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Initramfs

From postmarketOS Wiki
Revision as of 12:46, 26 September 2024 by Exkc (talk | contribs)

Initramfs consists of busybox, a shell script and very few other command-line programs.

postmarketos-mkinitfs generates the initramfs .

After then , postmarketos-mkinitfs would run boot-deploy and boot-deploy would pack kernel and initramfs into a suitable format and generate config for the bootloader if necessary.

The initramfs takes care of many things during the early booting phase, such as setting up USB networking, resizing the root partition. Some of the tasks are listed in detail here, for others refer to the source code for now and consider expanding this article. Furthermore, the initramfs supports hooks (which may be used to work around device specific quirks, for example).

Here is a list of the boot process related to initramfs:

  • Enable set -x if the verbose-initfs hook exists
  • Load deviceinfo data
  • Create busybox symlinks
  • Mount /dev, /proc and /sys
  • Redirect stdout to /pmOS_init.log and to the kernel log (dmesg)
  • Modprobe modules listed in deviceinfo
  • Enable mdev (busybox replacement for udev)
  • Map subpartitions to block device
  • Wait for /dev/fb0 to appear (if not disabled in deviceinfo) and set the framebuffer mode
  • Execute hooks
  • Show loading splash
  • Start usb networking
  • Find and mount the partition with the pmOS_boot label if not show "Boot partition not found" splash
  • Unpack initramfs-extra from the boot partition if not show "Initramfs-extra not found" splash
  • Search for a partition with the label pmOS_root
  • Try to resize the partition containing the root filesystem to the maximum size
  • Start Unl0kr if the rootfs is a luks partition
  • Resize the root filesystem to match the partition it is in
  • Mount the root filesystem
  • Kill all processes running in the initramfs and pivot root to the real rootfs

Find the boot and root partitions

Due to the various possible partition layouts, the initramfs script has to look in various places for the postmarketOS boot and root partitions.

boot partition:

  • pmos_boot kernel parameter (from the device specific bootloader)
  • Any partition or subpartition with the pmOS_boot label

root partition:

  • pmos_root kernel parameter (from the device specific bootloader)
  • Any partition or subpartition with the pmOS_root label
  • Any partition or subpartition that is encrypted

Load initramfs-extra

Some Android devices require a very small initramfs, because otherwise the resulting boot.img file is too big for the Android boot partition. Therefore, we have split the initramfs into two parts, the tiny initramfs that is potentially placed in such a boot.img file (depending on the bootloader). And the initramfs-extra, which can hold bigger binaries, such as the UI to unlock an encrypted root partition (Unl0kr).

Switch root

Note It will look like this, once pmaports#664 is resolved.

At the end of the initramfs script, the root partition is mounted at /sysroot, and the boot partition is mounted at /sysroot/boot. The latter is done, so Init does not need to figure out the correct boot partition later on. Afterwards, the busybox command switch_root is called to switch over and pass control to Init.

See also

Internal pages