vmlinuz compression identifier

check-vmlinux

#!/bin/sh
# SPDX-License-Identifier: GPL-2.0-only
# ----------------------------------------------------------------------
# check-vmlinux - Identify compression used for a kernel image
#
# Inspired from extract-ikconfig
# (c) 2009,2010 Dick Streefland <dick@streefland.net>
# (c) 2011      Corentin Chary <corentin.chary@gmail.com>
# © 2019        Stephen Kitt <steve@sk2.org>
#
# ----------------------------------------------------------------------

check_vmlinux()
{
	# Use readelf to check if it's a valid ELF
	# TODO: find a better to way to check that it's really vmlinux
	#       and not just an elf
	readelf -h $1 > /dev/null 2>&1 || return 1

	echo The image was compressed using $2
	exit 0
}

try_decompress()
{
	# The obscure use of the "tr" filter is to work around older versions of
	# "grep" that report the byte offset of the line instead of the pattern.

	# Try to find the header ($1) and decompress from here
	for	pos in `tr "$1\n$2" "\n$2=" < "$img" | grep -abo "^$2"`
	do
		pos=${pos%%:*}
		tail -c+$pos "$img" | $3 > $tmp 2> /dev/null
		check_vmlinux $tmp $4
	done
}

# Check invocation:
me=${0##*/}
img=$1
if	[ $# -ne 1 -o ! -s "$img" ]
then
	echo "Usage: $me <kernel-image>" >&2
	exit 2
fi

# Prepare temp files:
tmp=$(mktemp /tmp/vmlinux-XXX)
trap "rm -f $tmp" 0

# That didn't work, so retry after decompression.
try_decompress '\037\213\010' xy    gunzip    gzip
try_decompress '\3757zXZ\000' abcde unxz      xz
try_decompress 'BZh'          xy    bunzip2   bzip2
try_decompress '\135\0\0\0'   xxx   unlzma    lzma
try_decompress '\211\114\132' xy    'lzop -d' lzop
try_decompress '\002!L\030'   xxx   'lz4 -d'  lz4
try_decompress '(\265/\375'   xxx   unzstd    zstd

# Finally check for uncompressed images or objects:
check_vmlinux $img none

# Bail out:
echo "$me: Cannot find vmlinux." >&2