diff --git a/Makefile b/Makefile
index d5b37391195f80..f64d96867cb3c8 100644
--- a/Makefile
+++ b/Makefile
@@ -143,7 +143,8 @@ $(filter-out _all sub-make $(CURDIR)/Makefile, $(MAKECMDGOALS)) _all: sub-make
 	@:
 
 sub-make: FORCE
-	$(Q)$(MAKE) -C $(KBUILD_OUTPUT) KBUILD_SRC=$(CURDIR) \
+	$(Q)$(MAKE) -C $(KBUILD_OUTPUT) \
+	KBUILD_SRC=$(shell realpath --relative-to=$(KBUILD_OUTPUT) $(CURDIR)) \
 	-f $(CURDIR)/Makefile $(filter-out _all sub-make,$(MAKECMDGOALS))
 
 # Leave processing to above invocation of make