linux-kernel - Re: [PATCH v3 1/5] arm64: vdso32: Introduce COMPAT_CC_IS

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Date:   Tue, 1 Oct 2019 15:43:54 +0100
From:   Will Deacon <will@...nel.org>
To:     Vincenzo Frascino <vincenzo.frascino@....com>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        ard.biesheuvel@...aro.org, ndesaulniers@...gle.com,
        tglx@...utronix.de
Subject: Re: [PATCH v3 1/5] arm64: vdso32: Introduce COMPAT_CC_IS_GCC

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.

Will