fuse: expose BackingFdMap

This prepares for extension to the passthrough support, in particular,
the ops_mask to be added to `struct fuse_backing_map`.

Change-Id: If4d7c321a5d09e9b2a3b804bd5e434aab394fe30

fuse/passthrough_linux.go

@@ -14,29 +14,20 @@ const (
 	_DEV_IOC_BACKING_CLOSE = 0x4004e502
 )
 
-type backingMap struct {
-	Fd      int32
-	Flags   uint32
-	padding uint64
-}
-
 // RegisterBackingFd registers the given file descriptor in the
 // kernel, so the kernel can bypass FUSE and access the backing file
 // directly for read and write calls. On success a backing ID is
 // returned. The backing ID should unregistered using
-// UnregisterBackingFd() once the file is released. For now, the flags
-// argument is unused, and should be 0.
-func (ms *Server) RegisterBackingFd(fd int, flags uint32) (int32, syscall.Errno) {
-	m := backingMap{
-		Fd:    int32(fd),
-		Flags: flags,
-	}
-
+// UnregisterBackingFd() once the file is released.  Within the
+// kernel, an inode can only have a single backing file, so multiple
+// Open/Create calls should coordinate to return a consistent backing
+// ID.
+func (ms *Server) RegisterBackingFd(m *BackingMap) (int32, syscall.Errno) {
 	ms.writeMu.Lock()
-	id, _, errno := syscall.Syscall(syscall.SYS_IOCTL, uintptr(ms.mountFd), uintptr(_DEV_IOC_BACKING_OPEN), uintptr(unsafe.Pointer(&m)))
+	id, _, errno := syscall.Syscall(syscall.SYS_IOCTL, uintptr(ms.mountFd), uintptr(_DEV_IOC_BACKING_OPEN), uintptr(unsafe.Pointer(m)))
 	ms.writeMu.Unlock()
 	if ms.opts.Debug {
-		ms.opts.Logger.Printf("ioctl: BACKING_OPEN %d (flags %x): id %d (%v)", fd, flags, id, errno)
+		ms.opts.Logger.Printf("ioctl: BACKING_OPEN %v: id %d (%v)", m.string(), id, errno)
 	}
 	return int32(id), errno
 }

fuse/print.go

@@ -377,3 +377,7 @@ func (a *Attr) string() string {
 		a.Rdev, a.Ino, ft(a.Atime, a.Atimensec), ft(a.Mtime, a.Mtimensec),
 		ft(a.Ctime, a.Ctimensec))
 }
+
+func (m *BackingMap) string() string {
+	return fmt.Sprintf("{fd %d, flags 0x%x}", m.Fd, m.Flags)
+}

fuse/types.go

@@ -766,3 +766,10 @@ type WriteIn struct {
 	Flags      uint32
 	Padding    uint32
 }
+
+// Data for registering a file as backing an inode.
+type BackingMap struct {
+	Fd      int32
+	Flags   uint32
+	padding uint64
+}