Re: [PATCH v2 2/2] kbuild: warn objects shared among multiple modules

[Alexander Lobakin]

From: Masahiro Yamada <masahiroy@xxxxxxxxxx>
Date: Sat, 19 Nov 2022 04:15:51 +0900

> If an object is shared among multiple modules, amd some of them are
> configured as 'm', but the others as 'y', the shared object is built
> as modular, then linked to the modules and vmlinux. This is a potential
> issue because the expected CFLAGS are different between modules and
> builtins.
>
> Commit 637a642f5ca5 ("zstd: Fixing mixed module-builtin objects")
> reported that this could be even more fatal in some cases such as
> Clang LTO.
>
> That commit fixed lib/zlib/zstd_{compress,decompress}, but there are
> still more instances of breakage.
>
> This commit adds a W=1 warning for shared objects, so that the kbuild
> test robot, which provides build tests with W=1, will avoid a new
> breakage slipping in.
>
> Quick compile tests on v6.1-rc4 detected the following:

For the series:

Reviewed-and-tested-by: Alexander Lobakin <alobakin@xxxxx>

[...]

> Once all the warnings are fixed, it can become an error irrespective of
> W= option.

BTW I've fixed most of these (the ones that get build on typical
x86_64 distroconfig) during my local tests, basing off your WIP
tree. I could send a series if you wish.

>
> Signed-off-by: Masahiro Yamada <masahiroy@xxxxxxxxxx>
> ---
>
> Changes in v2:
>   - Add drivers/block/rnbd/Makefile for commit log
>
>  scripts/Makefile.build | 6 ++++++
>  1 file changed, 6 insertions(+)
>
> diff --git a/scripts/Makefile.build b/scripts/Makefile.build
> index 37cf88d076e8..799df12b53f3 100644
> --- a/scripts/Makefile.build
> +++ b/scripts/Makefile.build
> @@ -222,6 +222,10 @@ endif
>
>  cmd_check_local_export = $(srctree)/scripts/check-local-export $@
>
> +ifneq ($(findstring 1, $(KBUILD_EXTRA_WARN)),)
> +cmd_warn_shared_object = $(if $(word 2, $(modname-multi)),$(warning $(kbuild-file): $*.o is added to multiple modules: $(modname-multi)))
> +endif
> +
>  define rule_cc_o_c
>  	$(call cmd_and_fixdep,cc_o_c)
>  	$(call cmd,gen_ksymdeps)
> @@ -231,6 +235,7 @@ define rule_cc_o_c
>  	$(call cmd,gen_objtooldep)
>  	$(call cmd,gen_symversions_c)
>  	$(call cmd,record_mcount)
> +	$(call cmd,warn_shared_object)
>  endef
>
>  define rule_as_o_S
> @@ -239,6 +244,7 @@ define rule_as_o_S
>  	$(call cmd,check_local_export)
>  	$(call cmd,gen_objtooldep)
>  	$(call cmd,gen_symversions_S)
> +	$(call cmd,warn_shared_object)
>  endef
>
>  # Built-in and composite module parts
> --
> 2.34.1

Thanks,
Olek