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@xxxxxxxxxxxxxx>
> ---
> 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
