Re: "arm64: vdso: Substitute gettimeofday() with C implementation" breaks clang build

From: Vincenzo Frascino <vincenzo.frascino@arm.com>
To: Nick Desaulniers <ndesaulniers@google.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will.deacon@arm.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Qian Cai <cai@lca.pw>, Thomas Gleixner <tglx@linutronix.de>,
	Nathan Chancellor <natechancellor@gmail.com>,
	Linux ARM <linux-arm-kernel@lists.infradead.org>
Subject: Re: "arm64: vdso: Substitute gettimeofday() with C implementation" breaks clang build
Date: Tue, 25 Jun 2019 18:00:27 +0100	[thread overview]
Message-ID: <193c179e-16ca-4b4e-2584-75e8f6c1819f@arm.com> (raw)
In-Reply-To: <CAKwvOdmCFjunXRbninTdqoDGPNJ6b7npgXLAPYGqFZas5ofNjw@mail.gmail.com>

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Hi Nick,

On 25/06/2019 17:26, Nick Desaulniers wrote:
> On Tue, Jun 25, 2019 at 7:54 AM Vincenzo Frascino
> <vincenzo.frascino@arm.com> wrote:
>>
>> Hi Qian,
>>
>> ...
>>
>>>
>>> but clang 7.0 is still use in many distros by default, so maybe this commit can
>>> be fixed by adding a conditional check to use "small" if clang version < 8.0.
>>>
>>
>> Could you please verify that the patch below works for you?
> 
> Should it be checking against CONFIG_CLANG_VERSION, or better yet be
> using cc-option macro?
> 

This is what I did in my proposed patch, but I was surprised that clang-7
generates relocations that clang-8 does not.

  LD      arch/arm64/kernel/vdso/vdso.so.dbg
  VDSOCHK arch/arm64/kernel/vdso/vdso.so.dbg
00000000000009d0 R_AARCH64_JUMP_SLOT  _mcount

arch/arm64/kernel/vdso/vdso.so.dbg: dynamic relocations are not supported
make[1]: *** [arch/arm64/kernel/vdso/Makefile:59:
arch/arm64/kernel/vdso/vdso.so.dbg] Error 1
make: *** [arch/arm64/Makefile:180: vdso_prepare] Error 2

This is the the result of the macro I introduced in lib/vdso/Makefile.

And I just found out why. I forgot to add a "+" in the patch provided :)

@Qian: Could you please retry with the one provided below?

-- 
Regards,
Vincenzo

--->8----




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From eed9ea23cf999d31b87db4b98a8e9de209706132 Mon Sep 17 00:00:00 2001
From: Vincenzo Frascino <vincenzo.frascino@arm.com>
Date: Tue, 25 Jun 2019 15:49:37 +0100
Subject: [PATCH] arm64: vdso: Fix compilation with clang < 8

clang versions previous to 8 do not support -mcmodel=tiny.

Add a check to the vDSO Makefile for arm64 to remove the flag when these
versions of the compiler are detected.

Reported-by: Qian Cai <cai@lca.pw>
Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
---
 arch/arm64/kernel/vdso/Makefile | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/arch/arm64/kernel/vdso/Makefile b/arch/arm64/kernel/vdso/Makefile
index ec81d28aeb5d..5154f50aff2d 100644
--- a/arch/arm64/kernel/vdso/Makefile
+++ b/arch/arm64/kernel/vdso/Makefile
@@ -38,6 +38,11 @@ else
 CFLAGS_vgettimeofday.o = -O2 -mcmodel=tiny -include $(c-gettimeofday-y)
 endif
 
+# Clang versions less than 8 do not support -mcmodel=tiny
+ifeq ($(shell test $(CONFIG_CLANG_VERSION) -lt 80000; echo $$?),0)
+CFLAGS_REMOVE_vgettimeofday.o += -mcmodel=tiny
+endif
+
 # Disable gcov profiling for VDSO code
 GCOV_PROFILE := n
 
-- 
2.22.0


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