[Bug 1878413] [NEW] /proc/sys/fs/binfmt_misc/ empty even though binfmt_misc is loaded

[Hans-Christoph Steiner]

Public bug reported:

_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

** Affects: binfmt-support
     Importance: Undecided
         Status: New

** Project changed: qemu => binfmt-support

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https://bugs.launchpad.net/bugs/1878413

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

Status in binfmt-support:
  New

Bug description:
  _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

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