linux-kernel - Re: [PATCH] rcu: Fixup noinstr warnings

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Date:   Wed, 3 Jun 2020 12:52:06 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     Marco Elver <elver@...gle.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Will Deacon <will@...nel.org>, Borislav Petkov <bp@...en8.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Alexander Potapenko <glider@...gle.com>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        kasan-dev <kasan-dev@...glegroups.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] rcu: Fixup noinstr warnings

On Wed, Jun 03, 2020 at 02:59:32AM -0700, Paul E. McKenney wrote:
> On Wed, Jun 03, 2020 at 10:48:18AM +0200, Peter Zijlstra wrote:
> > On Tue, Jun 02, 2020 at 09:38:53PM +0200, Peter Zijlstra wrote:
> > 
> > > That said; noinstr's __no_sanitize combined with atomic_t might be
> > > 'interesting', because the regular atomic things have explicit
> > > annotations in them. That should give validation warnings for the right
> > > .config, I'll have to go try -- so far I've made sure to never enable
> > > the *SAN stuff.
> > 
> > ---
> > Subject: rcu: Fixup noinstr warnings
> > 
> > A KCSAN build revealed we have explicit annoations through atomic_t
> > usage, switch to arch_atomic_*() for the respective functions.
> > 
> > vmlinux.o: warning: objtool: rcu_nmi_exit()+0x4d: call to __kcsan_check_access() leaves .noinstr.text section
> > vmlinux.o: warning: objtool: rcu_dynticks_eqs_enter()+0x25: call to __kcsan_check_access() leaves .noinstr.text section
> > vmlinux.o: warning: objtool: rcu_nmi_enter()+0x4f: call to __kcsan_check_access() leaves .noinstr.text section
> > vmlinux.o: warning: objtool: rcu_dynticks_eqs_exit()+0x2a: call to __kcsan_check_access() leaves .noinstr.text section
> > vmlinux.o: warning: objtool: __rcu_is_watching()+0x25: call to __kcsan_check_access() leaves .noinstr.text section
> > 
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> 
> This one does not apply cleanly onto the -rcu tree's "dev" branch, so
> I am guessing that it is intended to be carried in -tip with yours and
> Thomas's patch series.

Right, I've not played patch tetris yet so see how it should all fit
together. I also didn't know you feel about loosing the instrumentation
in these functions.

One option would be do add explicit: instrument_atomic_write() calls
before instrument_end() / after instrument_begin() in
the respective callers that have that.

Anyway, I'll shortly be posting a pile of patches resulting from various
KCSAN and KASAN builds. The good news is that GCC-KASAN seems to behave
quite well with Marco's patches, the bad news is that GCC-KASAN is
retarded wrt inline and needs a bunch of kicks.

That is, it out-of-lines:

static inline bool foo(..)
{
	return false;
}

just because..