Date:   Wed, 28 Sep 2022 15:17:36 +0800
From:   Zeng Heng <zengheng4@...wei.com>
To:     Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>
CC:     <masahiroy@...nel.org>, <michal.lkml@...kovi.net>,
        <akpm@...ux-foundation.org>, <peterz@...radead.org>,
        <keescook@...omium.org>, <davidgow@...gle.com>,
        <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 2022/9/28 1:02, Nathan Chancellor wrote:
> On Tue, Sep 27, 2022 at 09:45:17AM -0700, Nick Desaulniers wrote:
>> 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?
> With clang, -Wenum-conversion is just default enabled, not even behind
> -Wall:
>
> $ cat test.c
> enum enum1 { A = 1 };
> enum enum2 { B = 2 };
>
> enum enum1 foo(enum enum2 bar)
> {
>      return bar;
> }
>
> $ clang -fsyntax-only test.c
> test.c:11:9: warning: implicit conversion from enumeration type 'enum enum2' to different enumeration type 'enum enum1' [-Wenum-conversion]
>          return bar;
>          ~~~~~~ ^~~
> 1 warning generated.
>
> On the other hand, GCC does have it under -Wextra:
>
> $ gcc -fsyntax-only test.c
>
> $ gcc -Wextra -fsyntax-only test.c
> test.c: In function ‘foo’:
> test.c:6:16: warning: implicit conversion from ‘enum enum2’ to ‘enum enum1’ [-Wenum-conversion]
>      6 |         return bar;
>        |                ^~~
>
> But the kernel does not build with -Wextra aside from W=[123], hence
> this warning has to be explicitly requested for GCC.

Thanks to your replenish about clang.


>>> 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".
> But there is no dependency to avoid this getting enabled by allmodconfig
> (such as 'depends on !COMPILE_TEST') so I don't see the point in the
> current form; 'default n' does nothing to prevent it. Regardless, I
> agree with Nick's sentiment below.
>
>>> 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.
Got it.
> Agreed. If there is some reason -Wenum-conversion cannot be enabled for
> GCC right now (such as existing warnings, which the commit message
> appears to alude to), they should be cleaned up first then
> -Wenum-conversion should just be unconditionally enabled for all
> compilers that support it via cc-option, not half enabled via Kconfig so
> that maybe people will clean up the warnings. That is not how enabling
> warnings works:
>
> https://lore.kernel.org/CAHk-=wg-mH-_GYpkhz_psjBWG6ZcjKnPo83fg7YMj_by+-LRTQ@mail.gmail.com/

Have sent the patches to fix the involving warnings.

If all the concerned warnings were repaired, I would send the v2 again.

Thanks all.

>>> +
>>>   # 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
>>

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