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Date:   Wed, 17 Jun 2020 17:19:59 +0200
From:   Marco Elver <elver@google.com>
To:     Peter Zijlstra <peterz@infradead.org>
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
Subject: Re: [PATCH -tip v3 1/2] kcov: Make runtime functions
 noinstr-compatible
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On Wed, Jun 17, 2020 at 04:49PM +0200, Peter Zijlstra wrote:
> On Wed, Jun 17, 2020 at 04:32:08PM +0200, Marco Elver wrote:
> > On Mon, Jun 15, 2020 at 05:20PM +0200, Peter Zijlstra wrote:
> > > On Mon, Jun 15, 2020 at 05:03:27PM +0200, Peter Zijlstra wrote:
> > > 
> > > > Yes, I think so. x86_64 needs lib/memcpy_64.S in .noinstr.text then. For
> > > > i386 it's an __always_inline inline-asm thing.
> > > 
> > > Bah, I tried writing it without memcpy, but clang inserts memcpy anyway
> > > :/
> > 
> > Hmm, __builtin_memcpy() won't help either.
> > 
> > Turns out, Clang 11 got __builtin_memcpy_inline(): https://reviews.llvm.org/D73543
> > 
> > The below works, no more crash on either KASAN or KCSAN with Clang. We
> > can test if we have it with __has_feature(__builtin_memcpy_inline)
> > (although that's currently not working as expected, trying to fix :-/).
> > 
> > Would a memcpy_inline() be generally useful? It's not just Clang but
> > also GCC that isn't entirely upfront about which memcpy is inlined and
> > which isn't. If the compiler has __builtin_memcpy_inline(), we can use
> > it, otherwise the arch likely has to provide the implementation.
> > 
> > Thoughts?
> 
> I had the below, except of course that yields another objtool
> complaint, and I was still looking at that.
> 
> 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?

Thanks,
-- Marco