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o5gmmob8
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Joined: 17 Oct 2003
Posts: 555
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 Posted: Thu Mar 13, 2025 12:41 pm Post subject: custom initramfs |
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I was trying to create a custom init that
1. unlocks a LUKS volume
2. copies squashfs image into memory
3. closes LUKS volume
I generated an init and it appears that it is loading, so I think the file format is correct, but the system reboots and I'm unable to debug. The contents of the init are:
1. lddtree -l /usr/bin/{cat,cp,cryptsetup,dd,find,grep,insmod,ls,mkdir,modprobe,mount,printf,rm,sh,sleep,switch_root,umount}
2. kernel modules at their respective locations
3. /init
The /init script is a simple shell script which loads /proc, /dev, /sys, and /run. Then the kernel modules are loaded based on what is loaded on the machine when the init is generated. From there, I basically call cryptsetup, mount, umount, and cryptsetup again.
Since I'm using sh instead of bash, I included the printf command (and any necessary libraries).
I'm wondering what I may be missing since it reboots fairly quickly. I don't see any output and I call printf early on to help with debugging. |
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NeddySeagoon
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Joined: 05 Jul 2003
Posts: 55015
Location: 56N 3W
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| Posted: Thu Mar 13, 2025 12:50 pm Post subject: |
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o5gmmob8,
We need an least the the init script and the file you feed to | Code: | | /usr/src/linux/usr/gen_init_cpio /root/initramfs/initramfs_list > /boot/<initramfs_name> |
to assemble the cpio archive.
Early reboots sound like illegal instruction exceptions.
Have you seen https://wiki.gentoo.org/wiki/Initramfs_-_make_your_own ?
Its not exactly what you need but its a start.
_________________
Regards,
NeddySeagoon
Computer users fall into two groups:-
those that do backups
those that have never had a hard drive fail. |
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o5gmmob8
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Joined: 17 Oct 2003
Posts: 555
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| Posted: Thu Mar 13, 2025 1:02 pm Post subject: |
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Hi NeddySeagoon,
Yes, I saw many pages, some of which appear out-of-date.
Here is my init script:
| Code: | #!/bin/sh
export LD_LIBRARY_PATH=/lib:/lib64:/usr/lib64:/usr/lib/gcc/x86_64-pc-linux-gnu/14
export PATH=/bin:/sbin:/usr/bin:/usr/sbin
printf '### init ###\n'
OVERLAYFS_SIZE=
LUKS_DEVICE_UUID=
# setup some basic mounts, is this needed?
mount -t proc proc /proc
mount -t sysfs sysfs /sys
mount -t devtmpfs devtmpfs /dev
mount -t tmpfs -o rw,nosuid,nodev,relatime,mode=755 none /run
printf '### mounted filesystems ###\n'
# load modules
modprobe -a KERNEL_MODULES
printf '### loaded modules ###\n'
cryptsetup luksOpen /dev/disk/by-uuid/$LUKS_DEVICE_UUID luks-$LUKS_DEVICE_UUID
printf '### unlocked device ###\n'
mkdir -p /run/root-volume /run/root-image
mount /dev/mapper/luks-$LUKS_DEVICE_UUID /run/root-volume
# copy squashfs image into memory
printf 'Copying image into memory\n'
dd if=/run/root-volume/root-squashfs.img of=/run/root-image/root-squashfs.img
printf 'Copied image into memory\n'
umount /run/root-volume
cryptsetup luksClose luks-$LUKS_DEVICE_UUID
# mount squashfs image to new root
mkdir -p /mnt/overlayfs/rw /mnt/overlayfs/work /mnt/root
mount /run/root-image/root-squashfs.img /mnt/root
mount -t tmpfs -o size=$OVERLAYFS_SIZE tmpfs /mnt/overlay
mount -t overlay -o lowerdir=/mnt/root,upperdir=/mnt/overlay/rw,workdir=/mnt/overlay/work overlay /mnt/root
mkdir -p /mnt/root/rw
mount -o bind /mnt/overlayfs/rw /mnt/root/rw
umount /dev /proc /sys
exec /sbin/switch_root /mnt/root /sbin/init
printf '### init done ###\n' |
I'm using this script to generate an init:
| Code: | #!/bin/sh
_REQUIRED_ARGUMENTS="LUKS_DEVICE_UUID:LUKS Device UUID"
# @see: https://wiki.gentoo.org/wiki/Custom_Initramfs#Prerequisites
_info "Preparing init"
rm -rf /tmp/init && mkdir -p /tmp/init
cd /tmp/init
KERNEL_VERSION=$(uname -r)
: ${OVERLAYFS_SIZE:=1G}
mkdir -p {dev,etc,mnt/root,proc,root,sys,run,usr/bin,usr/lib,usr/lib64,usr/sbin}
cp --archive /dev/{null,console,tty} dev/
ln -s usr/bin bin
ln -s usr/sbin sbin
ln -s usr/lib lib
ln -s usr/lib64 lib64
#mkdir -p lib/modules/$KERNEL_VERSION
# copy binaries and dependencies
_info "Copying dependencies"
tar cp -C / $(lddtree -l /usr/bin/{cat,cp,cryptsetup,dd,find,grep,insmod,ls,mkdir,modprobe,mount,printf,rm,sh,sleep,switch_root,umount} | tr '\n' ' ') 2>/dev/null | tar xp -C /tmp/init
_info "Copying init"
cp $_CONF_APPLICATION_LIBRARY_PATH/init.sh init
chmod +x init
_info "Copying kernel modules"
# kernel modules
for module_name in $(lsmod | awk {'print$1'} | sort -u); do
module_file=$(find /lib/modules/$KERNEL_VERSION -type f -name $module_name.ko)
[ -z "$module_file" ] && {
_warn "No module file found for $module_name"
continue
}
tar cp -C / $module_file 2>/dev/null | tar xp -C /tmp/init
done
tar cp -C / /lib/modules/$KERNEL_VERSION/modules.* 2>/dev/null | tar xp -C /tmp/init
