linux-kernel - Re: [PATCH] ARM: Emit __gnu_mcount_nc when using Clang 10.0.0 or newer

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Date:   Thu, 29 Aug 2019 09:57:10 +0200
From:   Stefan Agner <stefan@...er.ch>
To:     Nathan Chancellor <natechancellor@...il.com>
Cc:     Russell King <linux@...linux.org.uk>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Arnd Bergmann <arnd@...db.de>,
        Matthias Kaehlcke <mka@...omium.org>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        clang-built-linux@...glegroups.com
Subject: Re: [PATCH] ARM: Emit __gnu_mcount_nc when using Clang 10.0.0 or
 newer

On 2019-08-29 08:26, Nathan Chancellor wrote:
> Currently, multi_v7_defconfig + CONFIG_FUNCTION_TRACER fails to build
> with clang:
> 
> arm-linux-gnueabi-ld: kernel/softirq.o: in function `_local_bh_enable':
> softirq.c:(.text+0x504): undefined reference to `mcount'
> arm-linux-gnueabi-ld: kernel/softirq.o: in function `__local_bh_enable_ip':
> softirq.c:(.text+0x58c): undefined reference to `mcount'
> arm-linux-gnueabi-ld: kernel/softirq.o: in function `do_softirq':
> softirq.c:(.text+0x6c8): undefined reference to `mcount'
> arm-linux-gnueabi-ld: kernel/softirq.o: in function `irq_enter':
> softirq.c:(.text+0x75c): undefined reference to `mcount'
> arm-linux-gnueabi-ld: kernel/softirq.o: in function `irq_exit':
> softirq.c:(.text+0x840): undefined reference to `mcount'
> arm-linux-gnueabi-ld: kernel/softirq.o:softirq.c:(.text+0xa50): more
> undefined references to `mcount' follow
> 
> clang can emit a working mcount symbol, __gnu_mcount_nc, when
> '-meabi gnu' is passed to it. Until r369147 in LLVM, this was
> broken and caused the kernel not to boot because the calling
> convention was not correct. Now that it is fixed, add this to
> the command line when clang is 10.0.0 or newer so everything
> works properly.

Cool, finally function tracing with Clang :-)

> 
> Link: https://github.com/ClangBuiltLinux/linux/issues/35
> Link: https://bugs.llvm.org/show_bug.cgi?id=33845
> Link:
> https://github.com/llvm/llvm-project/commit/16fa8b09702378bacfa3d07081afe6b353b99e60
> Signed-off-by: Nathan Chancellor <natechancellor@...il.com>

Reviewed-by: Stefan Agner <stefan@...er.ch>

--
Stefan

> ---
>  arch/arm/Makefile | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/arch/arm/Makefile b/arch/arm/Makefile
> index c3624ca6c0bc..7b5a26a866fc 100644
> --- a/arch/arm/Makefile
> +++ b/arch/arm/Makefile
> @@ -112,6 +112,12 @@ ifeq ($(CONFIG_ARM_UNWIND),y)
>  CFLAGS_ABI	+=-funwind-tables
>  endif
>  
> +ifeq ($(CONFIG_CC_IS_CLANG),y)
> +ifeq ($(shell test $(CONFIG_CLANG_VERSION) -ge 100000; echo $$?),0)
> +CFLAGS_ABI	+=-meabi gnu
> +endif
> +endif
> +
>  # Accept old syntax despite ".syntax unified"
>  AFLAGS_NOWARN	:=$(call as-option,-Wa$(comma)-mno-warn-deprecated,-Wa$(comma)-W)