Re: [PATCH 1/1] mnt: add support for non-rootfs initramfs

[Randy Dunlap <rdunlap@infradead.org>]

On 3/30/20 6:14 AM, Ignat Korchagin 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@cloudflare.com>
> ---
>  fs/namespace.c | 47 +++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 47 insertions(+)

Hi,
Please document "nonroot_initramfs" in
Documentation/admin-guide/kernel-parameters.txt.

> 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)
> 

thanks.
-- 
~Randy