Append MediaTek header to boot.img files for Fairphone 1

mtk_mkimage.sh

#!/bin/sh -e
# shellcheck disable=SC2039,SC2155
# Copyright 2018 Oliver Smith
# SPDX-License-Identifier: GPL-3.0-or-later

script="$(basename "$0")"
label="$1"
input="$2"
output="$3"
temp=""


# $1: argument count
check_arguments() {
	if [ "$1" -ne 3 ]; then
		echo "usage: $script LABEL INPUT OUTPUT"
		exit 1
	fi
	if ! [ -e "$input" ]; then
		echo "File not found: $input"
		exit 1
	fi
	if [ -e "$output" ]; then
		echo "Output file already exists: $output"
		exit 1
	fi
}


write_identifier() {
	printf '\x88\x16\x88\x58' >> "$temp"
}


# $1: new file size
# $2: padding character code (377 for 0xFF, 000 for 0x00)
write_padding() {
	# Missing byte count
	local old="$(stat -c%s "$temp")"
	local count="$(($1 - old))"

	# Write to file
	# This uses printf to generate $count white spaces, then replaces them with
	# the desired character. shellcheck disable=SC2059
	printf "%0${count}s" "" | tr " " "\\$2" >> "$temp"
}


write_size() {
	# File size as hexadecimal number
	local int="$(stat -c%s "$input")"
	local hex="$(printf '%x' "$int")"

	# Format string with reversed byte order
	local split="$(echo "$hex" | sed 's/.\{2\}/& /g')"
	local formatstring=""
	for byte in $split; do
		formatstring="\\x$byte$formatstring"
	done

	# shellcheck disable=SC2059
	printf "$formatstring" >> "$temp"
	write_padding "8" "000"
}


write_label() {
	printf "%s" "$label" >> "$temp"
	write_padding "40" "000"
}


write_header() {
	write_identifier
	write_size
	write_label
	write_padding "512" "377"
}


# $1: argument count
main() {
	check_arguments "$1"

	temp="$(mktemp -t mtkheaderXXXXXX)"
	write_header

	cat "$temp" "$input" > "$output"
	rm "$temp"
}


main "$#"