Add support for USB writeable, MicroPython read-only volumes.

This prevents file system corruption due to two systems mutating
it at once.

2 files changed, 9 insertions, 4 deletions

diff --git a/extmod/fsusermount.h b/extmod/fsusermount.h
index 3eb54f8bd..7f2912b7f 100644
--- a/extmod/fsusermount.h
+++ b/extmod/fsusermount.h
@@ -28,9 +28,11 @@
 #include "py/obj.h"
 
 // these are the values for fs_user_mount_t.flags
-#define FSUSER_NATIVE       (0x0001) // readblocks[2]/writeblocks[2] contain native func
-#define FSUSER_FREE_OBJ     (0x0002) // fs_user_mount_t obj should be freed on umount
-#define FSUSER_HAVE_IOCTL   (0x0004) // new protocol with ioctl
+#define FSUSER_NATIVE        (0x0001) // readblocks[2]/writeblocks[2] contain native func
+#define FSUSER_FREE_OBJ      (0x0002) // fs_user_mount_t obj should be freed on umount
+#define FSUSER_HAVE_IOCTL    (0x0004) // new protocol with ioctl
+// Device is write-able over USB and read-only to MicroPython.
+#define FSUSER_USB_WRITEABLE (0x0008)
 
 // constants for block protocol ioctl
 #define BP_IOCTL_INIT           (1)
diff --git a/extmod/vfs_fat_diskio.c b/extmod/vfs_fat_diskio.c
index 5608e0645..4d776f6b4 100644
--- a/extmod/vfs_fat_diskio.c
+++ b/extmod/vfs_fat_diskio.c
@@ -98,7 +98,10 @@ DSTATUS disk_status (
         return STA_NOINIT;
     }
 
-    if (vfs->writeblocks[0] == MP_OBJ_NULL) {
+    // This is used to determine the writeability of the disk from MicroPython.
+    // So, if its USB writeable we make it read-only from MicroPython.
+    if (vfs->writeblocks[0] == MP_OBJ_NULL ||
+        (vfs->flags & FSUSER_USB_WRITEABLE) != 0) {
         return STA_PROTECT;
     } else {
         return 0;