diff --git a/core/main.mk b/core/main.mk
index fa56b83920..3c64d95551 100644
--- a/core/main.mk
+++ b/core/main.mk
@@ -52,6 +52,11 @@ endif
 endif
 endif
 
+# Absolute path of the present working direcotry.
+# This overrides the shell variable $PWD, which does not necessarily points to
+# the top of the source tree, for example when "make -C" is used in m/mm/mmm.
+PWD := $(shell pwd)
+
 TOP := .
 TOPDIR :=