[PATCH] r/o bind mounts: track numbers of writers to mounts

[linux-2.6] / 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/slab.h>
#include <linux/sched.h>
#include <linux/smp_lock.h>
#include <linux/init.h>
#include <linux/kernel.h>
#include <linux/quotaops.h>
#include <linux/acct.h>
#include <linux/capability.h>
#include <linux/cpumask.h>
#include <linux/module.h>
#include <linux/sysfs.h>
#include <linux/seq_file.h>
#include <linux/mnt_namespace.h>
#include <linux/namei.h>
#include <linux/security.h>
#include <linux/mount.h>
#include <linux/ramfs.h>
#include <linux/log2.h>
#include <asm/uaccess.h>
#include <asm/unistd.h>
#include "pnode.h"
#include "internal.h"

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

/* spinlock for vfsmount related operations, inplace of dcache_lock */
__cacheline_aligned_in_smp DEFINE_SPINLOCK(vfsmount_lock);

static int event;

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);

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)

struct vfsmount *alloc_vfsmnt(const char *name)
{
        struct vfsmount *mnt = kmem_cache_zalloc(mnt_cache, GFP_KERNEL);
        if (mnt) {
                atomic_set(&mnt->mnt_count, 1);
                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);
                atomic_set(&mnt->__mnt_writers, 0);
                if (name) {
                        int size = strlen(name) + 1;
                        char *newname = kmalloc(size, GFP_KERNEL);
                        if (newname) {
                                memcpy(newname, name, size);
                                mnt->mnt_devname = newname;
                        }
                }
        }
        return 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_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)
{
        return (mnt->mnt_sb->s_flags & MS_RDONLY);
}
EXPORT_SYMBOL_GPL(__mnt_is_readonly);

struct mnt_writer {
        /*
         * If holding multiple instances of this lock, they
         * must be ordered by cpu number.
         */
        spinlock_t lock;
        struct lock_class_key lock_class; /* compiles out with !lockdep */
        unsigned long count;
        struct vfsmount *mnt;
} ____cacheline_aligned_in_smp;
static DEFINE_PER_CPU(struct mnt_writer, mnt_writers);

static int __init init_mnt_writers(void)
{
        int cpu;
        for_each_possible_cpu(cpu) {
                struct mnt_writer *writer = &per_cpu(mnt_writers, cpu);
                spin_lock_init(&writer->lock);
                lockdep_set_class(&writer->lock, &writer->lock_class);
                writer->count = 0;
        }
        return 0;
}
fs_initcall(init_mnt_writers);

static void unlock_mnt_writers(void)
{
        int cpu;
        struct mnt_writer *cpu_writer;

        for_each_possible_cpu(cpu) {
                cpu_writer = &per_cpu(mnt_writers, cpu);
                spin_unlock(&cpu_writer->lock);
        }
}

static inline void __clear_mnt_count(struct mnt_writer *cpu_writer)
{
        if (!cpu_writer->mnt)
                return;
        /*
         * This is in case anyone ever leaves an invalid,
         * old ->mnt and a count of 0.
         */
        if (!cpu_writer->count)
                return;
        atomic_add(cpu_writer->count, &cpu_writer->mnt->__mnt_writers);
        cpu_writer->count = 0;
}
 /*
 * must hold cpu_writer->lock
 */
static inline void use_cpu_writer_for_mount(struct mnt_writer *cpu_writer,
                                          struct vfsmount *mnt)
{
        if (cpu_writer->mnt == mnt)
                return;
        __clear_mnt_count(cpu_writer);
        cpu_writer->mnt = 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
 * @mnt: 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 *mnt)
{
        int ret = 0;
        struct mnt_writer *cpu_writer;

        cpu_writer = &get_cpu_var(mnt_writers);
        spin_lock(&cpu_writer->lock);
        if (__mnt_is_readonly(mnt)) {
                ret = -EROFS;
                goto out;
        }
        use_cpu_writer_for_mount(cpu_writer, mnt);
        cpu_writer->count++;
out:
        spin_unlock(&cpu_writer->lock);
        put_cpu_var(mnt_writers);
        return ret;
}
EXPORT_SYMBOL_GPL(mnt_want_write);

static void lock_mnt_writers(void)
{
        int cpu;
        struct mnt_writer *cpu_writer;

        for_each_possible_cpu(cpu) {
                cpu_writer = &per_cpu(mnt_writers, cpu);
                spin_lock(&cpu_writer->lock);
                __clear_mnt_count(cpu_writer);
                cpu_writer->mnt = NULL;
        }
}

