linux-kernel - [PATCH v2 01/10] compiler/gcc: Emit build-time warning for GCC prior to version 4.8

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Message-Id: <20200123153341.19947-2-will@kernel.org>
Date:   Thu, 23 Jan 2020 15:33:32 +0000
From:   Will Deacon <will@...nel.org>
To:     linux-kernel@...r.kernel.org
Cc:     linux-arch@...r.kernel.org, kernel-team@...roid.com,
        Will Deacon <will@...nel.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Peter Zijlstra <peterz@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Segher Boessenkool <segher@...nel.crashing.org>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Luc Van Oostenryck <luc.vanoostenryck@...il.com>,
        Arnd Bergmann <arnd@...db.de>,
        Peter Oberparleiter <oberpar@...ux.ibm.com>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>
Subject: [PATCH v2 01/10] compiler/gcc: Emit build-time warning for GCC prior to version 4.8

Prior to version 4.8, GCC may miscompile READ_ONCE() by erroneously
discarding the 'volatile' qualifier:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58145

We've been working around this using some nasty hacks which make
READ_ONCE() both horribly complicated and also prevent us from enforcing
that it is only used on scalar types. Since GCC 4.8 is pretty old for
kernel builds now, emit a warning if we detect it during the build.

Suggested-by: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Michael Ellerman <mpe@...erman.id.au>
Cc: Arnd Bergmann <arnd@...db.de>
Cc: Christian Borntraeger <borntraeger@...ibm.com>
Cc: Masahiro Yamada <masahiroy@...nel.org>
Signed-off-by: Will Deacon <will@...nel.org>
---
 init/Kconfig            | 4 ++--
 scripts/Kconfig.include | 6 ++++++
 2 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/init/Kconfig b/init/Kconfig
index a34064a031a5..bdc2f1b1667b 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -10,11 +10,11 @@ config DEFCONFIG_LIST
 	default "arch/$(ARCH)/defconfig"
 
 config CC_IS_GCC
-	def_bool $(success,$(CC) --version | head -n 1 | grep -q gcc)
+	def_bool $(cc-is-gcc)
 
 config GCC_VERSION
 	int
-	default $(shell,$(srctree)/scripts/gcc-version.sh $(CC)) if CC_IS_GCC
+	default $(gcc-version) if CC_IS_GCC
 	default 0
 
 config CC_IS_CLANG
diff --git a/scripts/Kconfig.include b/scripts/Kconfig.include
index d4adfbe42690..4e645a798b56 100644
--- a/scripts/Kconfig.include
+++ b/scripts/Kconfig.include
@@ -40,3 +40,9 @@ $(error-if,$(success, $(LD) -v | grep -q gold), gold linker '$(LD)' not supporte
 
 # gcc version including patch level
 gcc-version := $(shell,$(srctree)/scripts/gcc-version.sh $(CC))
+
+# Return y if the compiler is GCC, n otherwise
+cc-is-gcc := $(success,$(CC) --version | head -n 1 | grep -q gcc)
+
+# Warn if the compiler is GCC prior to 4.8
+$(warning-if,$(if-success,[ $(gcc-version) -lt 40800 ],$(cc-is-gcc),n),"Your compiler is old and may miscompile the kernel due to https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58145 - please upgrade it.")
-- 
2.25.0.341.g760bfbb309-goog