[PATCH v2] kbuild: remove trailing slashes from $(KBUILD_EXTMOD)

[Masahiro Yamada <masahiroy@kernel.org>]

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'.

The following commands correctly build and install 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 adds more slashes 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@sipsolutions.net/

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

Changes in v2:
  - Use $(filter %/, ) so that the shell invocation is avoided
    if M= is already good.

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

diff --git a/Makefile b/Makefile
index b79e0e8acbe3..8018b8adbcaf 100644
--- a/Makefile
+++ b/Makefile
@@ -129,6 +129,11 @@ endif
 $(if $(word 2, $(KBUILD_EXTMOD)), \
 	$(error building multiple external modules is not supported))
 
+# Remove trailing slashes
+ifneq ($(filter %/, $(KBUILD_EXTMOD)),)
+KBUILD_EXTMOD := $(shell dirname $(KBUILD_EXTMOD).)
+endif
+
 export KBUILD_EXTMOD
 
 # Kbuild will save output files in the current working directory.
-- 
2.27.0