brlocks/lglocks: API cleanups

[deliverable/linux.git] / fs / namespace.c

/*
 *  linux/fs/namespace.c
 *
 * (C) Copyright Al Viro 2000, 2001
 *      Released under GPL v2.
 *
 * Based on code from fs/super.c, copyright Linus Torvalds and others.
 * Heavily rewritten.
 */

#include <linux/syscalls.h>
#include <linux/export.h>
#include <linux/capability.h>
#include <linux/mnt_namespace.h>
#include <linux/namei.h>
#include <linux/security.h>
#include <linux/idr.h>
#include <linux/acct.h>         /* acct_auto_close_mnt */
#include <linux/ramfs.h>        /* init_rootfs */
#include <linux/fs_struct.h>    /* get_fs_root et.al. */
#include <linux/fsnotify.h>     /* fsnotify_vfsmount_delete */
#include <linux/uaccess.h>
#include "pnode.h"
#include "internal.h"

#define HASH_SHIFT ilog2(PAGE_SIZE / sizeof(struct list_head))
#define HASH_SIZE (1UL << HASH_SHIFT)

static int event;
static DEFINE_IDA(mnt_id_ida);
static DEFINE_IDA(mnt_group_ida);
static DEFINE_SPINLOCK(mnt_id_lock);
static int mnt_id_start = 0;
static int mnt_group_start = 1;

static struct list_head *mount_hashtable __read_mostly;
static struct kmem_cache *mnt_cache __read_mostly;
static struct rw_semaphore namespace_sem;

/* /sys/fs */
struct kobject *fs_kobj;
EXPORT_SYMBOL_GPL(fs_kobj);

/*
 * vfsmount lock may be taken for read to prevent changes to the
 * vfsmount hash, ie. during mountpoint lookups or walking back
 * up the tree.
 *
 * It should be taken for write in all cases where the vfsmount
 * tree or hash is modified or when a vfsmount structure is modified.
 */
DEFINE_BRLOCK(vfsmount_lock);

static inline unsigned long hash(struct vfsmount *mnt, struct dentry *dentry)
{
        unsigned long tmp = ((unsigned long)mnt / L1_CACHE_BYTES);
        tmp += ((unsigned long)dentry / L1_CACHE_BYTES);
        tmp = tmp + (tmp >> HASH_SHIFT);
        return tmp & (HASH_SIZE - 1);
}

#define MNT_WRITER_UNDERFLOW_LIMIT -(1<<16)

/*
 * allocation is serialized by namespace_sem, but we need the spinlock to
 * serialize with freeing.
 */
static int mnt_alloc_id(struct mount *mnt)
{
        int res;

retry:
        ida_pre_get(&mnt_id_ida, GFP_KERNEL);
        spin_lock(&mnt_id_lock);
        res = ida_get_new_above(&mnt_id_ida, mnt_id_start, &mnt->mnt_id);
        if (!res)
                mnt_id_start = mnt->mnt_id + 1;
        spin_unlock(&mnt_id_lock);
        if (res == -EAGAIN)
                goto retry;

        return res;
}

static void mnt_free_id(struct mount *mnt)
{
        int id = mnt->mnt_id;
        spin_lock(&mnt_id_lock);
        ida_remove(&mnt_id_ida, id);
        if (mnt_id_start > id)
                mnt_id_start = id;
        spin_unlock(&mnt_id_lock);
}

/*
 * Allocate a new peer group ID
 *
 * mnt_group_ida is protected by namespace_sem
 */
static int mnt_alloc_group_id(struct mount *mnt)
{
        int res;

        if (!ida_pre_get(&mnt_group_ida, GFP_KERNEL))
                return -ENOMEM;

        res = ida_get_new_above(&mnt_group_ida,
                                mnt_group_start,
                                &mnt->mnt_group_id);
        if (!res)
                mnt_group_start = mnt->mnt_group_id + 1;

        return res;
}

/*
 * Release a peer group ID
 */
void mnt_release_group_id(struct mount *mnt)
{
        int id = mnt->mnt_group_id;
        ida_remove(&mnt_group_ida, id);
        if (mnt_group_start > id)
                mnt_group_start = id;
        mnt->mnt_group_id = 0;
}

/*
 * vfsmount lock must be held for read
 */
static inline void mnt_add_count(struct mount *mnt, int n)
{
#ifdef CONFIG_SMP
        this_cpu_add(mnt->mnt_pcp->mnt_count, n);
#else
        preempt_disable();
        mnt->mnt_count += n;
        preempt_enable();
#endif
}

/*
 * vfsmount lock must be held for write
 */
unsigned int mnt_get_count(struct mount *mnt)
{
#ifdef CONFIG_SMP
        unsigned int count = 0;
        int cpu;

        for_each_possible_cpu(cpu) {
                count += per_cpu_ptr(mnt->mnt_pcp, cpu)->mnt_count;
        }

        return count;
#else
        return mnt->mnt_count;
#endif
}

static struct mount *alloc_vfsmnt(const char *name)
{
        struct mount *mnt = kmem_cache_zalloc(mnt_cache, GFP_KERNEL);
        if (mnt) {
                int err;

                err = mnt_alloc_id(mnt);
                if (err)
                        goto out_free_cache;

                if (name) {
                        mnt->mnt_devname = kstrdup(name, GFP_KERNEL);
                        if (!mnt->mnt_devname)
                                goto out_free_id;
                }

#ifdef CONFIG_SMP
                mnt->mnt_pcp = alloc_percpu(struct mnt_pcp);
                if (!mnt->mnt_pcp)
                        goto out_free_devname;

                this_cpu_add(mnt->mnt_pcp->mnt_count, 1);
#else
                mnt->mnt_count = 1;
                mnt->mnt_writers = 0;
#endif

                INIT_LIST_HEAD(&mnt->mnt_hash);
                INIT_LIST_HEAD(&mnt->mnt_child);
                INIT_LIST_HEAD(&mnt->mnt_mounts);
                INIT_LIST_HEAD(&mnt->mnt_list);
                INIT_LIST_HEAD(&mnt->mnt_expire);
                INIT_LIST_HEAD(&mnt->mnt_share);
                INIT_LIST_HEAD(&mnt->mnt_slave_list);
                INIT_LIST_HEAD(&mnt->mnt_slave);
#ifdef CONFIG_FSNOTIFY
                INIT_HLIST_HEAD(&mnt->mnt_fsnotify_marks);
#endif
        }
        return mnt;

#ifdef CONFIG_SMP
out_free_devname:
        kfree(mnt->mnt_devname);
#endif
out_free_id:
        mnt_free_id(mnt);
out_free_cache:
        kmem_cache_free(mnt_cache, mnt);
        return NULL;
}

/*
 * Most r/o checks on a fs are for operations that take
 * discrete amounts of time, like a write() or unlink().
 * We must keep track of when those operations start
 * (for permission checks) and when they end, so that
 * we can determine when writes are able to occur to
 * a filesystem.
 */
/*
 * __mnt_is_readonly: check whether a mount is read-only
 * @mnt: the mount to check for its write status
 *
 * This shouldn't be used directly ouside of the VFS.
 * It does not guarantee that the filesystem will stay
 * r/w, just that it is right *now*.  This can not and
 * should not be used in place of IS_RDONLY(inode).
 * mnt_want/drop_write() will _keep_ the filesystem
 * r/w.
 */
int __mnt_is_readonly(struct vfsmount *mnt)
{
        if (mnt->mnt_flags & MNT_READONLY)
                return 1;
        if (mnt->mnt_sb->s_flags & MS_RDONLY)
                return 1;
        return 0;
}
EXPORT_SYMBOL_GPL(__mnt_is_readonly);

static inline void mnt_inc_writers(struct mount *mnt)
{
#ifdef CONFIG_SMP
        this_cpu_inc(mnt->mnt_pcp->mnt_writers);
#else
        mnt->mnt_writers++;
#endif
}

static inline void mnt_dec_writers(struct mount *mnt)
{
#ifdef CONFIG_SMP
        this_cpu_dec(mnt->mnt_pcp->mnt_writers);
#else
        mnt->mnt_writers--;
#endif
}

static unsigned int mnt_get_writers(struct mount *mnt)
{
#ifdef CONFIG_SMP
        unsigned int count = 0;
        int cpu;

        for_each_possible_cpu(cpu) {
                count += per_cpu_ptr(mnt->mnt_pcp, cpu)->mnt_writers;
        }

        return count;
#else
        return mnt->mnt_writers;
#endif
}

static int mnt_is_readonly(struct vfsmount *mnt)
{
        if (mnt->mnt_sb->s_readonly_remount)
                return 1;
        /* Order wrt setting s_flags/s_readonly_remount in do_remount() */
        smp_rmb();
        return __mnt_is_readonly(mnt);
}

/*
 * Most r/o checks on a fs are for operations that take
 * discrete amounts of time, like a write() or unlink().
 * We must keep track of when those operations start
 * (for permission checks) and when they end, so that
 * we can determine when writes are able to occur to
 * a filesystem.
 */
/**
 * mnt_want_write - get write access to a mount
 * @m: the mount on which to take a write
 *
 * This tells the low-level filesystem that a write is
 * about to be performed to it, and makes sure that
 * writes are allowed before returning success.  When
 * the write operation is finished, mnt_drop_write()
 * must be called.  This is effectively a refcount.
 */
int mnt_want_write(struct vfsmount *m)
{
        struct mount *mnt = real_mount(m);
        int ret = 0;

        preempt_disable();
        mnt_inc_writers(mnt);
        /*
         * The store to mnt_inc_writers must be visible before we pass
         * MNT_WRITE_HOLD loop below, so that the slowpath can see our
         * incremented count after it has set MNT_WRITE_HOLD.
         */
        smp_mb();
        while (mnt->mnt.mnt_flags & MNT_WRITE_HOLD)
                cpu_relax();
        /*
         * After the slowpath clears MNT_WRITE_HOLD, mnt_is_readonly will
         * be set to match its requirements. So we must not load that until
         * MNT_WRITE_HOLD is cleared.
         */
        smp_rmb();
        if (mnt_is_readonly(m)) {
                mnt_dec_writers(mnt);
                ret = -EROFS;
        }
        preempt_enable();
        return ret;
}
EXPORT_SYMBOL_GPL(mnt_want_write);