/*
 * These per-cpu write counts are not guaranteed to have
 * matched increments and decrements on any given cpu.
 * A file open()ed for write on one cpu and close()d on
 * another cpu will imbalance this count.  Make sure it
 * does not get too far out of whack.
 */
static void handle_write_count_underflow(struct vfsmount *mnt)
{
        if (atomic_read(&mnt->__mnt_writers) >=
            MNT_WRITER_UNDERFLOW_LIMIT)
                return;
        /*
         * It isn't necessary to hold all of the locks
         * at the same time, but doing it this way makes
         * us share a lot more code.
         */
        lock_mnt_writers();
        /*
         * vfsmount_lock is for mnt_flags.
         */
        spin_lock(&vfsmount_lock);
        /*
         * If coalescing the per-cpu writer counts did not
         * get us back to a positive writer count, we have
         * a bug.
         */
        if ((atomic_read(&mnt->__mnt_writers) < 0) &&
            !(mnt->mnt_flags & MNT_IMBALANCED_WRITE_COUNT)) {
                printk(KERN_DEBUG "leak detected on mount(%p) writers "
                                "count: %d\n",
                        mnt, atomic_read(&mnt->__mnt_writers));
                WARN_ON(1);
                /* use the flag to keep the dmesg spam down */
                mnt->mnt_flags |= MNT_IMBALANCED_WRITE_COUNT;
        }
        spin_unlock(&vfsmount_lock);
        unlock_mnt_writers();
}

/**
 * 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)
{
        int must_check_underflow = 0;
        struct mnt_writer *cpu_writer;

        cpu_writer = &get_cpu_var(mnt_writers);
        spin_lock(&cpu_writer->lock);

        use_cpu_writer_for_mount(cpu_writer, mnt);
        if (cpu_writer->count > 0) {
                cpu_writer->count--;
        } else {
                must_check_underflow = 1;
                atomic_dec(&mnt->__mnt_writers);
        }

        spin_unlock(&cpu_writer->lock);
        /*
         * Logically, we could call this each time,
         * but the __mnt_writers cacheline tends to
         * be cold, and makes this expensive.
         */
        if (must_check_underflow)
                handle_write_count_underflow(mnt);
        /*
         * This could be done right after the spinlock
         * is taken because the spinlock keeps us on
         * the cpu, and disables preemption.  However,
         * putting it here bounds the amount that
         * __mnt_writers can underflow.  Without it,
         * we could theoretically wrap __mnt_writers.
         */
        put_cpu_var(mnt_writers);
}
EXPORT_SYMBOL_GPL(mnt_drop_write);

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

        lock_mnt_writers();
        /*
         * With all the locks held, this value is stable
         */
        if (atomic_read(&mnt->__mnt_writers) > 0) {
                ret = -EBUSY;
                goto out;
        }
        /*
         * actually set mount's r/o flag here to make
         * __mnt_is_readonly() true, which keeps anyone
         * from doing a successful mnt_want_write().
         */
out:
        unlock_mnt_writers();
        return ret;
}

int simple_set_mnt(struct vfsmount *mnt, struct super_block *sb)
{
        mnt->mnt_sb = sb;
        mnt->mnt_root = dget(sb->s_root);
        return 0;
}

EXPORT_SYMBOL(simple_set_mnt);

void free_vfsmnt(struct vfsmount *mnt)
{
        kfree(mnt->mnt_devname);
        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.
 */
struct vfsmount *__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 vfsmount *p, *found = NULL;

        for (;;) {
                tmp = dir ? tmp->next : tmp->prev;
                p = NULL;
                if (tmp == head)
                        break;
                p = list_entry(tmp, struct vfsmount, mnt_hash);
                if (p->mnt_parent == 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 vfsmount *mnt, struct dentry *dentry)
{
        struct vfsmount *child_mnt;
        spin_lock(&vfsmount_lock);
        if ((child_mnt = __lookup_mnt(mnt, dentry, 1)))
                mntget(child_mnt);
        spin_unlock(&vfsmount_lock);
        return child_mnt;
}

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

static void touch_mnt_namespace(struct mnt_namespace *ns)
{
        if (ns) {
                ns->event = ++event;
                wake_up_interruptible(&ns->poll);
        }
}

static void __touch_mnt_namespace(struct mnt_namespace *ns)
{
        if (ns && ns->event != event) {
                ns->event = event;
                wake_up_interruptible(&ns->poll);
        }
}

static void detach_mnt(struct vfsmount *mnt, struct path *old_path)
{
        old_path->dentry = mnt->mnt_mountpoint;
        old_path->mnt = mnt->mnt_parent;
        mnt->mnt_parent = mnt;
        mnt->mnt_mountpoint = mnt->mnt_root;
        list_del_init(&mnt->mnt_child);
        list_del_init(&mnt->mnt_hash);
        old_path->dentry->d_mounted--;
}

void mnt_set_mountpoint(struct vfsmount *mnt, struct dentry *dentry,
                        struct vfsmount *child_mnt)
{
        child_mnt->mnt_parent = mntget(mnt);
        child_mnt->mnt_mountpoint = dget(dentry);
        dentry->d_mounted++;
}

static void attach_mnt(struct vfsmount *mnt, struct path *path)
{
        mnt_set_mountpoint(path->mnt, path->dentry, mnt);
        list_add_tail(&mnt->mnt_hash, mount_hashtable +
                        hash(path->mnt, path->dentry));
        list_add_tail(&mnt->mnt_child, &path->mnt->mnt_mounts);
}

