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

[Zeng Heng]

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@xxxxxxxxxx> 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@xxxxxxxxxxxxxx
> > >
> > > "-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@xxxxxxxxxx>
> > > ---
> > >   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@xxxxxxxxxxxxxx/

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