GNU bug report logs - #65992
readutmp should check for sd_booted() and not if /run/utmp exists

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From: Thorsten Kukuk <kukuk <at> suse.com>
To: 65992 <at> debbugs.gnu.org
Cc: Paul Eggert <eggert <at> cs.ucla.edu>, Bruno Haible <bruno <at> clisp.org>
Subject: bug#65992: readutmp should check for sd_booted() and not if /run/utmp exists
Date: Fri, 15 Sep 2023 10:00:35 +0000

Hi,

if there is no /run/utmp file, /usr/bin/who falls back correctly to the
systemd-logind interface and shows correct data.

But there are applications, which don't use the libc interface for
reading/writing utmp entries, they have their own implementation. And
this implementations create the file /run/utmp if this does not exist
(glibc does not). Which means, /usr/bin/who shows sometimes wrong/incomplete
data.

procps (especially w) and other packages don't check if /run/utmp exist
or not, but use sd_booted(). e.g.:

 if (sd_booted() > 0) {
    numuser = sd_get_sessions(NULL);
  } else {
    setutent();
    while ((ut = getutent())) {
      if ((ut->ut_type == USER_PROCESS) && (ut->ut_name[0] != '\0'))
        numuser++;
    }
    endutent();
  }

So if logind is running, logind is used, else /run/utmp.
I think gnulib/coreutils should do the same. It's impossible to find and
fix all the code writing utmp entries at their own, especially if this
is 3rd party software and you don't have the source code. 

  Thorsten

-- 
Thorsten Kukuk, Distinguished Engineer, Senior Architect, Future Technologies
SUSE Software Solutions Germany GmbH, Frankenstraße 146, 90461 Nuernberg, Germany
Managing Director: Ivo Totev, Andrew McDonald, Werner Knoblich
(HRB 36809, AG Nürnberg)

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