/*
 * the caller must hold vfsmount_lock
 */
static void commit_tree(struct vfsmount *mnt)
{
        struct vfsmount *parent = mnt->mnt_parent;
        struct vfsmount *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;
        list_splice(&head, n->list.prev);

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

static struct vfsmount *next_mnt(struct vfsmount *p, struct vfsmount *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 vfsmount, mnt_child);
}

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

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

        if (mnt) {
                mnt->mnt_flags = old->mnt_flags;
                atomic_inc(&sb->s_active);
                mnt->mnt_sb = sb;
                mnt->mnt_root = dget(root);
                mnt->mnt_mountpoint = mnt->mnt_root;
                mnt->mnt_parent = mnt;

                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_PROPAGATION) || 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;
}

static inline void __mntput(struct vfsmount *mnt)
{
        int cpu;
        struct super_block *sb = mnt->mnt_sb;
        /*
         * We don't have to hold all of the locks at the
         * same time here because we know that we're the
         * last reference to mnt and that no new writers
         * can come in.
         */
        for_each_possible_cpu(cpu) {
                struct mnt_writer *cpu_writer = &per_cpu(mnt_writers, cpu);
                if (cpu_writer->mnt != mnt)
                        continue;
                spin_lock(&cpu_writer->lock);
                atomic_add(cpu_writer->count, &mnt->__mnt_writers);
                cpu_writer->count = 0;
                /*
                 * Might as well do this so that no one
                 * ever sees the pointer and expects
                 * it to be valid.
                 */
                cpu_writer->mnt = NULL;
                spin_unlock(&cpu_writer->lock);
        }
        /*
         * 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.
         */
        WARN_ON(atomic_read(&mnt->__mnt_writers));
        dput(mnt->mnt_root);
        free_vfsmnt(mnt);
        deactivate_super(sb);
}

void mntput_no_expire(struct vfsmount *mnt)
{
repeat:
        if (atomic_dec_and_lock(&mnt->mnt_count, &vfsmount_lock)) {
                if (likely(!mnt->mnt_pinned)) {
                        spin_unlock(&vfsmount_lock);
                        __mntput(mnt);
                        return;
                }
                atomic_add(mnt->mnt_pinned + 1, &mnt->mnt_count);
                mnt->mnt_pinned = 0;
                spin_unlock(&vfsmount_lock);
                acct_auto_close_mnt(mnt);
                security_sb_umount_close(mnt);
                goto repeat;
        }
}

EXPORT_SYMBOL(mntput_no_expire);

void mnt_pin(struct vfsmount *mnt)
{
        spin_lock(&vfsmount_lock);
        mnt->mnt_pinned++;
        spin_unlock(&vfsmount_lock);
}

EXPORT_SYMBOL(mnt_pin);

void mnt_unpin(struct vfsmount *mnt)
{
        spin_lock(&vfsmount_lock);
        if (mnt->mnt_pinned) {
                atomic_inc(&mnt->mnt_count);
                mnt->mnt_pinned--;
        }
        spin_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 vfsmount *mnt)
{
        const char *options = mnt->mnt_sb->s_options;

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

        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)
{
        kfree(sb->s_options);
        sb->s_options = kstrdup(options, GFP_KERNEL);
}
EXPORT_SYMBOL(save_mount_options);

/* iterator */
static void *m_start(struct seq_file *m, loff_t *pos)
{
        struct mnt_namespace *n = m->private;

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

static void *m_next(struct seq_file *m, void *v, loff_t *pos)
{
        struct mnt_namespace *n = m->private;

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

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

static int show_vfsmnt(struct seq_file *m, void *v)
{
        struct vfsmount *mnt = list_entry(v, struct vfsmount, mnt_list);
        int err = 0;
        static struct proc_fs_info {
                int flag;
                char *str;
        } fs_info[] = {
                { MS_SYNCHRONOUS, ",sync" },
                { MS_DIRSYNC, ",dirsync" },
                { MS_MANDLOCK, ",mand" },
                { 0, NULL }
        };
        static struct proc_fs_info mnt_info[] = {
                { MNT_NOSUID, ",nosuid" },
                { MNT_NODEV, ",nodev" },
                { MNT_NOEXEC, ",noexec" },
                { MNT_NOATIME, ",noatime" },
                { MNT_NODIRATIME, ",nodiratime" },
                { MNT_RELATIME, ",relatime" },
                { 0, NULL }
        };
        struct proc_fs_info *fs_infop;
        struct path mnt_path = { .dentry = mnt->mnt_root, .mnt = mnt };

