On 2020-03-31, Ignat Korchagin <ignat@xxxxxxxxxxxxxx> wrote: > The main need for this is to support container runtimes on stateless Linux > system (pivot_root system call from initramfs). > > Normally, the task of initramfs is to mount and switch to a "real" root > filesystem. However, on stateless systems (booting over the network) it is just > convenient to have your "real" filesystem as initramfs from the start. > > This, however, breaks different container runtimes, because they usually use > pivot_root system call after creating their mount namespace. But pivot_root does > not work from initramfs, because initramfs runs form rootfs, which is the root > of the mount tree and can't be unmounted. > > One workaround is to do: > > mount --bind / / > > However, that defeats one of the purposes of using pivot_root in the cloned > containers: get rid of host root filesystem, should the code somehow escapes the > chroot. > > There is a way to solve this problem from userspace, but it is much more > cumbersome: > * either have to create a multilayered archive for initramfs, where the outer > layer creates a tmpfs filesystem and unpacks the inner layer, switches root > and does not forget to properly cleanup the old rootfs > * or we need to use keepinitrd kernel cmdline option, unpack initramfs to > rootfs, run a script to create our target tmpfs root, unpack the same > initramfs there, switch root to it and again properly cleanup the old root, > thus unpacking the same archive twice and also wasting memory, because > the kernel stores compressed initramfs image indefinitely. > > With this change we can ask the kernel (by specifying nonroot_initramfs kernel > cmdline option) to create a "leaf" tmpfs mount for us and switch root to it > before the initramfs handling code, so initramfs gets unpacked directly into > the "leaf" tmpfs with rootfs being empty and no need to clean up anything. > > This also bring the behaviour in line with the older style initrd, where the > initrd is located on some leaf filesystem in the mount tree and rootfs remaining > empty. > > Signed-off-by: Ignat Korchagin <ignat@xxxxxxxxxxxxxx> I know this is a bit of a stretch, but I thought I'd ask -- is it possible to solve the problem with pivot_root(2) without requiring this workaround (and an additional cmdline option)? From the container runtime side of things, most runtimes do support working on initramfs but it requires disabling pivot_root(2) support (in the runc world this is --no-pivot-root). We would love to be able to remove support for disabling pivot_root(2) because lots of projects have been shipping with pivot_root(2) disabled (such as minikube until recently[1]) -- which opens such systems to quite a few breakout and other troubling exploits (obviously they also ship without using user namespaces *sigh*). But requiring a new cmdline option might dissuade people from switching. If there was a way to fix the underlying restriction on pivot_root(2), I'd be much happier with that as a solution. Thanks. [1]: https://github.com/kubernetes/minikube/issues/3512 -- Aleksa Sarai Senior Software Engineer (Containers) SUSE Linux GmbH <https://www.cyphar.com/>
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