linux-kernel - Re: [PATCH -tip v3 1/2] kcov: Make runtime functions noinstr-compatible

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Message-ID: <20200617155517.GB576905@hirez.programming.kicks-ass.net>
Date:   Wed, 17 Jun 2020 17:55:17 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Marco Elver <elver@...gle.com>
Cc:     Dmitry Vyukov <dvyukov@...gle.com>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        Mark Rutland <mark.rutland@....com>,
        Borislav Petkov <bp@...en8.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Alexander Potapenko <glider@...gle.com>,
        kasan-dev <kasan-dev@...glegroups.com>,
        LKML <linux-kernel@...r.kernel.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>, ndesaulniers@...gle.com,
        Andy Lutomirski <luto@...capital.net>
Subject: Re: [PATCH -tip v3 1/2] kcov: Make runtime functions
 noinstr-compatible

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?