EINVAL for certain utime invocations when inside a chroot

[ihucos]

When unionfs-fuse is used from inside an chroot, utime fails for some invocations:

$mkdir /tmp/rootfs
$ pip3 install plash # use https://github.com/ihucos/plash/ to get an alpine root filesystem with unionfs-fuse installed on it.
$ plash export-tar --from alpine --apk unionfs-fuse | tar -x -C /tmp/rootfs
$ sudo su
# mount -t proc proc /tmp/rootfs/proc/
# mount -o bind /dev /tmp/rootfs/dev/
# chroot /tmp/rootfs ash
# mkdir /tmp/a /tmp/aa /tmp/aaa
# unionfs  -o cow /tmp/a=RW:/tmp/aa=RO /tmp/aaa
# touch /tmp/aaa/filex
# touch -m /tmp/aaa/filex
touch: /tmp/aaa/filex: Invalid argument

This would work with e.g ubuntus touch binary, it depends on how exactly utime-like system calls are called.

Here an example for something that works as expected

utimensat(AT_FDCWD, "/", [UTIME_OMIT, UTIME_NOW], 0) = 0

And something that does fail with EINVAL

utimensat(AT_FDCWD, "/", NULL, 0) = -1

I am still trying to get into it. In case anyone wonder, I need this for a container engine. Here is the issue, where I write some notes to it: ihucos/plash#69

Note that this is not about mounting an unionfs and then chrooting into it but calling unionfs from inside a chroot.

EDIT:
I think the snippet I posted makes you run chroot as root on top of an user-owned file system. Its's really just for debugging.