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SubjectRe: RFC: time_namespaces(7) manual page
Date
Andrei Vagin <avagin@gmail.com> writes:
> On Sat, Apr 04, 2020 at 01:08:50PM +0200, Michael Kerrisk (man-pages) wrote:
>> /proc/PID/timens_offsets
>> Associated with each time namespace are offsets, expressed with
>> respect to the initial time namespace, that define the values of
>> the monotonic and boot clocks in that namespace. These offsets
>> are exposed via the file /proc/PID/timens_offsets. Within this
>> file, the offsets are expressed as lines consisting of three
>> space-delimited fields:
>>
>> <clock-id> <offset-secs> <offset-nanosecs>
>>
>> The clock-id identifies the clock whose offsets are being shown.
>> This field is either 1, for CLOCK_MONOTONIC, or 7, for CLOCK_BOOT‐
>> TIME. The remaining fields express the offset (seconds plus
>> nanoseconds) for the clock in this time namespace. These offsets
>> are expressed relative to the clock values in the initial time
>> namespace. In the initial time namespace, the contents of this
>> file are as follows:
>
> I think we can mention that offset-secs can be negative, but
> offset-nanosleep has to be 0 or positive.

I assume you meant offset-nanosecs :)

That aside, there are also limitations in place.

1) Negative offsets which would offset time into negative space are
rejected, i.e. its enforced that

now(CLOCK) + offset[CLOCK] >= 0

This is necessary as the kernel expects and also enforces that time
cannot be negative.

2) Positive offsets which would offset time above KTTIME_SEC_MAX / 2 are
rejected, i.e. it's enforced that

now(CLOCK) + offset[CLOCK] <= KTIME_SEC_MAX / 2

That is done to prevent that clocks wrap around if the offset would
bring it close enough to the wrap around point.

The cutoff value is a pretty arbitrary choice (~146 years). So to
hit this you'd need a system which has an uptime of > 146 years,
which is pretty much unrealistic.

Thanks,

tglx




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