Re: [PATCH v5 2/3] init/do_mounts.c: create second mount for initramfs

[Christian Brauner]

On Fri, Jun 04, 2021 at 05:07:26AM -0700, 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 | 49 +++++++++++++++++++++++++++++++++++++++++++++---
>  init/do_mounts.h | 17 ++++++++++++++++-
>  init/initramfs.c |  9 ++++++++-
>  usr/Kconfig      | 10 ++++++++++
>  4 files changed, 80 insertions(+), 5 deletions(-)
> 
> diff --git a/init/do_mounts.c b/init/do_mounts.c
> index a78e44ee6adb..b13a412b8d94 100644
> --- a/init/do_mounts.c
> +++ b/init/do_mounts.c
> @@ -617,6 +617,51 @@ void __init prepare_namespace(void)
>  	init_chroot(".");
>  }
>  
> +static inline __init bool is_tmpfs_enabled(void)
> +{
> +	return IS_ENABLED(CONFIG_TMPFS) && !saved_root_name[0] &&
> +		(!root_fs_names || strstr(root_fs_names, "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 = is_tmpfs_enabled() ? "tmpfs" : "ramfs";

I think we can simplify this further and drop the third patch and
further simplify this one.

You have:
start_kernel()
-> vfs_caches_init()
   -> mnt_init()
      -> init_rootfs()

At this point

is_tmpfs is either true or false because init_rootfs() has been called.
So by the time we hit:

kernel_init()
-> kernel_init_freeable()
   -> do_basic_setup()
      -> do_initcalls()

which calls populate_rootfs() is_tmpfs is already initialized. So how
about you move up the is_tmpfs variable above the CONFIG_INITRAMFS_MOUNT
ifdef. The you can drop the third patch and this whole patch becomes sm
like (__completely untested__):

---
 init/do_mounts.c | 43 +++++++++++++++++++++++++++++++++++++++++++
 init/do_mounts.h | 14 ++++++++++++++
 init/initramfs.c |  9 ++++++++-
 usr/Kconfig      | 10 ++++++++++
 4 files changed, 75 insertions(+), 1 deletion(-)

diff --git a/init/do_mounts.c b/init/do_mounts.c
index a78e44ee6adb..8a0b59bc4c1b 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();
+}
+#endif
+
 static int rootfs_init_fs_context(struct fs_context *fc)
 {
 	if (IS_ENABLED(CONFIG_TMPFS) && is_tmpfs)
diff --git a/init/do_mounts.h b/init/do_mounts.h
index 7a29ac3e427b..94c9c757725d 100644
--- a/init/do_mounts.h
+++ b/init/do_mounts.h
@@ -14,6 +14,20 @@ 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)
 {
 	init_unlink(name);
diff --git a/init/initramfs.c b/init/initramfs.c
index af27abc59643..f49d29de4fd9 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)
 {
@@ -682,15 +684,20 @@ 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:
 	/*
 	 * If the initrd region is overlapped with crashkernel reserved region,
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
-- 
2.27.0