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From:   Thomas Garnier <thgarnie@chromium.org>
Date:   Mon, 8 Apr 2019 08:58:54 -0700
Message-ID: <CAJcbSZETNhr=Pv+0daEWUsD2=qnZxT9rbmFaE+y7ag-xKOs89w@mail.gmail.com>
Subject: Re: [PATCH v6 14/27] x86/percpu: Adapt percpu for PIE support
To:     Christopher Lameter <cl@linux.com>
Cc:     Kernel Hardening <kernel-hardening@lists.openwall.com>,
        Kristen Carlson Accardi <kristen@linux.intel.com>,
        Andy Lutomirski <luto@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        Dennis Zhou <dennis@kernel.org>, Tejun Heo <tj@kernel.org>,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        Juergen Gross <jgross@suse.com>,
        Stefano Stabellini <sstabellini@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Andi Kleen <ak@linux.intel.com>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Michal Hocko <mhocko@suse.com>,
        Mike Rapoport <rppt@linux.vnet.ibm.com>,
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On Fri, Feb 1, 2019 at 9:13 AM Thomas Garnier <thgarnie@chromium.org> wrote:
>
> On Thu, Jan 31, 2019 at 6:31 PM Christopher Lameter <cl@linux.com> wrote:
> >
> > On Thu, 31 Jan 2019, Thomas Garnier wrote:
> >
> > > The per-cpu symbols are in a section that is zero based to create
> > > offsets. The compiler doesn't see them as offsets but as relative
> > > symbol and try to relocate them. Given the distance between zero and
> > > the mapped kernel is much larger than the instruction offset range, it
> > > fails to do it.
> >
> > We switch that off in the linker. If that does not work with your
> > modifications then you need to figure out how to update the link
> > configuration.
> >
>
> It didn't work originally but I will revisit to see if I missed something.

I revisited and couldn't find a way to prevent relocations to the
percpu section. Without PIE, you can reference absolute address which
was convenient for percpu.

Christopher: Did you have something specific in mind?

I checked the following:
 - Changing the FLAGS() on the PHDRS.
 - using -z noreloc-overflow which actually doesn't seem to apply to
PC32 relocations.
 - Look at all linker options and script format for anything around that.