/**
 * mnt_clone_write - get write access to a mount
 * @mnt: the mount on which to take a write
 *
 * This is effectively like mnt_want_write, except
 * it must only be used to take an extra write reference
 * on a mountpoint that we already know has a write reference
 * on it. This allows some optimisation.
 *
 * After finished, mnt_drop_write must be called as usual to
 * drop the reference.
 */
int mnt_clone_write(struct vfsmount *mnt)
{
        /* superblock may be r/o */
        if (__mnt_is_readonly(mnt))
                return -EROFS;
        preempt_disable();
        mnt_inc_writers(real_mount(mnt));
        preempt_enable();
        return 0;
}
EXPORT_SYMBOL_GPL(mnt_clone_write);

/**
 * mnt_want_write_file - get write access to a file's mount
 * @file: the file who's mount on which to take a write
 *
 * This is like mnt_want_write, but it takes a file and can
 * do some optimisations if the file is open for write already
 */
int mnt_want_write_file(struct file *file)
{
        struct inode *inode = file->f_dentry->d_inode;
        if (!(file->f_mode & FMODE_WRITE) || special_file(inode->i_mode))
                return mnt_want_write(file->f_path.mnt);
        else
                return mnt_clone_write(file->f_path.mnt);
}
EXPORT_SYMBOL_GPL(mnt_want_write_file);

/**
 * mnt_drop_write - give up write access to a mount
 * @mnt: the mount on which to give up write access
 *
 * Tells the low-level filesystem that we are done
 * performing writes to it.  Must be matched with
 * mnt_want_write() call above.
 */
void mnt_drop_write(struct vfsmount *mnt)
{
        preempt_disable();
        mnt_dec_writers(real_mount(mnt));
        preempt_enable();
}
EXPORT_SYMBOL_GPL(mnt_drop_write);

void mnt_drop_write_file(struct file *file)
{
        mnt_drop_write(file->f_path.mnt);
}
EXPORT_SYMBOL(mnt_drop_write_file);

static int mnt_make_readonly(struct mount *mnt)
{
        int ret = 0;

        br_write_lock(&vfsmount_lock);
        mnt->mnt.mnt_flags |= MNT_WRITE_HOLD;
        /*
         * After storing MNT_WRITE_HOLD, we'll read the counters. This store
         * should be visible before we do.
         */
        smp_mb();

        /*
         * With writers on hold, if this value is zero, then there are
         * definitely no active writers (although held writers may subsequently
         * increment the count, they'll have to wait, and decrement it after
         * seeing MNT_READONLY).
         *
         * It is OK to have counter incremented on one CPU and decremented on
         * another: the sum will add up correctly. The danger would be when we
         * sum up each counter, if we read a counter before it is incremented,
         * but then read another CPU's count which it has been subsequently
         * decremented from -- we would see more decrements than we should.
         * MNT_WRITE_HOLD protects against this scenario, because
         * mnt_want_write first increments count, then smp_mb, then spins on
         * MNT_WRITE_HOLD, so it can't be decremented by another CPU while
         * we're counting up here.
         */
        if (mnt_get_writers(mnt) > 0)
                ret = -EBUSY;
        else
                mnt->mnt.mnt_flags |= MNT_READONLY;
        /*
         * MNT_READONLY must become visible before ~MNT_WRITE_HOLD, so writers
         * that become unheld will see MNT_READONLY.
         */
        smp_wmb();
        mnt->mnt.mnt_flags &= ~MNT_WRITE_HOLD;
        br_write_unlock(&vfsmount_lock);
        return ret;
}

static void __mnt_unmake_readonly(struct mount *mnt)
{
        br_write_lock(&vfsmount_lock);
        mnt->mnt.mnt_flags &= ~MNT_READONLY;
        br_write_unlock(&vfsmount_lock);
}

int sb_prepare_remount_readonly(struct super_block *sb)
{
        struct mount *mnt;
        int err = 0;

        /* Racy optimization.  Recheck the counter under MNT_WRITE_HOLD */
        if (atomic_long_read(&sb->s_remove_count))
                return -EBUSY;

        br_write_lock(&vfsmount_lock);
        list_for_each_entry(mnt, &sb->s_mounts, mnt_instance) {
                if (!(mnt->mnt.mnt_flags & MNT_READONLY)) {
                        mnt->mnt.mnt_flags |= MNT_WRITE_HOLD;
                        smp_mb();
                        if (mnt_get_writers(mnt) > 0) {
                                err = -EBUSY;
                                break;
                        }
                }
        }
        if (!err && atomic_long_read(&sb->s_remove_count))
                err = -EBUSY;

        if (!err) {
                sb->s_readonly_remount = 1;
                smp_wmb();
        }
        list_for_each_entry(mnt, &sb->s_mounts, mnt_instance) {
                if (mnt->mnt.mnt_flags & MNT_WRITE_HOLD)
                        mnt->mnt.mnt_flags &= ~MNT_WRITE_HOLD;
        }
        br_write_unlock(&vfsmount_lock);

        return err;
}

static void free_vfsmnt(struct mount *mnt)
{
        kfree(mnt->mnt_devname);
        mnt_free_id(mnt);
#ifdef CONFIG_SMP
        free_percpu(mnt->mnt_pcp);
#endif
        kmem_cache_free(mnt_cache, mnt);
}

/*
 * find the first or last mount at @dentry on vfsmount @mnt depending on
 * @dir. If @dir is set return the first mount else return the last mount.
 * vfsmount_lock must be held for read or write.
 */
struct mount *__lookup_mnt(struct vfsmount *mnt, struct dentry *dentry,
                              int dir)
{
        struct list_head *head = mount_hashtable + hash(mnt, dentry);
        struct list_head *tmp = head;
        struct mount *p, *found = NULL;

        for (;;) {
                tmp = dir ? tmp->next : tmp->prev;
                p = NULL;
                if (tmp == head)
                        break;
                p = list_entry(tmp, struct mount, mnt_hash);
                if (&p->mnt_parent->mnt == mnt && p->mnt_mountpoint == dentry) {
                        found = p;
                        break;
                }
        }
        return found;
}

/*
 * lookup_mnt increments the ref count before returning
 * the vfsmount struct.
 */
struct vfsmount *lookup_mnt(struct path *path)
{
        struct mount *child_mnt;

        br_read_lock(&vfsmount_lock);
        child_mnt = __lookup_mnt(path->mnt, path->dentry, 1);
        if (child_mnt) {
                mnt_add_count(child_mnt, 1);
                br_read_unlock(&vfsmount_lock);
                return &child_mnt->mnt;
        } else {
                br_read_unlock(&vfsmount_lock);
                return NULL;
        }
}

static inline int check_mnt(struct mount *mnt)
{
        return mnt->mnt_ns == current->nsproxy->mnt_ns;
}

/*
 * vfsmount lock must be held for write
 */
static void touch_mnt_namespace(struct mnt_namespace *ns)
{
        if (ns) {
                ns->event = ++event;
                wake_up_interruptible(&ns->poll);
        }
}

/*
 * vfsmount lock must be held for write
 */
static void __touch_mnt_namespace(struct mnt_namespace *ns)
{
        if (ns && ns->event != event) {
                ns->event = event;
                wake_up_interruptible(&ns->poll);
        }
}

/*
 * Clear dentry's mounted state if it has no remaining mounts.
 * vfsmount_lock must be held for write.
 */
static void dentry_reset_mounted(struct dentry *dentry)
{
        unsigned u;

        for (u = 0; u < HASH_SIZE; u++) {
                struct mount *p;

                list_for_each_entry(p, &mount_hashtable[u], mnt_hash) {
                        if (p->mnt_mountpoint == dentry)
                                return;
                }
        }
        spin_lock(&dentry->d_lock);
        dentry->d_flags &= ~DCACHE_MOUNTED;
        spin_unlock(&dentry->d_lock);
}

/*
 * vfsmount lock must be held for write
 */
static void detach_mnt(struct mount *mnt, struct path *old_path)
{
        old_path->dentry = mnt->mnt_mountpoint;
        old_path->mnt = &mnt->mnt_parent->mnt;
        mnt->mnt_parent = mnt;
        mnt->mnt_mountpoint = mnt->mnt.mnt_root;
        list_del_init(&mnt->mnt_child);
        list_del_init(&mnt->mnt_hash);
        dentry_reset_mounted(old_path->dentry);
}

/*
 * vfsmount lock must be held for write
 */
void mnt_set_mountpoint(struct mount *mnt, struct dentry *dentry,
                        struct mount *child_mnt)
{
        mnt_add_count(mnt, 1);  /* essentially, that's mntget */
        child_mnt->mnt_mountpoint = dget(dentry);
        child_mnt->mnt_parent = mnt;
        spin_lock(&dentry->d_lock);
        dentry->d_flags |= DCACHE_MOUNTED;
        spin_unlock(&dentry->d_lock);
}

/*
 * vfsmount lock must be held for write
 */
static void attach_mnt(struct mount *mnt, struct path *path)
{
        mnt_set_mountpoint(real_mount(path->mnt), path->dentry, mnt);
        list_add_tail(&mnt->mnt_hash, mount_hashtable +
                        hash(path->mnt, path->dentry));
        list_add_tail(&mnt->mnt_child, &real_mount(path->mnt)->mnt_mounts);
}

static inline void __mnt_make_longterm(struct mount *mnt)
{
#ifdef CONFIG_SMP
        atomic_inc(&mnt->mnt_longterm);
#endif
}

/* needs vfsmount lock for write */
static inline void __mnt_make_shortterm(struct mount *mnt)
{
#ifdef CONFIG_SMP
        atomic_dec(&mnt->mnt_longterm);
#endif
}

/*
 * vfsmount lock must be held for write
 */
static void commit_tree(struct mount *mnt)
{
        struct mount *parent = mnt->mnt_parent;
        struct mount *m;
        LIST_HEAD(head);
        struct mnt_namespace *n = parent->mnt_ns;

