Merge pull request #83 from containers/docs

docs: add doc for lsinitrd and centos auto qcow2

docs/devel/LISTING-FILES-INITRAMFS.md

@@ -0,0 +1,100 @@
+# Reading content of initramfs image
+
+During the development of initoverlayfs might be useful to identify files included in the initramfs.  
+For this example, let's use a real use case from CentOS Automotive Stream Distribution (qcow2 image).
+
+Build the CentOS Automotive Stream Distribution image
+```bash
+git clone https://gitlab.com/CentOS/automotive/sample-images
+cd sample-images/osbuild-manifests
+make cs9-qemu-qm-minimal-ostree.x86_64.qcow2
+```
+
+Enable NBD on the Host
+```bash
+modprobe nbd max_part=8
+```
+
+Connect the QCOW2 as network block device
+```bash
+qemu-nbd --connect=/dev/nbd0 ./cs9-qemu-qm-minimal-ostree.x86_64.qcow2
+```
+
+Understanding the output from fdisk
+```
+# fdisk /dev/nbd0 -l
+Disk /dev/nbd0: 8 GiB, 8589934592 bytes, 16777216 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: gpt
+Disk identifier: D209C89E-EA5E-4FBD-B161-B461CCE297E0
+
+Device       Start      End  Sectors  Size Type
+/dev/nbd0p1   2048   206847   204800  100M EFI System           <--- EFI boot partition
+/dev/nbd0p2 206848   821247   614400  300M Linux filesystem     <--- initramfs partition
+/dev/nbd0p3 821248 16775167 15953920  7.6G Linux filesystem     <--- rootfs partition (main fs partition)
+```
+
+Mount the initramfs partition
+```bash
+mount /dev/nbd0p2 /mnt
+```
+
+find the initramfs file
+```bash
+# find . -name initramfs*
+./ostree/centos-082d9caaac6cb51dc22cd7d9881692d44c9d7cbc6b3d8edf8709286cb8bd12f9/initramfs-5.14.0-438.391.el9iv.x86_64.img
+```
+
+now use lsinitrd tool to list the content of the initramfs image
+```
+lsinitrd ./ostree/centos-082d9caaac6cb51dc22cd7d9881692d44c9d7cbc6b3d8edf8709286cb8bd12f9/initramfs-5.14.0-438.391.el9iv.x86_64.img
+Image: ./ostree/centos-082d9caaac6cb51dc22cd7d9881692d44c9d7cbc6b3d8edf8709286cb8bd12f9/initramfs-5.14.0-438.391.el9iv.x86_64.img: 17M
+========================================================================
+dracut modules:
+systemd
+systemd-initrd
+kernel-modules
+kernel-modules-extra
+rootfs-block
+udev-rules
+dracut-systemd
+ostree
+usrmount
+base
+fs-lib
+microcode_ctl-fw_dir_override
+shutdown
+========================================================================
+drwxr-xr-x   1 root     root            0 Jan  3 20:49 .
+lrwxrwxrwx   1 root     root            7 Jan  3 20:49 bin -> usr/bin
+drwxr-xr-x   1 root     root            0 Jan  3 20:49 dev
+drwxr-xr-x   1 root     root            0 Jan  3 20:49 etc
+drwxr-xr-x   1 root     root            0 Jan  3 20:49 etc/cmdline.d
+drwxr-xr-x   1 root     root            0 Jan  3 20:49 etc/conf.d
+-rw-r--r--   1 root     root          124 Jan  3 20:49 etc/conf.d/systemd.conf
+-rw-r--r--   1 root     root            0 Jan  3 20:49 etc/fstab.empty
+-rw-r--r--   1 root     root          203 Jan  3 20:49 etc/group
+lrwxrwxrwx   1 root     root           25 Jan  3 20:49 etc/initrd-release -> ../usr/lib/initrd-release
+-rw-r--r--   1 root     root         2431 Jan  3 20:49 etc/ld.so.cache
+-rw-r--r--   1 root     root           28 Aug  2  2021 etc/ld.so.conf
+-rw-r--r--   1 root     root            0 Jan  3 20:49 etc/machine-id
+lrwxrwxrwx   1 root     root           17 Jan  3 20:49 etc/mtab -> /proc/self/mounts
+lrwxrwxrwx   1 root     root           14 Jan  3 20:49 etc/os-release -> initrd-release
+drwxr-xr-x   1 root     root            0 Jan  3 20:49 etc/ostree
+-rw-r--r--   1 root     root           44 Jan  3 20:49 etc/ostree/initramfs-root-binding.key
+-rw-r--r--   1 root     root          121 Jan  3 20:49 etc/passwd
+drwxr-xr-x   1 root     root            0 Jan  3 20:49 etc/systemd
+
+....
+snip
+....
+```
+
+unmount and disconnect as soon as you are done
+
+```
+umount /mnt
+qemu-nbd -d /dev/nbd0
+```