raid-sb

#!/bin/bash
LC_ALL=C
optstring_long="force,metadata:,write,verbose,dry-run,help"
optstring_short="fe:wvnh"

opts=$(getopt -o "${optstring_short}" --long "${optstring_long}" --name "$0" -- "$@") ||
    exit $?
eval set -- "$opts"

unset force
metadata=0
unset verbose
unset dryrun

usage()
{
    cat <<EOF
Usage: $0 device output-file
EOF
}

bin2hex()
{
    # FIXME: this cannot read zero-byte, it is treated as EOF
    while read -n 1 byte
    do
        printf "%x" "'$byte"
    done
    printf "\n"
}

while true
do
    case "$1" in
        -f|--force)
            force=y
            shift;;
        -e|--metadata)
            metadata=$2
            shift 2;;
        -w|--write)
            write=y
            shift;;
        -v|--verbose)
            verbose=y
            shift;;
         -n|--dry-run)
            dryrun=echo
            shift;;
        -h|--help)
            usage
            exit;;
        --) shift; break;;
    esac
done

dev=$1
bak=$2

if [ -z "$dev" -o -z "$bak" ]; then
    usage
    exit
fi

if [ "$write" ]; then
    if [ ! -e "$bak" ]; then
        echo "$bak not exists!" >&2
        exit 1
    fi
elif [ -z "$force" -a -e "$bak" ]; then
    echo "$bak exists!" >&2
    exit 1
fi

# As told in https://raid.wiki.kernel.org/index.php/RAID_superblock_formats
#
# and https://raid.wiki.kernel.org/index.php/Superblock
#
# Version 0.9:
# The superblock is 4K long and is written into a 64K aligned block that starts at 
# least 64K and less than 128K from the end of the device (i.e. to get the address 
# of the superblock round the size of the device down to a multiple of 64K and 
# then subtract 64K). The available size of each device is the amount of space 
# before the super block, so between 64K and 128K is lost when a device in 
# incorporated into an MD array.
#
# Version 1.0:
# Stored near the end of the device (at least 8K, and less than 12K, from the end).
#
# Version 1.1 stored at the start of the device.
#
# Version 1.2 like version 1.1 but stores the superblock 4K from the device start. 
#

size=$(blockdev --getsize64 "$dev")
if [ $((size + 0)) -lt 65536 ]; then
    echo "Unusable ${dev} size ${size}" >&2
    exit 1
fi

case "$metadata" in
    0|0.90)
        super_addr=$(((size / 65536 - 1) * 16))
        ;;
    1.0)
        # FIXME: test
        super_addr=$(((size / 4096 - 3)))
        ;;
    1.1)
        # FIXME: test
        super_addr=0
        ;;
    1|1.2)
        super_addr=1
        ;;
    *)
        echo "Wrong metadata version: $metadata" >&2
        exit 1
        ;;
esac

get_magic()
{
    magic=$(head -c4 | bin2hex)
    case "$magic" in
        a92b4efc)
            magic_type=big-endian
            ;;
        fc4e2ba9)
            magic_type=little-endian
            ;;
        *)
            magic_type=wrong
            ;;
    esac
    echo ${1}magic=$magic
    echo ${1}magic_type=$magic_type
}

check_bak_magic()
{
    eval $(head -c4 $bak|get_magic bak_)
    if [ -z "$force" -a "$bak_magic_type" = wrong ]; then
        echo "Wrong $bak superblock backup magic${bak_magic:+: $bak_magic}!" >&2
        exit 2
    fi
}

check_dev_magic()
{
    eval $(dd if=$dev bs=4096 count=1 skip=$super_addr status=none|get_magic)
    if [ -z "$force" -a "$magic_type" = wrong ]; then
        echo "Wrong $dev superblock $metadata magic${magic:+: $magic}!" >&2
        exit 2
    fi
}

set -e

if [ "$write" ]; then
    bak_size=$(blockdev --getsize64 "$bak")
    if [ -z "$force" -a "$bak_size" != 4096 ]; then
        echo "Wrong backup size $bak_size (expected 4096)!" >&2
        exit 1
    fi
    check_bak_magic
    check_dev_magic
    $dryrun dd of=$dev if=$bak bs=4096 count=1 seek=$super_addr status=none
    if [ -z "$dryrun" ]; then
        echo "Written $dev superblock from $bak"
        magic=$bak_magic
        magic_type=$bak_magic_type
    fi
else
    check_dev_magic
    $dryrun dd if=$dev of=$bak bs=4096 count=1 skip=$super_addr status=none
    if [ -z "$dryrun" ]; then
        echo "Backed up $dev superblock to $bak"
    fi
fi

if [ "$verbose" ]; then
    echo "Device size:        $size"
    echo "Superblock address: $((super_addr * 4096))"
    echo "Superblock magic:   $magic ($magic_type)"
fi