        mangle(m, mnt->mnt_devname ? mnt->mnt_devname : "none");
        seq_putc(m, ' ');
        seq_path(m, &mnt_path, " \t\n\\");
        seq_putc(m, ' ');
        mangle(m, mnt->mnt_sb->s_type->name);
        if (mnt->mnt_sb->s_subtype && mnt->mnt_sb->s_subtype[0]) {
                seq_putc(m, '.');
                mangle(m, mnt->mnt_sb->s_subtype);
        }
        seq_puts(m, mnt->mnt_sb->s_flags & MS_RDONLY ? " ro" : " rw");
        for (fs_infop = fs_info; fs_infop->flag; fs_infop++) {
                if (mnt->mnt_sb->s_flags & fs_infop->flag)
                        seq_puts(m, fs_infop->str);
        }
        for (fs_infop = mnt_info; fs_infop->flag; fs_infop++) {
                if (mnt->mnt_flags & fs_infop->flag)
                        seq_puts(m, fs_infop->str);
        }
        if (mnt->mnt_sb->s_op->show_options)
                err = mnt->mnt_sb->s_op->show_options(m, mnt);
        seq_puts(m, " 0 0\n");
        return err;
}

struct seq_operations mounts_op = {
        .start  = m_start,
        .next   = m_next,
        .stop   = m_stop,
        .show   = show_vfsmnt
};

static int show_vfsstat(struct seq_file *m, void *v)
{
        struct vfsmount *mnt = list_entry(v, struct vfsmount, mnt_list);
        struct path mnt_path = { .dentry = mnt->mnt_root, .mnt = mnt };
        int err = 0;

        /* device */
        if (mnt->mnt_devname) {
                seq_puts(m, "device ");
                mangle(m, mnt->mnt_devname);
        } else
                seq_puts(m, "no device");

        /* mount point */
        seq_puts(m, " mounted on ");
        seq_path(m, &mnt_path, " \t\n\\");
        seq_putc(m, ' ');

        /* file system type */
        seq_puts(m, "with fstype ");
        mangle(m, mnt->mnt_sb->s_type->name);

        /* optional statistics */
        if (mnt->mnt_sb->s_op->show_stats) {
                seq_putc(m, ' ');
                err = mnt->mnt_sb->s_op->show_stats(m, mnt);
        }

        seq_putc(m, '\n');
        return err;
}

struct seq_operations mountstats_op = {
        .start  = m_start,
        .next   = m_next,
        .stop   = m_stop,
        .show   = show_vfsstat,
};

/**
 * 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 *mnt)
{
        int actual_refs = 0;
        int minimum_refs = 0;
        struct vfsmount *p;

        spin_lock(&vfsmount_lock);
        for (p = mnt; p; p = next_mnt(p, mnt)) {
                actual_refs += atomic_read(&p->mnt_count);
                minimum_refs += 2;
        }
        spin_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;
        spin_lock(&vfsmount_lock);
        if (propagate_mount_busy(mnt, 2))
                ret = 0;
        spin_unlock(&vfsmount_lock);
        return ret;
}

EXPORT_SYMBOL(may_umount);

void release_mounts(struct list_head *head)
{
        struct vfsmount *mnt;
        while (!list_empty(head)) {
                mnt = list_first_entry(head, struct vfsmount, mnt_hash);
                list_del_init(&mnt->mnt_hash);
                if (mnt->mnt_parent != mnt) {
                        struct dentry *dentry;
                        struct vfsmount *m;
                        spin_lock(&vfsmount_lock);
                        dentry = mnt->mnt_mountpoint;
                        m = mnt->mnt_parent;
                        mnt->mnt_mountpoint = mnt->mnt_root;
                        mnt->mnt_parent = mnt;
                        m->mnt_ghosts--;
                        spin_unlock(&vfsmount_lock);
                        dput(dentry);
                        mntput(m);
                }
                mntput(mnt);
        }
}

void umount_tree(struct vfsmount *mnt, int propagate, struct list_head *kill)
{
        struct vfsmount *p;

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

        if (propagate)
                propagate_umount(kill);

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

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

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

        retval = security_sb_umount(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 == current->fs->root.mnt ||
                    flags & (MNT_FORCE | MNT_DETACH))
                        return -EINVAL;

                if (atomic_read(&mnt->mnt_count) != 2)
                        return -EBUSY;

                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.
         */

        lock_kernel();
        if (sb->s_op->umount_begin)
                sb->s_op->umount_begin(mnt, flags);
        unlock_kernel();

        /*
         * 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 == 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)) {
                        lock_kernel();
                        DQUOT_OFF(sb);
                        retval = do_remount_sb(sb, MS_RDONLY, NULL, 0);
                        unlock_kernel();
                }
                up_write(&sb->s_umount);
                return retval;
        }

        down_write(&namespace_sem);
        spin_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;
        }
        spin_unlock(&vfsmount_lock);
        if (retval)
                security_sb_umount_busy(mnt);
        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
 */

asmlinkage long sys_umount(char __user * name, int flags)
{
        struct nameidata nd;
        int retval;

        retval = __user_walk(name, LOOKUP_FOLLOW, &nd);
        if (retval)
                goto out;
        retval = -EINVAL;
        if (nd.path.dentry != nd.path.mnt->mnt_root)
                goto dput_and_out;
        if (!check_mnt(nd.path.mnt))
                goto dput_and_out;

