> though sadly sometimes chroot() is needed because pivot_root() doesn't work on initramfs.
Are people actually attempting to boot a super-minimalist system that just has a kernel and an initramfs with something like docker into it where they don't bother with a rootfs at all and just start running containers directly from the initramfs? That's kinda cool, if that's the case.
You could do that (though one could argue that there's no real benefit to using containers in that case), but the issue is sadly more general than that. You cannot use pivot_root() if the current root is on initramfs. The reason is fairly historic, and boils down to "you cannot unmount initramfs" in the same way that "you cannot kill pid1".
This means that setups where you have the entire OS image in initramfs, and you try to run a container (even if it has a different filesystem as its rootfs) it will fail with pivot_root(). There are solutions for this but they require changing how the system is started (which can be a bit complicated depending on what system you're using to build your initramfs). From memory, minikube has used --no-pivot-root for a while precisely for this reason, though I believe they have switched away from it sometime recently.
Are people actually attempting to boot a super-minimalist system that just has a kernel and an initramfs with something like docker into it where they don't bother with a rootfs at all and just start running containers directly from the initramfs? That's kinda cool, if that's the case.