linux-kernel - Re: [PATCH] kbuild: do not automatically add -w option to modpost

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Message-ID: <Y88NX11/LcnfloYk@google.com>
Date:   Mon, 23 Jan 2023 14:42:39 -0800
From:   William McVicker <willmcvicker@...gle.com>
To:     Masahiro Yamada <masahiroy@...nel.org>
Cc:     linux-kbuild@...r.kernel.org, linux-kernel@...r.kernel.org,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Nicolas Schier <nicolas@...sle.eu>
Subject: Re: [PATCH] kbuild: do not automatically add -w option to modpost

On 01/23/2023, Masahiro Yamada wrote:
> When there is a missing input file (vmlinux.o or Module.symvers), you
> are likely to get a ton of unresolved symbols.
> 
> Currently, Kbuild automatically adds the -w option to allow module builds
> to continue with warnings instead of errors.
> 
> This may not be what the user expects because it is generally more useful
> to catch all possible issues at build time instead of at run time.
> 
> Let's not do what the user did not ask.
> 
> If you still want to build modules anyway, you can proceed by explicitly
> setting KBUILD_MODPOST_WARN=1. Since you may miss a real issue, you need
> to be aware of what you are doing.
> 
> Suggested-by: William McVicker <willmcvicker@...gle.com>
> Signed-off-by: Masahiro Yamada <masahiroy@...nel.org>
> ---
> 
>  scripts/Makefile.modpost | 8 +++-----
>  1 file changed, 3 insertions(+), 5 deletions(-)
> 
> diff --git a/scripts/Makefile.modpost b/scripts/Makefile.modpost
> index 43343e13c542..9254ed811ddd 100644
> --- a/scripts/Makefile.modpost
> +++ b/scripts/Makefile.modpost
> @@ -121,16 +121,14 @@ modpost-args += -e $(addprefix -i , $(KBUILD_EXTRA_SYMBOLS))
>  
>  endif # ($(KBUILD_EXTMOD),)
>  
> -ifneq ($(missing-input),)
> -modpost-args += -w
> -endif
> -
>  quiet_cmd_modpost = MODPOST $@
>        cmd_modpost = \
>  	$(if $(missing-input), \
>  		echo >&2 "WARNING: $(missing-input) is missing."; \
>  		echo >&2 "         Modules may not have dependencies or modversions."; \
> -		echo >&2 "         You may get many unresolved symbol warnings.";) \
> +		echo >&2 "         You may get many unresolved symbol errors.";) \

You need to move the closing parenthesis to come at the end of these
echo messages. Otherwise you get this new message unconditionally.

I also found during testing that the refactoring in commit f73edc8951b2
("kbuild: unify two modpost invocations") dropped the check for missing
KBUILD_EXTRA_SYMBOLS. That means if an external module depends on
another external module and sets:

  KBUILD_EXTRA_SYMBOLS=/path/to/ext_module/Module.symvers

... then make will fail even with KBUILD_MODPOST_WARN=1 since we
unconditionally add KBUILD_EXTRA_SYMBOLS to the modpost-args like this:

  modpost-args += -e $(addprefix -i , $(KBUILD_EXTRA_SYMBOLS))

To fix this, I suggest you also take the following patch so that
KBUILD_MODPOST_WARN=1 will allow you to skip those unresolved symbols as
well:

diff --git a/scripts/Makefile.modpost b/scripts/Makefile.modpost
index 931a3272a4ba..0e2f7fa58056 100644
--- a/scripts/Makefile.modpost
+++ b/scripts/Makefile.modpost
@@ -149,14 +149,12 @@ include $(kbuild-file)

 output-symdump := $(KBUILD_EXTMOD)/Module.symvers

-ifeq ($(wildcard Module.symvers),)
-missing-input := Module.symvers
-else
-modpost-args += -i Module.symvers
-modpost-deps += Module.symvers
-endif
+input-symdump := Module.symvers $(KBUILD_EXTRA_SYMBOLS)
+existing-input-symdump := $(wildcard $(input-symdump))
+missing-input := $(filter-out $(existing-input-symdump), $(input-symdump))

-modpost-args += -e $(addprefix -i , $(KBUILD_EXTRA_SYMBOLS))
+modpost-deps += $(existing-input-symdumps)
+modpost-args += -e $(addprefix -i , $(existing-input-symdump))

 endif # ($(KBUILD_EXTMOD),)


With those changes, the patch works for me as expected. I verified I get
a build time error when referencing unresolved symbols without setting
KBUILD_MODPOST_WARN=1. And then I verified setting KBUILD_MODPOST_WARN=1
treated those errors as warnings.

Thanks,
Will

> +		echo >&2 "         You can set KBUILD_MODPOST_WARN=1 to turn errors into warning"; \
> +		echo >&2 "         if you want to proceed at your own risk."; \
>  	$(MODPOST) $(modpost-args)
>  
>  targets += $(output-symdump)
> -- 
> 2.34.1
>