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

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

#ifdef __ARCH_WANT_SYS_OLDUMOUNT

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

#endif

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

static int lives_below_in_same_fs(struct dentry *d, struct dentry *dentry)
{
        while (1) {
                if (d == dentry)
                        return 1;
                if (d == NULL || d == d->d_parent)
                        return 0;
                d = d->d_parent;
        }
}

struct vfsmount *copy_tree(struct vfsmount *mnt, struct dentry *dentry,
                                        int flag)
{
        struct vfsmount *res, *p, *q, *r, *s;
        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) {
                if (!lives_below_in_same_fs(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;
                        path.dentry = p->mnt_mountpoint;
                        q = clone_mnt(p, p->mnt_root, flag);
                        if (!q)
                                goto Enomem;
                        spin_lock(&vfsmount_lock);
                        list_add_tail(&q->mnt_list, &res->mnt_list);
                        attach_mnt(q, &path);
                        spin_unlock(&vfsmount_lock);
                }
        }
        return res;
Enomem:
        if (res) {
                LIST_HEAD(umount_list);
                spin_lock(&vfsmount_lock);
                umount_tree(res, 0, &umount_list);
                spin_unlock(&vfsmount_lock);
                release_mounts(&umount_list);
        }
        return NULL;
}

struct vfsmount *collect_mounts(struct vfsmount *mnt, struct dentry *dentry)
{
        struct vfsmount *tree;
        down_read(&namespace_sem);
        tree = copy_tree(mnt, dentry, CL_COPY_ALL | CL_PRIVATE);
        up_read(&namespace_sem);
        return tree;
}

void drop_collected_mounts(struct vfsmount *mnt)
{
        LIST_HEAD(umount_list);
        down_read(&namespace_sem);
        spin_lock(&vfsmount_lock);
        umount_tree(mnt, 0, &umount_list);
        spin_unlock(&vfsmount_lock);
        up_read(&namespace_sem);
        release_mounts(&umount_list);
}

/*
 *  @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 vfsmount *source_mnt,
                        struct path *path, struct path *parent_path)
{
        LIST_HEAD(tree_list);
        struct vfsmount *dest_mnt = path->mnt;
        struct dentry *dest_dentry = path->dentry;
        struct vfsmount *child, *p;

        if (propagate_mnt(dest_mnt, dest_dentry, source_mnt, &tree_list))
                return -EINVAL;

        if (IS_MNT_SHARED(dest_mnt)) {
                for (p = source_mnt; p; p = next_mnt(p, source_mnt))
                        set_mnt_shared(p);
        }

        spin_lock(&vfsmount_lock);
        if (parent_path) {
                detach_mnt(source_mnt, parent_path);
                attach_mnt(source_mnt, path);
                touch_mnt_namespace(current->nsproxy->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);
        }
        spin_unlock(&vfsmount_lock);
        return 0;
}

static int graft_tree(struct vfsmount *mnt, struct nameidata *nd)
{
        int err;
        if (mnt->mnt_sb->s_flags & MS_NOUSER)
                return -EINVAL;

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

        err = -ENOENT;
        mutex_lock(&nd->path.dentry->d_inode->i_mutex);
        if (IS_DEADDIR(nd->path.dentry->d_inode))
                goto out_unlock;

        err = security_sb_check_sb(mnt, nd);
        if (err)
                goto out_unlock;

        err = -ENOENT;
        if (IS_ROOT(nd->path.dentry) || !d_unhashed(nd->path.dentry))
                err = attach_recursive_mnt(mnt, &nd->path, NULL);
out_unlock:
        mutex_unlock(&nd->path.dentry->d_inode->i_mutex);
        if (!err)
                security_sb_post_addmount(mnt, nd);
        return err;
}

/*
 * recursively change the type of the mountpoint.
 * noinline this do_mount helper to save do_mount stack space.
 */
static noinline int do_change_type(struct nameidata *nd, int flag)
{
        struct vfsmount *m, *mnt = nd->path.mnt;
        int recurse = flag & MS_REC;
        int type = flag & ~MS_REC;

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

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

        down_write(&namespace_sem);
        spin_lock(&vfsmount_lock);
        for (m = mnt; m; m = (recurse ? next_mnt(m, mnt) : NULL))
                change_mnt_propagation(m, type);
        spin_unlock(&vfsmount_lock);
        up_write(&namespace_sem);
        return 0;
}

/*
 * do loopback mount.
 * noinline this do_mount helper to save do_mount stack space.
 */
static noinline int do_loopback(struct nameidata *nd, char *old_name,
                                int recurse)
{
        struct nameidata old_nd;
        struct vfsmount *mnt = NULL;
        int err = mount_is_safe(nd);
        if (err)
                return err;
        if (!old_name || !*old_name)
                return -EINVAL;
        err = path_lookup(old_name, LOOKUP_FOLLOW, &old_nd);
        if (err)
                return err;

        down_write(&namespace_sem);
        err = -EINVAL;
        if (IS_MNT_UNBINDABLE(old_nd.path.mnt))
                goto out;

        if (!check_mnt(nd->path.mnt) || !check_mnt(old_nd.path.mnt))
                goto out;

