Initial remote commit

test/testoffset.sh

@@ -0,0 +1,52 @@
+python3 -c "
+import struct
+
+# Read what's actually at the start of our NBD device
+with open('/dev/nbd0','rb') as f:
+    # Block 0 (should be boot block, all zeros for non-bootable)
+    block0 = f.read(4096)
+    # Superblock at byte 1024
+    f.seek(1024)
+    sb = f.read(256)
+
+# Check block 0
+nonzero = sum(1 for b in block0 if b != 0)
+print(f'Block 0 non-zero bytes: {nonzero} (should be 0 for ext4)')
+print(f'Block 0 first 16: {block0[:16].hex()}')
+
+# Check superblock
+magic = struct.unpack_from('<H', sb, 56)[0]
+uuid  = sb[104:120].hex()
+first_data_block = struct.unpack_from('<I', sb, 20)[0]
+blocks_per_group = struct.unpack_from('<I', sb, 40)[0]
+print(f'SB magic:          0x{magic:04x} (want 0xef53)')
+print(f'SB uuid:           {uuid}')
+print(f'first_data_block:  {first_data_block} (0 for bsize>1024, 1 for bsize=1024)')
+print(f'blocks_per_group:  {blocks_per_group}')
+
+# The GDT should be at block 1 = byte 4096
+# But if first_data_block=1, GDT is at block 2 = byte 8192
+f2 = open('/dev/nbd0','rb')
+f2.seek(4096)
+gdt0 = f2.read(64)
+bb = struct.unpack_from('<I',gdt0,0)[0]
+ib = struct.unpack_from('<I',gdt0,4)[0]
+it = struct.unpack_from('<I',gdt0,8)[0]
+cs = struct.unpack_from('<H',gdt0,30)[0]
+print(f'At byte 4096 (block 1): bb={bb} ib={ib} it={it} csum=0x{cs:04x}')
+
+f2.seek(8192)
+gdt0b = f2.read(64)
+bb = struct.unpack_from('<I',gdt0b,0)[0]
+ib = struct.unpack_from('<I',gdt0b,4)[0]
+it = struct.unpack_from('<I',gdt0b,8)[0]
+cs = struct.unpack_from('<H',gdt0b,30)[0]
+print(f'At byte 8192 (block 2): bb={bb} ib={ib} it={it} csum=0x{cs:04x}')
+
+# Check what libext2fs would see as the device size
+import os
+f2.seek(0,2)
+size = f2.tell()
+print(f'NBD device size: {size} bytes = {size//4096} blocks')
+f2.close()
+"