Re: PROBLEM: monotonic clock going backwards on ppc64

[Daniel Axtens <dja@axtens.net>]

Mathieu Malaterre <malat@debian.org> writes:

> On Tue, Feb 26, 2019 at 9:36 PM Jakub Drnec <jaydee@email.cz> wrote:
>>
>> Hi all,
>>
>> I think I observed a potential problem, is this the correct place to report it? (CC me, not on list)
>>
>> [1.] One line summary: monotonic clock can be made to decrease on ppc64
>> [2.] Full description:
>> Setting the realtime clock can sometimes make the monotonic clock go back by over a hundred years.
>> Decreasing the realtime clock across the y2k38 threshold is one reliable way to reproduce.
>> Allegedly this can also happen just by running ntpd, I have not managed to reproduce that other
>> than booting with rtc at >2038 and then running ntp.
>
> Isn't it the expected behavior. Here is what I see for powermac:
>
> $ git show 22db552b50fa
> ...
>     This changes the logic to cast to an unsigned 32-bit number first for
>     the Macintosh time and then convert that to the Unix time, which then
>     gives us a time in the documented 1904..2040 year range. I decided not
>     to use the longer 1970..2106 range that other drivers use, for
>     consistency with the literal interpretation of the register, but that
>     could be easily changed if we decide we want to support any Mac after
>     2040.
> ...
>

My interpretation of that commit is that it relates to the kernel
reading the hardware RTC on a powermac, but this issue relates to
userspace fetching the time from the vDSO. I'm also not running on a
powermac, so my hardware should be able to deal with times > 2040...


Regards,
Daniel

>> When this happens, anything with timers (e.g. openjdk) breaks rather badly.
>>
>> [3.] Keywords: gettimeofday, ppc64, vdso
>> [4.] Kernel information
>> [4.1.] Kernel version: any (tested on 4.19)
>> [4.2.] Kernel .config file: any
>> [5.] Most recent kernel version which did not have the bug: not a regression
>> [6.] Output of Oops..: not applicable
>> [7.] Example program which triggers the problem
>> --- testcase.c
>> #include <stdio.h>
>> #include <time.h>
>> #include <stdlib.h>
>> #include <unistd.h>
>>
>> long get_time() {
>>   struct timespec tp;
>>   if (clock_gettime(CLOCK_MONOTONIC, &tp) != 0) {
>>     perror("clock_gettime failed");
>>     exit(1);
>>   }
>>   long result = tp.tv_sec + tp.tv_nsec / 1000000000;
>>   return result;
>> }
>>
>> int main() {
>>   printf("monitoring monotonic clock...\n");
>>   long last = get_time();
>>   while(1) {
>>     long now = get_time();
>>     if (now < last) {
>>       printf("clock went backwards by %ld seconds!\n",
>>         last - now);
>>     }
>>     last = now;
>>     sleep(1);
>>   }
>>   return 0;
>> }
>> ---
>> when running
>> # date -s 2040-1-1
>> # date -s 2037-1-1
>> program outputs: clock went backwards by 4294967295 seconds!
>>
>> [8.] Environment: any ppc64, currently reproducing on qemu-system-ppc64le running debian unstable
>> [X.] Other notes, patches, fixes, workarounds:
>> The problem seems to be in vDSO code in arch/powerpc/kernel/vdso64/gettimeofday.S.
>> (possibly because some values used in the calculation are only 32 bit?)
>> Slightly silly workaround:
>> nuke the "cmpwi cr1,r3,CLOCK_MONOTONIC" in __kernel_clock_gettime
>> Now it always goes through the syscall fallback which does not have the same problem.
>>
>> Regards,
>> Jakub Drnec