Operating Systems 2018F Lecture 12

Video

Video from the lecture given on October 17, 2018 is now available.

Notes

Today: Kernel modules

First, Anil's ssh setup:

student@anil-vm: // openstack

• why is it named anil-vm?
  • edit /etc/hosts and /etc/hostname

anilclass@sutherland: // local machine

• edit /etc/hosts
  • add the IP of your VM and give it name
  • distributed DNS does this for URLs
• login without password
  • ssh key pair, private and public
  • files in ~/.ssh/

Public keys?

• sharing won't compromise security

Running X-windows programs on vm:

• ssh -X student@anil-vm // works natively in linux
• on macOS, you can run X-server separately
• on Windows, use x2go

Whats a kernel module?

• kernel code, that you load, into the kernel
• same privileges as other kernel code
• dangerous! load kernel module = anything can happen
  • only superuser (root) can load
• NOT a system call

Kernel Oops:

• kernel catches the bad memory access

Kernel crash:

• didn't catch it

Build kernel from scratch? Not usually - everything is in modules now

What modules are running? lsmod

• gets info from /proc/modules
• /proc/modules is reading a kernel data structure
• strace lsmod:
  • shows that it starts with /proc/modules
  • but then goes into /sys/module/
• /proc : human readable
• /sys : machine readable
  • wouldn't find a list of modules here, but rather deep directory structure

Where are kernel modules stored?

• /lib/modules/<kernel version>/
• why so many kernel versions?
  • ex. 4.15.0-33
    • 4.15.0 is version.
    • -33 = package release (patches etc)

Why so many modules? Why not build them in?

• originally, everything was built in - now we basically never do that
  • some may conflict
  • many are unnecessary
  • memory used by kernel cannot be swapped to disk
    • so we minimize this by only loading necessary kernel modules

.ko: kernel object .so: shared object (ex. C libraries)

Where is kernel itself? /boot/vmlinuz-4.15.0-33-generic

• naming is traditional. 'z' is for compressed.
• variants may have different suffix, ex. 'rt' for realtime, instead of 'generic'
• kernel is around 8mb

Review tutorial

• sudo insmod simple.ko // load kernel module
• dmesg | tail // shows that kernel is loaded
• sudo rmmod simple.ko // removes kernel module

simple.c

• Why #include <linux/ ...>?
• No headers that are familiar, ex. stdlib, etc.
  • You can't use these in kernel modules. Why?
    • You can't use any C library that makes a system call
  • What if you wanted to printf?
    • printk() is a reimplementation, because
      • printf would "bottom out" by making a system call to write()
      • printk is used to generate logs
      • how does it do that?
        • keeps a buffer until they can be printed somewhere
        • bootup messages? produced by printk
    • kernel has its own weird world of standard libraries
• Some special syntax around declaring init and exit functions..
• How does kernel actually load a module? (ie. through module_init() call in simple.c)
  • don't worry about it. always something you don't understand...
  • and this is one of these things few people need to understand!
  • you just needs to know how to use it.
• Why doesn't simple_exit return anything? Nobody cares what it returns!
  • vs. if init failed... we would care

Rule of kernel programming: do as little as possible

• nicer to write in userspace

ones.c

• kernel module that implements a device driver for /dev/ones
  • from linux kernels perspective.. same as any other device
  • a module might be a device driver, but might not be
  • and a driver might be baked into the kernel (not a module)
  • but usually, device drivers are implemented as modules
• makes a character device: register_chrdev()
  • every device has a major and a minor number: the above function returns major number
• ones_fops { }: important. struct containing function pointers.
  • we never call these functions in ones.c
  • these implement the file operations on /dev/ones
    • after normal syscall dispatcher stuff, it will want to know HOW to open the device, read, etc
    • what if we wanted to do a write?
      • well, /dev/ones is opened read-only. let's chmod a+w /dev/ones.
      • echo 5 > /dev/ones // write error
      • we only have the operations that we've implemented
  • who calls them?
• class_create() ?
  • can't just create a device from the major number
  • need a class first before running device_create()
  • grouping of all devices that use the same driver
  • see /sys/class/comp3000
• /sys/devices/virtual/comp3000/ones
  • what's this about? metadata.
  • thankfully we didn't have to create this ourself - device_create() took care of all that
• failed_devreg, etc:
  • handling failures is really important in kernel programming.
  • if something fails and you don't manually deregister / deallocate memory, it never gets "cleaned up"
    • stays in physical memory until you reboot
• How did we come up with this code, generally?
  • no manpages
  • the linux kernel source is the ultimate reference for how this works
  • best way to figure it out is to look at existing device drivers, modify to your purpose, trial & error...
• ones_read():
  • similar to read() in userspace
  • in userspace you take a file descriptor; in kernel you take a pointer to a file struct
  • buf, len just as in read()..
  • offset? point to specific part of the file
  • the file is always going to be /dev/ones in this case (since it is a device driver)
  • why put_user('1', buf++) instead of *buf++ = 1?
    • The 1's are going out into userspace. This is what put_user() is for.

aside: find . -name "ones"

• will find files named "ones" in current directory (.)

Look at newgetpid.c