate the program or shut down. © 2024 Ethan Blanton, Carl Alphonce, & Eric Mikida / CSE 220: Systems Programming 13 Introduction Protection Domains Exceptions System Calls Summary References Fault Recovery Some faults are not true errors: e.g., page faults to bring in new pages. Other faults may be recoverable by the program: Divide by zero Segmentation fault Bus error … Each of these is an error, but might not be fatal. For example, a calculation might usefully return some concrete value if it reaches a divide-by-zero. © 2024 Ethan Blanton, Carl Alphonce, & Eric Mikida / CSE 220: Systems Programming 14 Introduction Protection Domains Exceptions System Calls Summary References Aborts Aborts are relatively uninteresting to us. They represent some unrecoverable error that often ends in: Rebooting the computer Shutting down the computer Terminating some or all processes etc. Aborts are handled by the kernel. We will not discuss them more. © 2024 Ethan Blanton, Carl Alphonce, & Eric Mikida / CSE 220: Systems Programming 15 Introduction Protection Domains Exceptions System Calls Summary References Traps Traps are software-generated exceptions. (They are sometimes called software interrupts.) They are generated by special instructions run by a program. Their critical feature is: Trap handlers are run by the kernel in supervisor mode. This means that a user mode program can call into the kernel. This provides a safe method of changing protection domains. © 2024 Ethan Blanton, Carl Alphonce, & Eric Mikida / CSE 220: Systems Programming 16 Introduction Protection Domains Exceptions System Calls Summary References Lecture Question Ask a lecture question! © 2024 Ethan Blanton, Carl Alphonce, & Eric Mikida / CSE 220: Systems Programming 17 Introduction Protection Domains Exceptions System Calls Summary References System Calls System calls are: traps used by user-mode programs to invoke kernel functions Many platforms have a dedicated hardware instruction for this: ARM: svc x86-64: syscall © 2024 Ethan Blanton, Carl Alphonce, & Eric Mikida / CSE 220: Systems Programming 18 Introduction Protection Domains Exceptions System Calls Summary References System Call Handling When the system call instruction runs, the hardware: Switches to supervisor mode Invokes a specific kernel routine When the kernel receives a system call, it: Identifies what the program wants Verifies the program arguments Authenticates the request Performs the operation (or indicates failure) This allows the kernel to decide whether a program can access something outside its “dedicated computer”. © 2024 Ethan Blanton, Carl Alphonce, & Eric Mikida / CSE 220: Systems Programming 19 Introduction Protection Domains Exceptions System Calls Summary References The Implications of the Trap User mode cannot control what code the kernel runs. This is: controlled by the hardware configured by the kernel This is how modern operating systems protect themselves from malicious or buggy programs. © 2024 Ethan Blanton, Carl Alphonce, & Eric Mikida / CSE 220: Systems Programming 20 Introduction Protection Domains Exceptions System Calls Summary References Invoking a System call We have invoked system calls! open(), sbrk(), mmap(), etc. are system calls! Anything outside the dedicated computer needs a system call. We never used the syscall instruction. The C library makes system calls look like a C function. All functions in manual section 2 are system calls. © 2024 Ethan Blanton, Carl Alphonce, & Eric Mikida / CSE 220: Systems Programming 21 Introduction Protection Domains Exceptions System Calls Summary References Overhead System calls are very slow. They can take tens to hundreds of thousands of clock cycles. This is due to: Changing protection domains Validating arguments Adjusting memory mappings Cache effects … Programs should make fewer system calls when practical. © 2024 Ethan Blanton, Carl Alphonce, & Eric Mikida / CSE 220: Systems Programming 22 Introduction Protection Domains Exceptions System Calls Summary References Summary Exceptions are special control flow Protection domains control access to hardware resources Exception handlers run in supervisor mode in the kernel Special trap exceptions can be used to implement system calls System calls allow user mode programs to request access to the kernel © 2024 Ethan Blanton, Carl Alphonce, & Eric Mikida / CSE 220: Systems Programming 23 Introduction Protection Domains Exceptions System Calls Summary References References I Required Readings Ian Weinand. Computer Science from the Bottom Up. Chapter 4: part 1; part 2 except 2.1 and its subsections; part 3 through 3.1. URL: https://www.bottomupcs.com/index.html. Optional Readings Randal E. Bryant and David R. O’Hallaron. Computer Science: A Programmer’s Perspective. Third Edition. Chapter 8: Intro, 8.1, 8.2. Pearson, 2016. Ian Weinand. Computer Science from the Bottom Up. Chapter 4. URL: https://www.bottomupcs.com/index.html. © 2024 Ethan Blanton, Carl Alphonce, & Eric Mikida / CSE 220: Systems Programming 24 Introduction Protection Domains Exceptions System Calls Summary References License Copyright 2019–2024 Ethan Blanton, All Rights Reserved. Copyright 2024–2024 Eric Mikida, All Rights Reserved. Copyright 2022–2024 Carl Alphonce, All Rights Reserved. Copyright 2019 Karthik Dantu, All Rights Reserved. Reproduction of this material without written consent of the author is prohibited. To retrieve a copy of this material, or related materials, see https://www.cse.buffalo.edu/~eblanton/. © 2024 Ethan Blanton, Carl Alphonce, & Eric Mikida / CSE 220: Systems Programming 25