fix-grub.sh

set -e -o pipefail

file=/boot/coreos/grub/i386-pc/linux

# in case something goes horribly wrong
if [[ ! -e "${file}.bak" ]]; then
	cp ${file}.mod ${file}.bak
fi

gunzip -k -S .mod ${file}.mod

# Values derived from https://gist.github.com/ajeddeloh/365cfed1d3a326362e05f78720baf4df
declare -A offsets
offsets=(
[1e11052c144ae483cba4f70efe278070c50daa80d1a5febe7c0d08e401baf0ab16a542b7a34da00d7aea1591238f01d9902ac6fbfcce8d82eebcf09d97d132cd]='3378'
[2fd5f0fade7c4c986259524f148f79ee1d1353d7ab83d1bdd0d50e52d393d8d896c32ab64eb714ba08861b8ba4f113d19f940a04889fa407784f010f119c8c19]='3378'
[6d60e369c1b4b484c7221e91d80f03a782b5286137f8087b2bf22b9f54e3507c4947d2a456ba46a6ef3c0c7216dc8251c017e9122f44cba89e32f23a0542afd3]='3383'
[6e9e5ebb6cd1a15d5a570d9a06a56c9bf60cb047d858c7220dd9dcfd54ebb87c8e0ea4611ac98a0ab51fec9e87b265b2207973f3e4882d87f62d887da72f87ab]='3404'
[82b37fa4b305cab33277d2cf0249008731a69575b5689a47e72fe2a35be4440e0e116bc02191f9b0066ea3ae278327fe3409f28d25d13bae88c5f347dba6a254]='3383'
[8a7b03d92a8b115943e7f004820fadd2dc6ab125c077a48fb232a1e9ac77fdb27fbb01d52fd33a6ddf65a9f58ce981244c99bcca821030511caa277bc2f68239]='3378'
[a3e9dadfe3cc34189b5fee83bfc01c3c5b42e04ba19cdcd84f8301c42566617b5916294e2414348139d8c5e557a7ccf6c0d3dca0661f2d10c0c0077345630b1d]='3298'
[c127d7c1dbd5d11cf7af627e37808ea16166b6430ddd8e96111e503cc78ae1fd78083d474495951743fa1b489140be63178a4bb65dabb0d719c5d0ad9c57eb78]='3298'
[f1f9abefa49eeba6a3fe46ba3d254dfc3fa6e2cd8823835e2d982e9cbcd0d82c298e3896dc79d234305d2262a828a7398526906022f8ed7407368725d95e08d8]='3375'
)

declare -A correctvals
correctvals=(
[1e11052c144ae483cba4f70efe278070c50daa80d1a5febe7c0d08e401baf0ab16a542b7a34da00d7aea1591238f01d9902ac6fbfcce8d82eebcf09d97d132cd]='\x74\x02\x00\x00'
[2fd5f0fade7c4c986259524f148f79ee1d1353d7ab83d1bdd0d50e52d393d8d896c32ab64eb714ba08861b8ba4f113d19f940a04889fa407784f010f119c8c19]='\x88\x02\x00\x00'
[6d60e369c1b4b484c7221e91d80f03a782b5286137f8087b2bf22b9f54e3507c4947d2a456ba46a6ef3c0c7216dc8251c017e9122f44cba89e32f23a0542afd3]='\x74\x02\x00\x00'
[6e9e5ebb6cd1a15d5a570d9a06a56c9bf60cb047d858c7220dd9dcfd54ebb87c8e0ea4611ac98a0ab51fec9e87b265b2207973f3e4882d87f62d887da72f87ab]='\x88\x02\x00\x00'
[82b37fa4b305cab33277d2cf0249008731a69575b5689a47e72fe2a35be4440e0e116bc02191f9b0066ea3ae278327fe3409f28d25d13bae88c5f347dba6a254]='\x74\x02\x00\x00'
[8a7b03d92a8b115943e7f004820fadd2dc6ab125c077a48fb232a1e9ac77fdb27fbb01d52fd33a6ddf65a9f58ce981244c99bcca821030511caa277bc2f68239]='\x74\x02\x00\x00'
[a3e9dadfe3cc34189b5fee83bfc01c3c5b42e04ba19cdcd84f8301c42566617b5916294e2414348139d8c5e557a7ccf6c0d3dca0661f2d10c0c0077345630b1d]='\x74\x02\x00\x00'
[c127d7c1dbd5d11cf7af627e37808ea16166b6430ddd8e96111e503cc78ae1fd78083d474495951743fa1b489140be63178a4bb65dabb0d719c5d0ad9c57eb78]='\x74\x02\x00\x00'
[f1f9abefa49eeba6a3fe46ba3d254dfc3fa6e2cd8823835e2d982e9cbcd0d82c298e3896dc79d234305d2262a828a7398526906022f8ed7407368725d95e08d8]='\x74\x02\x00\x00'
)

filesum=$(sha512sum $file | cut -d' ' -f1)
if [[ -z "${offsets[$filesum]}" ]]; then
	echo "Nothing to do"
	rm ${file}
	exit
fi
printf "${correctvals[$filesum]}" | dd of=$file bs=1 seek="${offsets[$filesum]}" conv=notrunc status=none

# rezip as atomically as possible so ENOSPC can't cause problems
gzip -S .tmp ${file}
mv ${file}.tmp ${file}.mod

echo "linux.mod updated successfully"