linux-kernel - Re: [PATCH 1/2] [RFC] rtc: expose direct access to hardware alarm time in debugfs

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Date:   Thu, 31 Mar 2022 22:59:58 +0200
From:   Alexandre Belloni <alexandre.belloni@...tlin.com>
To:     Mateusz Jończyk <mat.jonczyk@...pl>
Cc:     linux-kernel@...r.kernel.org, linux-rtc@...r.kernel.org,
        linux-kselftest@...r.kernel.org,
        Alessandro Zummo <a.zummo@...ertech.it>,
        Shuah Khan <shuah@...nel.org>
Subject: Re: [PATCH 1/2] [RFC] rtc: expose direct access to hardware alarm
 time in debugfs

On 31/03/2022 21:52:09+0200, Mateusz Jończyk wrote:
> 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.
> 

Really, it is nice to see someone wanting to improve testing but I
really believe that we would benefit more from unit tests for the
actually userspace API.

> Would you point to these race conditions reported by syzbot? I cannot find them.
> 

It was that one:
https://groups.google.com/g/syzkaller-bugs/c/K1FV5LBwSgM/m/hhC_DciwBAAJ?pli=1

> Greetings,
> 
> Mateusz
> 

-- 
Alexandre Belloni, co-owner and COO, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com