[PATCH] kbuild: warn a relative path used for M= in out-of-tree builds

[Masahiro Yamada <masahiroy@kernel.org>]

Surprising to me, Kbuild can handle a relative M= path while having
in-kernel objects in a separate directory. In this case, the M= option
actually defines:

 - the path to external module sources relative to $(srctree)
 - the path to external module objects relative to $(objtree)

For example,

  make O=foo/bar M=baz

... will find the source files in baz, and create objects in foo/bar/baz.

Currently, it works like that because of the presence of VPATH and
--include-dir=$(abs_srctree), but this behavior is subtle and cannot
be guaranteed in the future. I do not know if it was even officially
supported ever.

I do not recommend this usage.

Reported-by: Will McVicker <willmcvicker@google.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
---

 Makefile | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/Makefile b/Makefile
index b0e4767735dc..e42a27447eae 100644
--- a/Makefile
+++ b/Makefile
@@ -1699,6 +1699,13 @@ else # KBUILD_EXTMOD
 #                      Install the modules built in the module directory
 #                      Assumes install directory is already created
 
+ifdef building_out_of_srctree
+ifneq ($(filter-out /%, $(KBUILD_EXTMOD)),)
+$(warning The combination of out-of-tree build and relative M= path is not well supported.)
+$(warning Please do not complain about any trouble happening under this circumstance.)
+endif
+endif
+
 # We are always building only modules.
 KBUILD_BUILTIN :=
 KBUILD_MODULES := 1
-- 
2.27.0