Initial remote commit

test/patch_gdt.py

@@ -0,0 +1,54 @@
+#!/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")