Hi Jens,
On 05/13/2018 01:14 PM, Michael Kerrisk (man-pages) wrote:
> On 05/12/2018 04:21 PM, Jens Thoms Toerring wrote:
>> Hi,
>>
>> as observed recently settimeofday(2) may fail when called
>> with reasonably looking values for the time/date if they're
>> near to the Epoch. The reason is that it's not possible to
>> set the system time to a value that is lower than the number
>> of seconds since the system was booted - otherwise the time
>> of boot reported by 'uptime -s' would predate the Epoch.
>>
>> As this took some time to figure out I think it might be
>> worth to mention it in the NOTES section of the man page
>> of gettimeofday/settimeofday, see the attached patch.
>
> Hi Jens,
>
> It's pretty clear that there are some missing EINVAL cases
> in the manual page. However, I think the case should rather
> be documented as
>
> (tv.tv_set < 0 || tv.tv_usec < 0 || tv.tv_usec >= 1000000)
>
> giving EINVAL. But, perhaps I miss your point. What values
> were you specifying to settimeofday() that led to EINVAL?
Okay -- an update. The text should I think be phrased in terms of the
monotonic clock.
In the kernel source file kernel/time/timekeeping.c::do_settimeofday64(),
there is this check:
if (timespec64_compare(&tk->wall_to_monotonic, &ts_delta) > 0) {
ret = -EINVAL;
goto out;
}
(It took me a little too long too find that check :-/)
It appears that the check was added in Linux 4.3:
commit e1d7ba8735551ed79c7a0463a042353574b96da3
Author: Wang YanQing <udknight@xxxxxxxxx>
Date: Tue Jun 23 18:38:54 2015 +0800
time: Always make sure wall_to_monotonic isn't positive
I applied the patch below (which included a related change in the
documentation of clock_settime(2)). Does it seem okay?
Thanks for the report!
Cheers,
Michael
diff --git a/man2/clock_getres.2 b/man2/clock_getres.2
index c32f9e183..240b87276 100644
--- a/man2/clock_getres.2
+++ b/man2/clock_getres.2
@@ -212,6 +212,19 @@ is negative or
.I tp.tv_nsec
is outside the range [0..999,999,999].
.TP
+.BR EINVAL " (since Linux 4.3)"
+.\" commit e1d7ba8735551ed79c7a0463a042353574b96da3
+A call to
+.BR clock_settime ()
+with a
+.I clk_id
+of
+.B CLOCK_REALTIME
+attempted to set the time to a value less than
+the current value of the
+.B CLOCK_MONOTONIC
+clock.
+.TP
.B EPERM
.BR clock_settime ()
does not have permission to set the clock indicated.
diff --git a/man2/gettimeofday.2 b/man2/gettimeofday.2
index 05efb67d1..e1c3905e0 100644
--- a/man2/gettimeofday.2
+++ b/man2/gettimeofday.2
@@ -166,6 +166,15 @@ is negative or
.I tv.tv_usec
is outside the range [0..999,999].
.TP
+.BR EINVAL " (since Linux 4.3)"
+.\" commit e1d7ba8735551ed79c7a0463a042353574b96da3
+.RB ( settimeofday ()):
+An attempt was made to set the time to a value less than
+the current value of the
+.B CLOCK_MONOTONIC
+clock (see
+.BR clock_gettime (2)).
+.TP
.B EPERM
The calling process has insufficient privilege to call
.BR settimeofday ();
--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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