# kernel modules to load
$_CONF_GNU_SED -i "s/KERNEL_MODULES/$(lsmod | awk {'print$1'} | sort -u | tr '\n' ' ')/" init
$_CONF_GNU_SED -i "s/^OVERLAYFS_SIZE=.*$/OVERLAYFS_SIZE=$OVERLAYFS_SIZE/" init
$_CONF_GNU_SED -i "s/^LUKS_DEVICE_UUID=.*$/LUKS_DEVICE_UUID=$LUKS_DEVICE_UUID/" init
# generate init
find . -print0 | cpio --null --create --verbose --format=newc | zstd --ultra -o /tmp/initramfs.cpio.zstd |
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NeddySeagoon
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Joined: 05 Jul 2003
Posts: 55015
Location: 56N 3W
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| Posted: Thu Mar 13, 2025 1:11 pm Post subject: |
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o5gmmob8,
is a set oy symbolic links created by udev
Do you have udev in the initrd and started?
You could do with a rescue_shell() function, so that when something fails, you get a shell to poke about. e.g.
| Code: | | /sbin/mdadm --assemble /dev/md1 --uuid=de8f2cbc-17ca3275-0b69db3c-b9f91a6b || rescue_shell "The host RAID set failed to assemble" |
_________________
Regards,
NeddySeagoon
Computer users fall into two groups:-
those that do backups
those that have never had a hard drive fail. |
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o5gmmob8
Guru
Joined: 17 Oct 2003
Posts: 555
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| Posted: Thu Mar 13, 2025 1:15 pm Post subject: |
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Hi NeddySeagoon,
No, I don't have udev, let me see about that. I suppose I can add the rescue shell, but my thought process was I have a printf statement very early, I should see that, right? |
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NeddySeagoon
Administrator
Joined: 05 Jul 2003
Posts: 55015
Location: 56N 3W
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| Posted: Thu Mar 13, 2025 1:25 pm Post subject: |
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o5gmmob8,
You may need some sleep statements too, to have time to read the print statements.
The rescue shell will do noting in normal operation, as it would never be called. It may save debug time though.
| Code: | | cryptsetup luksOpen /dev/disk/by-uuid/$LUKS_DEVICE_UUID luks-$LUKS_DEVICE_UUID || rescueshell "luksOpen failed" |
would have allowed to to poke about in /dev and maybe spot that /dev/disk/by-* was missing.
It also allows you to complete the boot manually.
Is there any reason not to use busybox in the initrd?
_________________
Regards,
NeddySeagoon
Computer users fall into two groups:-
those that do backups
those that have never had a hard drive fail. |
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o5gmmob8
Guru
Joined: 17 Oct 2003
Posts: 555
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| Posted: Thu Mar 13, 2025 1:28 pm Post subject: |
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Hi NeddySeagoon,
I suppose I'm trying to understand what is actually required to make an init  . The less 'stuff' there is in it, the easier it is for me to understand.
I saw the notes on busybox and historically, I think all of my inits probably utilized that.
Good points about the rescue shell, I can include that earlier on to have an idea that something did go wrong.
Let me explore those options and report back. |
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NeddySeagoon
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Joined: 05 Jul 2003
Posts: 55015
Location: 56N 3W
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| Posted: Thu Mar 13, 2025 1:35 pm Post subject: |
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o5gmmob8,
Its just the kernel, the initrd and the init script.
Everything must be provided.
busybox gives you cut down versions of most of the commands you need and some that that you don't.
e.g. busybox mount did not understand filesystem UUID, so I needed the user space mount.
_________________
Regards,
NeddySeagoon
Computer users fall into two groups:-
those that do backups
those that have never had a hard drive fail. |
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o5gmmob8
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Joined: 17 Oct 2003
Posts: 555
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| Posted: Thu Mar 13, 2025 2:00 pm Post subject: |
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Ok, now I'm getting somewhere.
I think you're right, I installed busybox (not static because I didn't want to rebuild other libraries) and was able to capture a segfault. It is unable to load a library.
So, I think I will stick with busybox and statically compile it to simplify that. It only requires a rebuild of a few other packages, so it isn't a huge deal. |
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o5gmmob8
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Joined: 17 Oct 2003
Posts: 555
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| Posted: Thu Mar 13, 2025 3:30 pm Post subject: |
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Ok, my cryptsetup is not static and if I build it with static-libs, then it wants me to remove udev support which I suppose I can do locally.
Hmm, getting somewhere. |
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