Fix ActualLength=0 for OUT transfers in BuildRetSubmit

internal/usbip/protocol.go

@@ -267,8 +267,11 @@ func BuildImportReply(status uint32, dev *DeviceDescriptor) ([]byte, error) {
 	return buf.Bytes(), nil
 }
 
-// BuildRetSubmit builds a RET_SUBMIT message
-func BuildRetSubmit(seqNum, devID, direction, endpoint uint32, status int32, data []byte) ([]byte, error) {
+// BuildRetSubmit builds a RET_SUBMIT message.
+// actualLength must be set for BOTH directions: it reports how many bytes
+// were actually transferred. For OUT transfers the kernel driver checks this
+// (e.g. UVC probe control expects actualLength == 26).
+func BuildRetSubmit(seqNum, devID, direction, endpoint uint32, status int32, actualLength uint32, data []byte) ([]byte, error) {
 	buf := &bytes.Buffer{}
 
 	hdr := &URBHeader{
@@ -282,14 +285,9 @@ func BuildRetSubmit(seqNum, devID, direction, endpoint uint32, status int32, dat
 		return nil, err
 	}
 
-	actualLen := uint32(0)
-	if direction == DirIn && data != nil {
-		actualLen = uint32(len(data))
-	}
-
 	body := &RetSubmitBody{
 		Status:          status,
-		ActualLength:    actualLen,
+		ActualLength:    actualLength,
 		NumberOfPackets: 0xFFFFFFFF,
 	}
 	if err := WriteRetSubmit(buf, body); err != nil {
@@ -307,7 +305,7 @@ func BuildRetSubmit(seqNum, devID, direction, endpoint uint32, status int32, dat
 // BuildRetSubmitISO builds a RET_SUBMIT message for isochronous transfers.
 // The transfer data must already be packed (compact, no gaps).
 func BuildRetSubmitISO(seqNum, devID, direction, endpoint uint32, status int32,
-	packedData []byte, startFrame uint32, numPackets int32, errorCount int32,
+	actualLength uint32, packedData []byte, startFrame uint32, numPackets int32, errorCount int32,
 	isoDescs []ISOPacketDescriptor) ([]byte, error) {
 
 	buf := &bytes.Buffer{}
@@ -323,14 +321,9 @@ func BuildRetSubmitISO(seqNum, devID, direction, endpoint uint32, status int32,
 		return nil, err
 	}
 
-	actualLen := uint32(0)
-	if direction == DirIn && packedData != nil {
-		actualLen = uint32(len(packedData))
-	}
-
 	body := &RetSubmitBody{
 		Status:          status,
-		ActualLength:    actualLen,
+		ActualLength:    actualLength,
 		StartFrame:      startFrame,
 		NumberOfPackets: uint32(numPackets),
 		ErrorCount:      uint32(errorCount),