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

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Date:   Wed, 20 Jan 2021 11:46:11 -0800
From:   Will McVicker <willmcvicker@...gle.com>
To:     Masahiro Yamada <masahiroy@...nel.org>
Cc:     Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "Cc: Android Kernel" <kernel-team@...roid.com>
Subject: Re: [PATCH] kbuild: warn a relative path used for M= in out-of-tree builds

CC: kernel-team@...roid.com

Thanks Masahiro!

I'd like to also note that the Android kernel build system is
leveraging this behaviour as sort of an "O=" for external modules [1].
This way the external module's build artifacts are contained separate
from the kernel build artifacts and the module's source code.

--Will

[1] https://android.googlesource.com/kernel/build/+/refs/heads/master/build.sh#748

On Wed, Jan 20, 2021 at 11:31 AM Masahiro Yamada <masahiroy@...nel.org> wrote:
>
> 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@...gle.com>
> Signed-off-by: Masahiro Yamada <masahiroy@...nel.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
>