        BUG_ON(parent == mnt);

        list_add_tail(&head, &mnt->mnt_list);
        list_for_each_entry(m, &head, mnt_list) {
                m->mnt_ns = n;
                __mnt_make_longterm(m);
        }

        list_splice(&head, n->list.prev);

        list_add_tail(&mnt->mnt_hash, mount_hashtable +
                                hash(&parent->mnt, mnt->mnt_mountpoint));
        list_add_tail(&mnt->mnt_child, &parent->mnt_mounts);
        touch_mnt_namespace(n);
}

static struct mount *next_mnt(struct mount *p, struct mount *root)
{
        struct list_head *next = p->mnt_mounts.next;
        if (next == &p->mnt_mounts) {
                while (1) {
                        if (p == root)
                                return NULL;
                        next = p->mnt_child.next;
                        if (next != &p->mnt_parent->mnt_mounts)
                                break;
                        p = p->mnt_parent;
                }
        }
        return list_entry(next, struct mount, mnt_child);
}

static struct mount *skip_mnt_tree(struct mount *p)
{
        struct list_head *prev = p->mnt_mounts.prev;
        while (prev != &p->mnt_mounts) {
                p = list_entry(prev, struct mount, mnt_child);
                prev = p->mnt_mounts.prev;
        }
        return p;
}

struct vfsmount *
vfs_kern_mount(struct file_system_type *type, int flags, const char *name, void *data)
{
        struct mount *mnt;
        struct dentry *root;

        if (!type)
                return ERR_PTR(-ENODEV);

        mnt = alloc_vfsmnt(name);
        if (!mnt)
                return ERR_PTR(-ENOMEM);

        if (flags & MS_KERNMOUNT)
                mnt->mnt.mnt_flags = MNT_INTERNAL;

        root = mount_fs(type, flags, name, data);
        if (IS_ERR(root)) {
                free_vfsmnt(mnt);
                return ERR_CAST(root);
        }

        mnt->mnt.mnt_root = root;
        mnt->mnt.mnt_sb = root->d_sb;
        mnt->mnt_mountpoint = mnt->mnt.mnt_root;
        mnt->mnt_parent = mnt;
        br_write_lock(&vfsmount_lock);
        list_add_tail(&mnt->mnt_instance, &root->d_sb->s_mounts);
        br_write_unlock(&vfsmount_lock);
        return &mnt->mnt;
}
EXPORT_SYMBOL_GPL(vfs_kern_mount);

static struct mount *clone_mnt(struct mount *old, struct dentry *root,
                                        int flag)
{
        struct super_block *sb = old->mnt.mnt_sb;
        struct mount *mnt = alloc_vfsmnt(old->mnt_devname);

        if (mnt) {
                if (flag & (CL_SLAVE | CL_PRIVATE))
                        mnt->mnt_group_id = 0; /* not a peer of original */
                else
                        mnt->mnt_group_id = old->mnt_group_id;

                if ((flag & CL_MAKE_SHARED) && !mnt->mnt_group_id) {
                        int err = mnt_alloc_group_id(mnt);
                        if (err)
                                goto out_free;
                }

                mnt->mnt.mnt_flags = old->mnt.mnt_flags & ~MNT_WRITE_HOLD;
                atomic_inc(&sb->s_active);
                mnt->mnt.mnt_sb = sb;
                mnt->mnt.mnt_root = dget(root);
                mnt->mnt_mountpoint = mnt->mnt.mnt_root;
                mnt->mnt_parent = mnt;
                br_write_lock(&vfsmount_lock);
                list_add_tail(&mnt->mnt_instance, &sb->s_mounts);
                br_write_unlock(&vfsmount_lock);

                if (flag & CL_SLAVE) {
                        list_add(&mnt->mnt_slave, &old->mnt_slave_list);
                        mnt->mnt_master = old;
                        CLEAR_MNT_SHARED(mnt);
                } else if (!(flag & CL_PRIVATE)) {
                        if ((flag & CL_MAKE_SHARED) || IS_MNT_SHARED(old))
                                list_add(&mnt->mnt_share, &old->mnt_share);
                        if (IS_MNT_SLAVE(old))
                                list_add(&mnt->mnt_slave, &old->mnt_slave);
                        mnt->mnt_master = old->mnt_master;
                }
                if (flag & CL_MAKE_SHARED)
                        set_mnt_shared(mnt);

                /* stick the duplicate mount on the same expiry list
                 * as the original if that was on one */
                if (flag & CL_EXPIRE) {
                        if (!list_empty(&old->mnt_expire))
                                list_add(&mnt->mnt_expire, &old->mnt_expire);
                }
        }
        return mnt;

 out_free:
        free_vfsmnt(mnt);
        return NULL;
}

static inline void mntfree(struct mount *mnt)
{
        struct vfsmount *m = &mnt->mnt;
        struct super_block *sb = m->mnt_sb;

        /*
         * This probably indicates that somebody messed
         * up a mnt_want/drop_write() pair.  If this
         * happens, the filesystem was probably unable
         * to make r/w->r/o transitions.
         */
        /*
         * The locking used to deal with mnt_count decrement provides barriers,
         * so mnt_get_writers() below is safe.
         */
        WARN_ON(mnt_get_writers(mnt));
        fsnotify_vfsmount_delete(m);
        dput(m->mnt_root);
        free_vfsmnt(mnt);
        deactivate_super(sb);
}

static void mntput_no_expire(struct mount *mnt)
{
put_again:
#ifdef CONFIG_SMP
        br_read_lock(&vfsmount_lock);
        if (likely(atomic_read(&mnt->mnt_longterm))) {
                mnt_add_count(mnt, -1);
                br_read_unlock(&vfsmount_lock);
                return;
        }
        br_read_unlock(&vfsmount_lock);

        br_write_lock(&vfsmount_lock);
        mnt_add_count(mnt, -1);
        if (mnt_get_count(mnt)) {
                br_write_unlock(&vfsmount_lock);
                return;
        }
#else
        mnt_add_count(mnt, -1);
        if (likely(mnt_get_count(mnt)))
                return;
        br_write_lock(&vfsmount_lock);
#endif
        if (unlikely(mnt->mnt_pinned)) {
                mnt_add_count(mnt, mnt->mnt_pinned + 1);
                mnt->mnt_pinned = 0;
                br_write_unlock(&vfsmount_lock);
                acct_auto_close_mnt(&mnt->mnt);
                goto put_again;
        }

        list_del(&mnt->mnt_instance);
        br_write_unlock(&vfsmount_lock);
        mntfree(mnt);
}

void mntput(struct vfsmount *mnt)
{
        if (mnt) {
                struct mount *m = real_mount(mnt);
                /* avoid cacheline pingpong, hope gcc doesn't get "smart" */
                if (unlikely(m->mnt_expiry_mark))
                        m->mnt_expiry_mark = 0;
                mntput_no_expire(m);
        }
}
EXPORT_SYMBOL(mntput);

struct vfsmount *mntget(struct vfsmount *mnt)
{
        if (mnt)
                mnt_add_count(real_mount(mnt), 1);
        return mnt;
}
EXPORT_SYMBOL(mntget);

void mnt_pin(struct vfsmount *mnt)
{
        br_write_lock(&vfsmount_lock);
        real_mount(mnt)->mnt_pinned++;
        br_write_unlock(&vfsmount_lock);
}
EXPORT_SYMBOL(mnt_pin);

void mnt_unpin(struct vfsmount *m)
{
        struct mount *mnt = real_mount(m);
        br_write_lock(&vfsmount_lock);
        if (mnt->mnt_pinned) {
                mnt_add_count(mnt, 1);
                mnt->mnt_pinned--;
        }
        br_write_unlock(&vfsmount_lock);
}
EXPORT_SYMBOL(mnt_unpin);

static inline void mangle(struct seq_file *m, const char *s)
{
        seq_escape(m, s, " \t\n\\");
}

/*
 * Simple .show_options callback for filesystems which don't want to
 * implement more complex mount option showing.
 *
 * See also save_mount_options().
 */
int generic_show_options(struct seq_file *m, struct dentry *root)
{
        const char *options;

        rcu_read_lock();
        options = rcu_dereference(root->d_sb->s_options);

        if (options != NULL && options[0]) {
                seq_putc(m, ',');
                mangle(m, options);
        }
        rcu_read_unlock();

        return 0;
}
EXPORT_SYMBOL(generic_show_options);

/*
 * If filesystem uses generic_show_options(), this function should be
 * called from the fill_super() callback.
 *
 * The .remount_fs callback usually needs to be handled in a special
 * way, to make sure, that previous options are not overwritten if the
 * remount fails.
 *
 * Also note, that if the filesystem's .remount_fs function doesn't
 * reset all options to their default value, but changes only newly
 * given options, then the displayed options will not reflect reality
 * any more.
 */
void save_mount_options(struct super_block *sb, char *options)
{
        BUG_ON(sb->s_options);
        rcu_assign_pointer(sb->s_options, kstrdup(options, GFP_KERNEL));
}
EXPORT_SYMBOL(save_mount_options);

void replace_mount_options(struct super_block *sb, char *options)
{
        char *old = sb->s_options;
        rcu_assign_pointer(sb->s_options, options);
        if (old) {
                synchronize_rcu();
                kfree(old);
        }
}
EXPORT_SYMBOL(replace_mount_options);

#ifdef CONFIG_PROC_FS
/* iterator; we want it to have access to namespace_sem, thus here... */
static void *m_start(struct seq_file *m, loff_t *pos)
{
        struct proc_mounts *p = container_of(m, struct proc_mounts, m);

        down_read(&namespace_sem);
        return seq_list_start(&p->ns->list, *pos);
}

static void *m_next(struct seq_file *m, void *v, loff_t *pos)
{
        struct proc_mounts *p = container_of(m, struct proc_mounts, m);

        return seq_list_next(v, &p->ns->list, pos);
}

static void m_stop(struct seq_file *m, void *v)
{
        up_read(&namespace_sem);
}

static int m_show(struct seq_file *m, void *v)
{
        struct proc_mounts *p = container_of(m, struct proc_mounts, m);
        struct mount *r = list_entry(v, struct mount, mnt_list);
        return p->show(m, &r->mnt);
}

const struct seq_operations mounts_op = {
        .start  = m_start,
        .next   = m_next,
        .stop   = m_stop,
        .show   = m_show,
};
#endif  /* CONFIG_PROC_FS */

