§2023-10-16

• Creating Initramfs by Kiky Tokamuro

• Gentoo Custom Initramfs

Initramfs, short for “initial RAM file system”, is the receiver of initrd (initial ramdisk). This is the cpio (copy in and out) archive of the source file system, which is loaded into memory during the Linux startup process. Linux copies the contents of the archive to rootfs (which can be based on ramfs or tmpfs), and then runs init. Init is designed to perform certain tasks before the real or final file system is installed on top of rootfs. Thus, initramfs should contain all the device drivers and tools needed to install the final root file system.

1. Download BusyBox

$ wget https://busybox.net/downloads/busybox-1.36.1.tar.bz2
$ cd ..
$ tar xvf ./src/busybox-1.36.1.tar.bz2 

2. make defconfig and menuconfig

cd busybox-1.26.2
make defconfig
make menuconfig

• in make menuconfig
  • In the Busybox Settings menu
    • select Build Options, and check the box next to Build BusyBox as a static binary (no shared libs).
    • Next, specify the output folder for binaries and collect busybox:

     --- Build Options                                                                       [*] Build static binary (no shared libs)    
     Destination path for 'make install'  (./_install) 

3. make

time make -j8
...
  LINK    busybox_unstripped
Static linking against glibc, can't use --gc-sections
Trying libraries: m resolv rt
 Library m is needed, can't exclude it (yet)
 Library resolv is needed, can't exclude it (yet)
 Library rt is not needed, excluding it
 Library m is needed, can't exclude it (yet)
 Library resolv is needed, can't exclude it (yet)
Final link with: m resolv

real	0m31.466s
user	2m21.103s
sys	0m32.328s

4. make install

$ make install
...
 ./_install//usr/sbin/telnetd -> ../../bin/busybox
  ./_install//usr/sbin/tftpd -> ../../bin/busybox
  ./_install//usr/sbin/ubiattach -> ../../bin/busybox
  ./_install//usr/sbin/ubidetach -> ../../bin/busybox
  ./_install//usr/sbin/ubimkvol -> ../../bin/busybox
  ./_install//usr/sbin/ubirename -> ../../bin/busybox
  ./_install//usr/sbin/ubirmvol -> ../../bin/busybox
  ./_install//usr/sbin/ubirsvol -> ../../bin/busybox
  ./_install//usr/sbin/ubiupdatevol -> ../../bin/busybox
  ./_install//usr/sbin/udhcpd -> ../../bin/busybox


--------------------------------------------------
You will probably need to make your busybox binary
setuid root to ensure all configured applets will
work properly.
--------------------------------------------------

[alexlai@orgpi58G busybox-1.36.1]$ ls _install/
bin  linuxrc  sbin  usr

5. make CONFIG_PREFIX=./../busybox_rootfs install

$ make CONFIG_PREFIX=./../busybox_rootfs install
...
 ./../busybox_rootfs//usr/sbin/ubirename -> ../../bin/busybox
  ./../busybox_rootfs//usr/sbin/ubirmvol -> ../../bin/busybox
  ./../busybox_rootfs//usr/sbin/ubirsvol -> ../../bin/busybox
  ./../busybox_rootfs//usr/sbin/ubiupdatevol -> ../../bin/busybox
  ./../busybox_rootfs//usr/sbin/udhcpd -> ../../bin/busybox


--------------------------------------------------
You will probably need to make your busybox binary
setuid root to ensure all configured applets will
work properly.
--------------------------------------------------

$ ls ../busybox_rootfs/
bin  linuxrc  sbin  usr

6. Create a directory hierarchy for initramfs

[alexlai@orgpi58G build]$ pwd
/home/alexlai/build
[alexlai@orgpi58G build]$ cd initramfs/dev
[alexlai@orgpi58G dev]$ sudo mknod sda b 8 0 
[sudo] password for alexlai: 
[alexlai@orgpi58G dev]$ sudo mknod console c 5 1
[alexlai@orgpi58G dev]$ ls -l
total 0
crw-r--r-- 1 root root 5, 1 10月 16 16:02 console
brw-r--r-- 1 root root 8, 0 10月 16 16:02 sda