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McKenney" , Alexander Potapenko , Borislav Petkov , Dmitry Vyukov , Ingo Molnar , Josh Poimboeuf , Mark Rutland , Peter Zijlstra , Thomas Gleixner , Waiman Long , Will Deacon , kasan-dev@googlegroups.com, linux-arch@vger.kernel.org, linux-doc@vger.kernel.org, linux-kbuild@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, x86@kernel.org Subject: Re: [PATCH v2 03/23] kcsan: Avoid checking scoped accesses from nested contexts Message-ID: References: <20211118081027.3175699-1-elver@google.com> <20211118081027.3175699-4-elver@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/2.0.5 (2021-01-21) X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: 4F5B990000AD Authentication-Results: imf23.hostedemail.com; dkim=pass header.d=google.com header.s=20210112 header.b=FpAGCEzp; dmarc=pass (policy=reject) header.from=google.com; spf=pass (imf23.hostedemail.com: domain of elver@google.com designates 209.85.221.41 as permitted sender) smtp.mailfrom=elver@google.com X-Stat-Signature: 9ru176g8abnbcjxox9xgzy9eji1oos18 X-HE-Tag: 1638183451-593647 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Mon, Nov 29, 2021 at 04:47PM +0800, Boqun Feng wrote: > Hi Marco, > > On Thu, Nov 18, 2021 at 09:10:07AM +0100, Marco Elver wrote: > > Avoid checking scoped accesses from nested contexts (such as nested > > interrupts or in scheduler code) which share the same kcsan_ctx. > > > > This is to avoid detecting false positive races of accesses in the same > > Could you provide an example for a false positive? > > I think we do want to detect the following race: > > static int v = SOME_VALUE; // a percpu variable. > static int other_v = ... ; > > void foo(..) > { > int tmp; > int other_tmp; > > preempt_disable(); > { > ASSERT_EXCLUSIVE_ACCESSS_SCOPED(v); > tmp = v; > > other_tmp = other_v; // int_handler() may run here > > v = tmp + 2; > } > preempt_enabled(); > } > > void int_handler() // an interrupt handler > { > v++; > } > > , if I understand correctly, we can detect this currently, but with this > patch, we cannot detect this if the interrupt happens while we're doing > the check for "other_tmp = other_v;", right? Of course, running tests > multiple times may eventually catch this, but I just want to understand > what's this patch for, thanks! The above will still be detected. Task and interrupt contexts in this case are distinct, i.e. kcsan_ctx differ (see get_ctx()). But there are rare cases where kcsan_ctx is shared, such as nested interrupts (NMI?), or when entering scheduler code -- which currently has a KCSAN_SANITIZE := n, but I occasionally test it, which is how I found this problem. The problem occurs frequently when enabling KCSAN in kernel/sched and placing a random ASSERT_EXCLUSIVE_ACCESS_SCOPED() in task context, or just enable "weak memory modeling" without this fix. You also need CONFIG_PREEMPT=y + CONFIG_KCSAN_INTERRUPT_WATCHER=y. The emphasis here really is on _shared kcsan_ctx_, which is not too common. As noted in the commit description, we need to "[...] setting up a watchpoint for a non-scoped (normal) access that also "conflicts" with a current scoped access." Consider this: static int v; int foo(..) { ASSERT_EXCLUSIVE_ACCESS_SCOPED(v); v++; // preempted during watchpoint for 'v++' } Here we set up a scoped_access to be checked for v. Then on v++, a watchpoint is set up for the normal access. While the watchpoint is set up, the task is preempted and upon entering scheduler code, we're still in_task() and 'current' is still the same, thus get_ctx() returns a kcsan_ctx where the scoped_accesses list is non-empty containing the scoped access for foo()'s ASSERT_EXCLUSIVE. That means, when instrumenting scheduler code or any other code called by scheduler code or nested interrupts (anything where get_ctx() still returns the same as parent context), it'd now perform checks based on the parent context's scoped access, and because the parent context also has a watchpoint set up on the variable that conflicts with the scoped access we'd report a nonsensical race. This case is also possible: static int v; static int x; int foo(..) { ASSERT_EXCLUSIVE_ACCESS_SCOPED(v); x++; // preempted during watchpoint for 'v' after checking x++ } Here, all we need is for the scoped access to be checked after x++, end up with a watchpoint for it, then enter scheduler code, which then checked 'v', sees the conflicting watchpoint, and reports a nonsensical race again. By disallowing scoped access checking for a kcsan_ctx, we simply make sure that in such nested contexts where kcsan_ctx is shared, none of these nonsensical races would be detected nor reported. Hopefully that clarifies what this is about. Thanks, -- Marco