From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-8.6 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY, SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id DA9ABC35669 for ; Sat, 22 Feb 2020 01:31:46 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 9043E20722 for ; Sat, 22 Feb 2020 01:31:46 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1582335106; bh=XR2WubHswawxz/gISzXfQkBX54WDc547uMhBtaF/ZjM=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:List-ID: From; b=XrStUDGLfv9M8fxFieE5FKLs8WAixnUBqKD/ihbYtDktwrA5ecYFDIa+cMpjkrxMg euIQsNAEM0uwYc0RjaiGzu0QIW6xDHCdAUeqMIEmDkBWHeu9vRCApE5x4yn1RaoE4R vWRxxMbz84IykWSgQOr8gK7WZYtzDlnBZxl82cuc= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727799AbgBVBbp (ORCPT ); Fri, 21 Feb 2020 20:31:45 -0500 Received: from mail.kernel.org ([198.145.29.99]:50302 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726842AbgBVBbp (ORCPT ); Fri, 21 Feb 2020 20:31:45 -0500 Received: from paulmck-ThinkPad-P72.home (50-39-105-78.bvtn.or.frontiernet.net [50.39.105.78]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id B73AD20675; Sat, 22 Feb 2020 01:31:43 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1582335103; bh=XR2WubHswawxz/gISzXfQkBX54WDc547uMhBtaF/ZjM=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=QebuZBPucT7ImK6hJ2jnyt3ePATnQIy0DyNQbUFhK8eHLy3WNn1p/9NKFTB4AkaR3 dfg37gMhMiMd7MRNxA7FmJMSAowrIR9BTdMNas1Dj9XjowXGYi0AwcASM9+rTfRfck eJvPdocrckX0eNixSzzD/AF7Y4wr6msCEe7Ualr4= Received: by paulmck-ThinkPad-P72.home (Postfix, from userid 1000) id 90F5335226DB; Fri, 21 Feb 2020 17:31:43 -0800 (PST) Date: Fri, 21 Feb 2020 17:31:43 -0800 From: "Paul E. McKenney" To: Marco Elver Cc: andreyknvl@google.com, glider@google.com, dvyukov@google.com, kasan-dev@googlegroups.com, linux-kernel@vger.kernel.org Subject: Re: [PATCH v2] kcsan: Add option to allow watcher interruptions Message-ID: <20200222013143.GP2935@paulmck-ThinkPad-P72> Reply-To: paulmck@kernel.org References: <20200221220209.164772-1-elver@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200221220209.164772-1-elver@google.com> User-Agent: Mutt/1.9.4 (2018-02-28) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Feb 21, 2020 at 11:02:09PM +0100, Marco Elver wrote: > Add option to allow interrupts while a watchpoint is set up. This can be > enabled either via CONFIG_KCSAN_INTERRUPT_WATCHER or via the boot > parameter 'kcsan.interrupt_watcher=1'. > > Note that, currently not all safe per-CPU access primitives and patterns > are accounted for, which could result in false positives. For example, > asm-generic/percpu.h uses plain operations, which by default are > instrumented. On interrupts and subsequent accesses to the same > variable, KCSAN would currently report a data race with this option. > > Therefore, this option should currently remain disabled by default, but > may be enabled for specific test scenarios. > > To avoid new warnings, changes all uses of smp_processor_id() to use the > raw version (as already done in kcsan_found_watchpoint()). The exact SMP > processor id is for informational purposes in the report, and > correctness is not affected. > > Signed-off-by: Marco Elver Applied in place of V1, thank you! Thanx, Paul > --- > v2: > * Change smp_processor_id() to raw_smp_processor_id() as already used in > kcsan_found_watchpoint() to avoid warnings. > --- > kernel/kcsan/core.c | 34 ++++++++++------------------------ > lib/Kconfig.kcsan | 11 +++++++++++ > 2 files changed, 21 insertions(+), 24 deletions(-) > > diff --git a/kernel/kcsan/core.c b/kernel/kcsan/core.c > index 589b1e7f0f253..e7387fec66795 100644 > --- a/kernel/kcsan/core.c > +++ b/kernel/kcsan/core.c > @@ -21,6 +21,7 @@ static bool kcsan_early_enable = IS_ENABLED(CONFIG_KCSAN_EARLY_ENABLE); > static unsigned int kcsan_udelay_task = CONFIG_KCSAN_UDELAY_TASK; > static unsigned int kcsan_udelay_interrupt = CONFIG_KCSAN_UDELAY_INTERRUPT; > static long kcsan_skip_watch = CONFIG_KCSAN_SKIP_WATCH; > +static bool kcsan_interrupt_watcher = IS_ENABLED(CONFIG_KCSAN_INTERRUPT_WATCHER); > > #ifdef MODULE_PARAM_PREFIX > #undef MODULE_PARAM_PREFIX > @@ -30,6 +31,7 @@ module_param_named(early_enable, kcsan_early_enable, bool, 0); > module_param_named(udelay_task, kcsan_udelay_task, uint, 0644); > module_param_named(udelay_interrupt, kcsan_udelay_interrupt, uint, 0644); > module_param_named(skip_watch, kcsan_skip_watch, long, 0644); > +module_param_named(interrupt_watcher, kcsan_interrupt_watcher, bool, 0444); > > bool kcsan_enabled; > > @@ -354,7 +356,7 @@ kcsan_setup_watchpoint(const volatile void *ptr, size_t size, int type) > unsigned long access_mask; > enum kcsan_value_change value_change = KCSAN_VALUE_CHANGE_MAYBE; > unsigned long ua_flags = user_access_save(); > - unsigned long irq_flags; > + unsigned long irq_flags = 0; > > /* > * Always reset kcsan_skip counter in slow-path to avoid underflow; see > @@ -370,26 +372,9 @@ kcsan_setup_watchpoint(const volatile void *ptr, size_t size, int type) > goto out; > } > > - /* > - * Disable interrupts & preemptions to avoid another thread on the same > - * CPU accessing memory locations for the set up watchpoint; this is to > - * avoid reporting races to e.g. CPU-local data. > - * > - * An alternative would be adding the source CPU to the watchpoint > - * encoding, and checking that watchpoint-CPU != this-CPU. There are > - * several problems with this: > - * 1. we should avoid stealing more bits from the watchpoint encoding > - * as it would affect accuracy, as well as increase performance > - * overhead in the fast-path; > - * 2. if we are preempted, but there *is* a genuine data race, we > - * would *not* report it -- since this is the common case (vs. > - * CPU-local data accesses), it makes more sense (from a data race > - * detection point of view) to simply disable preemptions to ensure > - * as many tasks as possible run on other CPUs. > - * > - * Use raw versions, to avoid lockdep recursion via IRQ flags tracing. > - */ > - raw_local_irq_save(irq_flags); > + if (!kcsan_interrupt_watcher) > + /* Use raw to avoid lockdep recursion via IRQ flags tracing. */ > + raw_local_irq_save(irq_flags); > > watchpoint = insert_watchpoint((unsigned long)ptr, size, is_write); > if (watchpoint == NULL) { > @@ -507,7 +492,7 @@ kcsan_setup_watchpoint(const volatile void *ptr, size_t size, int type) > if (is_assert && value_change == KCSAN_VALUE_CHANGE_TRUE) > kcsan_counter_inc(KCSAN_COUNTER_ASSERT_FAILURES); > > - kcsan_report(ptr, size, type, value_change, smp_processor_id(), > + kcsan_report(ptr, size, type, value_change, raw_smp_processor_id(), > KCSAN_REPORT_RACE_SIGNAL); > } else if (value_change == KCSAN_VALUE_CHANGE_TRUE) { > /* Inferring a race, since the value should not have changed. */ > @@ -518,13 +503,14 @@ kcsan_setup_watchpoint(const volatile void *ptr, size_t size, int type) > > if (IS_ENABLED(CONFIG_KCSAN_REPORT_RACE_UNKNOWN_ORIGIN) || is_assert) > kcsan_report(ptr, size, type, KCSAN_VALUE_CHANGE_TRUE, > - smp_processor_id(), > + raw_smp_processor_id(), > KCSAN_REPORT_RACE_UNKNOWN_ORIGIN); > } > > kcsan_counter_dec(KCSAN_COUNTER_USED_WATCHPOINTS); > out_unlock: > - raw_local_irq_restore(irq_flags); > + if (!kcsan_interrupt_watcher) > + raw_local_irq_restore(irq_flags); > out: > user_access_restore(ua_flags); > } > diff --git a/lib/Kconfig.kcsan b/lib/Kconfig.kcsan > index f0b791143c6ab..081ed2e1bf7b1 100644 > --- a/lib/Kconfig.kcsan > +++ b/lib/Kconfig.kcsan > @@ -88,6 +88,17 @@ config KCSAN_SKIP_WATCH_RANDOMIZE > KCSAN_WATCH_SKIP. If false, the chosen value is always > KCSAN_WATCH_SKIP. > > +config KCSAN_INTERRUPT_WATCHER > + bool "Interruptible watchers" > + help > + If enabled, a task that set up a watchpoint may be interrupted while > + delayed. This option will allow KCSAN to detect races between > + interrupted tasks and other threads of execution on the same CPU. > + > + Currently disabled by default, because not all safe per-CPU access > + primitives and patterns may be accounted for, and therefore could > + result in false positives. > + > config KCSAN_REPORT_ONCE_IN_MS > int "Duration in milliseconds, in which any given race is only reported once" > default 3000 > -- > 2.25.0.265.gbab2e86ba0-goog >