linux-kernel - Re: [PATCH] x86: vdso: Fix leaky vdso link with CC=clang

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Message-Id: <5AB78E47-BC62-4DA5-920B-00E9731AAE04@amacapital.net>
Date:   Thu, 12 Jul 2018 16:20:06 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     "H. Peter Anvin" <hpa@...or.com>
Cc:     Alistair Strachan <astrachan@...gle.com>,
        linux-kernel@...r.kernel.org, luto@...nel.org, tglx@...utronix.de,
        mingo@...hat.com, Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        x86@...nel.org, kernel-team@...roid.com, joel@...lfernandes.com
Subject: Re: [PATCH] x86: vdso: Fix leaky vdso link with CC=clang


> On Jul 12, 2018, at 3:06 PM, H. Peter Anvin <hpa@...or.com> wrote:
> 
>> On 07/12/18 13:37, Alistair Strachan wrote:
>>> On Thu, Jul 12, 2018 at 1:25 PM H. Peter Anvin <hpa@...or.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.

How about $(CC)?

> 
>    -hpa