Message-Id: <20210517142542.187574-1-dong.menglong@zte.com.cn>
Date:   Mon, 17 May 2021 07:25:42 -0700
From:   menglong8.dong@...il.com
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        linux-kernel@...r.kernel.org
Subject: [PATCH] init/initramfs.c: add a new mount as root file system

From: Menglong Dong <dong.menglong@....com.cn>

During the kernel initialization, the mount tree, which is used by the
kernel, is created, and 'rootfs' is mounted as the root file system.

While using initramfs as the root file system, cpio file is unpacked
into the rootfs. Thus, this rootfs is exactly what users see in user
space, and some problems arose: this rootfs has no parent mount,
which make it can't be umounted or pivot_root.

'pivot_root' is used to change the rootfs and clean the old mountpoints,
and it is essential for some users, such as docker. Docker use
'pivot_root' to change the root fs of a process if the current root
fs is a block device of initrd. However, when it comes to initramfs,
things is different: docker has to use 'chroot()' to change the root
fs, as 'pivot_root()' is not supported in initramfs.

The usage of 'chroot()' to create root fs for a container introduced
a lot problems.

First, 'chroot()' can't clean the old mountpoints which inherited
from the host. It means that the mountpoints in host will have a
duplicate in every container. Let's image that there are 100
containers in host, and there will be 100 duplicates of every
mountpoints, which makes resource release an issue. User may
remove a USB after he (or she) umount it successfully in the
host. However, the USB may still be mounted in containers, although
it can't be seen by the 'mount' commond in the container. This
means the USB is not released yet, and data may not write back.
Therefore, data lose arise.

Second, net-namespace leak is another problem. The net-namespace
of containers will be mounted in /var/run/docker/netns/ in host
by dockerd. It means that the net-namespace of a container will
be mounted in containers which are created after it. Things
become worse now, as the net-namespace can't be remove after
the destroy of that container, as it is still mounted in other
containers. If users want to recreate that container, he will
fail if a certain mac address is to be binded with the container,
as it is not release yet.

Maybe dockerd can umount the unnecessary mountpoints that inherited
for the host before do 'chroot()', but that is not a graceful way.
I think the best way is to make 'pivot_root()' support initramfs.

After this patch, initramfs is supported by 'pivot_root()' perfectly.
I just create a new rootfs and mount it to the mount-tree before
unpack cpio. Therefore, the rootfs used by users has a parent mount,
and can use 'pivot_root()'.

Signed-off-by: Menglong Dong <dong.menglong@....com.cn>
---
 init/do_mounts.c | 53 +++++++++++++++++++++++++++++++++++++++---------
 init/do_mounts.h |  1 +
 init/initramfs.c | 32 +++++++++++++++++++++++++++++
 init/main.c      | 17 +++++++++++++++-
 4 files changed, 92 insertions(+), 11 deletions(-)

diff --git a/init/do_mounts.c b/init/do_mounts.c
index a78e44ee6adb..a156b0d28b43 100644
--- a/init/do_mounts.c
+++ b/init/do_mounts.c
@@ -459,7 +459,7 @@ void __init mount_block_root(char *name, int flags)
 out:
 	put_page(page);
 }
- 
+
 #ifdef CONFIG_ROOT_NFS
 
 #define NFSROOT_TIMEOUT_MIN	5
@@ -617,24 +617,57 @@ void __init prepare_namespace(void)
 	init_chroot(".");
 }
 
-static bool is_tmpfs;
-static int rootfs_init_fs_context(struct fs_context *fc)
+#ifdef CONFIG_TMPFS
+static __init bool is_tmpfs_enabled(void)
+{
+	return (!root_fs_names || strstr(root_fs_names, "tmpfs")) &&
+	       !saved_root_name[0];
+}
+#endif
+
+static __init bool is_ramfs_enabled(void)
 {
-	if (IS_ENABLED(CONFIG_TMPFS) && is_tmpfs)
-		return shmem_init_fs_context(fc);
+	return true;
+}
+
+struct fs_user_root {
+	bool (*enabled)(void);
+	char *dev_name;
+	char *fs_name;
+};
 
