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

[Menglong Dong]

Hello,

On Sat, Jun 5, 2021 at 7:50 PM Christian Brauner
<christian.brauner@xxxxxxxxxx> wrote:
>
> 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?

I think it's necessary, as I explained in the third patch. When the rootfs
is a block device, ramfs is used in init_mount_tree() unconditionally,
which can be seen from the enable of is_tmpfs.

That makes sense, because rootfs will not become the root if a block
device is specified by 'root' in boot cmd, so it makes no sense to use
tmpfs, because ramfs is more simple.

Here, I make rootfs as ramfs for the same reason: the first mount is not
used as the root, so make it ramfs which is more simple.

Thanks!
Menglong Dong

>
> > +#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
> >