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

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Date:   Thu, 12 Jul 2018 17:01:06 -0700
From:   Alistair Strachan <astrachan@...gle.com>
To:     luto@...capital.net
Cc:     hpa@...or.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 Thu, Jul 12, 2018 at 4:20 PM Andy Lutomirski <luto@...capital.net> wrote:
>
> > 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)?

I'm guessing, but I think this wasn't done originally because CC is
something the user might reasonably specify on the command line (the
other bit comes from CROSS_COMPILE), so doing this via CC would
require us to override the CC passed in on the command line. Not sure
how to do that, since the vdso makefile is executed with a submake, so
the usual "override CC := $(CC) something else" followed by "export
CC" doesn't work.