linux-kernel - Re: [PATCH v4 1/6] arm64: vdso32: Fix syncconfig errors.

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Date:   Thu, 3 Oct 2019 18:40:02 +0100
From:   Vincenzo Frascino <vincenzo.frascino@....com>
To:     Catalin Marinas <catalin.marinas@....com>
Cc:     ard.biesheuvel@...aro.org, ndesaulniers@...gle.com,
        linux-kernel@...r.kernel.org, luto@...nel.org, tglx@...utronix.de,
        will@...nel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v4 1/6] arm64: vdso32: Fix syncconfig errors.

Hi Catalin,

On 03/10/2019 17:16, Catalin Marinas wrote:
> On Thu, Oct 03, 2019 at 05:00:41PM +0100, Catalin Marinas wrote:
>> On Wed, Oct 02, 2019 at 03:41:51PM +0100, Vincenzo Frascino wrote:
>>> diff --git a/arch/arm64/Makefile b/arch/arm64/Makefile
>>> index 84a3d502c5a5..dfa6a5cb99e4 100644
>>> --- a/arch/arm64/Makefile
>>> +++ b/arch/arm64/Makefile
>>> @@ -53,20 +53,12 @@ $(warning Detected assembler with broken .inst; disassembly will be unreliable)
>>>    endif
>>>  endif
>>>  
>>> +COMPATCC ?= $(CROSS_COMPILE_COMPAT)gcc
>>> +export COMPATCC
>>> +
>>>  ifeq ($(CONFIG_GENERIC_COMPAT_VDSO), y)
>>> -  CROSS_COMPILE_COMPAT ?= $(CONFIG_CROSS_COMPILE_COMPAT_VDSO:"%"=%)
>>> -
>>> -  ifeq ($(CONFIG_CC_IS_CLANG), y)
>>> -    $(warning CROSS_COMPILE_COMPAT is clang, the compat vDSO will not be built)
>>> -  else ifeq ($(strip $(CROSS_COMPILE_COMPAT)),)
>>> -    $(warning CROSS_COMPILE_COMPAT not defined or empty, the compat vDSO will not be built)
>>> -  else ifeq ($(shell which $(CROSS_COMPILE_COMPAT)gcc 2> /dev/null),)
>>> -    $(error $(CROSS_COMPILE_COMPAT)gcc not found, check CROSS_COMPILE_COMPAT)
>>> -  else
>>> -    export CROSS_COMPILE_COMPAT
>>> -    export CONFIG_COMPAT_VDSO := y
>>> -    compat_vdso := -DCONFIG_COMPAT_VDSO=1
>>> -  endif
>>> +  export CONFIG_COMPAT_VDSO := y
>>> +  compat_vdso := -DCONFIG_COMPAT_VDSO=1
>>>  endif
>>
>> With this change, if I don't have any CROSS_COMPILE_COMPAT in my
>> environment, the kernel fails to build because COMPATCC becomes gcc
>> which cannot build the vdso32. What I really want is not to warn me, nor
>> fail to build the kernel when I don't care about the compat vDSO (e.g. I
>> have a 64-bit only machine).
>>
>> What saved us before was the COMPATCC_IS_ARM_GCC check and a selection
>> of the GENERIC_COMPAT_VDSO dependent on this check. This was now dropped
>> from the previous version of the patch. We could add something like
>> COMPATCC_CAN_LINK.
> 
> Ah, the COMPATCC_CAN_LINK idea doesn't help because an x86 gcc can still
> link. Maybe only enable CONFIG_COMPAT_VDSO above if CROSS_COMPILE_COMPAT
> was set. You could move the COMPATCC setting and export under the
> same 'if' block in the Makefile.
> 

The snippet below the scissors solves the problem for me. I am going to re-post
a new version of the patches that contains it.

--->8---

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index 37c610963eee..b4cc8fb5344c 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -110,7 +110,7 @@ config ARM64
 	select GENERIC_STRNLEN_USER
 	select GENERIC_TIME_VSYSCALL
 	select GENERIC_GETTIMEOFDAY
-	select GENERIC_COMPAT_VDSO if (!CPU_BIG_ENDIAN && COMPAT)
+	select GENERIC_COMPAT_VDSO if (!CPU_BIG_ENDIAN && COMPAT && !COMPATCC_EMPTY)
 	select HANDLE_DOMAIN_IRQ
 	select HARDIRQS_SW_RESEND
 	select HAVE_PCI
@@ -313,6 +313,9 @@ config KASAN_SHADOW_OFFSET
 	default 0xeffffff900000000 if ARM64_VA_BITS_36 && KASAN_SW_TAGS
 	default 0xffffffffffffffff

+config COMPATCC_EMPTY
+	def_bool $(success,test -z "${CROSS_COMPILE_COMPAT}")
+
 source "arch/arm64/Kconfig.platforms"

 menu "Kernel Features"


-- 
Regards,
Vincenzo

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