linux-kernel - Re: [PATCH v6 14/27] x86/percpu: Adapt percpu for PIE support

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Message-ID: <CAJcbSZFVnJ6qYNsGipf6foa8dtsX_RDykRWK-edP85ySQacsVg@mail.gmail.com>
Date:   Mon, 8 Apr 2019 11:08:58 -0700
From:   Thomas Garnier <thgarnie@...omium.org>
To:     Christopher Lameter <cl@...ux.com>
Cc:     Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        Kristen Carlson Accardi <kristen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Dennis Zhou <dennis@...nel.org>, Tejun Heo <tj@...nel.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andi Kleen <ak@...ux.intel.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Michal Hocko <mhocko@...e.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Cao jin <caoj.fnst@...fujitsu.com>,
        Brijesh Singh <brijesh.singh@....com>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Joerg Roedel <jroedel@...e.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Kees Cook <keescook@...omium.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        LKML <linux-kernel@...r.kernel.org>,
        xen-devel <xen-devel@...ts.xenproject.org>
Subject: Re: [PATCH v6 14/27] x86/percpu: Adapt percpu for PIE support

On Mon, Apr 8, 2019 at 10:56 AM Christopher Lameter <cl@...ux.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.