linux-kernel - Re: [PATCH v6 02/27] x86: Use symbol name in jump table for PIE support

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Message-ID: <CAJcbSZFTiHcRM8gy8LbVMQYM2b7H=Gngbsf6bMGKmkDo7mKbjg@mail.gmail.com>
Date:   Thu, 7 Feb 2019 15:55:35 -0800
From:   Thomas Garnier <thgarnie@...omium.org>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        Kristen Carlson Accardi <kristen@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Nadav Amit <namit@...are.com>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v6 02/27] x86: Use symbol name in jump table for PIE support

On Thu, Feb 7, 2019 at 9:11 AM Borislav Petkov <bp@...en8.de> wrote:
>
> On Thu, Feb 07, 2019 at 09:04:45AM -0800, Thomas Garnier wrote:
> > I assume that's an optimisation done by gcc later.
>
> So why is that change even needed? Where does it break?
>
> > The P modifier in the documentation does state that it is used to
> > generate PIC code.
>
> The documentation says:
>
> "If used for a function, print the PLT suffix and generate PIC code. For
> example, emit foo@PLT instead of ’foo’ for the function foo()."
>
> when you use %P for a function. Which is not how it is used here.

I did more checks about that. I think Ard's patch to make jump label
relative actually fixed the issue I had with them.

Thanks for spotting this, I will do additional checks and look at
removing this change.

>
> --
> Regards/Gruss,
>     Boris.
>
> Good mailing practices for 400: avoid top-posting and trim the reply.