linux-kernel - Re: [PATCH RESEND] gcc-plugins: avoid errors with -std=gnu++11 on old gcc

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Message-ID: <202103172251.F9D770D@keescook>
Date:   Wed, 17 Mar 2021 22:52:56 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Valdis Klētnieks <valdis.kletnieks@...edu>
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        linux-hardening@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RESEND] gcc-plugins: avoid errors with -std=gnu++11 on
 old gcc

On Mon, Mar 08, 2021 at 03:40:21AM -0500, Valdis Klētnieks wrote:
> It turns out that older gcc (4.9 and 5.4) have gnu++11 support, but
> due to a gcc bug fixed in gcc6, throw errors during the build.
> The relevant gcc bug is https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69959
> 
> Version the option based on what gcc we're using.

Is there a better way to detect this than with version checking?

-Kees

> 
> Signed-off-by: Valdis Kletnieks <valdis.kletnieks@...edu>
> Fixes: 27c287b41659 ("gcc-plugins: fix gcc 11 indigestion with plugins...")
> ---
> diff --git a/scripts/gcc-plugins/Makefile b/scripts/gcc-plugins/Makefile
> index b5487cce69e8..cc779973724a 100644
> --- a/scripts/gcc-plugins/Makefile
> +++ b/scripts/gcc-plugins/Makefile
> @@ -21,8 +21,11 @@ always-y += $(GCC_PLUGIN)
>  
>  GCC_PLUGINS_DIR = $(shell $(CC) -print-file-name=plugin)
>  
> +# need gnu++11 for gcc 11, but 4.9 and 5.4 need gnu++98
> +GCC_FLAVOR = $(call cc-ifversion, -ge, 1100, 11, 98)
> +
>  plugin_cxxflags	= -Wp,-MMD,$(depfile) $(KBUILD_HOSTCXXFLAGS) -fPIC \
> -		   -I $(GCC_PLUGINS_DIR)/include -I $(obj) -std=gnu++11 \
> +		   -I $(GCC_PLUGINS_DIR)/include -I $(obj) -std=gnu++$(GCC_FLAVOR) \
>  		   -fno-rtti -fno-exceptions -fasynchronous-unwind-tables \
>  		   -ggdb -Wno-narrowing -Wno-unused-variable \
>  		   -Wno-format-diag
> 
> 

-- 
Kees Cook