autopkgtest in a Sid schroot environment: how to set up subuid/subgid

Hello,

I have issues for running autopkgtest in Sid schroot environment.

The command

$ autopkgtest hello -- unshare -r unstable

gives

autopkgtest [06:30:34]: starting date and time: 2026-05-15 06:30:34+0000
autopkgtest [06:30:34]: version 5.55
autopkgtest [06:30:34]: host XXXXXXX; command line: /usr/bin/autopkgtest hello -- unshare -r unstable
['mmdebstrap', '--mode=unshare', '--variant=apt', '--skip=output/mknod,cleanup/apt/lists', '--customize-hook=sed "s/^deb /deb-src /" "$1/etc/apt/sources.list" > "$1/etc/apt/sources.list.d/src.list" && APT_CONFIG=$MMDEBSTRAP_APT_CONFIG apt-get update', 'unstable', '/tmp/ridpmt/1000/tmpy8lvswrj/chroot.tar'] failed (exit status 25, stderr "E: unshare syscall failed: Operation not permitted\nI: chroot architecture amd64 is equal to the host's architecture\nI: finding correct signed-by value...\nI: automatically chosen format: tar\nI: using /tmp/ridpmt/1000/mmdebstrap.pjvxruusU2 as tempdir\nE: unshare() failed: Operation not permitted\nnewuidmap: write to uid_map failed: Operation not permitted\nE: newuidmap 10196  0 1000 1 1 493216 1 failed: \nE: chown failed\n")
autopkgtest [06:30:34]: ERROR: testbed failure: eof from the virtualisation server
Exception ignored in: <_io.TextIOWrapper name=5 encoding='utf-8'>
BrokenPipeError: [Errno 32] Broken pipe




I suspect that my /etc/subuid and /etc/subgid are not properly setup.

Both on them look coherent on my box and within the schroot environment.
However I guess that the set-ups on my box and within the schroot environment
are interfering badly.

Any clue is welcome.

Thanks in advance,
Jerome