/**
 * may_umount_tree - check if a mount tree is busy
 * @mnt: root of mount tree
 *
 * This is called to check if a tree of mounts has any
 * open files, pwds, chroots or sub mounts that are
 * busy.
 */
int may_umount_tree(struct vfsmount *m)
{
        struct mount *mnt = real_mount(m);
        int actual_refs = 0;
        int minimum_refs = 0;
        struct mount *p;
        BUG_ON(!m);

        /* write lock needed for mnt_get_count */
        br_write_lock(&vfsmount_lock);
        for (p = mnt; p; p = next_mnt(p, mnt)) {
                actual_refs += mnt_get_count(p);
                minimum_refs += 2;
        }
        br_write_unlock(&vfsmount_lock);

        if (actual_refs > minimum_refs)
                return 0;

        return 1;
}

EXPORT_SYMBOL(may_umount_tree);

/**
 * may_umount - check if a mount point is busy
 * @mnt: root of mount
 *
 * This is called to check if a mount point has any
 * open files, pwds, chroots or sub mounts. If the
 * mount has sub mounts this will return busy
 * regardless of whether the sub mounts are busy.
 *
 * Doesn't take quota and stuff into account. IOW, in some cases it will
 * give false negatives. The main reason why it's here is that we need
 * a non-destructive way to look for easily umountable filesystems.
 */
int may_umount(struct vfsmount *mnt)
{
        int ret = 1;
        down_read(&namespace_sem);
        br_write_lock(&vfsmount_lock);
        if (propagate_mount_busy(real_mount(mnt), 2))
                ret = 0;
        br_write_unlock(&vfsmount_lock);
        up_read(&namespace_sem);
        return ret;
}

EXPORT_SYMBOL(may_umount);

void release_mounts(struct list_head *head)
{
        struct mount *mnt;
        while (!list_empty(head)) {
                mnt = list_first_entry(head, struct mount, mnt_hash);
                list_del_init(&mnt->mnt_hash);
                if (mnt_has_parent(mnt)) {
                        struct dentry *dentry;
                        struct mount *m;

                        br_write_lock(&vfsmount_lock);
                        dentry = mnt->mnt_mountpoint;
                        m = mnt->mnt_parent;
                        mnt->mnt_mountpoint = mnt->mnt.mnt_root;
                        mnt->mnt_parent = mnt;
                        m->mnt_ghosts--;
                        br_write_unlock(&vfsmount_lock);
                        dput(dentry);
                        mntput(&m->mnt);
                }
                mntput(&mnt->mnt);
        }
}

/*
 * vfsmount lock must be held for write
 * namespace_sem must be held for write
 */
void umount_tree(struct mount *mnt, int propagate, struct list_head *kill)
{
        LIST_HEAD(tmp_list);
        struct mount *p;

        for (p = mnt; p; p = next_mnt(p, mnt))
                list_move(&p->mnt_hash, &tmp_list);

        if (propagate)
                propagate_umount(&tmp_list);

        list_for_each_entry(p, &tmp_list, mnt_hash) {
                list_del_init(&p->mnt_expire);
                list_del_init(&p->mnt_list);
                __touch_mnt_namespace(p->mnt_ns);
                p->mnt_ns = NULL;
                __mnt_make_shortterm(p);
                list_del_init(&p->mnt_child);
                if (mnt_has_parent(p)) {
                        p->mnt_parent->mnt_ghosts++;
                        dentry_reset_mounted(p->mnt_mountpoint);
                }
                change_mnt_propagation(p, MS_PRIVATE);
        }
        list_splice(&tmp_list, kill);
}

static void shrink_submounts(struct mount *mnt, struct list_head *umounts);

static int do_umount(struct mount *mnt, int flags)
{
        struct super_block *sb = mnt->mnt.mnt_sb;
        int retval;
        LIST_HEAD(umount_list);

        retval = security_sb_umount(&mnt->mnt, flags);
        if (retval)
                return retval;

        /*
         * Allow userspace to request a mountpoint be expired rather than
         * unmounting unconditionally. Unmount only happens if:
         *  (1) the mark is already set (the mark is cleared by mntput())
         *  (2) the usage count == 1 [parent vfsmount] + 1 [sys_umount]
         */
        if (flags & MNT_EXPIRE) {
                if (&mnt->mnt == current->fs->root.mnt ||
                    flags & (MNT_FORCE | MNT_DETACH))
                        return -EINVAL;

                /*
                 * probably don't strictly need the lock here if we examined
                 * all race cases, but it's a slowpath.
                 */
                br_write_lock(&vfsmount_lock);
                if (mnt_get_count(mnt) != 2) {
                        br_write_unlock(&vfsmount_lock);
                        return -EBUSY;
                }
                br_write_unlock(&vfsmount_lock);

                if (!xchg(&mnt->mnt_expiry_mark, 1))
                        return -EAGAIN;
        }

        /*
         * If we may have to abort operations to get out of this
         * mount, and they will themselves hold resources we must
         * allow the fs to do things. In the Unix tradition of
         * 'Gee thats tricky lets do it in userspace' the umount_begin
         * might fail to complete on the first run through as other tasks
         * must return, and the like. Thats for the mount program to worry
         * about for the moment.
         */

        if (flags & MNT_FORCE && sb->s_op->umount_begin) {
                sb->s_op->umount_begin(sb);
        }

        /*
         * No sense to grab the lock for this test, but test itself looks
         * somewhat bogus. Suggestions for better replacement?
         * Ho-hum... In principle, we might treat that as umount + switch
         * to rootfs. GC would eventually take care of the old vfsmount.
         * Actually it makes sense, especially if rootfs would contain a
         * /reboot - static binary that would close all descriptors and
         * call reboot(9). Then init(8) could umount root and exec /reboot.
         */
        if (&mnt->mnt == current->fs->root.mnt && !(flags & MNT_DETACH)) {
                /*
                 * Special case for "unmounting" root ...
                 * we just try to remount it readonly.
                 */
                down_write(&sb->s_umount);
                if (!(sb->s_flags & MS_RDONLY))
                        retval = do_remount_sb(sb, MS_RDONLY, NULL, 0);
                up_write(&sb->s_umount);
                return retval;
        }

        down_write(&namespace_sem);
        br_write_lock(&vfsmount_lock);
        event++;

        if (!(flags & MNT_DETACH))
                shrink_submounts(mnt, &umount_list);

        retval = -EBUSY;
        if (flags & MNT_DETACH || !propagate_mount_busy(mnt, 2)) {
                if (!list_empty(&mnt->mnt_list))
                        umount_tree(mnt, 1, &umount_list);
                retval = 0;
        }
        br_write_unlock(&vfsmount_lock);
        up_write(&namespace_sem);
        release_mounts(&umount_list);
        return retval;
}

/*
 * Now umount can handle mount points as well as block devices.
 * This is important for filesystems which use unnamed block devices.
 *
 * We now support a flag for forced unmount like the other 'big iron'
 * unixes. Our API is identical to OSF/1 to avoid making a mess of AMD
 */

SYSCALL_DEFINE2(umount, char __user *, name, int, flags)
{
        struct path path;
        struct mount *mnt;
        int retval;
        int lookup_flags = 0;

        if (flags & ~(MNT_FORCE | MNT_DETACH | MNT_EXPIRE | UMOUNT_NOFOLLOW))
                return -EINVAL;

        if (!(flags & UMOUNT_NOFOLLOW))
                lookup_flags |= LOOKUP_FOLLOW;

        retval = user_path_at(AT_FDCWD, name, lookup_flags, &path);
        if (retval)
                goto out;
        mnt = real_mount(path.mnt);
        retval = -EINVAL;
        if (path.dentry != path.mnt->mnt_root)
                goto dput_and_out;
        if (!check_mnt(mnt))
                goto dput_and_out;

        retval = -EPERM;
        if (!capable(CAP_SYS_ADMIN))
                goto dput_and_out;

        retval = do_umount(mnt, flags);
dput_and_out:
        /* we mustn't call path_put() as that would clear mnt_expiry_mark */
        dput(path.dentry);
        mntput_no_expire(mnt);
out:
        return retval;
}

#ifdef __ARCH_WANT_SYS_OLDUMOUNT

/*
 *      The 2.0 compatible umount. No flags.
 */
SYSCALL_DEFINE1(oldumount, char __user *, name)
{
        return sys_umount(name, 0);
}

#endif

static int mount_is_safe(struct path *path)
{
        if (capable(CAP_SYS_ADMIN))
                return 0;
        return -EPERM;
#ifdef notyet
        if (S_ISLNK(path->dentry->d_inode->i_mode))
                return -EPERM;
        if (path->dentry->d_inode->i_mode & S_ISVTX) {
                if (current_uid() != path->dentry->d_inode->i_uid)
                        return -EPERM;
        }
        if (inode_permission(path->dentry->d_inode, MAY_WRITE))
                return -EPERM;
        return 0;
#endif
}

struct mount *copy_tree(struct mount *mnt, struct dentry *dentry,
                                        int flag)
{
        struct mount *res, *p, *q, *r;
        struct path path;

        if (!(flag & CL_COPY_ALL) && IS_MNT_UNBINDABLE(mnt))
                return NULL;

        res = q = clone_mnt(mnt, dentry, flag);
        if (!q)
                goto Enomem;
        q->mnt_mountpoint = mnt->mnt_mountpoint;

        p = mnt;
        list_for_each_entry(r, &mnt->mnt_mounts, mnt_child) {
                struct mount *s;
                if (!is_subdir(r->mnt_mountpoint, dentry))
                        continue;

                for (s = r; s; s = next_mnt(s, r)) {
                        if (!(flag & CL_COPY_ALL) && IS_MNT_UNBINDABLE(s)) {
                                s = skip_mnt_tree(s);
                                continue;
                        }
                        while (p != s->mnt_parent) {
                                p = p->mnt_parent;
                                q = q->mnt_parent;
                        }
                        p = s;
                        path.mnt = &q->mnt;
                        path.dentry = p->mnt_mountpoint;
                        q = clone_mnt(p, p->mnt.mnt_root, flag);
                        if (!q)
                                goto Enomem;
                        br_write_lock(&vfsmount_lock);
                        list_add_tail(&q->mnt_list, &res->mnt_list);
                        attach_mnt(q, &path);
                        br_write_unlock(&vfsmount_lock);
                }
        }
        return res;
Enomem:
        if (res) {
                LIST_HEAD(umount_list);
                br_write_lock(&vfsmount_lock);
                umount_tree(res, 0, &umount_list);
                br_write_unlock(&vfsmount_lock);
                release_mounts(&umount_list);
        }
        return NULL;
}

