[PATCH] kbuild: remove trailing slashes from $(KBUILD_EXTMOD)

[Masahiro Yamada]

M= (or KBUILD_EXTMOD) generally expects a directory path without any
trailing slashes, like M=a/b/c.

If you add a trailing slash, like M=a/b/c/, you will get ugly build
logs (two slashes in a series), but it still works fine as long as it
is consistent between 'make modules' and 'make modules_install'.

So, the following code correctly builds and installs the modules.

  $ make M=a/b/c/ modules
  $ sudo make M=a/b/c/ modules_install

Since commit ccae4cfa7bfb ("kbuild: refactor scripts/Makefile.modinst"),
a problem happens if you add a trailing slash only for modules_install.

  $ make M=a/b/c modules
  $ sudo make M=a/b/c/ modules_install

No module is installed in this case, Johannes Berg reported. [1]

Trim any trailing slashes from $(KBUILD_EXTMOD).

I used the 'dirname' command to remove all the trailing slashes in
case someone plays with a crazy directory path like M=a/b/c/////.
The Make's built-in function, $(dir ...) cannot take care of such
a case.

[1]: https://lore.kernel.org/lkml/10cc8522b27a051e6a9c3e158a4c4b6414fd04a0.camel@xxxxxxxxxxxxxxxx/

Fixes: ccae4cfa7bfb ("kbuild: refactor scripts/Makefile.modinst")
Reported-by: Johannes Berg <johannes@xxxxxxxxxxxxxxxx>
Signed-off-by: Masahiro Yamada <masahiroy@xxxxxxxxxx>
---

 Makefile | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/Makefile b/Makefile
index b79e0e8acbe3..6dabf3a1f635 100644
--- a/Makefile
+++ b/Makefile
@@ -129,6 +129,11 @@ endif
 $(if $(word 2, $(KBUILD_EXTMOD)), \
 	$(error building multiple external modules is not supported))
 
+ifneq ($(KBUILD_EXTMOD),)
+# remove trailing slashes
+KBUILD_EXTMOD := $(shell dirname $(KBUILD_EXTMOD)/.)
+endif
+
 export KBUILD_EXTMOD
 
 # Kbuild will save output files in the current working directory.
-- 
2.27.0