linux-kernel - Re: [PATCH -next] Makefile: add implicit enum-conversion check for compile build

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Date:   Tue, 27 Sep 2022 09:45:17 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Zeng Heng <zengheng4@...wei.com>
Cc:     masahiroy@...nel.org, michal.lkml@...kovi.net,
        akpm@...ux-foundation.org, peterz@...radead.org,
        keescook@...omium.org, davidgow@...gle.com, nathan@...nel.org,
        jpoimboe@...nel.org, dan.j.williams@...el.com, ojeda@...nel.org,
        isabbasso@...eup.net, dmitrii.bundin.a@...il.com, vbabka@...e.cz,
        linux@...musvillemoes.dk, linux-kbuild@...r.kernel.org,
        linux-kernel@...r.kernel.org, liwei391@...wei.com,
        weiyongjun1@...wei.com, clang-built-linux <llvm@...ts.linux.dev>,
        linux-toolchains <linux-toolchains@...r.kernel.org>
Subject: Re: [PATCH -next] Makefile: add implicit enum-conversion check for
 compile build

On Tue, Sep 27, 2022 at 8:15 AM Zeng Heng <zengheng4@...wei.com> wrote:
>
> Provide implicit enum-conversion warning option
> in general build. When it set enabled, it can
> detect implicit enum type conversion and find
> potential conversion errors like below
> (use "allmodconfig"):
>
> drivers/gpu/drm/amd/amdgpu/../display/dc/dml/dcn20/display_mode_vba_20.c:3904:46:
> error: implicit conversion from ‘enum <anonymous>’ to ‘enum odm_combine_mode’ [-Werror=enum-conversion]
>  3904 |       locals->ODMCombineEnablePerState[i][k] = true;
>       |                                              ^
>
> The '-Wenum-conversion' could be regarded as
> effective check on compile runtime and
> call attention on potential mistakes.
>
> Anothor practical example could be referred to:
> https://lore.kernel.org/all/CADnq5_OE0yZvEYGu82QJHL9wvVcTFZrmeTgX7URgh7FVA=jqYg@mail.gmail.com
>
> "-Wenum-conversion" was firstly introduced from
> GNU gcc-10.

What about clang? ;)

>
> Although "-Wenum-conversion" could be enabled
> by "-Wextra" when compiling with 'W=1' option,
> there are many warnings generated by '-Wextra'
> that cause too much noise in a build.

With clang, I believe that -Wenum-conversion is part of -Wall or
-Wextra; so enabling this explicitly is only necessary for GCC.  I
wonder why it's not part of -Wall or -Wextra for GCC?  Perhaps worth a
bug report/feature request?

>
> Seeing the details from the following link:
> https://gcc.gnu.org/onlinedocs/gcc-11.3.0/gcc/Warning-Options.html
>
> Because there are still some concerned warnings
> exist, the patch marks the option disabled in default
> for avoiding compile failed like using "allmodconfig".
>
> Signed-off-by: Zeng Heng <zengheng4@...wei.com>
> ---
>  Makefile          | 5 +++++
>  lib/Kconfig.debug | 7 +++++++
>  2 files changed, 12 insertions(+)
>
> diff --git a/Makefile b/Makefile
> index ebd48fc956a3..1790a3624358 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -880,6 +880,11 @@ endif
>  KBUILD_CFLAGS += $(call cc-disable-warning, unused-but-set-variable)
>  KBUILD_CFLAGS += $(call cc-disable-warning, unused-const-variable)
>
> +# check implicit enum conversion
> +ifdef CONFIG_ENUM_CONVERSION
> +KBUILD_CFLAGS += -Wenum-conversion
> +endif

Having a kconfig for this is overkill.  cc-option with a comment about
the compiler default versions is the way to go.

> +
>  # These result in bogus false positives
>  KBUILD_CFLAGS += $(call cc-disable-warning, dangling-pointer)
>
> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> index 4f2b81229a2f..a64e06a747d8 100644
> --- a/lib/Kconfig.debug
> +++ b/lib/Kconfig.debug
> @@ -417,6 +417,13 @@ config FRAME_WARN
>           Setting this too low will cause a lot of warnings.
>           Setting it to 0 disables the warning.
>
> +config ENUM_CONVERSION
> +       bool "Warn for implicit enum conversion"
> +       depends on GCC_VERSION >= 100300
> +       default n
> +       help
> +         Tell gcc to warn at build time for implicit enum conversion.
> +
>  config STRIP_ASM_SYMS
>         bool "Strip assembler-generated symbols during link"
>         default n
> --
> 2.25.1
>


-- 
Thanks,
~Nick Desaulniers