diff '--color=auto' -Nur a/Makefile b/Makefile
--- a/Makefile	2025-03-12 10:37:08.581683046 +0000
+++ b/Makefile	2025-03-12 10:35:30.305692527 +0000
@@ -24,14 +24,17 @@
 # SPDX-License-Identifier: GPL-3.0-only
 #
 
-ifneq ($(KERNELRELEASE),)
+ifneq ($(KBUILD_EXTMOD),)
 
-# Building from kBuild (make -C <kernel_directory> M=`pwd`)
-# or inside a kernel source tree.
+# Building from kBuild (make -C <kernel_directory> M=`pwd`).
+# KBUILD_EXTMOD is set to $(M) in this case.
 
-obj-m = vboxguest/ vboxsf/
+obj-m = vboxguest/
+ ifneq ($(wildcard $(KBUILD_EXTMOD)/vboxsf/Makefile),)
+obj-m += vboxsf/
+ endif
 
-else # ! KERNELRELEASE
+else # ! KBUILD_EXTMOD
 
 KBUILD_VERBOSE =
  ifeq ($(KBUILD_VERBOSE),)
@@ -107,4 +110,4 @@
     install-vboxguest install-vboxsf \
     clean-vboxguest clean-vboxsf
 
-endif # ! KERNELRELEASE
+endif # ! KBUILD_EXTMOD