extract xzipped initramfs from uboot new FIT image format (itb)

extract_initramfs.sh

#!/bin/bash

XZMAGIC=fd377a58
STRUCT_POINTER_OFFSET=8

file=$1
declare -A word

map_word () {
    word[addr]=$1
    word[value]=$2
}

map_hex_word_at_cursor() {
    map_word $(hexdump -s$cursor -n4 -e '/4 "%_ad"' -e '/4 " %x"' $file)
}

map_uint_word_at_cursor() {
    map_word $(hexdump -s$cursor -n4 -e '/4 "%_ad"' -e '/4 " %u"' $file)
}

cursor_forward() {
    cursor=$(($cursor + $1))
}

# cursor = 0
cursor=0

# set cursor to location of struct pointer offset
cursor_forward $STRUCT_POINTER_OFFSET 
# read structure offset (4 bytes), and move cursor to structure
cursor=$(hexdump -s$cursor -n4 -e '/4 "%u"' $file)

map_uint_word_at_cursor # read value at cursor

# while true
while true; do
    # while 4-byte uint at cursor not 3
    #     cursor forward four bytes
    #     read 4-byte uint at cursor
    while [[ ${word[value]} -ne 3 ]]; do
        cursor_forward 4
        map_uint_word_at_cursor
    done
    cursor_forward 4    # cursor forward 4 bytes
    # read uint size (4 bytes)
    map_uint_word_at_cursor
    size=${word[value]} 
    cursor_forward 8    # cursor forward 8 bytes
    # if 4-bytes at cursor match XZ magic
    #     dd from cursor to (cursor + size)
    #     exit
    # else
    #     cursor forward size bytes
    map_hex_word_at_cursor
    if [[ "${word[value]}" = "$XZMAGIC" ]];  then
        dd if=$file of=initramfs bs=1 skip=${word[addr]} count=$size &>/dev/null
        exit
    else
        cursor_forward $(($size + 4 - $size % 4))
        map_uint_word_at_cursor
    fi
done