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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id DC641C433FE for ; Thu, 31 Mar 2022 19:52:20 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S240991AbiCaTyG (ORCPT ); Thu, 31 Mar 2022 15:54:06 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:40414 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S234828AbiCaTyF (ORCPT ); Thu, 31 Mar 2022 15:54:05 -0400 Received: from mx-out.tlen.pl (mx-out.tlen.pl [193.222.135.158]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 42EFB54BE1 for ; Thu, 31 Mar 2022 12:52:16 -0700 (PDT) Received: (wp-smtpd smtp.tlen.pl 7760 invoked from network); 31 Mar 2022 21:52:13 +0200 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=o2.pl; s=1024a; t=1648756333; bh=9DW+eSPUjcl4P7CH+RNH4XhlOlBnvOKNXj68YTnQ38U=; h=Subject:To:Cc:From; b=Ut2O/BMD03xK94n/BRPNygjHuhHC22WkujejJKrnvIX7AH9mHmxvgpBNaYYGx3W7k DGdrOQYHrPkn7ecXTg5ttN3j+KepXE7ttl6PZcx6fTF7Mlocn6L2jBI3zw5BI/XXdR qDQQbawSJIGnDUlagNYtVj5fvbXEc6khbBBj3Dco= Received: from aaew62.neoplus.adsl.tpnet.pl (HELO [192.168.1.22]) (mat.jonczyk@o2.pl@[83.4.126.62]) (envelope-sender ) by smtp.tlen.pl (WP-SMTPD) with ECDHE-RSA-AES256-GCM-SHA384 encrypted SMTP for ; 31 Mar 2022 21:52:13 +0200 Message-ID: <2d139619-455d-412f-d60b-e8d9259ed7e7@o2.pl> Date: Thu, 31 Mar 2022 21:52:09 +0200 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.7.0 Subject: Re: [PATCH 1/2] [RFC] rtc: expose direct access to hardware alarm time in debugfs Content-Language: en-GB To: Alexandre Belloni Cc: linux-kernel@vger.kernel.org, linux-rtc@vger.kernel.org, linux-kselftest@vger.kernel.org, Alessandro Zummo , Shuah Khan References: <20220331190612.22162-1-mat.jonczyk@o2.pl> From: =?UTF-8?Q?Mateusz_Jo=c5=84czyk?= In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-WP-MailID: a15a250fb30b6f30de58fc64e21921f2 X-WP-AV: skaner antywirusowy Poczty o2 X-WP-SPAM: NO 0000000 [0SMU] Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org W dniu 31.03.2022 o 21:36, Alexandre Belloni pisze: > Hello, > > On 31/03/2022 21:06:11+0200, Mateusz Jończyk wrote: >> Before Linux 5.17, there was a problem with the CMOS RTC driver: >> cmos_read_alarm() and cmos_set_alarm() did not check for the UIP (Update >> in progress) bit, which could have caused it to sometimes fail silently >> and read bogus values or do not set the alarm correctly. >> Luckily, this issue was masked by cmos_read_time() invocations in core >> RTC code - see https://marc.info/?l=linux-rtc&m=164858416511425&w=4 >> >> To avoid such a problem in the future in some other driver, I wrote a >> test unit that reads the alarm time many times in a row. As the alarm >> time is usually read once and cached by the RTC core, this requires a >> way for userspace to trigger direct alarm time read from hardware. I >> think that debugfs is the natural choice for this. >> >> So, introduce /sys/kernel/debug/rtc/rtcX/wakealarm_raw. This interface >> as implemented here does not seem to be that useful to userspace, so >> there is little risk that it will become kernel ABI. >> >> Is this approach correct and worth it? >> > I'm not really in favor of adding another interface for very little > gain, you want to use this interface to exercise the API in a way that > will never happen in the real world, especially since __rtc_read_alarm > is only called once, at registration time. > > I'm not sure the selftest is worth it then. You should better improve > the existing unit tests by exercising the ioctls a bit more. syzbot did > report interesting race conditions that were more severe. OK, I did not know if other RTC drivers are likely to suffer from this kind of bugs. I also thought that the bugs in cmos_read_alarm() / cmos_set_alarm() were more severe and likely to affect existing users. I had doubts if it's worth it, so I didn't finish the patches and sent it as RFC. It was a nice project, though. Would you point to these race conditions reported by syzbot? I cannot find them. Greetings, Mateusz