Re: [PATCH] mnt: add support for non-rootfs initramfs

[Ignat Korchagin <ignat@cloudflare.com>]

Following up on this: is there a way to move this forward somehow or
we're stuck with complex userspace code to achieve the same, like
https://git.kernel.org/pub/scm/libs/klibc/klibc.git/tree/usr/kinit/run-init/runinitlib.c,
but for tmpfs?

Just FYI, here is an example of a fix for a security issue, which is
caused by using chroot vs pivot_root in containers:
https://github.com/opencontainers/runc/commit/28a697cce3e4f905dca700eda81d681a30eef9cd

Alternatively, if the use-case is not generic enough, we could keep
the patch to ourselves - just would appreciate some advice/potential
concerns with this approach which we might have overlooked.

Thanks,
Ignat

On Thu, Mar 5, 2020 at 7:35 PM Ignat Korchagin <ignat@cloudflare.com> 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 can solve this problem from userspace, but it is much more cumbersome. We
> 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 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.
>
> Signed-off-by: Ignat Korchagin <ignat@cloudflare.com>
> ---
>  fs/namespace.c | 47 +++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 47 insertions(+)
>
> diff --git a/fs/namespace.c b/fs/namespace.c
> index 85b5f7bea82e..a1ec862e8146 100644
> --- a/fs/namespace.c
> +++ b/fs/namespace.c
> @@ -3701,6 +3701,49 @@ static void __init init_mount_tree(void)
>         set_fs_root(current->fs, &root);
>  }
>
> +#if IS_ENABLED(CONFIG_TMPFS)
> +static int __initdata nonroot_initramfs;
> +
> +static int __init nonroot_initramfs_param(char *str)
> +{
> +       if (*str)
> +               return 0;
> +       nonroot_initramfs = 1;
> +       return 1;
> +}
> +__setup("nonroot_initramfs", nonroot_initramfs_param);
> +
> +static void __init init_nonroot_initramfs(void)
> +{
> +       int err;
> +
> +       if (!nonroot_initramfs)
> +               return;
> +
> +       err = ksys_mkdir("/root", 0700);
> +       if (err < 0)
> +               goto out;
> +
> +       err = do_mount("tmpfs", "/root", "tmpfs", 0, NULL);
> +       if (err)
> +               goto out;
> +
> +       err = ksys_chdir("/root");
> +       if (err)
> +               goto out;
> +
> +       err = do_mount(".", "/", NULL, MS_MOVE, NULL);
> +       if (err)
> +               goto out;
> +
> +       err = ksys_chroot(".");
> +       if (!err)
> +               return;
> +out:
> +       printk(KERN_WARNING "Failed to create a non-root filesystem for initramfs\n");
> +}
> +#endif /* IS_ENABLED(CONFIG_TMPFS) */
> +
>  void __init mnt_init(void)
>  {
>         int err;
> @@ -3734,6 +3777,10 @@ void __init mnt_init(void)
>         shmem_init();
>         init_rootfs();
>         init_mount_tree();
> +
> +#if IS_ENABLED(CONFIG_TMPFS)
> +       init_nonroot_initramfs();
> +#endif
>  }
>
>  void put_mnt_ns(struct mnt_namespace *ns)
> --
> 2.20.1
>