定制 Ubuntu 18.04 UEFI 启动盘初探(1)
工作中需要制作定制化的Ubuntu启动盘，之前有用 buildroot 做过，但是在安装工具方面还是没有Ubuntu灵活方便，所以还是准备基于Ubuntu来制作。
网上的文章大部分是基于Ubuntu 安装盘来制作，server版本约1GB大小，desktop版本约需要1.9GB。它有以下优点和缺点，即修改目标。
优点：
- 使用 squash 文件系统，和initramrd相比，不需要全部加载进内存，对内存需求较低
- 有完整的启动脚本，很多改动需求只要修改这些脚本的配置参数就可以达成
缺点：
- 镜像太大无法满足频繁改动升级的需要
- 不需要安装功能，考虑剪除
- 不是所有应用都需要安装，考虑剪除
- 需要为不同项目灵活更换配套驱动及工具，需要另行安装
- 启动界面应按项目内容显示提示菜单
首先研究 boot/grub/grub.cfg中的启动参数，其中 boot=casper 将影响 init脚本中的 BOOT参数值。
menuentry "Try Ubuntu without installing" {
set gfxpayload=keep
linux /casper/vmlinuz file=/cdrom/preseed/ubuntu.seed boot=casper quiet splash ---
initrd /casper/initrd.lz
然后研究 casper/initrd.lz，它负责加载squash文件系统。注意“. /scripts/${BOOT}”会提供“mountroot()”函数。
. /scripts/local
. /scripts/nfs
. /scripts/${BOOT}
parse_numeric ${ROOT}
maybe_break mountroot
mount_top
mount_premount
mountroot
以下是“mountroot()”函数及其注释，看得出来，大部分casper启动功能都在这里完成。
| 执行下面3个脚本，都可不予支持 |
run_scripts /scripts/casper-premount |
| 10driver_updates 只有带启动参数 debian-installer/driver-update=* 才会真正执行（去找启动盘上的 /cdrom/ubuntu-drivers/$(uname -r)下的驱动更新） |
|
| 20iso_scan 只有带启动参数 iso-scan/filename=* 才会真正执行 |
|
| 30custom_installation 只有带启动参数 debian-installer/custom-installation=* 才会真正执行 |
|
| if [ ! -z "${NETBOOT}" ]; then | |
| 如果网络引导，可不予支持，不过支持这个功能调试比较方便 |
if do_netmount ; then |
| livefs_root="${mountpoint}" | |
| else | |
| panic "Unable to find a live file system on the network" | |
| fi | |
| else | |
| 本地引导 | # Scan local devices for the image |
| i=0 | |
| 尝试60次，每次间隔1秒 |
while [ "$i" -lt 60 ]; do |
| 》参考 find_livefs 的实现《 | livefs_root=$(find_livefs $i) |
| 搜索启动盘，优先搜启动参数live-media=*指定的设备， 如果没有指定或没能搜到，会在合适时机遍历所有设备搜索 |
if [ "${livefs_root}" ]; then |
| 其中 /bin/fstype 查看磁盘文件格式的功能不错，值得注意 | break |
| fi | |
| sleep 1 | |
| i="$(($i + 1))" | |
| done | |
| fi | |
| if [ -z "${livefs_root}" ]; then | |
| panic "Unable to find a medium containing a live file system" | |
| fi | |
| 如果启动参数指定 toram | if [ "${TORAM}" ]; then |
| 则将文件系统加载在内存 | live_dest="ram" |
| 如果启动参数指定 todisk=* | elif [ "${TODISK}" ]; then |
| 则将文件系统加载在磁盘 | live_dest="${TODISK}" |
| fi | |
| if [ "${live_dest}" ]; then | |
| log_begin_msg "Copying live_media to ${live_dest}" | |
| 》参考 copy_live_to 的实现《 | copy_live_to "${livefs_root}" "${live_dest}" |
| log_end_msg | |
| fi | |
| 挂载镜像到目录，会调用setup_unionfs() 会从/cdrom/casper/下匹配 "ext2" "squashfs" "dir" 后缀的文件或文件夹，并挂载 |
mount_images_in_directory "${livefs_root}" "${rootmnt}" |
| # initialize the /var/crash directory in overlayfs so that inotify for | |
| crash处理机制，并不理解实际工作机制。 | # /var/crash works and update-notifier will notify of crashes |
| touch /root/var/crash/crash.init | |
| rm /root/var/crash/crash.init | |
| log_end_msg | |
| # Allow to override USERNAME and HOST based on media information | |
| 从initrd的/etc/casper.conf提取到FLAVOUR变量 | # make it skipable by setting FLAVOUR= in casper.conf |
| 如果FLAVOUR为空，从启动盘的/.disk/info提取 | if [ -f /cdrom/.disk/info ] && [ -z "$FLAVOUR" ]; then |
| 它将影响 HOST，USERNAME，USERFULLNAME,BUILD_SYSTEM | FLAVOUR="$(cut -d' ' -f1 "/cdrom/.disk/info" 2>/dev/null | tr '[A-Z]' '[a-z]')" || FLAVOUR= |
| /etc/casper.conf里的值是 | if [ -n "$FLAVOUR" ]; then |
| export USERNAME="ubuntu" | HOST=$FLAVOUR |
| export USERFULLNAME="Live session user" | USERNAME=$FLAVOUR |
| export HOST="ubuntu" | export HOST USERNAME |
| export BUILD_SYSTEM="Ubuntu" | sed -i "s,USERNAME=.*,USERNAME=\"$FLAVOUR\",g; s,HOST=.*,HOST=\"$FLAVOUR\",g" /etc/casper.conf |
| fi | |
| fi | |
| # Apply command lines override of HOST, USERNAME and USERFULLNAME | |
| 如果命令行有传参设置， 则优先使用其设置 HOST，USERNAME，USERFULLNAME |
[ -n "$CMD_HOST" ] && HOST=$CMD_HOST && export HOST |
| [ -n "$CMD_USERNAME" ] && USERNAME=$CMD_USERNAME && export USERNAME | |
| [ -n "$CMD_USERFULLNAME" ] && USERFULLNAME=$CMD_USERFULLNAME && export USERFULLNAME | |