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

        if (!mnt)
                goto out;

        err = graft_tree(mnt, nd);
        if (err) {
                LIST_HEAD(umount_list);
                spin_lock(&vfsmount_lock);
                umount_tree(mnt, 0, &umount_list);
                spin_unlock(&vfsmount_lock);
                release_mounts(&umount_list);
        }

out:
        up_write(&namespace_sem);
        path_put(&old_nd.path);
        return err;
}

/*
 * 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.
 * noinline this do_mount helper to save do_mount stack space.
 */
static noinline int do_remount(struct nameidata *nd, int flags, int mnt_flags,
                      void *data)
{
        int err;
        struct super_block *sb = nd->path.mnt->mnt_sb;

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

        if (!check_mnt(nd->path.mnt))
                return -EINVAL;

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

        down_write(&sb->s_umount);
        err = do_remount_sb(sb, flags, data, 0);
        if (!err)
                nd->path.mnt->mnt_flags = mnt_flags;
        up_write(&sb->s_umount);
        if (!err)
                security_sb_post_remount(nd->path.mnt, flags, data);
        return err;
}

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

/*
 * noinline this do_mount helper to save do_mount stack space.
 */
static noinline int do_move_mount(struct nameidata *nd, char *old_name)
{
        struct nameidata old_nd;
        struct path parent_path;
        struct vfsmount *p;
        int err = 0;
        if (!capable(CAP_SYS_ADMIN))
                return -EPERM;
        if (!old_name || !*old_name)
                return -EINVAL;
        err = path_lookup(old_name, LOOKUP_FOLLOW, &old_nd);
        if (err)
                return err;

        down_write(&namespace_sem);
        while (d_mountpoint(nd->path.dentry) &&
               follow_down(&nd->path.mnt, &nd->path.dentry))
                ;
        err = -EINVAL;
        if (!check_mnt(nd->path.mnt) || !check_mnt(old_nd.path.mnt))
                goto out;

        err = -ENOENT;
        mutex_lock(&nd->path.dentry->d_inode->i_mutex);
        if (IS_DEADDIR(nd->path.dentry->d_inode))
                goto out1;

        if (!IS_ROOT(nd->path.dentry) && d_unhashed(nd->path.dentry))
                goto out1;

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

        if (old_nd.path.mnt == old_nd.path.mnt->mnt_parent)
                goto out1;

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

        err = attach_recursive_mnt(old_nd.path.mnt, &nd->path, &parent_path);
        if (err)
                goto out1;

        /* if the mount is moved, it should no longer be expire
         * automatically */
        list_del_init(&old_nd.path.mnt->mnt_expire);
out1:
        mutex_unlock(&nd->path.dentry->d_inode->i_mutex);
out:
        up_write(&namespace_sem);
        if (!err)
                path_put(&parent_path);
        path_put(&old_nd.path);
        return err;
}

/*
 * create a new mount for userspace and request it to be added into the
 * namespace's tree
 * noinline this do_mount helper to save do_mount stack space.
 */
static noinline int do_new_mount(struct nameidata *nd, char *type, int flags,
                        int mnt_flags, char *name, void *data)
{
        struct vfsmount *mnt;

        if (!type || !memchr(type, 0, PAGE_SIZE))
                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);

        return do_add_mount(mnt, nd, mnt_flags, NULL);
}

/*
 * add a mount into a namespace's mount tree
 * - provide the option of adding the new mount to an expiration list
 */
int do_add_mount(struct vfsmount *newmnt, struct nameidata *nd,
                 int mnt_flags, struct list_head *fslist)
{
        int err;

        down_write(&namespace_sem);
        /* Something was mounted here while we slept */
        while (d_mountpoint(nd->path.dentry) &&
               follow_down(&nd->path.mnt, &nd->path.dentry))
                ;
        err = -EINVAL;
        if (!check_mnt(nd->path.mnt))
                goto unlock;

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

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

        newmnt->mnt_flags = mnt_flags;
        if ((err = graft_tree(newmnt, nd)))
                goto unlock;

        if (fslist) /* add to the specified expiration list */
                list_add_tail(&newmnt->mnt_expire, fslist);

        up_write(&namespace_sem);
        return 0;

unlock:
        up_write(&namespace_sem);
        mntput(newmnt);
        return err;
}

EXPORT_SYMBOL_GPL(do_add_mount);

/*
 * 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 vfsmount *mnt, *next;
        LIST_HEAD(graveyard);
        LIST_HEAD(umounts);

        if (list_empty(mounts))
                return;

        down_write(&namespace_sem);
        spin_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 vfsmount, mnt_expire);
                touch_mnt_namespace(mnt->mnt_ns);
                umount_tree(mnt, 1, &umounts);
        }
        spin_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 vfsmount *parent, struct list_head *graveyard)
{
        struct vfsmount *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 vfsmount *mnt = list_entry(tmp, struct vfsmount, mnt_child);

                next = tmp->next;
                if (!(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
 */
static void shrink_submounts(struct vfsmount *mnt, struct list_head *umounts)
{
        LIST_HEAD(graveyard);
        struct vfsmount *m;

