Re: boot time on Linux

[Bruno Haible]

I wrote:
> So, all approaches that compute the boot time through a subtraction are
> going to be wrong in these scenarios.
> 
> The better approach is really to read the boot time from a time stamp —
> inside a file such as /var/run/utmp or /var/log/wtmp, or attached to
> a file such as
>   /var/lib/systemd/random-seed    (first file touched during boot)
>   /var/log/boot.log               (one of the last files touched during boot).
> 
> And on Alpine Linux, while /var/run/utmp is empty, its time stamp is
> essentially the boot time.

With the attached patches, the boot time returned by read_utmp() is stable
across 'sudo date MMDDhhmm' invocations.

On Linux distros, I found these files to be touched after boot:

/var/lib/systemd/random-seed   (seen on distros with systemd, e.g. CentOS 7,
                                Fedora 27, Debian 9, Ubuntu 17.10, Arch 19.11,
                                Manjaro 17)
/var/run/utmp                  (seen on distros with OpenRC, e.g. Alpine Linux)
/var/lib/random-seed           (seen on OpenSUSE 12.1, CentOS 5)

And, while at it, also on OpenBSD. It touches /var/db/host.random and
/var/run/utmp.

Bruno


2023-08-09  Bruno Haible  <bruno@clisp.org>

        readutmp: Return a boot time also on OpenBSD.
        * lib/readutmp.h (BOOT_TIME, USER_PROCESS): Provide fallback
        definitions.
        * lib/readutmp.c (read_utmp_from_file) [__OpenBSD__]: Fake a BOOT_TIME
        entry by looking at the time stamp of a specific file.

        readutmp: Return a boot time also on Alpine Linux.
        * lib/readutmp.c: Include stat-time.h.
        (SIZEOF): New macro.
        (read_utmp_from_file) [__linux__]: Fake a BOOT_TIME entry by looking
        at the time stamp of a specific file.
        * modules/readutmp (Depends-on): Add stat-time.

        readutmp: Fix boot time in VMs after sleep state and date update.
        * lib/readutmp.c (read_utmp_from_file): New function, extracted from
        read_utmp.
        (get_boot_time_uncached): Before all other approaches, try to find the
        boot time in the /var/run/utmp file.
        (read_utmp): Invoke read_utmp_from_file.

        readutmp: Make it easier to filter for/against the boot-time entry.
        * lib/readutmp.h (READ_UTMP_BOOT_TIME, READ_UTMP_NO_BOOT_TIME): New
        enum items.
        * lib/readutmp.c (desirable_utmp_entry, read_utmp_from_systemd):
        Implement them.
        (read_utmp): If no entries can match the given options, return
        immediately.

0001-readutmp-Make-it-easier-to-filter-for-against-the-bo.patch
Description: Text Data

0002-readutmp-Fix-boot-time-in-VMs-after-sleep-state-and-.patch
Description: Text Data

0003-readutmp-Return-a-boot-time-also-on-Alpine-Linux.patch
Description: Text Data

0004-readutmp-Return-a-boot-time-also-on-OpenBSD.patch
Description: Text Data