/proc/sys/fs/binfmt_misc/ empty even though binfmt_misc is loaded

[Hans-Christoph Steiner]

_apksigner_ uses binfmt to execute via _jarwrapper_, since it is a JAR. We have a test suite that relies on _apksigner_ working. It was running fine in Ubuntu/bionic. Since it was pegged to LTS, it got upgraded to Ubuntu/focal and it stopped working. This is likely because /proc/sys/fs/binfmt_misc/ is totally empty. The "binfmt_misc" kernel module shows as loaded:

$ grep binfmt /proc/modules
binfmt_misc 20480 1 - Live 0xffffffffc0452000

This relies on binfmt support in gitlab.com's CI runner setup, based on Docker. binfmt works in containers there, for example on Ubuntu/bionic:
https://gitlab.com/fdroid/fdroidserver/-/jobs/516857857

Something in Ubuntu/focal broke this when running focal in the container on the same Docker host runners:
https://gitlab.com/fdroid/fdroidserver/-/jobs/547148092

Debian's ci.debian.net lxc runners also have a similar problem, it might be related:
https://salsa.debian.org/ci-team/debian-ci-config/-/issues/1

[Laurent Vivier (laurent-vivier)]

The binfmt_misc filesystem must be mounted on /proc/sys/fs/binfmt_misc to work.

$ mount|grep ^binfmt_misc
binfmt_misc on /proc/sys/fs/binfmt_misc type binfmt_misc (rw,nosuid,nodev,noexec,relatime)

[Hans-Christoph Steiner (eighthave)]

From my experience, that mounting happens automatically once
binfmt-support is installed. At least that is the case on the
Ubuntu/bionic jobs and on my own Debian machines. Did something change
so that now it must be manually mounted?

[Hans-Christoph Steiner (eighthave)]

It seems in the focal container, binfmt_misc doesn't get setup properly:
https://gitlab.com/eighthave/fdroidserver/-/jobs/550962360

$ grep binfmt /proc/modules
binfmt_misc 20480 1 - Live 0xffffffffc0461000
$ mount | grep binfmt_misc || mount binfmt_misc /proc/sys/fs/binfmt_misc
mount: /proc/sys/fs/binfmt_misc: special device binfmt_misc does not exist.
$

[Hans-Christoph Steiner (eighthave)]

Ok, your hint lead me to the fix:

$ mount | grep binfmt_misc || mount -t binfmt_misc binfmt_misc /proc/sys/fs/binfmt_misc

I guess this mounting was somehow happening automatically before, but now it seems that it is handled by systemd in a user system. But a container usually doesn't run systemd.