Changes between Version 3 and Version 4 of linux/initramfs

Timestamp:

02/24/2024 12:14:58 AM (3 months ago)

Author:

Tim Harvey

Comment:

added example of modifying an existing initramfs

linux/initramfs

@@ -7 +7 @@
 
 Note that the Linux kernel runs {{{/init}}}, {{{/sbin/init}}}, or {{{/bin/sh}}} (searched in that order) as PID1 from the initramfs (if it exists). If you want to run a different executable as PID1 you can use the {{{init=}}} kernel parameter. The init application must be executable and statically linked or it will move on to the next item in the search path.
+
+The word 'initramfs' and 'initrd' can be used interchangeably here.
 
 see also:
@@ -170 +172 @@
 ls -l output/images/rootfs.cpio.gz
 }}}
+
+
+== rebuilding or modifying an initramfs
+You can modify an external initramfs fairly easily with the {{{cpio}}} tool. You merely need to uncompress it (if compressed) and use the extract parameter. You will want to do this as root in order to preserve file ownership of root files.
+
+Example:
+ 1. extract files from an existing gzipped cpio:
+{{{#!bash
+# uncompress it (if compressed) and use {{{cpio -i}}} to extract it:
+# -i extract
+# -d create directories
+# -m preserve mtime
+# -v verbose
+mkdir rootfs_mod
+(cd rootfs_mod; gzip -cd ../cpio | sudo cpio -idmv)
+}}}
+ 2. modify it; for example overwrite or create your own /init:
+cat <<EOF >rootfs_mod/init
+#!/bin/busybox sh
+mount -t devtmpfs  devtmpfs  /dev
+mount -t proc      proc      /proc
+mount -t sysfs     sysfs     /sys
+mount -t tmpfs     tmpfs     /tmp
+
+echo "Hello world!"
+sh
+EOF
+chmod +x rootfs_mod/init
+}}}
+ 1. re-create it
+{{{#!bash
+(cd rootfs_mod; find . | cpio -ov --format=newc) | gzip -9 > cpio
+}}}