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From:   Andy Lutomirski <luto@kernel.org>
Date:   Thu, 7 Jun 2018 17:47:14 -0700
Message-ID: <CALCETrXZjcTs6y8u5SvdUw_9itpb6EhLbMWpVJTAXVNkkvas_Q@mail.gmail.com>
Subject: Re: [PATCH 5/7] x86: Insert endbr32/endbr64 to vDSO
To:     "H. J. Lu" <hjl.tools@gmail.com>
Cc:     Andrew Lutomirski <luto@kernel.org>,
        Yu-cheng Yu <yu-cheng.yu@intel.com>,
        LKML <linux-kernel@vger.kernel.org>, linux-doc@vger.kernel.org,
        Linux-MM <linux-mm@kvack.org>,
        linux-arch <linux-arch@vger.kernel.org>, X86 ML <x86@kernel.org>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        "Shanbhogue, Vedvyas" <vedvyas.shanbhogue@intel.com>,
        "Ravi V. Shankar" <ravi.v.shankar@intel.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Oleg Nesterov <oleg@redhat.com>, Arnd Bergmann <arnd@arndb.de>,
        mike.kravetz@oracle.com
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On Thu, Jun 7, 2018 at 5:31 PM H.J. Lu <hjl.tools@gmail.com> wrote:
>
> On Thu, Jun 7, 2018 at 4:00 PM, Andy Lutomirski <luto@kernel.org> wrote:
> > On Thu, Jun 7, 2018 at 3:03 PM H.J. Lu <hjl.tools@gmail.com> wrote:
> >>
> >> On Thu, Jun 7, 2018 at 1:50 PM, Andy Lutomirski <luto@kernel.org> wrote:
> >> > On Thu, Jun 7, 2018 at 7:42 AM Yu-cheng Yu <yu-cheng.yu@intel.com> wrote:
> >> >>
> >> >> From: "H.J. Lu" <hjl.tools@gmail.com>
> >> >>
> >> >> When Intel indirect branch tracking is enabled, functions in vDSO which
> >> >> may be called indirectly should have endbr32 or endbr64 as the first
> >> >> instruction.  We try to compile vDSO with -fcf-protection=branch -mibt
> >> >> if possible.  Otherwise, we insert endbr32 or endbr64 by hand to assembly
> >> >> codes generated by the compiler.
> >> >
> >> > Wow, that's... a genuine abomination.  Do we really need to support
> >> > CET on kernels built with old toolchains?
> >> >
> >>
> >> Yes.  GCC 7 should be able to build CET kernel.
> >>
> >
> > Why?  Presumably people running distros that use CET are going to have
> > kernels build with a CET-supporting compiler.
> >
>
> Good point.  It was needed before GCC 8 was released.   We can drop
> arch/x86/entry/vdso/endbr.sh now.
>

Phew!  We'll still need a patch to prevent configuring CET on a
compiler that can't support it.