PROBLEM: tty devices with future timestamps

[Egmont Koblinger <egmont@gmail.com>]

Hi,

An active terminal line (/dev/pts/N), in about 6% of the cases, has a
future timestamp.

I noticed that the command
  ls -l $(tty)
occasionally prints the timestamp in "month day year" format, rather
than "month day hour:minute". This is what coreutils's "ls" does when
the timestamp is either 6+ months back in the past, or is in the
future. Let's test the "in the future" hypothesis:

  ls -l --full-time $(tty); date +%H:%M:%S.%N
occasionally produces something like
  crw--w---- 1 egmont tty 136, 8 2020-06-27 09:57:28.686473296 +0200 /dev/pts/8
  09:57:28.170717548
Yep, the timestamp of the tty line is a slightly higher value than the
system date at a later point.

Running
  ls -l --full-time $(tty)
in an endless loop causes the seconds to increment in steps of 8, but
the subseconds part remains unchanged, like:
crw--w---- 1 egmont tty 136, 8 2020-06-27 10:07:12.686473296 +0200 /dev/pts/8
crw--w---- 1 egmont tty 136, 8 2020-06-27 10:07:12.686473296 +0200 /dev/pts/8
crw--w---- 1 egmont tty 136, 8 2020-06-27 10:07:20.686473296 +0200 /dev/pts/8
crw--w---- 1 egmont tty 136, 8 2020-06-27 10:07:20.686473296 +0200 /dev/pts/8

drivers/tty/tty_io.c : tty_update_time() seems to be responsible for
this jump in 8 seconds, leaving the milliseconds intact.

I think zeroing out the milliseconds in this method would solve the
problem – although then it's maybe inconsistent that at creation time
the device would have subsecond precision which would be gone soon
afterwards. Maybe it's better to create the device with no subseconds
right away? I'll leave it to you guys to decide.

I'm experiencing this on two different machines with Ubuntu 20.04,
using their 5.4.0 kernel. The method setting the seconds but not the
nanoseconds is still present in latest kernel git.

Thanks a lot,
egmont