On 17/04/2026 14:54, Bruno Haible wrote:
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.
Excellent.
I'll apply that.
thanks,
Padraig
