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

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Date:   Thu, 12 Jul 2018 13:37:52 -0700
From:   Alistair Strachan <astrachan@...gle.com>
To:     hpa@...or.com
Cc:     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 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.