linux-kernel - Re: [PATCH v4 00/10] Add Kernel Concurrency Sanitizer (KCSAN)

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Date:   Wed, 20 Nov 2019 16:54:48 +0100
From:   Marco Elver <elver@...gle.com>
To:     Qian Cai <cai@....pw>
Cc:     LKMM Maintainers -- Akira Yokosawa <akiyks@...il.com>,
        Alan Stern <stern@...land.harvard.edu>,
        Alexander Potapenko <glider@...gle.com>,
        Andrea Parri <parri.andrea@...il.com>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Arnd Bergmann <arnd@...db.de>,
        Boqun Feng <boqun.feng@...il.com>,
        Borislav Petkov <bp@...en8.de>, Daniel Axtens <dja@...ens.net>,
        Daniel Lustig <dlustig@...dia.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        David Howells <dhowells@...hat.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        Jade Alglave <j.alglave@....ac.uk>,
        Joel Fernandes <joel@...lfernandes.org>,
        Jonathan Corbet <corbet@....net>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Luc Maranget <luc.maranget@...ia.fr>,
        Mark Rutland <mark.rutland@....com>,
        Nicholas Piggin <npiggin@...il.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Will Deacon <will@...nel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        kasan-dev <kasan-dev@...glegroups.com>,
        linux-arch <linux-arch@...r.kernel.org>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        linux-efi@...r.kernel.org,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux Memory Management List <linux-mm@...ck.org>,
        the arch/x86 maintainers <x86@...nel.org>
Subject: Re: [PATCH v4 00/10] Add Kernel Concurrency Sanitizer (KCSAN)

On Tue, 19 Nov 2019, Marco Elver wrote:

> On Tue, 19 Nov 2019 at 21:13, Qian Cai <cai@....pw> wrote:
> >
> > On Thu, 2019-11-14 at 19:02 +0100, 'Marco Elver' via kasan-dev wrote:
> > > This is the patch-series for the Kernel Concurrency Sanitizer (KCSAN).
> > > KCSAN is a sampling watchpoint-based *data race detector*. More details
> > > are included in **Documentation/dev-tools/kcsan.rst**. This patch-series
> > > only enables KCSAN for x86, but we expect adding support for other
> > > architectures is relatively straightforward (we are aware of
> > > experimental ARM64 and POWER support).
> >
> > This does not allow the system to boot. Just hang forever at the end.
> >
> > https://cailca.github.io/files/dmesg.txt
> >
> > the config (dselect KASAN and select KCSAN with default options):
> >
> > https://raw.githubusercontent.com/cailca/linux-mm/master/x86.config
> 
> Thanks! That config enables lots of other debug code. I could
> reproduce the hang. It's related to CONFIG_PROVE_LOCKING etc.
> 
> The problem is definitely not the fact that kcsan_setup_watchpoint
> disables interrupts (tested by removing that code). Although lockdep
> still complains here, and looking at the code in kcsan/core.c, I just
> can't see how local_irq_restore cannot be called before returning (in
> the stacktrace you provided, there is no kcsan function), and
> interrupts should always be re-enabled. (Interrupts are only disabled
> during delay in kcsan_setup_watchpoint.)
> 
> What I also notice is that this happens when the console starts
> getting spammed with data-race reports (presumably because some extra
> debug code has lots of data races according to KCSAN).
> 
> My guess is that some of the extra debug logic enabled in that config
> is incompatible with KCSAN. However, so far I cannot tell where
> exactly the problem is. For now the work-around would be not using
> KCSAN with these extra debug options.  I will investigate more, but
> nothing obviously wrong stands out..

It seems that due to spinlock_debug.c containing data races, the console
gets spammed with reports. However, it's also possible to encounter
deadlock, e.g.  printk lock -> spinlock_debug -> KCSAN detects data race
-> kcsan_print_report() -> printk lock -> deadlock.

So the best thing is to fix the data races in spinlock_debug. I will
send a patch separately for you to test.

The issue that lockdep still reports inconsistency in IRQ flags tracing
I cannot yet say what the problem is. It seems that lockdep IRQ flags
tracing may have an issue with KCSAN for numerous reasons: let's say
lockdep and IRQ flags tracing code is instrumented, which then calls
into KCSAN, which disables/enables interrupts, but due to tracing calls
back into lockdep code. In other words, there may be some recursion
which corrupts hardirqs_enabled.

Thanks,
-- Marco