Re: [PATCH v3 1/5] arm64: vdso32: Introduce COMPAT_CC_IS_GCC

From: Vincenzo Frascino <vincenzo.frascino@arm.com>
To: Will Deacon <will@kernel.org>
Cc: ard.biesheuvel@linaro.org,
	Catalin Marinas <catalin.marinas@arm.com>,
	ndesaulniers@google.com, linux-kernel@vger.kernel.org,
	tglx@linutronix.de, linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH v3 1/5] arm64: vdso32: Introduce COMPAT_CC_IS_GCC
Date: Tue, 1 Oct 2019 15:59:51 +0100	[thread overview]
Message-ID: <7306ead0-a9b6-98d0-e775-c677eeeb55a5@arm.com> (raw)
In-Reply-To: <20191001144353.5rn3bkcc6eyfclh7@willie-the-truck>

On 10/1/19 3:43 PM, Will Deacon wrote:
> On Tue, Oct 01, 2019 at 03:37:49PM +0100, Vincenzo Frascino wrote:
>> On 10/1/19 3:20 PM, Will Deacon wrote:
>>> On Tue, Oct 01, 2019 at 03:20:35PM +0100, Vincenzo Frascino wrote:
>>>> On 10/1/19 2:27 PM, Catalin Marinas wrote:
>>>>> On Tue, Oct 01, 2019 at 02:14:23PM +0100, Will Deacon wrote:
>>>>>> On Thu, Sep 26, 2019 at 10:43:38PM +0100, Vincenzo Frascino wrote:
>>>>>>> +config COMPATCC_IS_ARM_GCC
>>>>>>> +	def_bool $(success,$(COMPATCC) --version | head -n 1 | grep -q "arm-.*-gcc")
>>>>>>
>>>>>> I've seen toolchains where the first part of the tuple is "armv7-", so they
>>>>>> won't get detected here. However, do we really need to detect this? If
>>>>>> somebody passes a duff compiler, then the build will fail in the same way as
>>>>>> if they passed it to CROSS_COMPILE=.
>>>>>
>>>>> Not sure what happens if we pass an aarch64 compiler. Can we end up with
>>>>> a 64-bit compat vDSO?
>>>>>
>>>>
>>>> I agree with Catalin here. The problem is not only when you pass and aarch64
>>>> toolchain but even an x86 and so on.
>>>
>>> I disagree. What happens if you do:
>>>
>>> $ make ARCH=arm64 CROSS_COMPILE=x86_64-linux-gnu-
>>>
>>> on your x86 box?
>>>
>>
>> The kernel compilation breaks as follows:
>>
>> x86_64-linux-gnu-gcc: error: unrecognized command line option ‘-mlittle-endian’;
>> did you mean ‘-fconvert=little-endian’?
>> /data1/Projects/LinuxKernel/linux/scripts/Makefile.build:265: recipe for target
>> 'scripts/mod/empty.o' failed
>> make[2]: *** [scripts/mod/empty.o] Error 1
>> /data1/Projects/LinuxKernel/linux/Makefile:1128: recipe for target 'prepare0' failed
>> make[1]: *** [prepare0] Error 2
>> make[1]: Leaving directory '/data1/Projects/LinuxKernel/linux-out'
>> Makefile:179: recipe for target 'sub-make' failed
>> make: *** [sub-make] Error 2
>>
>> Similar issue in the compat vdso library compilation if I do (without the check):
>>
>> $ make ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu-
>> CROSS_COMPILE_COMPAT=x86_64-linux-gnu-
>>
>> With this check the compilation completes correctly but the compat vdso does not
>> get built (unless my environment is playing me tricks ;) ).
> 
> My point was that we don't attempt to sanitise the compiler passed via
> CROSS_COMPILE, so I don't think we should do anything special for COMPATCC
> either.
>

I agree on this, but the point I was trying to make is that the kernel should
still be able to build even if the compiler for compat vdso is not correct.

I do not have a strong opinion though.

> Will
> 

-- 
Regards,
Vincenzo

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