unpack_initramfs.sh

#! /bin/bash
set -e -o pipefail

. /etc/functions

TRACE_FUNC
# Unpack a Linux initramfs archive.
#
# In general, the initramfs archive is one or more cpio archives, optionally
# compressed, concatenated together.  Uncompressed and compressed segments can
# exist in the same file.  Zero bytes between segments are skipped.  To properly
# unpack such an archive, all segments must be unpacked.
#
# This script unpacks such an archive, but with a limitation that once a
# compressed segment is reached, no more segments can be read.  This works for
# common initrds on x86, where the microcode must be stored in an initial
# uncompressed segment, followed by the "real" initramfs content which is
# usually in one compressed segment.
#
# The limitation comes from gunzip/unzstd, there's no way to prevent them from
# consuming trailing data or tell us the member/frame length.  The script
# succeeds with whatever was extracted, since this is used to extract particular
# files and boot can proceed as long as those files were found.

INITRAMFS_ARCHIVE="$1"
DEST_DIR="$2"
shift
shift
# rest of args go to cpio, can specify filename patterns
CPIO_ARGS=("$@")

# Consume zero bytes, the first nonzero byte read (if any) is repeated on stdout
consume_zeros() {
	TRACE_FUNC
	next_byte='00'
	while [ "$next_byte" = "00" ]; do
		# if we reach EOF, next_byte becomes empty (dd does not fail)
		next_byte="$(dd bs=1 count=1 status=none | xxd -p | tr -d ' ')"
	done
	# if we finished due to nonzero byte (not EOF), then carry that byte
	if [ -n "$next_byte" ]; then
		echo -n "$next_byte" | xxd -p -r
	fi
}

unpack_cpio() {
	TRACE_FUNC
	(
		cd "$dest_dir"
		cpio -i "${CPIO_ARGS[@]}" 2>/dev/null
	)
}

# unpack the first segment of an archive, then write the rest to another file
unpack_first_segment() {
	TRACE_FUNC
	unpack_archive="$1"
	dest_dir="$2"
	rest_archive="$3"

	mkdir -p "$dest_dir"

	# peek the beginning of the file to determine what type of content is next
	magic="$(dd if="$unpack_archive" bs=6 count=1 status=none 2>/dev/null | xxd -p)"

	# read this segment of the archive, then write the rest to the next file
	(
		# Magic values correspond to Linux init/initramfs.c (zero, cpio) and
		# lib/decompress.c (gzip)
		case "$magic" in
		00*)
			DEBUG "archive segment $magic: uncompressed cpio"
			# Skip zero bytes and copy the first nonzero byte
			consume_zeros
			# Copy the remaining data
			cat
			;;
		303730373031* | 303730373032*) # plain cpio
			DEBUG "archive segment $magic: plain cpio"
			# Unpack the plain cpio, this stops reading after the trailer
			unpack_cpio
			# Copy the remaining data
			cat
			;;
		1f8b* | 1f9e*) # gzip
			DEBUG "archive segment $magic: gzip"
			# gunzip won't stop when reaching the end of the gzipped member,
			# so we can't read another segment after this.  We can't
			# reasonably determine the member length either, this requires
			# walking all the compressed blocks.
			gunzip | unpack_cpio
			;;
		fd37*) # xz
			DEBUG "archive segment $magic: xz"
			unxz | unpack_cpio
			;;
		28b5*) # zstd
			DEBUG "archive segment $magic: zstd"
			# Like gunzip, this will not stop when reaching the end of the
			# frame, and determining the frame length requires walking all
			# of its blocks.
			(zstd-decompress -d || true) | unpack_cpio
			;;
		*) # unknown
			die "Can't decompress initramfs archive, unknown type: $magic"
			# The following are magic values for other compression formats
			#  but not added because not tested.
			# TODO: open an issue for unsupported magic number reported on die.
			# 
			#425a*) # bzip2
			#	DEBUG "archive segment $magic: bzip2"
			#	bunzip2 | unpack_cpio
			#;;
			#5d00*) # lzma
			#	DEBUG "archive segment $magic: lzma"
			#	unlzma | unpack_cpio
			#;;
			#894c*) # lzo
			#	DEBUG "archive segment $magic: lzo"
			#	lzop -d | unpack_cpio
			#;;
			#0221*) # lz4
			#	DEBUG "archive segment $magic: lz4"
			#	lz4 -d | unpack_cpio
			#	;;
			;;
		esac
	) <"$unpack_archive" >"$rest_archive"

	orig_size="$(stat -c %s "$unpack_archive")"
	rest_size="$(stat -c %s "$rest_archive")"
	DEBUG "archive segment $magic: $((orig_size - rest_size)) bytes"
}

DEBUG "Unpacking $INITRAMFS_ARCHIVE to $DEST_DIR"

next_archive="$INITRAMFS_ARCHIVE"
rest_archive="/tmp/unpack_initramfs_rest"

# Break when there is no remaining data
while [ -s "$next_archive" ]; do
	unpack_first_segment "$next_archive" "$DEST_DIR" "$rest_archive"
	next_archive="/tmp/unpack_initramfs_next"
	mv "$rest_archive" "$next_archive"
done