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Date:   Wed, 17 Jun 2020 18:36:35 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Marco Elver <elver@google.com>
Cc:     Dmitry Vyukov <dvyukov@google.com>,
        Andrey Konovalov <andreyknvl@google.com>,
        Mark Rutland <mark.rutland@arm.com>,
        Borislav Petkov <bp@alien8.de>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@kernel.org>,
        clang-built-linux <clang-built-linux@googlegroups.com>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Alexander Potapenko <glider@google.com>,
        kasan-dev <kasan-dev@googlegroups.com>,
        LKML <linux-kernel@vger.kernel.org>,
        the arch/x86 maintainers <x86@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>, ndesaulniers@google.com,
        Andy Lutomirski <luto@amacapital.net>
Subject: Re: [PATCH -tip v3 1/2] kcov: Make runtime functions
 noinstr-compatible
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On Wed, Jun 17, 2020 at 05:55:17PM +0200, Peter Zijlstra wrote:
> On Wed, Jun 17, 2020 at 05:19:59PM +0200, Marco Elver wrote:
> 
> > > Does GCC (8, as per the new KASAN thing) have that
> > > __builtin_memcpy_inline() ?
> > 
> > No, sadly it doesn't. Only Clang 11. :-/
> > 
> > But using a call to __memcpy() somehow breaks with Clang+KCSAN. Yet,
> > it's not the memcpy that BUGs, but once again check_preemption_disabled
> > (which is noinstr!). Just adding calls anywhere here seems to results in
> > unpredictable behaviour. Are we running out of stack space?
> 
> Very likely, bad_iret is running on that entry_stack you found, and as
> you found, it is puny.
> 
> Andy wanted to make it a full page a while ago, so I suppose the
> question is do we do that now?

Andy suggested doing the full page; untested patches here:

  git://git.kernel.org/pub/scm/linux/kernel/git/peterz/queue.git x86/entry