-	return ramfs_init_fs_context(fc);
+static struct fs_user_root user_roots[] __initdata = {
+#ifdef CONFIG_TMPFS
+	{.fs_name = "tmpfs", .enabled = is_tmpfs_enabled },
+#endif
+	{.fs_name = "ramfs", .enabled = is_ramfs_enabled }
+};
+static struct fs_user_root * __initdata user_root;
+
+int __init mount_user_root(void)
+{
+	return do_mount_root(user_root->dev_name,
+			     user_root->fs_name,
+			     root_mountflags,
+			     root_mount_data);
 }
 
 struct file_system_type rootfs_fs_type = {
 	.name		= "rootfs",
-	.init_fs_context = rootfs_init_fs_context,
+	.init_fs_context = ramfs_init_fs_context,
 	.kill_sb	= kill_litter_super,
 };
 
 void __init init_rootfs(void)
 {
-	if (IS_ENABLED(CONFIG_TMPFS) && !saved_root_name[0] &&
-		(!root_fs_names || strstr(root_fs_names, "tmpfs")))
-		is_tmpfs = true;
+	struct fs_user_root *root;
+	int i;
+
+	for (i = 0; i < ARRAY_SIZE(user_roots); i++) {
+		root = &user_roots[i];
+		if (root->enabled()) {
+			user_root = root;
+			break;
+		}
+	}
 }
diff --git a/init/do_mounts.h b/init/do_mounts.h
index 7a29ac3e427b..34978b17454a 100644
--- a/init/do_mounts.h
+++ b/init/do_mounts.h
@@ -13,6 +13,7 @@
 void  mount_block_root(char *name, int flags);
 void  mount_root(void);
 extern int root_mountflags;
+int   mount_user_root(void);
 
 static inline __init int create_dev(char *name, dev_t dev)
 {
diff --git a/init/initramfs.c b/init/initramfs.c
index af27abc59643..c883379673c0 100644
--- a/init/initramfs.c
+++ b/init/initramfs.c
@@ -15,6 +15,11 @@
 #include <linux/mm.h>
 #include <linux/namei.h>
 #include <linux/init_syscalls.h>
+#include <uapi/linux/mount.h>
+
+#include "do_mounts.h"
+
+extern bool ramdisk_exec_exist(bool abs);
 
 static ssize_t __init xwrite(struct file *file, const char *p, size_t count,
 		loff_t *pos)
@@ -667,6 +672,27 @@ static void __init populate_initrd_image(char *err)
 }
 #endif /* CONFIG_BLK_DEV_RAM */
 
+/*
+ * This function is used to chroot to new initramfs root that
+ * we unpacked.
+ */
+static void __init end_mount_user_root(bool succeed)
+{
+	if (!succeed)
+		goto on_failed;
+
+	if (!ramdisk_exec_exist(false))
+		goto on_failed;
+
+	init_mount(".", "/", NULL, MS_MOVE, NULL);
+	init_chroot(".");
+	return;
+
+on_failed:
+	init_chdir("/");
+	init_umount("/root", 0);
+}
+
 static void __init do_populate_rootfs(void *unused, async_cookie_t cookie)
 {
 	/* Load the built in initramfs */
@@ -682,15 +708,21 @@ static void __init do_populate_rootfs(void *unused, async_cookie_t cookie)
 	else
 		printk(KERN_INFO "Unpacking initramfs...\n");
 
+	if (mount_user_root())
+		panic("Failed to create user root");
+
 	err = unpack_to_rootfs((char *)initrd_start, initrd_end - initrd_start);
 	if (err) {
+		end_mount_user_root(false);
 #ifdef CONFIG_BLK_DEV_RAM
 		populate_initrd_image(err);
 #else
 		printk(KERN_EMERG "Initramfs unpacking failed: %s\n", err);
 #endif
+		goto done;
 	}
 
+	end_mount_user_root(true);
 done:
 	/*
 	 * If the initrd region is overlapped with crashkernel reserved region,
diff --git a/init/main.c b/init/main.c
index eb01e121d2f1..431da5f01f11 100644
--- a/init/main.c
+++ b/init/main.c
@@ -607,6 +607,21 @@ static inline void setup_nr_cpu_ids(void) { }
 static inline void smp_prepare_cpus(unsigned int maxcpus) { }
 #endif
 
+bool __init ramdisk_exec_exist(bool abs)
+{
+	char *tmp_command = ramdisk_execute_command;
+
+	if (!tmp_command)
+		return false;
+
+	if (!abs) {
+		while (*tmp_command == '/' || *tmp_command == '.')
+			tmp_command++;
+	}
+
+	return init_eaccess(tmp_command) == 0;
+}
+
 /*
  * We need to store the untouched command line for future reference.
  * We also need to store the touched command line since the parameter
@@ -1568,7 +1583,7 @@ static noinline void __init kernel_init_freeable(void)
 	 * check if there is an early userspace init.  If yes, let it do all
 	 * the work
 	 */
-	if (init_eaccess(ramdisk_execute_command) != 0) {
+	if (!ramdisk_exec_exist(true)) {
 		ramdisk_execute_command = NULL;
 		prepare_namespace();
 	}
-- 
2.25.1

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