linux-kernel - Re: [PATCH] gcc-plugins: simplify GCC plugin-dev capability test

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Message-ID: <202012041402.97F582B00C@keescook>
Date:   Fri, 4 Dec 2020 14:02:25 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Masahiro Yamada <masahiroy@...nel.org>
Cc:     linux-kbuild@...r.kernel.org, Emese Revfy <re.emese@...il.com>,
        linux-hardening@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Nathan Chancellor <natechancellor@...il.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        clang-built-linux@...glegroups.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] gcc-plugins: simplify GCC plugin-dev capability test

On Thu, Dec 03, 2020 at 09:57:00PM +0900, Masahiro Yamada wrote:
> Linus pointed out a third of the time in the Kconfig parse stage comes
> from the single invocation of cc1plus in scripts/gcc-plugin.sh [1],
> and directly testing plugin-version.h for existence cuts down the
> overhead a lot. [2]
> 
> This commit takes one step further to kill the build test entirely.
> 
> The small piece of code was probably intended to test the C++ designated
> initializer, which was not supported until C++20.
> 
> In fact, with -pedantic option given, both GCC and Clang emit a warning.
> 
> $ echo 'class test { public: int test; } test = { .test = 1 };' | g++ -x c++ -pedantic - -fsyntax-only
> <stdin>:1:43: warning: C++ designated initializers only available with '-std=c++2a' or '-std=gnu++2a' [-Wpedantic]
> $ echo 'class test { public: int test; } test = { .test = 1 };' | clang++ -x c++ -pedantic - -fsyntax-only
> <stdin>:1:43: warning: designated initializers are a C++20 extension [-Wc++20-designator]
> class test { public: int test; } test = { .test = 1 };
>                                           ^
> 1 warning generated.
> 
> Otherwise, modern C++ compilers should be able to build the code, and
> hopefully skipping this test should not make any practical problem.
> 
> Checking the existence of plugin-version.h is still needed to ensure
> the plugin-dev package is installed. The test code is now small enough
> to be embedded in scripts/gcc-plugins/Kconfig.
> 
> [1] https://lore.kernel.org/lkml/CAHk-=wjU4DCuwQ4pXshRbwDCUQB31ScaeuDo1tjoZ0_PjhLHzQ@mail.gmail.com/
> [2] https://lore.kernel.org/lkml/CAHk-=whK0aQxs6Q5ijJmYF1n2ch8cVFSUzU5yUM_HOjig=+vnw@mail.gmail.com/
> 
> Reported-by: Linus Torvalds <torvalds@...ux-foundation.org>
> Signed-off-by: Masahiro Yamada <masahiroy@...nel.org>

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

-- 
Kees Cook