fix UNMAP data corruption by implementing block zeroing

The UNMAP command was a no-op in all backing stores, causing unmapped
blocks to retain stale data instead of returning zeros per SCSI spec.

- Implement Unmap in FileBackingStore to zero out unmapped blocks
- Implement Unmap in IOUringBackingStore to zero out unmapped blocks
- Enable Unmap in RemBackingStore (was commented out)
- Change UnmapBlockDescriptor.TL from uint32 to uint64 to prevent
  integer overflow when converting block count to byte length with
  large block shifts

Fixes #119

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

pkg/api/types.go

@@ -409,7 +409,7 @@ type LUNMap map[uint64]*SCSILu
 
 type UnmapBlockDescriptor struct {
 	Offset uint64
-	TL     uint32
+	TL     uint64
 }
 
 type ReaderWriterAt interface {

pkg/scsi/backingstore/common.go

@@ -159,6 +159,12 @@ func (bs *FileBackingStore) DataAdvise(offset, length int64, advise uint32) erro
 	return util.Fadvise(bs.file, offset, length, advise)
 }
 
-func (bs *FileBackingStore) Unmap([]api.UnmapBlockDescriptor) error {
+func (bs *FileBackingStore) Unmap(descriptors []api.UnmapBlockDescriptor) error {
+	for _, desc := range descriptors {
+		zeros := make([]byte, desc.TL)
+		if _, err := bs.file.WriteAt(zeros, int64(desc.Offset)); err != nil {
+			return err
+		}
+	}
 	return nil
 }

pkg/scsi/backingstore/iouring/iouring_linux.go

@@ -628,8 +628,14 @@ func (bs *IOUringBackingStore) DataAdvise(offset, length int64, advise uint32) e
 	return nil
 }
 
-// Unmap is a no-op for file-based storage
-func (bs *IOUringBackingStore) Unmap([]api.UnmapBlockDescriptor) error {
+// Unmap zeros out the specified blocks
+func (bs *IOUringBackingStore) Unmap(descriptors []api.UnmapBlockDescriptor) error {
+	for _, desc := range descriptors {
+		zeros := make([]byte, desc.TL)
+		if _, err := bs.file.WriteAt(zeros, int64(desc.Offset)); err != nil {
+			return err
+		}
+	}
 	return nil
 }
 

pkg/scsi/backingstore/remote/remote.go

@@ -113,6 +113,10 @@ func (bs *RemBackingStore) DataSync(offset, length int64) (err error) {
 }
 
 func (bs *RemBackingStore) Unmap(bd []api.UnmapBlockDescriptor) (err error) {
-	//_, err = bs.RemBs.Unmap(int64(bd[0].Offset), int64(bd[0].TL))
+	for _, desc := range bd {
+		if _, err = bs.RemBs.Unmap(int64(desc.Offset), int64(desc.TL)); err != nil {
+			return
+		}
+	}
 	return
 }

pkg/scsi/sbc.go

@@ -320,7 +320,7 @@ func SBCUnmap(host int, cmd *api.SCSICommand) api.SAMStat {
 		num := binary.BigEndian.Uint32(cmd.OutSDBBuffer.Buffer[off+8 : off+12])
 		blockDescs = append(blockDescs, api.UnmapBlockDescriptor{
 			Offset: lba << cmd.Device.BlockShift,
-			TL:     num << cmd.Device.BlockShift,
+			TL:     uint64(num) << cmd.Device.BlockShift,
 		})
 	}