linux-kernel - Re: [PATCH 0/9] x86/entry fixes

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Date:   Thu, 4 Jun 2020 08:00:30 +0200
From:   Marco Elver <elver@...gle.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        kasan-dev <kasan-dev@...glegroups.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Will Deacon <will@...nel.org>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Alexander Potapenko <glider@...gle.com>,
        Andrey Konovalov <andreyknvl@...gle.com>
Subject: Re: [PATCH 0/9] x86/entry fixes

On Wed, 3 Jun 2020 at 21:10, Marco Elver <elver@...gle.com> wrote:
>
> On Wed, 3 Jun 2020 at 20:16, Peter Zijlstra <peterz@...radead.org> wrote:
> >
> > On Wed, Jun 03, 2020 at 06:07:22PM +0200, Peter Zijlstra wrote:
> > > On Wed, Jun 03, 2020 at 04:47:54PM +0200, Marco Elver wrote:
> >
> > > > With that in mind, you could whitelist "__ubsan_handle"-prefixed
> > > > functions in objtool. Given the __always_inline+noinstr+__ubsan_handle
> > > > case is quite rare, it might be reasonable.
> > >
> > > Yes, I think so. Let me go have dinner and then I'll try and do a patch
> > > to that effect.
> >
> > Here's a slightly more radical patch, it unconditionally allows UBSAN.
> >
> > I've not actually boot tested this.. yet.
> >
> > ---
> > Subject: x86/entry, ubsan, objtool: Whitelist __ubsan_handle_*()
> > From: Peter Zijlstra <peterz@...radead.org>
> > Date: Wed Jun  3 20:09:06 CEST 2020
> >
> > The UBSAN instrumentation only inserts external CALLs when things go
> > 'BAD', much like WARN(). So treat them similar to WARN()s for noinstr,
> > that is: allow them, at the risk of taking the machine down, to get
> > their message out.
> >
> > Suggested-by: Marco Elver <elver@...gle.com>
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
>
> This is much cleaner, as it gets us UBSAN coverage back. Seems to work
> fine for me (only lightly tested), so
>
> Acked-by: Marco Elver <elver@...gle.com>
>
> Thanks!

I was thinking that if we remove __no_sanitize_undefined from noinstr,
we can lift the hard compiler restriction for UBSAN because
__no_sanitize_undefined isn't used anywhere. Turns out, that attribute
isn't broken on GCC <= 7, so I've sent v2 of my series:
https://lkml.kernel.org/r/20200604055811.247298-1-elver@google.com

Thanks,
-- Marco