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

/*
 * 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 nameidata nd;
        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 (dev_name && !memchr(dev_name, 0, PAGE_SIZE))
                return -EINVAL;

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

        /* 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_RELATIME)
                mnt_flags |= MNT_RELATIME;

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

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

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

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

/*
 * 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, *altrootmnt = NULL;
        struct vfsmount *p, *q;

        new_ns = kmalloc(sizeof(struct mnt_namespace), GFP_KERNEL);
        if (!new_ns)
                return ERR_PTR(-ENOMEM);

        atomic_set(&new_ns->count, 1);
        INIT_LIST_HEAD(&new_ns->list);
        init_waitqueue_head(&new_ns->poll);
        new_ns->event = 0;

        down_write(&namespace_sem);
        /* First pass: copy the tree topology */
        new_ns->root = copy_tree(mnt_ns->root, mnt_ns->root->mnt_root,
                                        CL_COPY_ALL | CL_EXPIRE);
        if (!new_ns->root) {
                up_write(&namespace_sem);
                kfree(new_ns);
                return ERR_PTR(-ENOMEM);;
        }
        spin_lock(&vfsmount_lock);
        list_add_tail(&new_ns->list, &new_ns->root->mnt_list);
        spin_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 = mnt_ns->root;
        q = new_ns->root;
        while (p) {
                q->mnt_ns = new_ns;
                if (fs) {
                        if (p == fs->root.mnt) {
                                rootmnt = p;
                                fs->root.mnt = mntget(q);
                        }
                        if (p == fs->pwd.mnt) {
                                pwdmnt = p;
                                fs->pwd.mnt = mntget(q);
                        }
                        if (p == fs->altroot.mnt) {
                                altrootmnt = p;
                                fs->altroot.mnt = mntget(q);
                        }
                }
                p = next_mnt(p, mnt_ns->root);
                q = next_mnt(q, new_ns->root);
        }
        up_write(&namespace_sem);

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

        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;
}

asmlinkage long sys_mount(char __user * dev_name, char __user * dir_name,
                          char __user * type, unsigned long flags,
                          void __user * data)
{
        int retval;
        unsigned long data_page;
        unsigned long type_page;
        unsigned long dev_page;
        char *dir_page;

        retval = copy_mount_options(type, &type_page);
        if (retval < 0)
                return retval;

        dir_page = getname(dir_name);
        retval = PTR_ERR(dir_page);
        if (IS_ERR(dir_page))
                goto out1;

        retval = copy_mount_options(dev_name, &dev_page);
        if (retval < 0)
                goto out2;

        retval = copy_mount_options(data, &data_page);
        if (retval < 0)
                goto out3;

        lock_kernel();
        retval = do_mount((char *)dev_page, dir_page, (char *)type_page,
                          flags, (void *)data_page);
        unlock_kernel();
        free_page(data_page);

out3:
        free_page(dev_page);
out2:
        putname(dir_page);
out1:
        free_page(type_page);
        return retval;
}

/*
 * Replace the fs->{rootmnt,root} with {mnt,dentry}. Put the old values.
 * It can block. Requires the big lock held.
 */
void set_fs_root(struct fs_struct *fs, struct path *path)
{
        struct path old_root;

        write_lock(&fs->lock);
        old_root = fs->root;
        fs->root = *path;
        path_get(path);
        write_unlock(&fs->lock);
        if (old_root.dentry)
                path_put(&old_root);
}

/*
 * Replace the fs->{pwdmnt,pwd} with {mnt,dentry}. Put the old values.
 * It can block. Requires the big lock held.
 */
void set_fs_pwd(struct fs_struct *fs, struct path *path)
{
        struct path old_pwd;

        write_lock(&fs->lock);
        old_pwd = fs->pwd;
        fs->pwd = *path;
        path_get(path);
        write_unlock(&fs->lock);

        if (old_pwd.dentry)
                path_put(&old_pwd);
}

static void chroot_fs_refs(struct path *old_root, struct path *new_root)
{
        struct task_struct *g, *p;
        struct fs_struct *fs;

        read_lock(&tasklist_lock);
        do_each_thread(g, p) {
                task_lock(p);
                fs = p->fs;
                if (fs) {
                        atomic_inc(&fs->count);
                        task_unlock(p);
                        if (fs->root.dentry == old_root->dentry
                            && fs->root.mnt == old_root->mnt)
                                set_fs_root(fs, new_root);
                        if (fs->pwd.dentry == old_root->dentry
                            && fs->pwd.mnt == old_root->mnt)
                                set_fs_pwd(fs, new_root);
                        put_fs_struct(fs);
                } else
                        task_unlock(p);
        } while_each_thread(g, p);
        read_unlock(&tasklist_lock);
}

/*
 * 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.
 */
asmlinkage long sys_pivot_root(const char __user * new_root,
                               const char __user * put_old)
{
        struct vfsmount *tmp;
        struct nameidata new_nd, old_nd, user_nd;
        struct path parent_path, root_parent;
        int error;

