import struct, binascii

# We need to recompute the GDT checksum for group 0
# after modifying its flags
# 
# ext4 GDT checksum = crc32c(uuid + group_num_le16 + gdt_entry_with_csum_zeroed)
# The checksum seed is stored in the superblock at offset 408

with open('/dev/nbd0','rb') as f:
    f.seek(1024)
    sb = f.read(1024)

uuid = sb[104:120]
csum_seed = struct.unpack_from('<I', sb, 408)[0]
print(f'UUID: {uuid.hex()}')
print(f'csum_seed: 0x{csum_seed:08x}')

# Install crcmod for crc32c
import crcmod
crc32c_fn = crcmod.predefined.mkCrcFun('crc-32c')

data = bytearray(open('/tmp/merged_gdt.bin','rb').read())

def compute_gdt_csum(g, entry):
    e = bytearray(entry)
    struct.pack_into('<H', e, 30, 0)  # zero checksum field
    grp_le = struct.pack('<H', g)
    csum_data = uuid + grp_le + bytes(e)
    csum = crc32c_fn(csum_data, csum_seed)
    return csum & 0xFFFF

# Recompute checksum for group 0 only
g = 0
entry = data[0:64]
csum = compute_gdt_csum(g, entry)
struct.pack_into('<H', data, 30, csum)
print(f'Group 0 new checksum: 0x{csum:04x}')

with open('/tmp/merged_gdt.bin','wb') as f:
    f.write(data)
print('Saved')