Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Wed, 17 Jun 2020 17:55:17 +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
Message-ID: <20200617155517.GB576905@hirez.programming.kicks-ass.net>
References: <20200612114900.GA187027@google.com>
 <CACT4Y+bBtCbEk2tg60gn5bgfBjARQFBgtqkQg8VnLLg5JwyL5g@mail.gmail.com>
 <CANpmjNM+Tcn40MsfFKvKxNTtev-TXDsosN+z9ATL8hVJdK1yug@mail.gmail.com>
 <20200615142949.GT2531@hirez.programming.kicks-ass.net>
 <20200615145336.GA220132@google.com>
 <20200615150327.GW2531@hirez.programming.kicks-ass.net>
 <20200615152056.GF2554@hirez.programming.kicks-ass.net>
 <20200617143208.GA56208@elver.google.com>
 <20200617144949.GA576905@hirez.programming.kicks-ass.net>
 <20200617151959.GB56208@elver.google.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20200617151959.GB56208@elver.google.com>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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?