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[66.111.4.227]) by smtp.gmail.com with ESMTPSA id q1sm11659103ilu.51.2021.11.29.06.27.46 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 29 Nov 2021 06:27:47 -0800 (PST) Received: from compute3.internal (compute3.nyi.internal [10.202.2.43]) by mailauth.nyi.internal (Postfix) with ESMTP id 241A527C005B; Mon, 29 Nov 2021 09:27:45 -0500 (EST) Received: from mailfrontend2 ([10.202.2.163]) by compute3.internal (MEProxy); Mon, 29 Nov 2021 09:27:46 -0500 X-ME-Sender: X-ME-Received: X-ME-Proxy-Cause: gggruggvucftvghtrhhoucdtuddrgedvuddrheelgdeihecutefuodetggdotefrodftvf curfhrohhfihhlvgemucfhrghsthforghilhdpqfgfvfdpuffrtefokffrpgfnqfghnecu uegrihhlohhuthemuceftddtnecusecvtfgvtghiphhivghnthhsucdlqddutddtmdenuc fjughrpeffhffvuffkfhggtggujgesthdtredttddtvdenucfhrhhomhepuehoqhhunhcu hfgvnhhguceosghoqhhunhdrfhgvnhhgsehgmhgrihhlrdgtohhmqeenucggtffrrghtth gvrhhnpedvleeigedugfegveejhfejveeuveeiteejieekvdfgjeefudehfefhgfegvdeg jeenucevlhhushhtvghrufhiiigvpedtnecurfgrrhgrmhepmhgrihhlfhhrohhmpegsoh hquhhnodhmvghsmhhtphgruhhthhhpvghrshhonhgrlhhithihqdeiledvgeehtdeigedq udejjeekheehhedvqdgsohhquhhnrdhfvghngheppehgmhgrihhlrdgtohhmsehfihigmh gvrdhnrghmvg X-ME-Proxy: Received: by mail.messagingengine.com (Postfix) with ESMTPA; Mon, 29 Nov 2021 09:27:44 -0500 (EST) Date: Mon, 29 Nov 2021 22:26:46 +0800 From: Boqun Feng To: Marco Elver Cc: "Paul E. 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: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Nov 29, 2021 at 11:57:30AM +0100, Marco Elver wrote: > 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()). > Ok, I was missing that. > 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. > Thanks for the background, it's now more clear that the problem is triggered ;-) > 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. > Agreed. > 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. > Just to be clear, in both examples, the assumption is that 'v' is a variable that scheduler code doesn't access, right? Because if scheduler code does access 'v', then it's a problem that KCSAN should report. Yes, I don't know any variable that scheduler exports, just to make sure here. > 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. > Make sense to me, thanks. Regards, Boqun > Thanks, > -- Marco