pivot_root from initramfs to new root - error "Invalid argument"

Question

You can not use pivot_root on an initramfs rootfs, you will get Invalid Argument. You can only pivot real filesystems.

Indeed:

1. Fedora Linux 28 - this uses the dracut initramfs.
2. Boot into an initramfs shell, by adding rd.break as an option on the kernel command line.
3. cd /sysroot
4. usr/bin/pivot_root . mnt

-> pivot_root fails with "Invalid argument", corresponding to an errno value of EINVAL.

There is no explanation for this in man 2 pivot_root:

EINVAL put_old is not underneath new_root.

Why does it fail? And as the next commenter replied, "Then how would Linux exit early user space?"

Answer 1

Unlike the initrd, Linux does not allow to unmount the initramfs. Apparently this helped keep the kernel code simple.

Instead of pivot_root, you can use the switch_root command. It implements the following procedure. Notice that switch_root deletes all the files on the old root, to free the initramfs memory, so you need to be careful where you run this command.

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.

Note the shell commands suggested are only rough equivalents to the C code. The commands won't really work unless they are all built in to your shell, because the first command deletes all the programs and other files from the initramfs :-).

---

Rootfs is a special instance of ramfs (or tmpfs, if that's enabled), which is always present in 2.6 systems. You can't unmount rootfs for approximately the same reason you can't kill the init process; rather than having special code to check for and handle an empty list, it's smaller and simpler for the kernel to just make sure certain lists can't become empty.

https://github.com/torvalds/linux/blob/v4.17/Documentation/filesystems/ramfs-rootfs-initramfs.txt

Answer 2

From the comment on the pivot_root syscall in fs/namespace.c:

Also, the current root cannot be on the 'rootfs' (initial ramfs) filesystem. See Documentation/filesystems/ramfs-rootfs-initramfs.txt for alternatives in this situation.

I submitted a patch to the man-pages project about this, so in future releases of it, man 2 pivot_root will mention this case.