struct vfsmount *collect_mounts(struct path *path)
{
        struct mount *tree;
        down_write(&namespace_sem);
        tree = copy_tree(real_mount(path->mnt), path->dentry,
                         CL_COPY_ALL | CL_PRIVATE);
        up_write(&namespace_sem);
        return tree ? &tree->mnt : NULL;
}

void drop_collected_mounts(struct vfsmount *mnt)
{
        LIST_HEAD(umount_list);
        down_write(&namespace_sem);
        br_write_lock(&vfsmount_lock);
        umount_tree(real_mount(mnt), 0, &umount_list);
        br_write_unlock(&vfsmount_lock);
        up_write(&namespace_sem);
        release_mounts(&umount_list);
}

int iterate_mounts(int (*f)(struct vfsmount *, void *), void *arg,
                   struct vfsmount *root)
{
        struct mount *mnt;
        int res = f(root, arg);
        if (res)
                return res;
        list_for_each_entry(mnt, &real_mount(root)->mnt_list, mnt_list) {
                res = f(&mnt->mnt, arg);
                if (res)
                        return res;
        }
        return 0;
}

static void cleanup_group_ids(struct mount *mnt, struct mount *end)
{
        struct mount *p;

        for (p = mnt; p != end; p = next_mnt(p, mnt)) {
                if (p->mnt_group_id && !IS_MNT_SHARED(p))
                        mnt_release_group_id(p);
        }
}

static int invent_group_ids(struct mount *mnt, bool recurse)
{
        struct mount *p;

        for (p = mnt; p; p = recurse ? next_mnt(p, mnt) : NULL) {
                if (!p->mnt_group_id && !IS_MNT_SHARED(p)) {
                        int err = mnt_alloc_group_id(p);
                        if (err) {
                                cleanup_group_ids(mnt, p);
                                return err;
                        }
                }
        }

        return 0;
}

/*
 *  @source_mnt : mount tree to be attached
 *  @nd         : place the mount tree @source_mnt is attached
 *  @parent_nd  : if non-null, detach the source_mnt from its parent and
 *                 store the parent mount and mountpoint dentry.
 *                 (done when source_mnt is moved)
 *
 *  NOTE: in the table below explains the semantics when a source mount
 *  of a given type is attached to a destination mount of a given type.
 * ---------------------------------------------------------------------------
 * |         BIND MOUNT OPERATION                                            |
 * |**************************************************************************
 * | source-->| shared        |       private  |       slave    | unbindable |
 * | dest     |               |                |                |            |
 * |   |      |               |                |                |            |
 * |   v      |               |                |                |            |
 * |**************************************************************************
 * |  shared  | shared (++)   |     shared (+) |     shared(+++)|  invalid   |
 * |          |               |                |                |            |
 * |non-shared| shared (+)    |      private   |      slave (*) |  invalid   |
 * ***************************************************************************
 * A bind operation clones the source mount and mounts the clone on the
 * destination mount.
 *
 * (++)  the cloned mount is propagated to all the mounts in the propagation
 *       tree of the destination mount and the cloned mount is added to
 *       the peer group of the source mount.
 * (+)   the cloned mount is created under the destination mount and is marked
 *       as shared. The cloned mount is added to the peer group of the source
 *       mount.
 * (+++) the mount is propagated to all the mounts in the propagation tree
 *       of the destination mount and the cloned mount is made slave
 *       of the same master as that of the source mount. The cloned mount
 *       is marked as 'shared and slave'.
 * (*)   the cloned mount is made a slave of the same master as that of the
 *       source mount.
 *
 * ---------------------------------------------------------------------------
 * |                    MOVE MOUNT OPERATION                                 |
 * |**************************************************************************
 * | source-->| shared        |       private  |       slave    | unbindable |
 * | dest     |               |                |                |            |
 * |   |      |               |                |                |            |
 * |   v      |               |                |                |            |
 * |**************************************************************************
 * |  shared  | shared (+)    |     shared (+) |    shared(+++) |  invalid   |
 * |          |               |                |                |            |
 * |non-shared| shared (+*)   |      private   |    slave (*)   | unbindable |
 * ***************************************************************************
 *
 * (+)  the mount is moved to the destination. And is then propagated to
 *      all the mounts in the propagation tree of the destination mount.
 * (+*)  the mount is moved to the destination.
 * (+++)  the mount is moved to the destination and is then propagated to
 *      all the mounts belonging to the destination mount's propagation tree.
 *      the mount is marked as 'shared and slave'.
 * (*)  the mount continues to be a slave at the new location.
 *
 * if the source mount is a tree, the operations explained above is
 * applied to each mount in the tree.
 * Must be called without spinlocks held, since this function can sleep
 * in allocations.
 */
static int attach_recursive_mnt(struct mount *source_mnt,
                        struct path *path, struct path *parent_path)
{
        LIST_HEAD(tree_list);
        struct mount *dest_mnt = real_mount(path->mnt);
        struct dentry *dest_dentry = path->dentry;
        struct mount *child, *p;
        int err;

        if (IS_MNT_SHARED(dest_mnt)) {
                err = invent_group_ids(source_mnt, true);
                if (err)
                        goto out;
        }
        err = propagate_mnt(dest_mnt, dest_dentry, source_mnt, &tree_list);
        if (err)
                goto out_cleanup_ids;

        br_write_lock(&vfsmount_lock);

        if (IS_MNT_SHARED(dest_mnt)) {
                for (p = source_mnt; p; p = next_mnt(p, source_mnt))
                        set_mnt_shared(p);
        }
        if (parent_path) {
                detach_mnt(source_mnt, parent_path);
                attach_mnt(source_mnt, path);
                touch_mnt_namespace(source_mnt->mnt_ns);
        } else {
                mnt_set_mountpoint(dest_mnt, dest_dentry, source_mnt);
                commit_tree(source_mnt);
        }

        list_for_each_entry_safe(child, p, &tree_list, mnt_hash) {
                list_del_init(&child->mnt_hash);
                commit_tree(child);
        }
        br_write_unlock(&vfsmount_lock);

        return 0;

 out_cleanup_ids:
        if (IS_MNT_SHARED(dest_mnt))
                cleanup_group_ids(source_mnt, NULL);
 out:
        return err;
}

static int lock_mount(struct path *path)
{
        struct vfsmount *mnt;
retry:
        mutex_lock(&path->dentry->d_inode->i_mutex);
        if (unlikely(cant_mount(path->dentry))) {
                mutex_unlock(&path->dentry->d_inode->i_mutex);
                return -ENOENT;
        }
        down_write(&namespace_sem);
        mnt = lookup_mnt(path);
        if (likely(!mnt))
                return 0;
        up_write(&namespace_sem);
        mutex_unlock(&path->dentry->d_inode->i_mutex);
        path_put(path);
        path->mnt = mnt;
        path->dentry = dget(mnt->mnt_root);
        goto retry;
}

static void unlock_mount(struct path *path)
{
        up_write(&namespace_sem);
        mutex_unlock(&path->dentry->d_inode->i_mutex);
}

static int graft_tree(struct mount *mnt, struct path *path)
{
        if (mnt->mnt.mnt_sb->s_flags & MS_NOUSER)
                return -EINVAL;

        if (S_ISDIR(path->dentry->d_inode->i_mode) !=
              S_ISDIR(mnt->mnt.mnt_root->d_inode->i_mode))
                return -ENOTDIR;

        if (d_unlinked(path->dentry))
                return -ENOENT;

        return attach_recursive_mnt(mnt, path, NULL);
}

/*
 * Sanity check the flags to change_mnt_propagation.
 */

static int flags_to_propagation_type(int flags)
{
        int type = flags & ~(MS_REC | MS_SILENT);

        /* Fail if any non-propagation flags are set */
        if (type & ~(MS_SHARED | MS_PRIVATE | MS_SLAVE | MS_UNBINDABLE))
                return 0;
        /* Only one propagation flag should be set */
        if (!is_power_of_2(type))
                return 0;
        return type;
}

/*
 * recursively change the type of the mountpoint.
 */
static int do_change_type(struct path *path, int flag)
{
        struct mount *m;
        struct mount *mnt = real_mount(path->mnt);
        int recurse = flag & MS_REC;
        int type;
        int err = 0;

        if (!capable(CAP_SYS_ADMIN))
                return -EPERM;

        if (path->dentry != path->mnt->mnt_root)
                return -EINVAL;

        type = flags_to_propagation_type(flag);
        if (!type)
                return -EINVAL;

        down_write(&namespace_sem);
        if (type == MS_SHARED) {
                err = invent_group_ids(mnt, recurse);
                if (err)
                        goto out_unlock;
        }

        br_write_lock(&vfsmount_lock);
        for (m = mnt; m; m = (recurse ? next_mnt(m, mnt) : NULL))
                change_mnt_propagation(m, type);
        br_write_unlock(&vfsmount_lock);

 out_unlock:
        up_write(&namespace_sem);
        return err;
}

/*
 * do loopback mount.
 */
static int do_loopback(struct path *path, char *old_name,
                                int recurse)
{
        LIST_HEAD(umount_list);
        struct path old_path;
        struct mount *mnt = NULL, *old;
        int err = mount_is_safe(path);
        if (err)
                return err;
        if (!old_name || !*old_name)
                return -EINVAL;
        err = kern_path(old_name, LOOKUP_FOLLOW|LOOKUP_AUTOMOUNT, &old_path);
        if (err)
                return err;

        err = lock_mount(path);
        if (err)
                goto out;

        old = real_mount(old_path.mnt);

        err = -EINVAL;
        if (IS_MNT_UNBINDABLE(old))
                goto out2;

        if (!check_mnt(real_mount(path->mnt)) || !check_mnt(old))
                goto out2;

        err = -ENOMEM;
        if (recurse)
                mnt = copy_tree(old, old_path.dentry, 0);
        else
                mnt = clone_mnt(old, old_path.dentry, 0);

        if (!mnt)
                goto out2;

        err = graft_tree(mnt, path);
        if (err) {
                br_write_lock(&vfsmount_lock);
                umount_tree(mnt, 0, &umount_list);
                br_write_unlock(&vfsmount_lock);
        }
out2:
        unlock_mount(path);
        release_mounts(&umount_list);
out:
        path_put(&old_path);
        return err;
}

static int change_mount_flags(struct vfsmount *mnt, int ms_flags)
{
        int error = 0;
        int readonly_request = 0;

        if (ms_flags & MS_RDONLY)
                readonly_request = 1;
        if (readonly_request == __mnt_is_readonly(mnt))
                return 0;

        if (readonly_request)
                error = mnt_make_readonly(real_mount(mnt));
        else
                __mnt_unmake_readonly(real_mount(mnt));
        return error;
}

