$ cat /proc/sys/kernel/unprivileged_userns_clone

1

$ unshare -mU /bin/sh -c "sleep 5" & /bin/sh -c "sleep 1; cd /proc/$!/cwd; realpath .; kill -9 $!"

[1] 12345
(unreachable)/home/user

TRACE_DEBUG=1  - Value for printing debug information (Default: 0)
STACK_SIZE=100 - Size output data of stack (Default: 100)

$ make
$ cp /bin/umount .
$ LD_PRELOAD="$(realpath tool.so)" TRACE_DEBUG=1 STACK_SIZE=100 ./umount /root