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

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Date:   Thu,  3 Dec 2020 21:57:00 +0900
From:   Masahiro Yamada <masahiroy@...nel.org>
To:     linux-kbuild@...r.kernel.org, Kees Cook <keescook@...omium.org>,
        Emese Revfy <re.emese@...il.com>,
        linux-hardening@...r.kernel.org
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Nathan Chancellor <natechancellor@...il.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        clang-built-linux@...glegroups.com, linux-kernel@...r.kernel.org
Subject: [PATCH] gcc-plugins: simplify GCC plugin-dev capability test

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>
---

 scripts/gcc-plugin.sh       | 19 -------------------
 scripts/gcc-plugins/Kconfig |  2 +-
 2 files changed, 1 insertion(+), 20 deletions(-)
 delete mode 100755 scripts/gcc-plugin.sh

diff --git a/scripts/gcc-plugin.sh b/scripts/gcc-plugin.sh
deleted file mode 100755
index b79fd0bea838..000000000000
--- a/scripts/gcc-plugin.sh
+++ /dev/null
@@ -1,19 +0,0 @@
-#!/bin/sh
-# SPDX-License-Identifier: GPL-2.0
-
-set -e
-
-srctree=$(dirname "$0")
-
-gccplugins_dir=$($* -print-file-name=plugin)
-
-# we need a c++ compiler that supports the designated initializer GNU extension
-$HOSTCC -c -x c++ -std=gnu++98 - -fsyntax-only -I $srctree/gcc-plugins -I $gccplugins_dir/include 2>/dev/null <<EOF
-#include "gcc-common.h"
-class test {
-public:
-	int test;
-} test = {
-	.test = 1
-};
-EOF
diff --git a/scripts/gcc-plugins/Kconfig b/scripts/gcc-plugins/Kconfig
index ae19fb0243b9..ab9eb4cbe33a 100644
--- a/scripts/gcc-plugins/Kconfig
+++ b/scripts/gcc-plugins/Kconfig
@@ -9,7 +9,7 @@ menuconfig GCC_PLUGINS
 	bool "GCC plugins"
 	depends on HAVE_GCC_PLUGINS
 	depends on CC_IS_GCC
-	depends on $(success,$(srctree)/scripts/gcc-plugin.sh $(CC))
+	depends on $(success,test -e $(shell,$(CC) -print-file-name=plugin)/include/plugin-version.h)
 	default y
 	help
 	  GCC plugins are loadable modules that provide extra features to the
-- 
2.27.0