/*
 * change filesystem flags. dir should be a physical root of filesystem.
 * If you've mounted a non-root directory somewhere and want to do remount
 * on it - tough luck.
 */
static int do_remount(struct path *path, int flags, int mnt_flags,
                      void *data)
{
        int err;
        struct super_block *sb = path->mnt->mnt_sb;
        struct mount *mnt = real_mount(path->mnt);

        if (!capable(CAP_SYS_ADMIN))
                return -EPERM;

        if (!check_mnt(mnt))
                return -EINVAL;

        if (path->dentry != path->mnt->mnt_root)
                return -EINVAL;

        err = security_sb_remount(sb, data);
        if (err)
                return err;

        down_write(&sb->s_umount);
        if (flags & MS_BIND)
                err = change_mount_flags(path->mnt, flags);
        else
                err = do_remount_sb(sb, flags, data, 0);
        if (!err) {
                br_write_lock(&vfsmount_lock);
                mnt_flags |= mnt->mnt.mnt_flags & MNT_PROPAGATION_MASK;
                mnt->mnt.mnt_flags = mnt_flags;
                br_write_unlock(&vfsmount_lock);
        }
        up_write(&sb->s_umount);
        if (!err) {
                br_write_lock(&vfsmount_lock);
                touch_mnt_namespace(mnt->mnt_ns);
                br_write_unlock(&vfsmount_lock);
        }
        return err;
}

static inline int tree_contains_unbindable(struct mount *mnt)
{
        struct mount *p;
        for (p = mnt; p; p = next_mnt(p, mnt)) {
                if (IS_MNT_UNBINDABLE(p))
                        return 1;
        }
        return 0;
}

static int do_move_mount(struct path *path, char *old_name)
{
        struct path old_path, parent_path;
        struct mount *p;
        struct mount *old;
        int err = 0;
        if (!capable(CAP_SYS_ADMIN))
                return -EPERM;
        if (!old_name || !*old_name)
                return -EINVAL;
        err = kern_path(old_name, LOOKUP_FOLLOW, &old_path);
        if (err)
                return err;

        err = lock_mount(path);
        if (err < 0)
                goto out;

        old = real_mount(old_path.mnt);
        p = real_mount(path->mnt);

        err = -EINVAL;
        if (!check_mnt(p) || !check_mnt(old))
                goto out1;

        if (d_unlinked(path->dentry))
                goto out1;

        err = -EINVAL;
        if (old_path.dentry != old_path.mnt->mnt_root)
                goto out1;

        if (!mnt_has_parent(old))
                goto out1;

        if (S_ISDIR(path->dentry->d_inode->i_mode) !=
              S_ISDIR(old_path.dentry->d_inode->i_mode))
                goto out1;
        /*
         * Don't move a mount residing in a shared parent.
         */
        if (IS_MNT_SHARED(old->mnt_parent))
                goto out1;
        /*
         * Don't move a mount tree containing unbindable mounts to a destination
         * mount which is shared.
         */
        if (IS_MNT_SHARED(p) && tree_contains_unbindable(old))
                goto out1;
        err = -ELOOP;
        for (; mnt_has_parent(p); p = p->mnt_parent)
                if (p == old)
                        goto out1;

        err = attach_recursive_mnt(old, path, &parent_path);
        if (err)
                goto out1;

        /* if the mount is moved, it should no longer be expire
         * automatically */
        list_del_init(&old->mnt_expire);
out1:
        unlock_mount(path);
out:
        if (!err)
                path_put(&parent_path);
        path_put(&old_path);
        return err;
}

static struct vfsmount *fs_set_subtype(struct vfsmount *mnt, const char *fstype)
{
        int err;
        const char *subtype = strchr(fstype, '.');
        if (subtype) {
                subtype++;
                err = -EINVAL;
                if (!subtype[0])
                        goto err;
        } else
                subtype = "";

        mnt->mnt_sb->s_subtype = kstrdup(subtype, GFP_KERNEL);
        err = -ENOMEM;
        if (!mnt->mnt_sb->s_subtype)
                goto err;
        return mnt;

 err:
        mntput(mnt);
        return ERR_PTR(err);
}

static struct vfsmount *
do_kern_mount(const char *fstype, int flags, const char *name, void *data)
{
        struct file_system_type *type = get_fs_type(fstype);
        struct vfsmount *mnt;
        if (!type)
                return ERR_PTR(-ENODEV);
        mnt = vfs_kern_mount(type, flags, name, data);
        if (!IS_ERR(mnt) && (type->fs_flags & FS_HAS_SUBTYPE) &&
            !mnt->mnt_sb->s_subtype)
                mnt = fs_set_subtype(mnt, fstype);
        put_filesystem(type);
        return mnt;
}

/*
 * add a mount into a namespace's mount tree
 */
static int do_add_mount(struct mount *newmnt, struct path *path, int mnt_flags)
{
        int err;

        mnt_flags &= ~(MNT_SHARED | MNT_WRITE_HOLD | MNT_INTERNAL);

        err = lock_mount(path);
        if (err)
                return err;

        err = -EINVAL;
        if (!(mnt_flags & MNT_SHRINKABLE) && !check_mnt(real_mount(path->mnt)))
                goto unlock;

        /* Refuse the same filesystem on the same mount point */
        err = -EBUSY;
        if (path->mnt->mnt_sb == newmnt->mnt.mnt_sb &&
            path->mnt->mnt_root == path->dentry)
                goto unlock;

        err = -EINVAL;
        if (S_ISLNK(newmnt->mnt.mnt_root->d_inode->i_mode))
                goto unlock;

        newmnt->mnt.mnt_flags = mnt_flags;
        err = graft_tree(newmnt, path);

unlock:
        unlock_mount(path);
        return err;
}

/*
 * create a new mount for userspace and request it to be added into the
 * namespace's tree
 */
static int do_new_mount(struct path *path, char *type, int flags,
                        int mnt_flags, char *name, void *data)
{
        struct vfsmount *mnt;
        int err;

        if (!type)
                return -EINVAL;

        /* we need capabilities... */
        if (!capable(CAP_SYS_ADMIN))
                return -EPERM;

        mnt = do_kern_mount(type, flags, name, data);
        if (IS_ERR(mnt))
                return PTR_ERR(mnt);

        err = do_add_mount(real_mount(mnt), path, mnt_flags);
        if (err)
                mntput(mnt);
        return err;
}

int finish_automount(struct vfsmount *m, struct path *path)
{
        struct mount *mnt = real_mount(m);
        int err;
        /* The new mount record should have at least 2 refs to prevent it being
         * expired before we get a chance to add it
         */
        BUG_ON(mnt_get_count(mnt) < 2);

        if (m->mnt_sb == path->mnt->mnt_sb &&
            m->mnt_root == path->dentry) {
                err = -ELOOP;
                goto fail;
        }

        err = do_add_mount(mnt, path, path->mnt->mnt_flags | MNT_SHRINKABLE);
        if (!err)
                return 0;
fail:
        /* remove m from any expiration list it may be on */
        if (!list_empty(&mnt->mnt_expire)) {
                down_write(&namespace_sem);
                br_write_lock(&vfsmount_lock);
                list_del_init(&mnt->mnt_expire);
                br_write_unlock(&vfsmount_lock);
                up_write(&namespace_sem);
        }
        mntput(m);
        mntput(m);
        return err;
}

/**
 * mnt_set_expiry - Put a mount on an expiration list
 * @mnt: The mount to list.
 * @expiry_list: The list to add the mount to.
 */
void mnt_set_expiry(struct vfsmount *mnt, struct list_head *expiry_list)
{
        down_write(&namespace_sem);
        br_write_lock(&vfsmount_lock);

        list_add_tail(&real_mount(mnt)->mnt_expire, expiry_list);

        br_write_unlock(&vfsmount_lock);
        up_write(&namespace_sem);
}
EXPORT_SYMBOL(mnt_set_expiry);

/*
 * process a list of expirable mountpoints with the intent of discarding any
 * mountpoints that aren't in use and haven't been touched since last we came
 * here
 */
void mark_mounts_for_expiry(struct list_head *mounts)
{
        struct mount *mnt, *next;
        LIST_HEAD(graveyard);
        LIST_HEAD(umounts);

        if (list_empty(mounts))
                return;

        down_write(&namespace_sem);
        br_write_lock(&vfsmount_lock);

        /* extract from the expiration list every vfsmount that matches the
         * following criteria:
         * - only referenced by its parent vfsmount
         * - still marked for expiry (marked on the last call here; marks are
         *   cleared by mntput())
         */
        list_for_each_entry_safe(mnt, next, mounts, mnt_expire) {
                if (!xchg(&mnt->mnt_expiry_mark, 1) ||
                        propagate_mount_busy(mnt, 1))
                        continue;
                list_move(&mnt->mnt_expire, &graveyard);
        }
        while (!list_empty(&graveyard)) {
                mnt = list_first_entry(&graveyard, struct mount, mnt_expire);
                touch_mnt_namespace(mnt->mnt_ns);
                umount_tree(mnt, 1, &umounts);
        }
        br_write_unlock(&vfsmount_lock);
        up_write(&namespace_sem);

        release_mounts(&umounts);
}

EXPORT_SYMBOL_GPL(mark_mounts_for_expiry);

