Update README.md

README.md

@@ -1,2 +1,132 @@
-# debug-linux-kernel
-debug linux kernel with gdb
+# Install the build toolchain
+```sh
+sudo apt-get install libncurses5-dev  build-essential git bison flex libssl-dev
+```
+## compile the kernel
+
+### 1. download linux source
+```sh
+wget https://cdn.kernel.org/pub/linux/kernel/v5.x/linux-5.10.129.tar.xz
+cd linux-5.10.129
+```
+
+### 2. configure the kernel
+```sh
+make defconfig ARCH=x86_64
+./scripts/config -e DEBUG_INFO -e DEBUG_KERNEL -e DEBUG_INFO_DWARF4
+```
+
+### 3. complie the kernel
+```sh
+make ARCH=x86_64 Image
+# make ARCH=arm64 Image -j8  CROSS_COMPILE=aarch64-linux-gnu-
+```
+
+# make root file system using busybox
+linux的启动需要配合根文件系统，这里我们利用busybox来制作一个简单的根文件系统
+
+## Compile busybox
+
+### 1. download busybox
+```sh
+wget https://busybox.net/downloads/busybox-1.35.0.tar.bz2
+tar -xjf busybox-1.35.0.tar.bz2
+cd busybox-1.35.0
+```
+### 2. config busybox
+```sh
+make defconfig ARCH=x86_64
+Settings --->
+ [*] Build static binary (no shared libs) 
+```
+
+### 3. Specify the arch and cross-toolchain
+```sh
+export ARCH=arm64
+export CROSS_COMPILE=aarch64-linux-gnu-
+```
+
+### 4. complie busybox
+After the compilation is completed, the _install directory is generated in the busybox directory
+```sh
+make && make install
+```
+
+## custom file system
+
+### 1. Add etc, dev and lib directories
+```sh
+cd _install
+mkdir etc dev lib mnt
+```
+
+### 2. etc/profile
+busybox 作为linuxrc启动后， 会读取/etc/profile, 这里面设置了一些环境变量和shell的属性
+```sh
+#!/bin/sh
+export HOSTNAME=lifan
+export USER=root
+export HOME=/home
+export PS1="[$USER@$HOSTNAME \W]\# "
+PATH=/bin:/sbin:/usr/bin:/usr/sbin
+LD_LIBRARY_PATH=/lib:/usr/lib:$LD_LIBRARY_PATH
+export PATH LD_LIBRARY_PATH
+```
+
+### 3. etc/inittab
+```sh
+#!/bin/sh
+::sysinit:/etc/init.d/rcS
+::respawn:-/bin/sh
+::askfirst:-/bin/sh
+::ctrlaltdel:/bin/umount -a -r
+```
+### 4. etc/fstab
+```sh
+#device  mount-point    type     options   dump   fsck order
+proc /proc proc defaults 0 0
+tmpfs /tmp tmpfs defaults 0 0
+sysfs /sys sysfs defaults 0 0
+tmpfs /dev tmpfs defaults 0 0
+debugfs /sys/kernel/debug debugfs defaults 0 0
+kmod_mount /mnt 9p trans=virtio 0 0
+```
+### 5. etc/init.d/rcS
+```sh
+mkdir -p /sys
+mkdir -p /tmp
+mkdir -p /proc
+mkdir -p /mnt
+/bin/mount -a
+mkdir -p /dev/pts
+mount -t devpts devpts /dev/pts
+echo /sbin/mdev > /proc/sys/kernel/hotplug
+mdev -s
+```
+
+#### update permissions
+```sh
+chmod +x etc/init.d/rcS
+```
+
+### 6. Add dev file 
+```sh
+sudo mknod console c 5 1
+sudo mknod -m 666 null c 1 3 
+```
+
+### 7. Add dynamic library
+```sh
+cp /usr/lib/gcc/x86_64-linux-gnu/9/*.so*  -a ./lib
+```
+
+### 8.compressed file system
+```sh
+find . -print0 | cpio --null -ov --format=newc | gzip -9 > ../initramfs.cpio.gz
+```
+
+这里对这几个文件做一点说明：
+1. busybox 作为linuxrc启动后， 会读取/etc/profile, 这里面设置了一些环境变量和shell的属性
+2. 根据/etc/fstab提供的挂载信息， 进行文件系统的挂载
+3. busybox会从/etc/inittab中读取sysinit并执行， 这里sysinit指向了/etc/init.d/rcS
+4. /etc/init.d/rcS 中 ，mdev -s 这条命令很重要， 它会扫描/sys目录，查找字符设备和块设备，并在/dev下mknod