Re: [PATCH] hw/rtc: fix crash caused by lost_clock >= 0 assertion

[Michael S. Tsirkin]

On Mon, Dec 05, 2022 at 04:23:35PM +0800, Yaowei Bai wrote:
> In our production environment a guest crashed with this log:
> 
>     qemu-kvm: 
> /home/abuild/rpmbuild/BUILD/qemu-5.0.0/hw/rtc/mc146818rtc.c:201: 
> periodic_timer_update: Assertion `lost_clock >= 0' failed.
>     2022-09-26 10:00:28.747+0000: shutting down, reason=crashed
> 
> This happened after the host synced time with the NTP server which
> we had adjusted backward the time because it mistakenly went faster
> than the real time. Other people also have this problem:
> 
>     https://bugzilla.redhat.com/show_bug.cgi?id=2054781
> 
> After the host adjusted backward the time, the guset reconfigured the
> period, this makes cur_clock smaller than last_periodic_clock in
> periodic_timer_update function. However, the code assumes that cur_clock
> is bigger than last_periodic_clock, which is not true in the situation
> above. So we need to make it clear by introducing a if statement. With
> this patch we can handle this crash situation to just reset the
> next_periodic_time.
> 
> Signed-off-by: Yaowei Bai <baiyw2@chinatelecom.cn>


Hmm not sure this is a good fix.  Paolo what's your take?

> ---
>  hw/rtc/mc146818rtc.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/hw/rtc/mc146818rtc.c b/hw/rtc/mc146818rtc.c
> index 1ebb412..a397949 100644
> --- a/hw/rtc/mc146818rtc.c
> +++ b/hw/rtc/mc146818rtc.c
> @@ -199,7 +199,9 @@ periodic_timer_update(RTCState *s, int64_t current_time, 
> uint32_t old_period, bo
>          next_periodic_clock = muldiv64(s->next_periodic_time,
>                                  RTC_CLOCK_RATE, NANOSECONDS_PER_SECOND);
>          last_periodic_clock = next_periodic_clock - old_period;
> -        lost_clock = cur_clock - last_periodic_clock;
> +        if (cur_clock > last_periodic_clock) {
> +            lost_clock = cur_clock - last_periodic_clock;
> +        }
>          assert(lost_clock >= 0);
>      }
>  
> -- 
> 2.7.4