Re: [PATCH v6 2/2] init/do_mounts.c: create second mount for initramfs

[Christian Brauner]

On Sat, Jun 05, 2021 at 11:44:47AM +0800, menglong8.dong@xxxxxxxxx wrote:
> From: Menglong Dong <dong.menglong@xxxxxxxxxx>
> 
> If using container platforms such as Docker, upon initialization it
> wants to use pivot_root() so that currently mounted devices do not
> propagate to containers. An example of value in this is that
> a USB device connected prior to the creation of a containers on the
> host gets disconnected after a container is created; if the
> USB device was mounted on containers, but already removed and
> umounted on the host, the mount point will not go away until all
> containers unmount the USB device.
> 
> Another reason for container platforms such as Docker to use pivot_root
> is that upon initialization the net-namspace is mounted under
> /var/run/docker/netns/ on the host by dockerd. Without pivot_root
> Docker must either wait to create the network namespace prior to
> the creation of containers or simply deal with leaking this to each
> container.
> 
> pivot_root is supported if the rootfs is a initrd or block device, but
> it's not supported if the rootfs uses an initramfs (tmpfs). This means
> container platforms today must resort to using block devices if
> they want to pivot_root from the rootfs. A workaround to use chroot()
> is not a clean viable option given every container will have a
> duplicate of every mount point on the host.
> 
> In order to support using container platforms such as Docker on
> all the supported rootfs types we must extend Linux to support
> pivot_root on initramfs as well. This patch does the work to do
> just that.
> 
> pivot_root will unmount the mount of the rootfs from its parent mount
> and mount the new root to it. However, when it comes to initramfs, it
> donesn't work, because the root filesystem has not parent mount, which
> makes initramfs not supported by pivot_root.
> 
> In order to make pivot_root supported on initramfs, we create a second
> mount with type of rootfs before unpacking cpio, and change root to
> this mount after unpacking.
> 
> While mounting the second rootfs, 'rootflags' is passed, and it means
> that we can set options for the mount of rootfs in boot cmd now.
> For example, the size of tmpfs can be set with 'rootflags=size=1024M'.
> 
> Signed-off-by: Menglong Dong <dong.menglong@xxxxxxxxxx>
> ---
>  init/do_mounts.c | 44 ++++++++++++++++++++++++++++++++++++++++++++
>  init/do_mounts.h | 17 ++++++++++++++++-
>  init/initramfs.c |  8 ++++++++
>  usr/Kconfig      | 10 ++++++++++
>  4 files changed, 78 insertions(+), 1 deletion(-)
> 
> diff --git a/init/do_mounts.c b/init/do_mounts.c
> index a78e44ee6adb..715bdaa89b81 100644
> --- a/init/do_mounts.c
> +++ b/init/do_mounts.c
> @@ -618,6 +618,49 @@ void __init prepare_namespace(void)
>  }
>  
>  static bool is_tmpfs;
> +#ifdef CONFIG_INITRAMFS_MOUNT
> +
> +/*
> + * Give systems running from the initramfs and making use of pivot_root a
> + * proper mount so it can be umounted during pivot_root.
> + */
> +int __init prepare_mount_rootfs(void)
> +{
> +	char *rootfs = "ramfs";
> +
> +	if (is_tmpfs)
> +		rootfs = "tmpfs";
> +
> +	return do_mount_root(rootfs, rootfs,
> +			     root_mountflags & ~MS_RDONLY,
> +			     root_mount_data);
> +}
> +
> +/*
> + * Revert to previous mount by chdir to '/' and unmounting the second
> + * mount.
> + */
> +void __init revert_mount_rootfs(void)
> +{
> +	init_chdir("/");
> +	init_umount(".", MNT_DETACH);
> +}
> +
> +/*
> + * Change root to the new rootfs that mounted in prepare_mount_rootfs()
> + * if cpio is unpacked successfully and 'ramdisk_execute_command' exist.
> + */
> +void __init finish_mount_rootfs(void)
> +{
> +	init_mount(".", "/", NULL, MS_MOVE, NULL);
> +	if (likely(ramdisk_exec_exist()))
> +		init_chroot(".");
> +	else
> +		revert_mount_rootfs();
> +}
> +
> +#define rootfs_init_fs_context ramfs_init_fs_context

Sorry, I think we're nearly there. What's the rationale for using ramfs
when unconditionally when a separate mount for initramfs is requested?
Meaning, why do we need this define at all?

