42 lines
1.1 KiB
Python
42 lines
1.1 KiB
Python
import struct, binascii
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# We need to recompute the GDT checksum for group 0
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# after modifying its flags
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#
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# ext4 GDT checksum = crc32c(uuid + group_num_le16 + gdt_entry_with_csum_zeroed)
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# The checksum seed is stored in the superblock at offset 408
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with open('/dev/nbd0','rb') as f:
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f.seek(1024)
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sb = f.read(1024)
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uuid = sb[104:120]
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csum_seed = struct.unpack_from('<I', sb, 408)[0]
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print(f'UUID: {uuid.hex()}')
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print(f'csum_seed: 0x{csum_seed:08x}')
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# Install crcmod for crc32c
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import crcmod
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crc32c_fn = crcmod.predefined.mkCrcFun('crc-32c')
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data = bytearray(open('/tmp/merged_gdt.bin','rb').read())
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def compute_gdt_csum(g, entry):
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e = bytearray(entry)
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struct.pack_into('<H', e, 30, 0) # zero checksum field
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grp_le = struct.pack('<H', g)
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csum_data = uuid + grp_le + bytes(e)
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csum = crc32c_fn(csum_data, csum_seed)
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return csum & 0xFFFF
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# Recompute checksum for group 0 only
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g = 0
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entry = data[0:64]
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csum = compute_gdt_csum(g, entry)
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struct.pack_into('<H', data, 30, csum)
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print(f'Group 0 new checksum: 0x{csum:04x}')
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with open('/tmp/merged_gdt.bin','wb') as f:
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f.write(data)
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print('Saved')
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