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

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Message-ID: <20200615150327.GW2531@hirez.programming.kicks-ass.net>
Date:   Mon, 15 Jun 2020 17:03:27 +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>
Subject: Re: [PATCH -tip v3 1/2] kcov: Make runtime functions
 noinstr-compatible

On Mon, Jun 15, 2020 at 04:53:36PM +0200, Marco Elver wrote:
> On Mon, 15 Jun 2020, Peter Zijlstra wrote:
> 
> > On Mon, Jun 15, 2020 at 09:53:06AM +0200, Marco Elver wrote:
> > > 
> > > Disabling KCOV for smp_processor_id now moves the crash elsewhere. In
> > > the case of KASAN into its 'memcpy' wrapper, called after
> > > __this_cpu_read in fixup_bad_iret. This is making me suspicious,
> > > because it shouldn't be called from the noinstr functions.
> > 
> > With your .config, objtool complains about exactly that though:
> > 
> > vmlinux.o: warning: objtool: fixup_bad_iret()+0x8e: call to memcpy() leaves .noinstr.text section
> > 
> > The utterly gruesome thing below 'cures' that.
> 
> Is __memcpy() generally available? I think that bypasses KASAN and
> whatever else.

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.