Build Alpine's Root Filesystem (Bootstrap)

tags: Alpine, Virtual Machine, QEMU, bootstrap, aarch64

Prepare RAW Disk

1. Create the disk image and have partitions: Boot and Root partition

$ dd if=/dev/zero of=~/qemu-images/simple-alpine.img bs=8M count=16
16+0 records in
16+0 records out
134217728 bytes (134 MB, 128 MiB) copied, 0.086382 s, 1.6 GB/s

$ fdisk -l ~/qemu-images/simple-alpine.img 
Disk /home/zack/qemu-images/simple-alpine.img: 128 MiB, 134217728 bytes, 262144 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: dos
Disk identifier: 0x7ac31012

Device                                     Boot  Start    End Sectors  Size Id Type
/home/zack/qemu-images/simple-alpine.img1 *      2048 206847  204800  100M  b W95 FAT32
/home/zack/qemu-images/simple-alpine.img2      206848 262143   55296   27M 83 Linux

2. Format the the Boot and Root partition

$ mkfs.vfat -v --offset=2048 ~/qemu-images/simple-alpine.img $((100*1024*1024/1024))
$ mkfs.ext4 -E offset=$((512*206848)) ~/qemu-images/simple-alpine.img

Have the Kernel and Initial RAM Disk from Alpine

Download the NETBOOT image from Alpine's download. And, decompress it.
Here is another place to download https://dl-cdn.alpinelinux.org/alpine/latest-stable/releases/aarch64/netboot/

Bootstrap with QEMU VM

1. Launch the QEMU aarch64 VM with the NETBOOT. We are going to take the normal aarch64 QEMU VM as the example. So, choose the NETBOOT image's vmlinuz-lts and initramfs-lts as the kernel and initial RAM disk

$ qemu-system-aarch64 \
    -smp 2 \
    -M virt \
    -cpu cortex-a57 \
    -m 1G \
    -kernel ~/alpine-image/temp/boot/vmlinuz-lts \
    -initrd ~/alpine-image/temp/boot/initramfs-lts \
    --append "console=ttyAMA0 ip=dhcp alpine_repo=http://dl-cdn.alpinelinux.org/alpine/latest-stable/main/ modloop=http://dl-cdn.alpinelinux.org/alpine/latest-stable/releases/aarch64/netboot/modloop-lts" \
    -hda ~/qemu-images/simple-alpine.img \
    -serial stdio

2. Mount the root partition of the disk image in the VM

# mount -t ext4 /dev/vda2 /mnt/

3. Build the root filesystem with Alpine's bootstrap. And, the target root path is /mnt/

# apk add apk-tools-static
# apk.static 
    --arch $(uname -m) \
    -X http://dl-cdn.alpinelinux.org/alpine/latest-stable/main/ \
    -U \
    --allow-untrusted \
    --root /mnt/ \
    --initdb add alpine-base

4. Add ttyAMA0::respawn:/sbin/getty -L 0 ttyAMA0 vt100 into /mnt/etc/inittab to get the VM's serial tty
5. Set /etc/fstab

# cat /mnt/etc/fstab
/dev/vda2	/		ext4	rw,relatime	0 0
/dev/vda1	/boot		vfat	rw,relatime	0 0
# mkdir /mnt/boot

6. Set Network
   • Set interface
   ​​​​# cat /mnt/etc/network/interfaces 
   ​​​​auto eth0
   ​​​​iface eth0 inet dhcp
   

   • Set hostname
   ​​​​# echo "alpine-arm64" > /mnt/etc/hostname
   

   • Enable networking service for each boot
   ​​​​# chroot /mnt/
   ​​​​# rc-update add networking
   

7. Exit the chroot and poweroff the VM
8. Can launch a new QEMU aarch64 VM with own built kernel and the disk image

$ qemu-system-aarch64 \
    -smp 2 \
    -M virt \
    -cpu cortex-a57 \
    -m 1G \
    -kernel ~/linux-stable/arch/arm64/boot/Image \
    --append "console=ttyAMA0 root=/dev/vda2 rw rootfstype=ext4" \
    -hda ~/qemu-images/simple-alpine.img \
    -serial stdio

Reference

• fdisk
• mkfs.vfat
• mkfs.ext4
• mount
• Bootstrapping Alpine Linux
• OpenRC in Alpine