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From:   Thomas Garnier <thgarnie@chromium.org>
Date:   Mon, 8 Apr 2019 11:08:58 -0700
Message-ID: <CAJcbSZFVnJ6qYNsGipf6foa8dtsX_RDykRWK-edP85ySQacsVg@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>,
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        "the arch/x86 maintainers" <x86@kernel.org>,
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        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        Juergen Gross <jgross@suse.com>,
        Stefano Stabellini <sstabellini@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
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On Mon, Apr 8, 2019 at 10:56 AM Christopher Lameter <cl@linux.com> wrote:
>
> On Mon, 8 Apr 2019, Thomas Garnier wrote:
>
> > > 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.
>
> Can you switch PIE off for the percpu section? If not maybe the linker
> needs to have an additional option?

I don't think so or I didn't find any option to do that. Changing the
linker might be a bit too much if we have a software solution which
doesn't impact performance.

>
> Cannot imagine that this is not possible. You neeed to be able to
> reference registers that are in fixed memory locations.
>
>
> > Christopher: Did you have something specific in mind?
>
> I thought that we just leave it as is.

I would like to as well. I will try couple things at the assembly
level instead of the linker and come back to this thread.