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Subject: Re: [PATCH] x86: vdso: Fix leaky vdso link with CC=clang
To:     Alistair Strachan <astrachan@google.com>
Cc:     linux-kernel@vger.kernel.org, luto@kernel.org, tglx@linutronix.de,
        mingo@redhat.com, Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        x86@kernel.org, kernel-team@android.com, joel@joelfernandes.com
References: <20180712201032.156997-1-astrachan@google.com>
 <47d4b24e-8c60-fb3b-3377-62d20c5f3dd1@zytor.com>
 <CANDihLEz3_fZ4r4g=VcxWps5DWSjMJdbJZKZWsLFKVp_c7FscA@mail.gmail.com>
From:   "H. Peter Anvin" <hpa@zytor.com>
Message-ID: <1432bfde-41b4-e8aa-037b-fca41a149cb8@zytor.com>
Date:   Thu, 12 Jul 2018 15:06:54 -0700
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On 07/12/18 13:37, Alistair Strachan wrote:
> On Thu, Jul 12, 2018 at 1:25 PM H. Peter Anvin <hpa@zytor.com> wrote:
>> On 07/12/18 13:10, Alistair Strachan wrote:
>>> The vdso{32,64}.so can fail to link with CC=clang when clang tries to
>>> find a suitable GCC toolchain to link these libraries with.
>>>
>>> /usr/bin/ld: arch/x86/entry/vdso/vclock_gettime.o:
>>>   access beyond end of merged section (782)
>>>
>>> This happens because the host environment leaked into the cross
>>> compiler environment due to the way clang searches for suitable GCC
>>> toolchains.
>>>
>>
>> Is this another clang bug that you want a workaround for in the kernel?
> 
> Clang is a retargetable compiler (specified with --target=<something>)
> and so it has a mechanism for searching for suitable binutils (from
> another "GCC toolchain") to perform assembly and linkage. This
> mechanism relies on both --target and --gcc-toolchain when
> cross-compiling, otherwise it will fall back to searching /usr.
> 
> The --target and --gcc-toolchain flags are already specified correctly
> in the top level Makefile, but the vdso Makefile rolls its own linker
> flags and doesn't use KBUILD_CFLAGS. Therefore, these flags get
> incorrectly dropped from the vdso $CC link command line, and an
> inconsistency is created between the "GCC toolchain" used to generate
> the objects for the vdso, and the linker used to link them.
> 

It sounds like there needs to be a more fundamental symbol than
KBUILD_CFLAGS to contain these kinds of things.

	-hpa