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File name : unmkinitramfs

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#!/bin/sh

set -eu

usage()
{
	cat << EOF

Usage: unmkinitramfs [-v] initramfs-file directory

Options:
  -v   Display verbose messages about extraction

See unmkinitramfs(8) for further details.

EOF
}

usage_error()
{
	usage >&2
	exit 2
}

# Extract a compressed cpio archive
xcpio()
{
	archive="$1"
	dir="$2"
	shift 2

if gzip -t "$archive" >/dev/null 2>&1 ; then
		gzip -c -d "$archive"
	elif zstd -q -c -t "$archive" >/dev/null 2>&1 ; then
		zstd -q -c -d "$archive"
	elif xzcat -t "$archive" >/dev/null 2>&1 ; then
		xzcat "$archive"
	elif lz4cat -t < "$archive" >/dev/null 2>&1 ; then
		lz4cat "$archive"
	elif bzip2 -t "$archive" >/dev/null 2>&1 ; then
		bzip2 -c -d "$archive"
	elif lzop -t "$archive" >/dev/null 2>&1 ; then
		lzop -c -d "$archive"
	# Ignoring other data, which may be garbage at the end of the file
	fi | (
		if [ -n "$dir" ]; then
			mkdir -p -- "$dir"
			cd -- "$dir"
		fi
		cpio "$@"
	)
}

# Read bytes out of a file, checking that they are valid hex digits
readhex()
{
	dd < "$1" bs=1 skip="$2" count="$3" 2> /dev/null | \
		LANG=C grep -E "^[0-9A-Fa-f]{$3}\$"
}

# Check for a zero byte in a file
checkzero()
{
	dd < "$1" bs=1 skip="$2" count=1 2> /dev/null | \
		LANG=C grep -q -z '^$'
}

# Split an initramfs into archives and call xcpio on each
splitinitramfs()
{
	initramfs="$1"
	dir="$2"
	shift 2

count=0
	start=0
	while true; do
		# There may be prepended uncompressed archives.  cpio
		# won't tell us the true size of these so we have to
		# parse the headers and padding ourselves.  This is
		# very roughly based on linux/lib/earlycpio.c
		end=$start
		while true; do
			if checkzero "$initramfs" $end; then
				# This is the EOF marker.  There might
				# be more zero padding before the next
				# archive, so read through all of it.
				end=$((end + 4))
				while checkzero "$initramfs" $end; do
					end=$((end + 4))
				done
				break
			fi
			magic="$(readhex "$initramfs" $end 6)" || break
			test "$magic" = 070701 || test "$magic" = 070702 || break
			namesize=0x$(readhex "$initramfs" $((end + 94)) 8)
			filesize=0x$(readhex "$initramfs" $((end + 54)) 8)
			end=$(((end + 110)))
			end=$(((end + namesize + 3) & ~3))
			end=$(((end + filesize + 3) & ~3))
		done
		if [ $end -eq $start ]; then
			break
		fi

# Extract to early, early2, ... subdirectories
		count=$((count + 1))
		if [ $count -eq 1 ]; then
			subdir=early
		else
			subdir=early$count
		fi
		dd < "$initramfs" skip=$start count=$((end - start)) iflag=skip_bytes 2> /dev/null |
		(
			if [ -n "$dir" ]; then
				mkdir -p -- "$dir/$subdir"
				cd -- "$dir/$subdir"
			fi
			cpio -i "$@"
		)
		start=$end
	done

if [ $end -gt 0 ]; then
		# Extract to main subdirectory
		subarchive=$(mktemp "${TMPDIR:-/var/tmp}/unmkinitramfs_XXXXXX")
		trap 'rm -f "$subarchive"' EXIT
		dd < "$initramfs" skip=$end iflag=skip_bytes 2> /dev/null \
			> "$subarchive"
		xcpio "$subarchive" "${dir:+$dir/main}" -i "$@"
	else
		# Don't use subdirectories (for backward compatibility)
		xcpio "$initramfs" "$dir" -i "$@"
	fi
}

OPTIONS=$(getopt -o hv --long help,list,verbose -n "$0" -- "$@") || usage_error

cpio_opts="--preserve-modification-time --no-absolute-filenames --quiet"
expected_args=2
eval set -- "$OPTIONS"

while true; do
	case "$1" in
        -h|--help)
		usage
		exit 0
	;;
	--list)
		# For lsinitramfs
		cpio_opts="${cpio_opts:+${cpio_opts} --list}"
		expected_args=1
		shift
	;;
	-v|--verbose)
		cpio_opts="${cpio_opts:+${cpio_opts} --verbose}"
		shift
	;;
	--)
		shift
		break
	;;
	*)
		echo "Internal error!" >&2
		exit 1
	esac
done

if [ $# -ne $expected_args ]; then
	usage_error
fi

# shellcheck disable=SC2086
splitinitramfs "$1" "${2:-}" $cpio_opts