I noticed a /core file in the root directory, dated around about when
my machine was upgraded to bullseye.  I checked another machine, and
it too had the same core file, dated from the upgrade:

24606,4> ls -lA /core
-rw-------   1 root     root     11542528 Sep 15 00:41 core
0-0-16:27:43, Fri Oct 01 tconnors@fs:/snapshots/dirac/latest (bash)
24607,5> sudo file core
core: ELF 64-bit LSB core file, x86-64, version 1 (SYSV), SVR4-style, from '/usr/libexec/qemu-binfmt/s390x-binfmt-P /check /check', real uid: 0, effective uid: 0, real gid: 0, effective gid: 0, execfn: '/check', platform: 'x86_64'

Working backwards, this was the exact minute this machine was last
rebooted, immediately after being dist-upgraded to bullseye.  I
haven't done anything deliberate to fire up qemu from this machine - I
only use it occasionally, and haven't configured it to autostart
anything.  Certainly never done anything with S390 on these amd64
boxen.

Control: tag -1 + moreinfo

This is apparently something checking for s390x availability,
so "not in use" is a bit interesting here in this context.
Neither binfmt-support nor systemd does that, and qemu-user-static
does not do this either.

It is interesting twice: the fact someone tried this s390x
abi, and the fact that qemu-s390-static (the binfmt-P is just
a symlink to it) crashed with a core dump.  It was run as root
too (well, or else it wouldn't be able to create /core).

It'd be interesting to see the core file and especially the
/check binary.

Did it happen again since that? Can you reinstall qemu-user-static,
or upgrade it again (installing the buster version first) and see
if it triggers the issue again?

I have no idea what is going on there...

Thanks!

/mjt

This bug report has been in 'moreinfo' state for quite some time,
with no more info available.  Nowhere else I see anything like this
happening.

Let's close this bug report now.  If you think it is incorrect, feel
free to reopen it and provide some requested information.

Thanks,

/mjt