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=unavailable 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 46FE8C10DCE for ; Tue, 24 Mar 2020 21:34:17 +0000 (UTC) Received: from mother.openwall.net (mother.openwall.net [195.42.179.200]) by mail.kernel.org (Postfix) with SMTP id 96F39206F6 for ; Tue, 24 Mar 2020 21:34:16 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=kernel.org header.i=@kernel.org header.b="iXKz/8wy" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 96F39206F6 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=kernel.org Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=kernel-hardening-return-18214-kernel-hardening=archiver.kernel.org@lists.openwall.com Received: (qmail 3495 invoked by uid 550); 24 Mar 2020 21:34:11 -0000 Mailing-List: contact kernel-hardening-help@lists.openwall.com; run by ezmlm Precedence: bulk List-Post: List-Help: List-Unsubscribe: List-Subscribe: List-ID: Received: (qmail 3458 invoked from network); 24 Mar 2020 21:34:10 -0000 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1585085638; bh=YSucMrRxtrynGJ/g9tDph2k08nv6kh3kh3Hl7TdUQPo=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=iXKz/8wy8p4KltD7XQjmmy52F9817kzLNOiw4klkv30Lpg690Ck3BR5pWPmOXvYTT 65/iPl6FiQNrWy8nDWzCtOh2lHQ2VCBp4DyOQADo7k+1qlTqsStcT0HN3HVZNR7MfY HZQqGk2p5wW29XzFJBh2XzyQ0tSzpezK2TzFy72Q= Date: Tue, 24 Mar 2020 21:33:53 +0000 From: Will Deacon To: Marco Elver Cc: LKML , Eric Dumazet , Jann Horn , Kees Cook , Maddie Stone , "Paul E . McKenney" , Peter Zijlstra , Thomas Gleixner , kernel-team@android.com, kernel-hardening@lists.openwall.com Subject: Re: [RFC PATCH 03/21] list: Annotate lockless list primitives with data_race() Message-ID: <20200324213352.GB21176@willie-the-truck> References: <20200324153643.15527-1-will@kernel.org> <20200324153643.15527-4-will@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.10.1 (2018-07-13) On Tue, Mar 24, 2020 at 05:23:30PM +0100, Marco Elver wrote: > On Tue, 24 Mar 2020 at 16:37, Will Deacon wrote: > > > > Some list predicates can be used locklessly even with the non-RCU list > > implementations, since they effectively boil down to a test against > > NULL. For example, checking whether or not a list is empty is safe even > > in the presence of a concurrent, tearing write to the list head pointer. > > Similarly, checking whether or not an hlist node has been hashed is safe > > as well. > > > > Annotate these lockless list predicates with data_race() and READ_ONCE() > > so that KCSAN and the compiler are aware of what's going on. The writer > > side can then avoid having to use WRITE_ONCE() in the non-RCU > > implementation. > > > > Cc: Paul E. McKenney > > Cc: Peter Zijlstra > > Cc: Marco Elver > > Signed-off-by: Will Deacon > > --- > > include/linux/list.h | 10 +++++----- > > include/linux/list_bl.h | 5 +++-- > > include/linux/list_nulls.h | 6 +++--- > > include/linux/llist.h | 2 +- > > 4 files changed, 12 insertions(+), 11 deletions(-) > > > > diff --git a/include/linux/list.h b/include/linux/list.h > > index 4fed5a0f9b77..4d9f5f9ed1a8 100644 > > --- a/include/linux/list.h > > +++ b/include/linux/list.h > > @@ -279,7 +279,7 @@ static inline int list_is_last(const struct list_head *list, > > */ > > static inline int list_empty(const struct list_head *head) > > { > > - return READ_ONCE(head->next) == head; > > + return data_race(READ_ONCE(head->next) == head); > > Double-marking should never be necessary, at least if you want to make > KCSAN happy. From what I gather there is an unmarked write somewhere, > correct? In that case, KCSAN will still complain because if it sees a > race between this read and the other write, then at least one is still > plain (the write). > > Then, my suggestion would be to mark the write with data_race() and > just leave this as a READ_ONCE(). Having a data_race() somewhere only > makes KCSAN stop reporting the race if the paired access is also > marked (be it with data_race() or _ONCE, etc.). > > Alternatively, if marking the write is impossible, you can surround > the access with kcsan_disable_current()/kcsan_enable_current(). Or, as > a last resort, just leaving as-is is fine too, because KCSAN's default > config (still) has KCSAN_ASSUME_PLAIN_WRITES_ATOMIC selected. Right, it looks like this is a bit of a smoking gun and we need to decide on whether list_empty() is actually usable without synchronisation first. Based on the outcome of that discussion, I'll update this patch accordingly. The main thing I want to avoid is marking parts of the non-RCU list implementation with data_race() or {READ,WRITE}_ONCE() because that's a sure-fire way to hide real bugs. Cheers, Will