Best way to manually decompress kernel

I'm on Raspberry Pi 4 usb booting ssd.
OS: Ubuntu desktop 20.04.2 64bit arm64

To boot on usb without sd card I configure my boot to boot on a decompress vmlinux.

This is due by the missing capability to boot on usb using a compressed kernel vmlinuz

so I use a script binding to apt event that decompress the kernel after it was update.
My script is like this:

#... controls
zcat vmlinuz > vmlinux
#... 

It works, but now, I have discover, relate to the kernel version updated, this script:
/usr/src/linux-raspi-headers-5.4.0-1028/scripts/extract-vmlinux

To me seems that this script decompress kernel too.

My question is: is better to use this script unless zcat?

like this command: # extract-vmlinux vmlinuz > vmlinux
is this command correct?

2 Answers

It is better to use extract-vmlinux for several reasons:

• You don't know what kind of compression is used. It could be gzip, LZ4, Bzip2, XZ, … The latest extract-vmlinux script knows how to extract every kind of compressed kernel, though you will still need the relevant command line utility.
• Compressed kernels aren't like simple compressed files. They are self-extracting so contain a binary header. The kernel image itself may have other binary objects before or after it also. I'm surprised that zcat worked for you; simply using lz4cat doesn't work for me with LZ4-compressed kernels.

For a different reason than yours I wrote a kernel install hook script that uses extract-vmlinux to decompress LZ4-compressed Ubuntu kernels. This should help you along in your quest to have the same for Raspberry Pi.

I will integrate your solution with mine.
I will execute on post update apt hook
but will think abaut to move in kernel post install hook
this is my code:

#!/bin/bash -e
#Set Variables
BTPATH=/boot/firmware
CKPATH=$BTPATH/vmlinuz
DKPATH=$BTPATH/vmlinux
EXTRACT_VMLINUX="/usr/src/linux-headers-$(uname -r)/scripts/extract-vmlinux"
if ! [ -x $EXTRACT_VMLINUX ]; then echo -e "\e[32m$EXTRACT_VMLINUX is not executable\e[0m" echo -e "\e[32mI will use zcat!\e[0m" EXTRACT_VMLINUX="zcat -qf"
fi
if [ -e $BTPATH/check.md5 ]; then if md5sum --status --ignore-missing -c $BTPATH/check.md5; then echo -e "\e[32mFiles have not changed, Decompression not needed\e[0m" exit 0 else echo -e "\e[31mHash check failed, kernel will be furter investigate\e[0m" fi
fi
mv $DKPATH $DKPATH.bak
if [ ! $? == 0 ]; then echo -e "\e[31mDECOMPRESSED KERNEL BACKUP FAILED!\e[0m" exit 1
else echo -e "\e[32mDecompressed kernel backup was successful\e[0m"
fi
# Decompress the new kernel
echo "Decompressing kernel: "$CKPATH".............."
$EXTRACT_VMLINUX $CKPATH > $DKPATH
if [ ! $? == 0 ]; then echo -e "\e[31mKERNEL FAILED TO DECOMPRESS!\e[0m" exit 1
else echo -e "\e[32mKernel Decompressed Succesfully\e[0m"
fi
#Hash the new kernel for checking
md5sum $CKPATH $DKPATH > $BTPATH/check.md5
if [ ! $? == 0 ]; then echo -e "\e[31mMD5 GENERATION FAILED!\e[0m"
else echo -e "\e[32mMD5 generated Succesfully\e[0m"
fi
#Exit
exit 0

UPDATE

there is a problem:
if I execute$EXTRACT_VMLINUX /boot/firmware/vmlinuz && echo 'ok'
the output will be:

extract-vmlinux: Cannot find vmlinux.
ok

extract-vmlinux if fail do not throw error: do not do exit 1
this is a bad new
if we use zcat -qf vmlinuz > vmlinux it work and if fail we can check it

so is better not use extract-vmlinux and use zcat instead.

for rpi user

on Ubuntu 20.04.2 there is no more need to decompress kernel on usb on raspberry pi 4:

in config.txt we can use kernel=vmlinuz and it work without decompression.

best regards