/*
 * Ripoff of 'select_parent()'
 *
 * search the list of submounts for a given mountpoint, and move any
 * shrinkable submounts to the 'graveyard' list.
 */
static int select_submounts(struct mount *parent, struct list_head *graveyard)
{
        struct mount *this_parent = parent;
        struct list_head *next;
        int found = 0;

repeat:
        next = this_parent->mnt_mounts.next;
resume:
        while (next != &this_parent->mnt_mounts) {
                struct list_head *tmp = next;
                struct mount *mnt = list_entry(tmp, struct mount, mnt_child);

                next = tmp->next;
                if (!(mnt->mnt.mnt_flags & MNT_SHRINKABLE))
                        continue;
                /*
                 * Descend a level if the d_mounts list is non-empty.
                 */
                if (!list_empty(&mnt->mnt_mounts)) {
                        this_parent = mnt;
                        goto repeat;
                }

                if (!propagate_mount_busy(mnt, 1)) {
                        list_move_tail(&mnt->mnt_expire, graveyard);
                        found++;
                }
        }
        /*
         * All done at this level ... ascend and resume the search
         */
        if (this_parent != parent) {
                next = this_parent->mnt_child.next;
                this_parent = this_parent->mnt_parent;
                goto resume;
        }
        return found;
}

/*
 * process a list of expirable mountpoints with the intent of discarding any
 * submounts of a specific parent mountpoint
 *
 * vfsmount_lock must be held for write
 */
static void shrink_submounts(struct mount *mnt, struct list_head *umounts)
{
        LIST_HEAD(graveyard);
        struct mount *m;

        /* extract submounts of 'mountpoint' from the expiration list */
        while (select_submounts(mnt, &graveyard)) {
                while (!list_empty(&graveyard)) {
                        m = list_first_entry(&graveyard, struct mount,
                                                mnt_expire);
                        touch_mnt_namespace(m->mnt_ns);
                        umount_tree(m, 1, umounts);
                }
        }
}

/*
 * Some copy_from_user() implementations do not return the exact number of
 * bytes remaining to copy on a fault.  But copy_mount_options() requires that.
 * Note that this function differs from copy_from_user() in that it will oops
 * on bad values of `to', rather than returning a short copy.
 */
static long exact_copy_from_user(void *to, const void __user * from,
                                 unsigned long n)
{
        char *t = to;
        const char __user *f = from;
        char c;

        if (!access_ok(VERIFY_READ, from, n))
                return n;

        while (n) {
                if (__get_user(c, f)) {
                        memset(t, 0, n);
                        break;
                }
                *t++ = c;
                f++;
                n--;
        }
        return n;
}

int copy_mount_options(const void __user * data, unsigned long *where)
{
        int i;
        unsigned long page;
        unsigned long size;

        *where = 0;
        if (!data)
                return 0;

        if (!(page = __get_free_page(GFP_KERNEL)))
                return -ENOMEM;

        /* We only care that *some* data at the address the user
         * gave us is valid.  Just in case, we'll zero
         * the remainder of the page.
         */
        /* copy_from_user cannot cross TASK_SIZE ! */
        size = TASK_SIZE - (unsigned long)data;
        if (size > PAGE_SIZE)
                size = PAGE_SIZE;

        i = size - exact_copy_from_user((void *)page, data, size);
        if (!i) {
                free_page(page);
                return -EFAULT;
        }
        if (i != PAGE_SIZE)
                memset((char *)page + i, 0, PAGE_SIZE - i);
        *where = page;
        return 0;
}

int copy_mount_string(const void __user *data, char **where)
{
        char *tmp;

        if (!data) {
                *where = NULL;
                return 0;
        }

        tmp = strndup_user(data, PAGE_SIZE);
        if (IS_ERR(tmp))
                return PTR_ERR(tmp);

        *where = tmp;
        return 0;
}

/*
 * Flags is a 32-bit value that allows up to 31 non-fs dependent flags to
 * be given to the mount() call (ie: read-only, no-dev, no-suid etc).
 *
 * data is a (void *) that can point to any structure up to
 * PAGE_SIZE-1 bytes, which can contain arbitrary fs-dependent
 * information (or be NULL).
 *
 * Pre-0.97 versions of mount() didn't have a flags word.
 * When the flags word was introduced its top half was required
 * to have the magic value 0xC0ED, and this remained so until 2.4.0-test9.
 * Therefore, if this magic number is present, it carries no information
 * and must be discarded.
 */
long do_mount(char *dev_name, char *dir_name, char *type_page,
                  unsigned long flags, void *data_page)
{
        struct path path;
        int retval = 0;
        int mnt_flags = 0;

        /* Discard magic */
        if ((flags & MS_MGC_MSK) == MS_MGC_VAL)
                flags &= ~MS_MGC_MSK;

        /* Basic sanity checks */

        if (!dir_name || !*dir_name || !memchr(dir_name, 0, PAGE_SIZE))
                return -EINVAL;

        if (data_page)
                ((char *)data_page)[PAGE_SIZE - 1] = 0;

        /* ... and get the mountpoint */
        retval = kern_path(dir_name, LOOKUP_FOLLOW, &path);
        if (retval)
                return retval;

        retval = security_sb_mount(dev_name, &path,
                                   type_page, flags, data_page);
        if (retval)
                goto dput_out;

        /* Default to relatime unless overriden */
        if (!(flags & MS_NOATIME))
                mnt_flags |= MNT_RELATIME;

        /* Separate the per-mountpoint flags */
        if (flags & MS_NOSUID)
                mnt_flags |= MNT_NOSUID;
        if (flags & MS_NODEV)
                mnt_flags |= MNT_NODEV;
        if (flags & MS_NOEXEC)
                mnt_flags |= MNT_NOEXEC;
        if (flags & MS_NOATIME)
                mnt_flags |= MNT_NOATIME;
        if (flags & MS_NODIRATIME)
                mnt_flags |= MNT_NODIRATIME;
        if (flags & MS_STRICTATIME)
                mnt_flags &= ~(MNT_RELATIME | MNT_NOATIME);
        if (flags & MS_RDONLY)
                mnt_flags |= MNT_READONLY;

        flags &= ~(MS_NOSUID | MS_NOEXEC | MS_NODEV | MS_ACTIVE | MS_BORN |
                   MS_NOATIME | MS_NODIRATIME | MS_RELATIME| MS_KERNMOUNT |
                   MS_STRICTATIME);

        if (flags & MS_REMOUNT)
                retval = do_remount(&path, flags & ~MS_REMOUNT, mnt_flags,
                                    data_page);
        else if (flags & MS_BIND)
                retval = do_loopback(&path, dev_name, flags & MS_REC);
        else if (flags & (MS_SHARED | MS_PRIVATE | MS_SLAVE | MS_UNBINDABLE))
                retval = do_change_type(&path, flags);
        else if (flags & MS_MOVE)
                retval = do_move_mount(&path, dev_name);
        else
                retval = do_new_mount(&path, type_page, flags, mnt_flags,
                                      dev_name, data_page);
dput_out:
        path_put(&path);
        return retval;
}

static struct mnt_namespace *alloc_mnt_ns(void)
{
        struct mnt_namespace *new_ns;

        new_ns = kmalloc(sizeof(struct mnt_namespace), GFP_KERNEL);
        if (!new_ns)
                return ERR_PTR(-ENOMEM);
        atomic_set(&new_ns->count, 1);
        new_ns->root = NULL;
        INIT_LIST_HEAD(&new_ns->list);
        init_waitqueue_head(&new_ns->poll);
        new_ns->event = 0;
        return new_ns;
}

void mnt_make_longterm(struct vfsmount *mnt)
{
        __mnt_make_longterm(real_mount(mnt));
}

void mnt_make_shortterm(struct vfsmount *m)
{
#ifdef CONFIG_SMP
        struct mount *mnt = real_mount(m);
        if (atomic_add_unless(&mnt->mnt_longterm, -1, 1))
                return;
        br_write_lock(&vfsmount_lock);
        atomic_dec(&mnt->mnt_longterm);
        br_write_unlock(&vfsmount_lock);
#endif
}

/*
 * Allocate a new namespace structure and populate it with contents
 * copied from the namespace of the passed in task structure.
 */
static struct mnt_namespace *dup_mnt_ns(struct mnt_namespace *mnt_ns,
                struct fs_struct *fs)
{
        struct mnt_namespace *new_ns;
        struct vfsmount *rootmnt = NULL, *pwdmnt = NULL;
        struct mount *p, *q;
        struct mount *old = mnt_ns->root;
        struct mount *new;

        new_ns = alloc_mnt_ns();
        if (IS_ERR(new_ns))
                return new_ns;

        down_write(&namespace_sem);
        /* First pass: copy the tree topology */
        new = copy_tree(old, old->mnt.mnt_root, CL_COPY_ALL | CL_EXPIRE);
        if (!new) {
                up_write(&namespace_sem);
                kfree(new_ns);
                return ERR_PTR(-ENOMEM);
        }
        new_ns->root = new;
        br_write_lock(&vfsmount_lock);
        list_add_tail(&new_ns->list, &new->mnt_list);
        br_write_unlock(&vfsmount_lock);

        /*
         * Second pass: switch the tsk->fs->* elements and mark new vfsmounts
         * as belonging to new namespace.  We have already acquired a private
         * fs_struct, so tsk->fs->lock is not needed.
         */
        p = old;
        q = new;
        while (p) {
                q->mnt_ns = new_ns;
                __mnt_make_longterm(q);
                if (fs) {
                        if (&p->mnt == fs->root.mnt) {
                                fs->root.mnt = mntget(&q->mnt);
                                __mnt_make_longterm(q);
                                mnt_make_shortterm(&p->mnt);
                                rootmnt = &p->mnt;
                        }
                        if (&p->mnt == fs->pwd.mnt) {
                                fs->pwd.mnt = mntget(&q->mnt);
                                __mnt_make_longterm(q);
                                mnt_make_shortterm(&p->mnt);
                                pwdmnt = &p->mnt;
                        }
                }
                p = next_mnt(p, old);
                q = next_mnt(q, new);
        }
        up_write(&namespace_sem);

        if (rootmnt)
                mntput(rootmnt);
        if (pwdmnt)
                mntput(pwdmnt);

        return new_ns;
}

struct mnt_namespace *copy_mnt_ns(unsigned long flags, struct mnt_namespace *ns,
                struct fs_struct *new_fs)
{
        struct mnt_namespace *new_ns;

