linux-kernel - Re: [PATCH] kbuild: allow making undefined symbols fatal for external modules

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Date:   Tue, 10 Oct 2017 01:25:38 +0900
From:   Masahiro Yamada <yamada.masahiro@...ionext.com>
To:     Jonas Gorski <jonas.gorski@...il.com>
Cc:     Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        Sam Ravnborg <sam@...nborg.org>
Subject: Re: [PATCH] kbuild: allow making undefined symbols fatal for external modules

2017-10-02 19:50 GMT+09:00 Jonas Gorski <jonas.gorski@...il.com>:
> By passing appropriate values in KBUILD_EXTRA_SYMBOLS it is possible to
> make modpost be able to resolve all symbols for external modules, even
> between to other external modules.
>
> Because of this, it might be desirable to make unresolved symbols an
> error on external module builds as well, to catch missing exports there
> as well.
>
> So add a new flag KBUILD_MODPOST_ERROR which can be set to enable this
> behaviour. It has no effect on non external module builds.
>
> Signed-off-by: Jonas Gorski <jonas.gorski@...il.com>
> ---
> The main target is to allow distributions (like LEDE) that make use of
> kernel backports and other out of tree drivers to be able to catch
> errors like missing EXPORT_SYMBOLs in the kernel early.
>
>  scripts/Makefile.modpost | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/scripts/Makefile.modpost b/scripts/Makefile.modpost
> index 16923ba4b5b1..29f787bf8c3a 100644
> --- a/scripts/Makefile.modpost
> +++ b/scripts/Makefile.modpost
> @@ -34,6 +34,8 @@
>
>  # KBUILD_MODPOST_WARN can be set to avoid error out in case of undefined
>  # symbols in the final module linking stage
> +# KBUILD_MODPOST_ERROR can be set to error out in case of undefined
> +# symbols in the final module linking stage for external modules
>  # KBUILD_MODPOST_NOFINAL can be set to skip the final link of modules.
>  # This is solely useful to speed up test compiles
>  PHONY := _modpost
> @@ -49,6 +51,8 @@ ifneq ($(KBUILD_EXTMOD),)
>  obj := $(KBUILD_EXTMOD)
>  src := $(obj)
>
> +KBUILD_MODPOST_WARN = $(if $(KBUILD_MODPOST_ERROR),,1)
> +
>  # Include the module's Makefile to find KBUILD_EXTRA_SYMBOLS
>  include $(if $(wildcard $(KBUILD_EXTMOD)/Kbuild), \
>               $(KBUILD_EXTMOD)/Kbuild, $(KBUILD_EXTMOD)/Makefile)
> @@ -78,7 +82,7 @@ modpost = scripts/mod/modpost                    \
>   $(if $(KBUILD_EXTMOD),-o $(modulesymfile))      \
>   $(if $(CONFIG_DEBUG_SECTION_MISMATCH),,-S)      \
>   $(if $(CONFIG_SECTION_MISMATCH_WARN_ONLY),,-E)  \
> - $(if $(KBUILD_EXTMOD)$(KBUILD_MODPOST_WARN),-w)
> + $(if $(KBUILD_MODPOST_WARN),-w)
>
>  MODPOST_OPT=$(subst -i,-n,$(filter -i,$(MAKEFLAGS)))
>


The motivation looks reasonable to me,
but having both KBUILD_MODPOST_WARN and KBUILD_MODPOST_ERROR
is a bit confusing, I think.

(Some people may wonder what would happen if both are set...)


So, here is my suggestion.


We have only KBUILD_MODPOST_WARN.

The default value is 1 for external module building.
                     0 for kernel building.


If you want to change the default behavior,
you can specify KBUILD_MODPOST_WARN=1 when building kernel,
or KBUILD_MODPOST_WARN=0 when building external module
from the command line.


Also, you need to update Documentation/kbuild/kbuild.txt





-- 
Best Regards
Masahiro Yamada