| if [ -n "$CMD_HOST" ] || [ -n "$CMD_USERNAME" ] || [ -n "$CMD_USERFULLNAME" ]; then | |
| sed -i "s,USERNAME=.*,USERNAME=\"$USERNAME\",g; s,USERFULLNAME=.*,USERFULLNAME=\"$USERFULLNAME\",g; s,HOST=.*,HOST=\"$HOST\",g" /etc/casper.conf | |
| fi | |
| # unionfs-fuse needs /dev to be bind-mounted for the duration of | |
| # casper-bottom; udev's init script will take care of things after that | |
| 如果是unionfs-fuse文件系统， | if [ "${UNIONFS}" = unionfs-fuse ]; then |
| 用“mount -n -o bind ...”挂载/dev设备目录 | mount -n -o bind /dev "${rootmnt}/dev" |
| fi | |
| # Open up two fifo's fd's for debconf-communicate to use. Speeds up | |
| # the Casper process considerably. | |
| log_begin_msg "Creating debconf-communicate fifo mechanism" | |
| 用 fifo 加速casper过程 | mkfifo /tmp/debconf-in.fifo |
| 结束前这些fifo会被释放删除 | mkfifo /tmp/debconf-out.fifo |
| # Make the template database read-only, so that passthrough debconf | |
| # instances can write to it directly; otherwise templates are only | |
| # passed through when necessary. Use temporary config databases as | |
| # well; we'll copy their contents back at the end. | |
| DEBCONF_TMPDIR="$(chroot /root mktemp -dt debconf.XXXXXX)" | |
| cp -a /root/var/cache/debconf/config.dat "/root$DEBCONF_TMPDIR/" | |
| cp -a /root/var/cache/debconf/passwords.dat "/root$DEBCONF_TMPDIR/" | |
| sed "s,^Filename: /var/cache/debconf/\(config\|passwords\).dat$,Filename: $DEBCONF_TMPDIR/\1.dat,; /^Name: templatedb/a\ | |
| Readonly: true" /root/etc/debconf.conf >"/root$DEBCONF_TMPDIR/debconf.conf" | |
| DEBCONF_SYSTEMRC="$DEBCONF_TMPDIR/debconf.conf" chroot /root debconf-communicate -fnoninteractive casper > /tmp/debconf-out.fifo < /tmp/debconf-in.fifo & | |
| debconfpid="$!" | |
| if [ ! -p /tmp/debconf-in.fifo ] || [ ! -p /tmp/debconf-out.fifo ]; then | |
| log_warning_msg "failed to setup debconf-communicate channel" | |
| fi | |
| log_end_msg | |
| # Order matters! | |
| # These file descriptors must stay open until we're finished with | |
| # debconf-communicate. | |
| exec 4/tmp/debconf-in.fifo | |
| maybe_break casper-bottom | |
| [ "$quiet" != "y" ] && log_begin_msg "Running /scripts/casper-bottom" | |
| 》参考对应脚本《 | run_scripts /scripts/casper-bottom |
| [ "$quiet" != "y" ] && log_end_msg | |
| if [ "${UNIONFS}" = unionfs-fuse ]; then | |
| umount "${rootmnt}/dev" | |
| fi | |
| # Close the fd's associated with debconf-communicate. | |
| exec 3>&- 4<&- | |
| rm -f /tmp/debconf-in.fifo | |
| rm -f /tmp/debconf-out.fifo | |
| wait $debconfpid | |
| # Copy config database changes back to the master files. | |
| chroot /root debconf-copydb tmpdb config \ | |
| --config=Name:tmpdb --config=Driver:File \ | |
| --config="Filename:$DEBCONF_TMPDIR/config.dat" | |
| chroot /root debconf-copydb tmpdb passwords \ | |
| --config=Name:tmpdb --config=Driver:File \ | |
| --config="Filename:$DEBCONF_TMPDIR/passwords.dat" | |
| rm -rf "$DEBCONF_TMPDIR" | |
| exec 1>&6 6>&- | |
| exec 2>&7 7>&- | |
| kill "$tailpid" | |
| cp casper.log "${rootmnt}/var/log/" | |
| if [ -f /etc/casper.conf ]; then | |
| cp /etc/casper.conf "${rootmnt}/etc/" | |
| fi |
要修改 casper/initrd.lz， 先了解怎么解压和重新打包
# 解压 initrd.lz
mkdir initrd-dir
cd initrd-dir
cp ../initrd.lz initrd.lzma
unlzma -d initrd.lzma
cpio -id < initrd
rm initrd
# 重新打包为 initrd2.lz
cd initrd-dir
find . | cpio --quiet --dereference -o -H newc > ../initrd2
cd ..
lzma -7 initrd2
mv initrd2.lzma initrd2.lz
这样我们可以做一些改动，比如升级busybox。
/scripts/casper会解析启动参数 live-media-path，我们可以通过指定该值来改变 squashfs文件的搜索路径$LIVE_MEDIA_PATH。
live-media-path=*)
LIVE_MEDIA_PATH="${x#live-media-path=}"
export LIVE_MEDIA_PATH
echo "export LIVE_MEDIA_PATH=\"$LIVE_MEDIA_PATH\"" >> /etc/casper.conf ;;
下一节尝试 用cubic 工具定制启动盘，因为使用这个工具可以将注意力集中在配置软件上。