#!/usr/bin/env python3
"""
Patch superblock to disable metadata_csum feature flag,
and zero GDT checksum fields for groups 0-12.
All writes go to dm-0 (overlay), nbd0 untouched.
"""
import struct

DEV = '/dev/dm-0'
BLOCK = 4096
BACKUP_SB_BLOCK = 32768

# Feature flag constants
INCOMPAT_64BIT        = 0x80
RO_COMPAT_METADATA_CSUM = 0x400

with open(DEV, 'r+b') as f:
    # Read primary superblock (already patched to block 0)
    f.seek(1024)
    sb = bytearray(f.read(1024))

    # Check current feature flags
    ro_compat = struct.unpack_from('<I', sb, 100)[0]
    print(f"ro_compat_features: {ro_compat:#010x}")
    print(f"metadata_csum set: {bool(ro_compat & RO_COMPAT_METADATA_CSUM)}")

    # Clear metadata_csum
    ro_compat &= ~RO_COMPAT_METADATA_CSUM
    struct.pack_into('<I', sb, 100, ro_compat)

    # Zero superblock checksum
    struct.pack_into('<I', sb, 1020, 0)

    # Write patched superblock back
    f.seek(1024)
    f.write(sb)
    print("Patched superblock: metadata_csum cleared")

    # Now zero bg_checksum in GDT entries for groups 0-12
    # bg_checksum is at offset 30 in each 64-byte descriptor
    f.seek(BLOCK)  # GDT starts at block 1
    gdt = bytearray(f.read(13 * 64))  # groups 0-12 only

    for grp in range(13):
        off = grp * 64
        old_csum = struct.unpack_from('<H', gdt, off + 30)[0]
        struct.pack_into('<H', gdt, off + 30, 0)
        print(f"  group {grp:2d}: cleared checksum {old_csum:#06x}")

    f.seek(BLOCK)
    f.write(gdt)
    print("Patched GDT checksums for groups 0-12")

print("Done - try debugfs again")