utmps

w will not work in future versions of shadow due to login not recording any utmp entries: https://github.com/shadow-maint/shadow/issues/945

Either link or preload using LD_PRELOAD=libutmps
We can rely on musl’s utmp.h (after patching) and only remove its utmpx.h
lh-bootstrap also has instructions on how to improve linking to utmps and skarnet (try it for toybox)

skarnet: “All in all, I believe the flexibility it offers overweighs the inconvenience of having to run services before providing utmp/wtmp.”

utmps is a solution to systems using s6, and won’t work for systemd, and it implements utmpx safely using a daemon

utmp.h has been deprecated in 2001 in favor of utmpx.h
On Glibc systems the former utmp.h is just an alias (or a symlink) for the latter utmpx.h
On musl libc, utmps can be used to provide utmpx functionality - but not utmp (This means that applications like procps-ng either don’t work properly (w shows nothing under musl with default no-op implementation), or fails to build (utmps provides utmpx.h but no utmp.h)
utmps also uses a daemon as the only authority to manage the utmp and wtmp data, which is an extra service to consider
we also need to remove the stub header utmpx.h (and maybe even utmp.h) provided by musl if you plan on using skarnet’s utmps
we also need to patch multiple software to support utmpx.h instead of utmp.h since utmps only provides the latter. Here’s a list of packages using utmp h (that might require patching) that I compiled based on my experiments in glaucus:
- autoconf
- gcc (in libsanitizer)
- musl
- procps-ng (for w and whattime)
- shadow
- toybox (some pending tools)
- util-linux (for agetty)

From the musl wiki FAQ:

Q: Why is the utmp/wtmp functionality only implemented as stubs?

if the feature is implemented, you need to take additional measures to protect your user’s privacy

in order to use the utmp/wtmp feature, you need a suid/sgid binary to modify the database, which opens the door for security issues:

if you compromise those binaries, you can inject arbitrary data into the db, that other programs might interpret in exploitable ways

that’s a HUGE risk to pay for the sake of a basically-useless and possibly-harmful “feature”

The only distribution (other than glaucus) that has a working utmps with s6, musl and other packages is Adélie Linux

For further reading:

https://github.com/dslm4515/Musl-LFS/issues/33

w, getty, tmux, libutempter, shadow, util-linux, procps-ng all use utmp/utmpx (For utmps)

DEPRECATE UTMP, UTMPX, UTMPS, WTMP, BTMP AND LASTLOG

When these are removed, distros are using systemd journal and logind to replace them: https://gitlab.archlinux.org/archlinux/packaging/packages/procps-ng/-/commit/749fd577af6fe8ca6218822039078a0c75e3d459