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
+// BuildRetSubmit builds a RET_SUBMIT message.
-func BuildRetSubmit(seqNum, devID, direction, endpoint uint32, status int32, data []byte) ([]byte, error) {
+// 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),

internal/usbip/server.go

@@ -372,7 +372,7 @@ func (s *Server) handleCmdSubmit(r io.Reader, hdr *URBHeader, retChan chan<- []b
 			status = -32 // -EPIPE
 			n = 0
 		}
-		resp, err := BuildRetSubmit(hdr.SeqNum, hdr.DevID, hdr.Direction, hdr.Endpoint, status, buf[:n])
+		resp, err := BuildRetSubmit(hdr.SeqNum, hdr.DevID, hdr.Direction, hdr.Endpoint, status, uint32(n), buf[:n])
 		if err != nil {
 			return err
 		}
@@ -387,6 +387,7 @@ func (s *Server) handleCmdSubmit(r io.Reader, hdr *URBHeader, retChan chan<- []b
 		wIndex := binary.LittleEndian.Uint16(body.Setup[4:6])
 
 		var status int32
+		var actualLength uint32
 
 		// Intercept standard USB requests that require special usbdevfs ioctls.
 		// Raw control transfers via USBDEVFS_CONTROL don't update kernel state.
@@ -415,7 +416,7 @@ func (s *Server) handleCmdSubmit(r io.Reader, hdr *URBHeader, retChan chan<- []b
 			if buf == nil {
 				buf = make([]byte, 0)
 			}
-			_, err := s.handle.ControlTransfer(
+			n, err := s.handle.ControlTransfer(
 				bmRequestType, bRequest, wValue, wIndex,
 				binary.LittleEndian.Uint16(body.Setup[6:8]),
 				5000, buf,
@@ -424,13 +425,14 @@ func (s *Server) handleCmdSubmit(r io.Reader, hdr *URBHeader, retChan chan<- []b
 				log.Printf("[usbip-server] CTRL OUT seq=%d failed: %v", hdr.SeqNum, err)
 				status = -32 // -EPIPE
 			} else {
-				log.Printf("[usbip-server] CTRL OUT seq=%d OK (bmReqType=0x%02x bReq=0x%02x wVal=0x%04x)",
+				actualLength = uint32(n)
-					hdr.SeqNum, bmRequestType, bRequest, wValue)
+				log.Printf("[usbip-server] CTRL OUT seq=%d OK actualLength=%d (bmReqType=0x%02x bReq=0x%02x wVal=0x%04x)",
+					hdr.SeqNum, n, bmRequestType, bRequest, wValue)
 			}
 		}
 
-		log.Printf("[usbip-server] CTRL OUT seq=%d → response status=%d", hdr.SeqNum, status)
+		log.Printf("[usbip-server] CTRL OUT seq=%d → response status=%d actualLength=%d", hdr.SeqNum, status, actualLength)
-		resp, err := BuildRetSubmit(hdr.SeqNum, hdr.DevID, hdr.Direction, hdr.Endpoint, status, nil)
+		resp, err := BuildRetSubmit(hdr.SeqNum, hdr.DevID, hdr.Direction, hdr.Endpoint, status, actualLength, nil)
 		if err != nil {
 			return err
 		}
@@ -468,7 +470,7 @@ func (s *Server) handleCmdSubmit(r io.Reader, hdr *URBHeader, retChan chan<- []b
 	})
 	if err != nil {
 		log.Printf("[usbip-server] SubmitURB(ep=0x%02x, type=%d, len=%d) FAILED: %v", ep, urbType, len(buf), err)
-		resp, _ := BuildRetSubmit(hdr.SeqNum, hdr.DevID, hdr.Direction, hdr.Endpoint, -32, nil)
+		resp, _ := BuildRetSubmit(hdr.SeqNum, hdr.DevID, hdr.Direction, hdr.Endpoint, -32, 0, nil)
 		retChan <- resp
 		return nil
 	}
@@ -535,7 +537,7 @@ func (s *Server) handleISOSubmit(hdr *URBHeader, body *CmdSubmitBody, transferBu
 	if err != nil {
 		log.Printf("[usbip-server] ISO submit FAILED: %v", err)
 		// Submit failed - send error response
-		resp, _ := BuildRetSubmit(hdr.SeqNum, hdr.DevID, hdr.Direction, hdr.Endpoint, -32, nil)
+		resp, _ := BuildRetSubmit(hdr.SeqNum, hdr.DevID, hdr.Direction, hdr.Endpoint, -32, 0, nil)
 		retChan <- resp
 		return nil
 	}
@@ -653,6 +655,7 @@ func (s *Server) reapLoop(retChan chan<- []byte, done <-chan struct{}) {
 				pending.direction,
 				pending.endpoint,
 				urbInfo.Status,
+				uint32(urbInfo.ActualLength),
 				data,
 			)
 		}
@@ -708,6 +711,7 @@ func (s *Server) buildISOResponse(urbInfo *usb.ReapedURBInfo, pending *pendingUR
 		pending.direction,
 		pending.endpoint,
 		urbInfo.Status,
+		uint32(urbInfo.ActualLength),
 		packedData,
 		uint32(urbInfo.StartFrame),
 		pending.numPackets,