grub-guess

#!/bin/sh

# Author: Maxim Suhanov

marker="sector sizes of %d bytes aren't supported yet" # A valid core image contains this string.
lzma_ver='--lzma1'
lzma_options='dict=65535,lc=3,lp=0,pb=2' # LZMA1/LZMA2 options used in "mkimage.c".

old_dd='no' # When 'yes', there is no iflag=skip_bytes support in dd.

usage()
{
	echo 'Usage: grub-guess [--lzma2] <input file>'
	echo
	echo 'Compression modes:'
	echo '  --lzma2: use LZMA2 (XZ) instead of LZMA1'
	echo
	echo 'Output: an uncompressed core image is written to stdout,'
	echo '        other messages go to stderr.'
	exit 0
}

fail()
{
	[ -n "$1" ] && echo "$1" >&2
	exit 1
}

exit_success()
{
	[ -n "$1" ] && echo "$1" >&2
	exit 0
}

checkdep()
{
	command -v "$1" > /dev/null || fail "Command \"$1\" not found!"
}

checkdd()
{
	dd if=/dev/zero of=/dev/null skip=1 count=1 iflag=skip_bytes 2> /dev/null
	dd_ret=$?
	[ $dd_ret -eq 0 ] || old_dd='yes'
}

if [ $# -eq 2 ] && [ "$1" = '--lzma2' ]; then
	lzma_ver='--lzma2'
	shift
fi

[ $# -eq 1 ] && [ -n "$1" ] && [ "$1" != '--lzma2' ] || usage
[ -e "$1" ] || fail "Input file \"$1\" does not exist!"
[ -r "$1" ] || fail "Input file \"$1\" has no read permission!"

checkdep "xz"
checkdep "grep"

# Let's start at zero offset.
xz --decompress --stdout --format=raw $lzma_ver=$lzma_options "$1" 2> /dev/null | grep -F -q -s "$marker"
ret=$?
if [ $ret -eq 0 ]; then
	xz --decompress --stdout --format=raw $lzma_ver=$lzma_options "$1" 2> /dev/null
	exit_success "\"$1\" is a compressed core image."
fi

# Try to guess the right offset. This is slow!
checkdep "seq"
checkdep "dd"
checkdd

if [ "$old_dd" != 'yes' ]; then
	for offset in `seq 2 2 20000`; do # Skip odd offsets.
		dd if="$1" skip=$offset iflag=skip_bytes 2> /dev/null | xz --decompress --stdout --format=raw $lzma_ver=$lzma_options 2> /dev/null | grep -F -q -s "$marker"
		ret=$?
		if [ $ret -eq 0 ]; then
			dd if="$1" skip=$offset iflag=skip_bytes 2> /dev/null | xz --decompress --stdout --format=raw $lzma_ver=$lzma_options 2> /dev/null
			exit_success "Found a compressed core image in \"$1\" at offset $offset bytes."
		fi
	done
else # $old_dd is 'yes'.
	for offset in `seq 2 2 20000`; do
		dd if="$1" bs=$offset skip=1 2> /dev/null | xz --decompress --stdout --format=raw $lzma_ver=$lzma_options 2> /dev/null | grep -F -q -s "$marker"
		ret=$?
		if [ $ret -eq 0 ]; then
			dd if="$1" bs=$offset skip=1 2> /dev/null | xz --decompress --stdout --format=raw $lzma_ver=$lzma_options 2> /dev/null
			exit_success "Found a compressed core image in \"$1\" at offset $offset bytes."
		fi
	done
fi

fail "Nothing was found in \"$1\"!"