I wrote:
> The same failure also occurs on Arch Linux 24.04.
> Reproducible as
> $ unshare -U src/logname
> bruno
> Both of these distros use glibc version 2.39.
Here are the results of "unshare -U logname" in various distros with
various glibc versions:
glibc Distro GUI/Console unshare -U logname unshare -U logname
< /dev/null
(KDE/GNOME/Mate/LXQt/Pantheon)
2.24 ubuntu1704 K bruno logname: no login
name
2.26 Fedora27 G logname: no login name
2.26 opensuse152 K bruno logname: no login
name
2.26 ubuntu1710 K bruno logname: no login
name
2.27 ElementaryOS50 P logname: no login name
2.27 ubuntu1804 K bruno logname: no login
name
2.28 centos8-64 G logname: no login name
2.30 arch1911 C logname: no login name
2.31 opensuse155 K logname: no login name
2.32 ubuntu2010 K logname: no login name
2.33 slackware15 C logname: no login name
2.33 guix14 M logname: no login name
2.34 alma9 G logname: no login name
2.34 centos9 G logname: no login name
2.34 ubuntu2110 K logname: no login name
2.35 ubuntu2204 K logname: no login name
2.35 popos2204 G logname: no login name
2.36 Fedora37 G logname: no login name
2.36 ubuntu2210 K logname: no login name
2.36 debian12 G logname: no login name
2.37 ubuntu2304 K logname: no login name
2.38 ubuntu2310 K logname: no login name
2.38 openeuler2403 C bruno logname: no login
name
2.39 arch2404 C bruno logname: no login
name
2.39 manjaro23 K bruno logname: no login
name
2.39 ubuntu2404 K logname: no login name
2.39 openmandriva50 K bruno logname: no login
name
2.39 alma10 G logname: no login name
2.40 openmamba L logname: no login name
2.40 opensuse160 K logname: no login name
2.41 Fedora42 K bruno logname: no login
name
2.41 ubuntu2504 G logname: no login name
2.42 Fedora43 C bruno logname: no login
name
In summary:
1) The problem of "unshare -U logname" returning the previous user name
existed in glibc < 2.28 and starts again in glibc 2.38.
2) 'strace' shows that glibc determines this previous user name by
resolving the /proc/self/fd/0 link to a tty, then doing some lookup
in the utmp / utmpx database.
3) Therefore "unshare -U logname </dev/null" provides a workaround.
4) The difference between distros (openeuler, arch, manjaro, openmandriva,
Fedora on one side, Ubuntu, openSUSE, RHEL, openmamba on the other side)
in glibc >= 2.38 is probably due to differences in the utmp / utmpx lookup.
I cannot find glibc patches regarding getlogin in Fedora nor Ubuntu.
> > If so, perhaps our replacement is falling back to a method that works.
>
> It could be related to this glibc bug from 2023:
> https://sourceware.org/bugzilla/show_bug.cgi?id=30235
> https://sourceware.org/git/?p=glibc.git;a=commitdiff;h=0d83b349fa7340475406b2fe933c7467e4584091
Indeed:
The change between glibc 2.27 and 2.28
https://sourceware.org/git/?p=glibc.git;a=commitdiff;h=cc8a1620eb97ccddd337d157263c13c57b39ab71
unintentionally broke the old behaviour, and
the change between glibc 2.37 and 2.38
https://sourceware.org/git/?p=glibc.git;a=commitdiff;h=0d83b349fa7340475406b2fe933c7467e4584091
restored it.
> > I'll comment out the problematic part of the test for now.
The attached proposed patch restores the test in a way that succeeds.
Tested on Manjaro 23.
>From 8a1d0541e9f437c52f9919ed10d20314b50a6f65 Mon Sep 17 00:00:00 2001
From: Bruno Haible <[email protected]>
Date: Fri, 17 Apr 2026 15:46:19 +0200
Subject: [PATCH] tests: Reenable test that was disabled yesterday
* tests/misc/user.sh: Modify the "unshare -U logname" test, so that it
does not fail on glibc versions (< 2.28, >= 2.38) that do a fallback
lookup based on the tty.
---
tests/misc/user.sh | 7 ++++---
1 file changed, 4 insertions(+), 3 deletions(-)
diff --git a/tests/misc/user.sh b/tests/misc/user.sh
index 8385d1058..931287e41 100755
--- a/tests/misc/user.sh
+++ b/tests/misc/user.sh
@@ -25,8 +25,9 @@ overflow_uid=$(cat /proc/sys/kernel/overflowuid) ||
test "$(unshare -U whoami)" = "$(id -un $overflow_uid)" || fail=1
-# FIXME: Fails with Manjaro Linux
-#returns_ 1 unshare -U logname 2>err || fail=1
-#test "$(cat err)" = "logname: no login name" || fail=1
+# The "</dev/null" disables a fallback lookup via utmp/utmpx,
+# that existed in glibc < 2.28 and exists again in glibc >= 2.38.
+returns_ 1 unshare -U logname </dev/null 2>err || fail=1
+test "$(cat err)" = "logname: no login name" || fail=1
Exit $fail
--
2.52.0
