jailer: pivot_root does not work with rootfs

[acipu-aws]

Per the comment in the syscall code, SYS_pivot_root will fail if the current root is rootfs. This is a problem for systems that do not mount anything over the rootfs.

[dhriti-rajan]

Hello! My name's Dhriti, I'm currently taking the CS 360V course at UT Austin. One of our projects for the class is to work on an issue in an open-source repository, and since I've been working on learning Rust recently, this repository looked pretty cool. Would it be alright if I worked on this issue? If so, I'd really appreciate any information on how to approach it, or where to get started.

[Manciukic]

 - When switching another root device, initrd would pivot_root and then
    umount the ramdisk.  But initramfs is rootfs: you can neither pivot_root
    rootfs, nor unmount it.  Instead delete everything out of rootfs to
    free up the space (find -xdev / -exec rm '{}' ';'), overmount rootfs
    with the new root (cd /newmount; mount --move . /; chroot .), attach
    stdin/stdout/stderr to the new /dev/console, and exec the new init.

Solving this is not trivial and requires changing from pivot_root to mount(MS_MOVE) in cases where / is rootfs (initramfs). We're not aware of any usecases where firecracker is run directly from an initramfs, so we're going to close this issue as not relevant,