> +#else
>  static int rootfs_init_fs_context(struct fs_context *fc)
>  {
>  	if (IS_ENABLED(CONFIG_TMPFS) && is_tmpfs)
> @@ -625,6 +668,7 @@ static int rootfs_init_fs_context(struct fs_context *fc)
>  
>  	return ramfs_init_fs_context(fc);
>  }
> +#endif
>  
>  struct file_system_type rootfs_fs_type = {
>  	.name		= "rootfs",
> diff --git a/init/do_mounts.h b/init/do_mounts.h
> index 7a29ac3e427b..ae4ab306caa9 100644
> --- a/init/do_mounts.h
> +++ b/init/do_mounts.h
> @@ -10,9 +10,24 @@
>  #include <linux/root_dev.h>
>  #include <linux/init_syscalls.h>
>  
> +extern int root_mountflags;
> +
>  void  mount_block_root(char *name, int flags);
>  void  mount_root(void);
> -extern int root_mountflags;
> +
> +#ifdef CONFIG_INITRAMFS_MOUNT
> +
> +int  prepare_mount_rootfs(void);
> +void finish_mount_rootfs(void);
> +void revert_mount_rootfs(void);
> +
> +#else
> +
> +static inline int  prepare_mount_rootfs(void) { return 0; }
> +static inline void finish_mount_rootfs(void) { }
> +static inline void revert_mount_rootfs(void) { }
> +
> +#endif
>  
>  static inline __init int create_dev(char *name, dev_t dev)
>  {
> diff --git a/init/initramfs.c b/init/initramfs.c
> index af27abc59643..1833de3cf04e 100644
> --- a/init/initramfs.c
> +++ b/init/initramfs.c
> @@ -16,6 +16,8 @@
>  #include <linux/namei.h>
>  #include <linux/init_syscalls.h>
>  
> +#include "do_mounts.h"
> +
>  static ssize_t __init xwrite(struct file *file, const char *p, size_t count,
>  		loff_t *pos)
>  {
> @@ -682,13 +684,19 @@ static void __init do_populate_rootfs(void *unused, async_cookie_t cookie)
>  	else
>  		printk(KERN_INFO "Unpacking initramfs...\n");
>  
> +	if (prepare_mount_rootfs())
> +		panic("Failed to mount rootfs");
> +
>  	err = unpack_to_rootfs((char *)initrd_start, initrd_end - initrd_start);
>  	if (err) {
> +		revert_mount_rootfs();
>  #ifdef CONFIG_BLK_DEV_RAM
>  		populate_initrd_image(err);
>  #else
>  		printk(KERN_EMERG "Initramfs unpacking failed: %s\n", err);
>  #endif
> +	} else {
> +		finish_mount_rootfs();
>  	}
>  
>  done:
> diff --git a/usr/Kconfig b/usr/Kconfig
> index 8bbcf699fe3b..4f6ac12eafe9 100644
> --- a/usr/Kconfig
> +++ b/usr/Kconfig
> @@ -52,6 +52,16 @@ config INITRAMFS_ROOT_GID
>  
>  	  If you are not sure, leave it set to "0".
>  
> +config INITRAMFS_MOUNT
> +	bool "Create second mount to make pivot_root() supported"
> +	default y
> +	help
> +	  Before unpacking cpio, create a second mount and make it become
> +	  the root filesystem. Therefore, initramfs will be supported by
> +	  pivot_root().
> +
> +	  If container platforms is used with initramfs, say Y.
> +
>  config RD_GZIP
>  	bool "Support initial ramdisk/ramfs compressed using gzip"
>  	default y
> -- 
> 2.32.0.rc0
>