fix: don't expose eMMC as USB mass storage when no image is mounted

When /boot/usb.disk0 exists but is empty (the default state), the USB
gadget init script (S03usbdev) sets /dev/mmcblk0p3 as the mass storage
backing file. This exposes the NanoKVM's raw eMMC partition (~162MB,
ext4/exfat) as a USB disk to the host computer.

On Legacy BIOS systems, the BIOS attempts to boot from this device,
reads invalid data (no MBR), and enters a HLT loop — completely hanging
the system with no keyboard input accepted. The only recovery is a
physical power cycle.

Similarly, the Go server (image.go) re-mounts /dev/mmcblk0p3 whenever
an ISO image is unmounted, re-exposing the eMMC partition.

Changes:

S03usbdev: Flip the empty-file check so that when usb.disk0 is empty,
no backing file is set. The mass_storage device (with removable=1)
reports "no media inserted" — which BIOS handles safely by skipping
to the next boot device. When usb.disk0 contains a path, that path
is used as before.

image.go: Remove the imageNone constant (/dev/mmcblk0p3). When
unmounting an image (empty request), the device stays unmounted instead
of falling back to the eMMC partition. GetMountedImage retains backward
compatibility by treating mmcblk0p3 as "no image" for devices that
haven't rebooted yet.

No changes needed to virtual-device.go — with this fix, toggling
"Virtual Disk" ON creates the mass_storage gadget with no media
(correct behavior), and users mount ISOs through the existing UI.

Refs: #633, #385, #438, #187

Author: MagnaCapax

kvmapp/system/init.d/S03usbdev

@@ -106,17 +106,15 @@ start_usb_dev(){
             fi
             echo "NanoKVM USB Mass Storage0520" > functions/mass_storage.disk0/lun.0/inquiry_string
             disk=$(cat /boot/usb.disk0)
-            if [ -z "${disk}" ]
+            if [ -n "${disk}" ]
             then
-                    # if [ ! -e /mnt/usbdisk.img ]
-                    # then
-                    #         fallocate -l 8G /mnt/usbdisk.img
-                    #         mkfs.vfat /mnt/usbdisk.img
-                    # fi
-                    echo /dev/mmcblk0p3 > functions/mass_storage.disk0/lun.0/file
-            else
                     cat /boot/usb.disk0 > functions/mass_storage.disk0/lun.0/file
             fi
+            # When usb.disk0 is empty, no backing file is set. The device
+            # reports "no media" (removable=1). Previously this defaulted to
+            # /dev/mmcblk0p3, exposing the NanoKVM's raw eMMC partition as a
+            # USB disk — causing Legacy BIOS systems to hang during boot.
+            # See: https://github.com/sipeed/NanoKVM/issues/633
     fi
 
     ls /sys/class/udc/ | cat > UDC

server/service/storage/image.go

@@ -18,7 +18,6 @@ import (
 
 const (
 	imageDirectory = "/data"
-	imageNone      = "/dev/mmcblk0p3"
 	cdromFlag      = "/sys/kernel/config/usb_gadget/g0/functions/mass_storage.disk0/lun.0/cdrom"
 	mountDevice    = "/sys/kernel/config/usb_gadget/g0/functions/mass_storage.disk0/lun.0/file"
 	inquiryString  = "/sys/kernel/config/usb_gadget/g0/functions/mass_storage.disk0/lun.0/inquiry_string"
@@ -109,16 +108,17 @@ func (s *Service) MountImage(c *gin.Context) {
 	}
 
 	// mount
-	image := req.File
-	if image == "" {
-		image = imageNone
-	}
-
-	if err := os.WriteFile(mountDevice, []byte(image), 0o666); err != nil {
-		log.Errorf("mount file %s failed: %s", image, err)
-		rsp.ErrRsp(c, -2, "mount image failed")
-		return
+	if req.File != "" {
+		if err := os.WriteFile(mountDevice, []byte(req.File), 0o666); err != nil {
+			log.Errorf("mount file %s failed: %s", req.File, err)
+			rsp.ErrRsp(c, -2, "mount image failed")
+			return
+		}
 	}
+	// When req.File is empty the device stays unmounted (no media).
+	// Previously this wrote /dev/mmcblk0p3, exposing the NanoKVM's raw
+	// eMMC partition as a USB disk — causing Legacy BIOS boot hangs.
+	// See: https://github.com/sipeed/NanoKVM/issues/633
 
 	h := hid.GetHid()
 	h.Lock()
@@ -156,8 +156,9 @@ func (s *Service) GetMountedImage(c *gin.Context) {
 		return
 	}
 
-	image := strings.ReplaceAll(string(content), "\n", "")
-	if image == imageNone {
+	image := strings.TrimSpace(string(content))
+	if image == "/dev/mmcblk0p3" {
+		// Backward compat: treat eMMC partition as "no image mounted"
 		image = ""
 	}