linux-kernel - Re: [PATCH v2 1/2] retpolines: Only enable retpoline support when compiler support it

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Date:   Thu, 1 Nov 2018 15:34:21 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     "Woodhouse, David" <dwmw@...zon.co.uk>,
        "zhenzhong.duan@...cle.com" <zhenzhong.duan@...cle.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "daniel@...earbox.net" <daniel@...earbox.net>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "bp@...e.de" <bp@...e.de>,
        "srinivas.eeda@...cle.com" <srinivas.eeda@...cle.com>,
        "konrad.wilk@...cle.com" <konrad.wilk@...cle.com>,
        "michal.lkml@...kovi.net" <michal.lkml@...kovi.net>,
        "hpa@...or.com" <hpa@...or.com>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "yamada.masahiro@...ionext.com" <yamada.masahiro@...ionext.com>,
        "luto@...nel.org" <luto@...nel.org>
Subject: Re: [PATCH v2 1/2] retpolines: Only enable retpoline support when
 compiler support it


* Thomas Gleixner <tglx@...utronix.de> wrote:

> On Thu, 1 Nov 2018, Woodhouse, David wrote:
> > On Thu, 2018-11-01 at 10:50 +0100, Ingo Molnar wrote:
> > > * Zhenzhong Duan <zhenzhong.duan@...cle.com> wrote:
> > > 
> > > > Since retpoline capable compilers are widely available, make
> > > > CONFIG_RETPOLINE hard depend on it.
> > > > 
> > > > The check of RETPOLINE is changed to CONFIG_RETPOLINE.
> > > > 
> > > > This change is based on suggestion in https://lkml.org/lkml/2018/9/18/1016
> > > > 
> > > > Signed-off-by: Zhenzhong Duan <zhenzhong.duan@...cle.com>
> > > > Cc: Thomas Gleixner <tglx@...utronix.de>
> > > > Cc: Peter Zijlstra <peterz@...radead.org>
> > > 
> > > Please turn such 'based on suggestions' into proper tags as well, i.e. 
> > > something like:
> > > 
> > >   Suggested-by: David Woodhouse <dwmw2@...radead.org>
> > 
> > I think the suggestion came from PeterZ; I just acked it.
> > 
> > Although on furthe reflection, I think I'd prefer a build break if
> > retpoline is enabled in the kernel config and the compiler doesn't
> > support it. This patch would make it silently fail to be secure.
> 
> Agreed.

Yeah, I agree that that's the best policy: if someone wants retpoline 
support it shouldn't silently turn off just because the wrong toolchain 
was used to build the kernel ...

Thanks,

	Ingo