        BUG_ON(!ns);
        get_mnt_ns(ns);

        if (!(flags & CLONE_NEWNS))
                return ns;

        new_ns = dup_mnt_ns(ns, new_fs);

        put_mnt_ns(ns);
        return new_ns;
}

/**
 * create_mnt_ns - creates a private namespace and adds a root filesystem
 * @mnt: pointer to the new root filesystem mountpoint
 */
static struct mnt_namespace *create_mnt_ns(struct vfsmount *m)
{
        struct mnt_namespace *new_ns = alloc_mnt_ns();
        if (!IS_ERR(new_ns)) {
                struct mount *mnt = real_mount(m);
                mnt->mnt_ns = new_ns;
                __mnt_make_longterm(mnt);
                new_ns->root = mnt;
                list_add(&new_ns->list, &mnt->mnt_list);
        } else {
                mntput(m);
        }
        return new_ns;
}

struct dentry *mount_subtree(struct vfsmount *mnt, const char *name)
{
        struct mnt_namespace *ns;
        struct super_block *s;
        struct path path;
        int err;

        ns = create_mnt_ns(mnt);
        if (IS_ERR(ns))
                return ERR_CAST(ns);

        err = vfs_path_lookup(mnt->mnt_root, mnt,
                        name, LOOKUP_FOLLOW|LOOKUP_AUTOMOUNT, &path);

        put_mnt_ns(ns);

        if (err)
                return ERR_PTR(err);

        /* trade a vfsmount reference for active sb one */
        s = path.mnt->mnt_sb;
        atomic_inc(&s->s_active);
        mntput(path.mnt);
        /* lock the sucker */
        down_write(&s->s_umount);
        /* ... and return the root of (sub)tree on it */
        return path.dentry;
}
EXPORT_SYMBOL(mount_subtree);

SYSCALL_DEFINE5(mount, char __user *, dev_name, char __user *, dir_name,
                char __user *, type, unsigned long, flags, void __user *, data)
{
        int ret;
        char *kernel_type;
        char *kernel_dir;
        char *kernel_dev;
        unsigned long data_page;

        ret = copy_mount_string(type, &kernel_type);
        if (ret < 0)
                goto out_type;

        kernel_dir = getname(dir_name);
        if (IS_ERR(kernel_dir)) {
                ret = PTR_ERR(kernel_dir);
                goto out_dir;
        }

        ret = copy_mount_string(dev_name, &kernel_dev);
        if (ret < 0)
                goto out_dev;

        ret = copy_mount_options(data, &data_page);
        if (ret < 0)
                goto out_data;

        ret = do_mount(kernel_dev, kernel_dir, kernel_type, flags,
                (void *) data_page);

        free_page(data_page);
out_data:
        kfree(kernel_dev);
out_dev:
        putname(kernel_dir);
out_dir:
        kfree(kernel_type);
out_type:
        return ret;
}

/*
 * Return true if path is reachable from root
 *
 * namespace_sem or vfsmount_lock is held
 */
bool is_path_reachable(struct mount *mnt, struct dentry *dentry,
                         const struct path *root)
{
        while (&mnt->mnt != root->mnt && mnt_has_parent(mnt)) {
                dentry = mnt->mnt_mountpoint;
                mnt = mnt->mnt_parent;
        }
        return &mnt->mnt == root->mnt && is_subdir(dentry, root->dentry);
}

int path_is_under(struct path *path1, struct path *path2)
{
        int res;
        br_read_lock(&vfsmount_lock);
        res = is_path_reachable(real_mount(path1->mnt), path1->dentry, path2);
        br_read_unlock(&vfsmount_lock);
        return res;
}
EXPORT_SYMBOL(path_is_under);

/*
 * pivot_root Semantics:
 * Moves the root file system of the current process to the directory put_old,
 * makes new_root as the new root file system of the current process, and sets
 * root/cwd of all processes which had them on the current root to new_root.
 *
 * Restrictions:
 * The new_root and put_old must be directories, and  must not be on the
 * same file  system as the current process root. The put_old  must  be
 * underneath new_root,  i.e. adding a non-zero number of /.. to the string
 * pointed to by put_old must yield the same directory as new_root. No other
 * file system may be mounted on put_old. After all, new_root is a mountpoint.
 *
 * Also, the current root cannot be on the 'rootfs' (initial ramfs) filesystem.
 * See Documentation/filesystems/ramfs-rootfs-initramfs.txt for alternatives
 * in this situation.
 *
 * Notes:
 *  - we don't move root/cwd if they are not at the root (reason: if something
 *    cared enough to change them, it's probably wrong to force them elsewhere)
 *  - it's okay to pick a root that isn't the root of a file system, e.g.
 *    /nfs/my_root where /nfs is the mount point. It must be a mountpoint,
 *    though, so you may need to say mount --bind /nfs/my_root /nfs/my_root
 *    first.
 */
SYSCALL_DEFINE2(pivot_root, const char __user *, new_root,
                const char __user *, put_old)
{
        struct path new, old, parent_path, root_parent, root;
        struct mount *new_mnt, *root_mnt;
        int error;

        if (!capable(CAP_SYS_ADMIN))
                return -EPERM;

        error = user_path_dir(new_root, &new);
        if (error)
                goto out0;

        error = user_path_dir(put_old, &old);
        if (error)
                goto out1;

        error = security_sb_pivotroot(&old, &new);
        if (error)
                goto out2;

        get_fs_root(current->fs, &root);
        error = lock_mount(&old);
        if (error)
                goto out3;

        error = -EINVAL;
        new_mnt = real_mount(new.mnt);
        root_mnt = real_mount(root.mnt);
        if (IS_MNT_SHARED(real_mount(old.mnt)) ||
                IS_MNT_SHARED(new_mnt->mnt_parent) ||
                IS_MNT_SHARED(root_mnt->mnt_parent))
                goto out4;
        if (!check_mnt(root_mnt) || !check_mnt(new_mnt))
                goto out4;
        error = -ENOENT;
        if (d_unlinked(new.dentry))
                goto out4;
        if (d_unlinked(old.dentry))
                goto out4;
        error = -EBUSY;
        if (new.mnt == root.mnt ||
            old.mnt == root.mnt)
                goto out4; /* loop, on the same file system  */
        error = -EINVAL;
        if (root.mnt->mnt_root != root.dentry)
                goto out4; /* not a mountpoint */
        if (!mnt_has_parent(root_mnt))
                goto out4; /* not attached */
        if (new.mnt->mnt_root != new.dentry)
                goto out4; /* not a mountpoint */
        if (!mnt_has_parent(new_mnt))
                goto out4; /* not attached */
        /* make sure we can reach put_old from new_root */
        if (!is_path_reachable(real_mount(old.mnt), old.dentry, &new))
                goto out4;
        br_write_lock(&vfsmount_lock);
        detach_mnt(new_mnt, &parent_path);
        detach_mnt(root_mnt, &root_parent);
        /* mount old root on put_old */
        attach_mnt(root_mnt, &old);
        /* mount new_root on / */
        attach_mnt(new_mnt, &root_parent);
        touch_mnt_namespace(current->nsproxy->mnt_ns);
        br_write_unlock(&vfsmount_lock);
        chroot_fs_refs(&root, &new);
        error = 0;
out4:
        unlock_mount(&old);
        if (!error) {
                path_put(&root_parent);
                path_put(&parent_path);
        }
out3:
        path_put(&root);
out2:
        path_put(&old);
out1:
        path_put(&new);
out0:
        return error;
}

static void __init init_mount_tree(void)
{
        struct vfsmount *mnt;
        struct mnt_namespace *ns;
        struct path root;

        mnt = do_kern_mount("rootfs", 0, "rootfs", NULL);
        if (IS_ERR(mnt))
                panic("Can't create rootfs");

        ns = create_mnt_ns(mnt);
        if (IS_ERR(ns))
                panic("Can't allocate initial namespace");

        init_task.nsproxy->mnt_ns = ns;
        get_mnt_ns(ns);

        root.mnt = mnt;
        root.dentry = mnt->mnt_root;

        set_fs_pwd(current->fs, &root);
        set_fs_root(current->fs, &root);
}

void __init mnt_init(void)
{
        unsigned u;
        int err;

        init_rwsem(&namespace_sem);

        mnt_cache = kmem_cache_create("mnt_cache", sizeof(struct mount),
                        0, SLAB_HWCACHE_ALIGN | SLAB_PANIC, NULL);

        mount_hashtable = (struct list_head *)__get_free_page(GFP_ATOMIC);

        if (!mount_hashtable)
                panic("Failed to allocate mount hash table\n");

        printk(KERN_INFO "Mount-cache hash table entries: %lu\n", HASH_SIZE);

        for (u = 0; u < HASH_SIZE; u++)
                INIT_LIST_HEAD(&mount_hashtable[u]);

        br_lock_init(&vfsmount_lock);

        err = sysfs_init();
        if (err)
                printk(KERN_WARNING "%s: sysfs_init error: %d\n",
                        __func__, err);
        fs_kobj = kobject_create_and_add("fs", NULL);
        if (!fs_kobj)
                printk(KERN_WARNING "%s: kobj create error\n", __func__);
        init_rootfs();
        init_mount_tree();
}

void put_mnt_ns(struct mnt_namespace *ns)
{
        LIST_HEAD(umount_list);

        if (!atomic_dec_and_test(&ns->count))
                return;
        down_write(&namespace_sem);
        br_write_lock(&vfsmount_lock);
        umount_tree(ns->root, 0, &umount_list);
        br_write_unlock(&vfsmount_lock);
        up_write(&namespace_sem);
        release_mounts(&umount_list);
        kfree(ns);
}

struct vfsmount *kern_mount_data(struct file_system_type *type, void *data)
{
        struct vfsmount *mnt;
        mnt = vfs_kern_mount(type, MS_KERNMOUNT, type->name, data);
        if (!IS_ERR(mnt)) {
                /*
                 * it is a longterm mount, don't release mnt until
                 * we unmount before file sys is unregistered
                */
                mnt_make_longterm(mnt);
        }
        return mnt;
}
EXPORT_SYMBOL_GPL(kern_mount_data);

void kern_unmount(struct vfsmount *mnt)
{
        /* release long term mount so mount point can be released */
        if (!IS_ERR_OR_NULL(mnt)) {
                mnt_make_shortterm(mnt);
                mntput(mnt);
        }
}
EXPORT_SYMBOL(kern_unmount);

bool our_mnt(struct vfsmount *mnt)
{
        return check_mnt(real_mount(mnt));
}