Re: utmps: database cursor position and pututxline(3)

From: Laurent Bercot <>
Date: Mon, 12 Apr 2021 11:14:47 +0000

>I only did some ad-hoc testing with the only 2 programs that both read
>and write to utmp and that I have integrated with utmps: util-linux
>login and OpenSSH. And I used the simple tests I attached earlier. All

  Cool, thanks. I'll still give it some more thought. The fact that my
earlier modification is broken is exactly why I didn't do it in the
first place, and I'm an idiot for having changed it without thinking,
and that's what happens when I'm looking at those things during the
week-end while doing something else. :P

  So now I'm going to think it through and find the correct way of
implementing that change. It may be your patch; it may be different.
And let me mention once more that I hate vague specifications that
sound simple but have hidden complexity like this. I will also use
the opportunity to understand what's going on with the ut_id field.

>Now that that's done, I'm having second-thoughts about this whole
>utmp/wtmp endeavor and wondering if it's worth the efforts. Don't get
>me wrong, I think your implementation fulfills its premises of
>security and robustness quite well, and I like how it fits within the
>s6 "ecosystem". But the POSIX API and data structures feel clunky and
>archaic. Take for example the id field, which is an arbitrary
>4-character string used to uniquely identify an entry in the database.
>That doesn't strike me as very robust nor secure, given that there is
>no mechanism to prevent id collisions.

  Well yes, it's not a secret that utmp sucks.

>Rich Felker may have made the right choice in leaving it on the
>cutting floor and expecting that no-one would miss it. Perhaps we
>should just let it die.

  Users want 'who' to work. musl-based distributions can't make
'who' work. Users blame the lack of 'who' on musl and stop using
musl-based distributions. Having a working utmp implementation makes
users happy and helps with musl adoption.

  The reason why Rich didn't implement utmp in musl is not that the
API and data structures are bad. There is *plenty* of suckage in the
POSIX standard - starting with something everyone uses: stdio! - and
he happily implemented 99% of it.
  The reason is that utmp is not implementable in a libc without
requiring suid/sgid binaries to modify the database, see
  So it's not "the APIs suck", it's "it's a security risk".

  utmps is the only existing *secure* utmp implementation, because it
relies on daemons and kernel-verified client credentials to actually
access the database, and removes the need for suid/sgid. The "needs
daemons to work securely" part is why it can't be done inside of a
libc; you could consider utmps as a companion package to musl for a
complete implementation of the standard.

>Isn't there a modern framework equivalent for user accounting on
>*nix-like systems? I mean beside systemd of course :)

  I have exactly zero doubt that any attempt at designing a "modern"
framework for user accounting would manage to do worse than utmp.
systemd has done exactly that. Other attempts would be similar.
People who would actually take time and energy to do this right are
not interested in doing it, because user accounting is 1. ultimately
user snooping, and 2. becoming useless by the day with the way Unix
is used now.

  As you said, it's best to let it die - not because utmp is bad, but
because *the concept of user accounting* is bad. I'm just providing
a utmp implementation that is as secure as possible for existing
programs that use it, because people want those programs to work; but
if you like user accounting, then you'll have to deal with all the
flaws of utmp. :P

Received on Mon Apr 12 2021 - 11:14:47 UTC

This archive was generated by hypermail 2.3.0 : Sun May 09 2021 - 19:38:49 UTC