Initial remote commit

test/patch4.py

@@ -0,0 +1,41 @@
+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')