[Feature] Setup parent mount before pivot_root if on initramfs

[aconz2]

I am using crun from initramfs and was running into pivot_root errors because the rootfs mount has no parent. I found a workaround in this comment which I'm reproducing below

unshare --mount
mount --rbind / /abc --mkdir
cd /abc
mount --move . /
chroot .
# bwrap will not get EINVAL from pivot_root() anymore

I've successfully used this (notes here).

I am wondering if you think this would be worthwhile to add as an option like --pivot-rootfs <dir> where it would do the above steps using the given <dir> to work from. I'm planning on writing a separate wrapper anyways but thought it might be useful to have builtin. I could work on putting together a PR too with some direction.

[giuseppe]

if you could open a PR, that would be great, so we can evaluate the complexity and if it is worth to add to crun itself

[aconz2]

verdict is doesn't make sense to include here, see #1508. I put a POC wrapper in a gist