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

        lock_kernel();

        error = __user_walk(new_root, LOOKUP_FOLLOW | LOOKUP_DIRECTORY,
                            &new_nd);
        if (error)
                goto out0;
        error = -EINVAL;
        if (!check_mnt(new_nd.path.mnt))
                goto out1;

        error = __user_walk(put_old, LOOKUP_FOLLOW | LOOKUP_DIRECTORY, &old_nd);
        if (error)
                goto out1;

        error = security_sb_pivotroot(&old_nd, &new_nd);
        if (error) {
                path_put(&old_nd.path);
                goto out1;
        }

        read_lock(&current->fs->lock);
        user_nd.path = current->fs->root;
        path_get(&current->fs->root);
        read_unlock(&current->fs->lock);
        down_write(&namespace_sem);
        mutex_lock(&old_nd.path.dentry->d_inode->i_mutex);
        error = -EINVAL;
        if (IS_MNT_SHARED(old_nd.path.mnt) ||
                IS_MNT_SHARED(new_nd.path.mnt->mnt_parent) ||
                IS_MNT_SHARED(user_nd.path.mnt->mnt_parent))
                goto out2;
        if (!check_mnt(user_nd.path.mnt))
                goto out2;
        error = -ENOENT;
        if (IS_DEADDIR(new_nd.path.dentry->d_inode))
                goto out2;
        if (d_unhashed(new_nd.path.dentry) && !IS_ROOT(new_nd.path.dentry))
                goto out2;
        if (d_unhashed(old_nd.path.dentry) && !IS_ROOT(old_nd.path.dentry))
                goto out2;
        error = -EBUSY;
        if (new_nd.path.mnt == user_nd.path.mnt ||
            old_nd.path.mnt == user_nd.path.mnt)
                goto out2; /* loop, on the same file system  */
        error = -EINVAL;
        if (user_nd.path.mnt->mnt_root != user_nd.path.dentry)
                goto out2; /* not a mountpoint */
        if (user_nd.path.mnt->mnt_parent == user_nd.path.mnt)
                goto out2; /* not attached */
        if (new_nd.path.mnt->mnt_root != new_nd.path.dentry)
                goto out2; /* not a mountpoint */
        if (new_nd.path.mnt->mnt_parent == new_nd.path.mnt)
                goto out2; /* not attached */
        /* make sure we can reach put_old from new_root */
        tmp = old_nd.path.mnt;
        spin_lock(&vfsmount_lock);
        if (tmp != new_nd.path.mnt) {
                for (;;) {
                        if (tmp->mnt_parent == tmp)
                                goto out3; /* already mounted on put_old */
                        if (tmp->mnt_parent == new_nd.path.mnt)
                                break;
                        tmp = tmp->mnt_parent;
                }
                if (!is_subdir(tmp->mnt_mountpoint, new_nd.path.dentry))
                        goto out3;
        } else if (!is_subdir(old_nd.path.dentry, new_nd.path.dentry))
                goto out3;
        detach_mnt(new_nd.path.mnt, &parent_path);
        detach_mnt(user_nd.path.mnt, &root_parent);
        /* mount old root on put_old */
        attach_mnt(user_nd.path.mnt, &old_nd.path);
        /* mount new_root on / */
        attach_mnt(new_nd.path.mnt, &root_parent);
        touch_mnt_namespace(current->nsproxy->mnt_ns);
        spin_unlock(&vfsmount_lock);
        chroot_fs_refs(&user_nd.path, &new_nd.path);
        security_sb_post_pivotroot(&user_nd, &new_nd);
        error = 0;
        path_put(&root_parent);
        path_put(&parent_path);
out2:
        mutex_unlock(&old_nd.path.dentry->d_inode->i_mutex);
        up_write(&namespace_sem);
        path_put(&user_nd.path);
        path_put(&old_nd.path);
out1:
        path_put(&new_nd.path);
out0:
        unlock_kernel();
        return error;
out3:
        spin_unlock(&vfsmount_lock);
        goto out2;
}

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 = kmalloc(sizeof(*ns), GFP_KERNEL);
        if (!ns)
                panic("Can't allocate initial namespace");
        atomic_set(&ns->count, 1);
        INIT_LIST_HEAD(&ns->list);
        init_waitqueue_head(&ns->poll);
        ns->event = 0;
        list_add(&mnt->mnt_list, &ns->list);
        ns->root = mnt;
        mnt->mnt_ns = ns;

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

        root.mnt = ns->root;
        root.dentry = ns->root->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 vfsmount),
                        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("Mount-cache hash table entries: %lu\n", HASH_SIZE);

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

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

void __put_mnt_ns(struct mnt_namespace *ns)
{
        struct vfsmount *root = ns->root;
        LIST_HEAD(umount_list);
        ns->root = NULL;
        spin_unlock(&vfsmount_lock);
        down_write(&namespace_sem);
        spin_lock(&vfsmount_lock);
        umount_tree(root, 0, &umount_list);
        spin_unlock(&vfsmount_lock);
        up_write(&namespace_sem);
        release_mounts(&umount_list);
        kfree(ns);
}