publish debug linux kernel with qemu and busybox

content/posts/debug_linux_kernel.md

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+---
+title: "How to Debug Linux Kernel (with `qemu` & `busybox`)"
+date: 2024-09-25T17:00:55-05:00
+draft: false
+toc: false
+images:
+tags:
+  - kernel
+---
+
+# How to Debug Linux Kernel (with `qemu` & `busybox`)
+
+## Outline
+
+- clone `linux` source code
+- clone `busybox`
+- check compile configs
+- generate `initramfs` by `busybox`
+- install `qemu`
+- boot kernel by `qemu` attached with user space from `busybox`
+- other configurations
+
+## Prepare Linux src
+
+> before get started, make sure you have `flex` & `bison` installed to config and compile linux kernel
+
+- clone [linux src](https://github.com/torvalds/linux.git)
+
+```bash
+git clone https://github.com/torvalds/linux.git
+cd linux
+```
+
+- config it for minimal:
+
+```bash
+make allnoconfig
+```
+
+- and enable following config by:
+
+```bash
+make menuconfig
+```
+
+> 64-bit kernel ---> yes
+> General setup ---> Initial RAM filesystem and RAM disk (initramfs/initrd) support ---> yes
+> General setup ---> Configure standard kernel features ---> Enable support for printk ---> yes
+> Executable file formats / Emulations ---> Kernel support for ELF binaries ---> yes
+> Executable file formats / Emulations ---> Kernel support for scripts starting with #! ---> yes
+> Device Drivers ---> Generic Driver Options ---> Maintain a devtmpfs filesystem to mount at /dev ---> yes
+> Device Drivers ---> Generic Driver Options ---> Automount devtmpfs at /dev, after the kernel mounted the rootfs ---> yes
+> Device Drivers ---> Character devices ---> Enable TTY ---> yes
+> Device Drivers ---> Character devices ---> Serial drivers ---> 8250/16550 and compatible serial support ---> yes
+> Device Drivers ---> Character devices ---> Serial drivers ---> Console on 8250/16550 and compatible serial port ---> yes
+> File systems ---> Pseudo filesystems ---> /proc file system support ---> yes
+> File systems ---> Pseudo filesystems ---> sysfs file system support ---> yes
+> Processor type and features ---> Linux guest support ---> Support for running PVH guests ---> yes
+
+you may check `linux/.config` to see whether its configured successfully
+
+- then compile it:
+
+```bash
+make -j
+```
+
+## Prepare BusyBox
+
+- clone [BusyBox](https://github.com/mirror/busybox.git)
+
+```bash
+git clone https://github.com/mirror/busybox.git
+```
+
+- config it:
+
+```bash
+make defconfig
+```
+
+- we're willing to compile it statically.
+  so we need to enable the following features by:
+
+```bash
+make menuconfig
+```
+
+> Settings ---> Build Options ---> Build static binary
+
+- then generate the rootfs by:
+
+```bash
+make install
+```
+
+- modify rootfs:
+  > create a file named `init` at the root of **\_install** under busybox with following content:
+
+```bash
+#!/bin/sh 
+mount -t proc proc /proc
+mount -t sysfs sysfs /sys
+mount -t devtmpfs devtmpfs /dev 
+
+echo "Booting BusyBox..."
+exec /bin/sh
+```
+
+- create initramfs by:
+
+```bash
+cd _install
+find . | cpio -H newc -o | gzip > ../ramdisk.img
+```
+
+## Install qemu
+
+```bash
+sudo apt install qemu-kvm
+```
+
+## Boot the kernel and user space
+
+```bash
+qemu-system-x86_64 -nographic --append "console=ttyS0" -kernel ./vmlinux -initrd ../busybox/ramdisk.img
+```