linux-kernel - Re: [PATCH] kbuild: enable unused-function warnings for W= build with Clang

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Message-ID: <201908190910.2E6A18A647@keescook>
Date:   Mon, 19 Aug 2019 09:10:45 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Masahiro Yamada <yamada.masahiro@...ionext.com>
Cc:     linux-kbuild@...r.kernel.org,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Nathan Chancellor <natechancellor@...il.com>,
        Miguel Ojeda <miguel.ojeda.sandonis@...il.com>,
        Borislav Petkov <bp@...e.de>,
        Luc Van Oostenryck <luc.vanoostenryck@...il.com>,
        Michal Marek <michal.lkml@...kovi.net>,
        Paul Burton <paul.burton@...s.com>,
        Xiaozhou Liu <liuxiaozhou@...edance.com>,
        clang-built-linux@...glegroups.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] kbuild: enable unused-function warnings for W= build
 with Clang

On Mon, Aug 19, 2019 at 07:51:38PM +0900, Masahiro Yamada wrote:
> GCC and Clang have different policy for -Wunused-function; GCC does
> not report unused-function warnings at all for the functions marked
> as 'static inline'. Clang does report unused-function warnings if they
> are defined in source files instead of headers.
> 
> We could use Clang for detecting unused functions, but it has been
> suppressed since commit abb2ea7dfd82 ("compiler, clang: suppress
> warning for unused static inline functions").
> 
> So, we never notice left-over code if functions in .c files are
> marked as inline.
> 
> Let's remove __maybe_unused from the inline macro. As always, it is
> not a good idea to sprinkle warnings for the normal build. So, these
> warnings will be shown for the W= build.
> 
> If you contribute to code clean-up, please run "make CC=clang W=1"
> and check -Wunused-function warnings. You will find lots of unused
> functions.
> 
> Some of them are false-positives because the call-sites are disabled
> by #ifdef. I do not like to abuse the inline keyword for suppressing
> unused-function warnings because it might affect the compiler's
> optimization. When I need to fix unused-functions warnings, I prefer
> adding #ifdef or __maybe_unused to function definitions.
> 
> Signed-off-by: Masahiro Yamada <yamada.masahiro@...ionext.com>

Reviewed-by: Kees Cook <keescook@...omium.org>

-Kees

> ---
> 
>  include/linux/compiler_types.h | 10 ++--------
>  scripts/Makefile.extrawarn     |  1 +
>  2 files changed, 3 insertions(+), 8 deletions(-)
> 
> diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
> index 599c27b56c29..14de8d0162fb 100644
> --- a/include/linux/compiler_types.h
> +++ b/include/linux/compiler_types.h
> @@ -130,10 +130,6 @@ struct ftrace_likely_data {
>  
>  /*
>   * Force always-inline if the user requests it so via the .config.
> - * GCC does not warn about unused static inline functions for
> - * -Wunused-function.  This turns out to avoid the need for complex #ifdef
> - * directives.  Suppress the warning in clang as well by using "unused"
> - * function attribute, which is redundant but not harmful for gcc.
>   * Prefer gnu_inline, so that extern inline functions do not emit an
>   * externally visible function. This makes extern inline behave as per gnu89
>   * semantics rather than c99. This prevents multiple symbol definition errors
> @@ -143,11 +139,9 @@ struct ftrace_likely_data {
>   * (which would break users of __always_inline).
>   */
>  #if !defined(CONFIG_OPTIMIZE_INLINING)
> -#define inline inline __attribute__((__always_inline__)) __gnu_inline \
> -	__maybe_unused notrace
> +#define inline inline __attribute__((__always_inline__)) __gnu_inline notrace
>  #else
> -#define inline inline                                    __gnu_inline \
> -	__maybe_unused notrace
> +#define inline inline                                    __gnu_inline notrace
>  #endif
>  
>  #define __inline__ inline
> diff --git a/scripts/Makefile.extrawarn b/scripts/Makefile.extrawarn
> index a74ce2e3c33e..92f542797e03 100644
> --- a/scripts/Makefile.extrawarn
> +++ b/scripts/Makefile.extrawarn
> @@ -70,5 +70,6 @@ KBUILD_CFLAGS += -Wno-initializer-overrides
>  KBUILD_CFLAGS += -Wno-format
>  KBUILD_CFLAGS += -Wno-sign-compare
>  KBUILD_CFLAGS += -Wno-format-zero-length
> +KBUILD_CFLAGS += -Wno-unused-function
>  endif
>  endif
> -- 
> 2.17.1
> 

-- 
Kees Cook