mirror of
https://github.com/cirosantilli/linux-kernel-module-cheat.git
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135 lines
3.4 KiB
Markdown
135 lines
3.4 KiB
Markdown
# Linux Kernel Module Cheat
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Run one command, get into QEMU Buildroot BusyBox with several minimal Linux kernel 4.9 module example tutorials. Tested in Ubuntu 14.04 - 16.10 hosts.
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Usage:
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sudo apt-get install qemu
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./run
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First build will take a while (GCC, Linux kernel).
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QEMU opens up, and you can run:
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root
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insmod /hello.ko
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insmod /hello2.ko
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rmmod hello
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rmmod hello2
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This should print to the screen:
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hello init
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hello2 init
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hello cleanup
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hello2 cleanup
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which are `printk` messages from `init` and `cleanup` methods of those modules.
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Each module comes from a C file under `kernel_module/`. For module usage do:
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head *. use Buildroot's default kernel version, you can confirm it after build with:
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grep BR2_LINUX_KERNEL_VERSION buildroot/.config
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After the first build, you can also run just:
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./runqemu
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to save a few seconds. `./run` wouldn't rebuild everything, but checking timestamps takes a few moments.
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We use `printk` a lot, and it shows on the QEMU terminal by default. If that annoys you (e.g. you want to see stdout separately), do:
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dmesg -n 1
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See also: <https://superuser.com/questions/351387/how-to-stop-kernel-messages-from-flooding-my-console>
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## Text mode
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Show serial output of QEMU directly on the current terminal, without opening a QEMU window:
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./run -n
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To exit, just do a regular:
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poweroff
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This is particularly useful to get full panic traces when you start making the kernel crashing :-)
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In case of a panic, you want your terminal back with `Ctrl + C, A` and type `quit`. See also: <http://stackoverflow.com/questions/14165158/how-to-switch-to-qemu-monitor-console-when-running-with-curses>
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See also: <https://unix.stackexchange.com/questions/208260/how-to-scroll-up-after-a-kernel-panic>
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## Debugging
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To GDB the Linux kernel, first run:
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./runqemu -d
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This starts QEMU on the background of the shell, to prepare for running GDB.
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If you want to break immediately at a symbol, e.g. `start_kernel` of the boot sequence, run:
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./rungdb start_kernel
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Now QEMU will stop there, and you can use the normal GDB commands:
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l
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n
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c
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To skip the boot, run just:
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./rungdb
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and when you want to break, do `Ctrl + C` from GDB.
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To have some fun, you can first run inside QEMU:
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/count.sh
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which counts to infinity to stdout, and then:
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Ctrl + C
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break sys_write
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And now you can control the counting from GDB:
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continue
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continue
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continue
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See also: <http://stackoverflow.com/questions/11408041/how-to-debug-the-linux-kernel-with-gdb-and-qemu>
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If you are using text mode:
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./runqemu -d -n
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QEMU cannot be put on the background of the current shell, so you will need to open a separate terminal and run:
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./rungdb
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manually.
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## Table of contents
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1. [Introduction](introduction.md)
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1. [Build](build.md)
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1. [kmod](kmod.md)
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1. [Bibliography](bibliography.md)
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1. Examples
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1. [Host](host/)
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1. Buildroot
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1. Debugging
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1. [hello](kernel_module/hello.c)
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1. [hello2](kernel_module/hello2.c)
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1. [debugfs](kernel_module/debugfs.c)
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1. [panic](kernel_module/panic.c)
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1. [fops](kernel_module/fops.c)
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1. Asynchronous
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1. [workqueue](kernel_module/workqueue.c)
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1. [sleep](kernel_module/sleep.c)
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1. [kthread](kernel_module/kthread.c)
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1. [timer](kernel_module/timer.c)
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1. [work_from_work](kernel_module/work_from_work.c)
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