root/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/config.h>
#include <linux/slab.h>
#include <linux/sched.h>
#include <linux/smp_lock.h>
#include <linux/init.h>
#include <linux/quotaops.h>
#include <linux/acct.h>
#include <linux/module.h>
#include <linux/seq_file.h>
#include <linux/namespace.h>
#include <linux/namei.h>
#include <linux/security.h>
#include <linux/mount.h>
#include <asm/uaccess.h>
#include <asm/unistd.h>

extern int __init init_rootfs(void);

#ifdef CONFIG_SYSFS
extern int __init sysfs_init(void);
#else
static inline int sysfs_init(void)
{
        return 0;
}
#endif

/* spinlock for vfsmount related operations, inplace of dcache_lock */
spinlock_t vfsmount_lock __cacheline_aligned_in_smp = SPIN_LOCK_UNLOCKED;

static struct list_head *mount_hashtable;
static int hash_mask, hash_bits;
static kmem_cache_t *mnt_cache; 

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_bits);
        return tmp & hash_mask;
}

struct vfsmount *alloc_vfsmnt(const char *name)
{
        struct vfsmount *mnt = kmem_cache_alloc(mnt_cache, GFP_KERNEL); 
        if (mnt) {
                memset(mnt, 0, sizeof(struct vfsmount));
                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);
                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;
}

void free_vfsmnt(struct vfsmount *mnt)
{
        kfree(mnt->mnt_devname);
        kmem_cache_free(mnt_cache, mnt);
}

/*
 * Now, lookup_mnt increments the ref count before returning
 * the vfsmount struct.
 */
struct vfsmount *lookup_mnt(struct vfsmount *mnt, struct dentry *dentry)
{
        struct list_head * head = mount_hashtable + hash(mnt, dentry);
        struct list_head * tmp = head;
        struct vfsmount *p, *found = NULL;

        spin_lock(&vfsmount_lock);
        for (;;) {
                tmp = tmp->next;
                p = NULL;
                if (tmp == head)
                        break;
                p = list_entry(tmp, struct vfsmount, mnt_hash);
                if (p->mnt_parent == mnt && p->mnt_mountpoint == dentry) {
                        found = mntget(p);
                        break;
                }
        }
        spin_unlock(&vfsmount_lock);
        return found;
}

EXPORT_SYMBOL(lookup_mnt);

static int check_mnt(struct vfsmount *mnt)
{
        spin_lock(&vfsmount_lock);
        while (mnt->mnt_parent != mnt)
                mnt = mnt->mnt_parent;
        spin_unlock(&vfsmount_lock);
        return mnt == current->namespace->root;
}

static void detach_mnt(struct vfsmount *mnt, struct nameidata *old_nd)
{
        old_nd->dentry = mnt->mnt_mountpoint;
        old_nd->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_nd->dentry->d_mounted--;
}

static void attach_mnt(struct vfsmount *mnt, struct nameidata *nd)
{
        mnt->mnt_parent = mntget(nd->mnt);
        mnt->mnt_mountpoint = dget(nd->dentry);
        list_add(&mnt->mnt_hash, mount_hashtable+hash(nd->mnt, nd->dentry));
        list_add_tail(&mnt->mnt_child, &nd->mnt->mnt_mounts);
        nd->dentry->d_mounted++;
}

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 *
clone_mnt(struct vfsmount *old, struct dentry *root)
{
        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;
        }
        return mnt;
}

void __mntput(struct vfsmount *mnt)
{
        struct super_block *sb = mnt->mnt_sb;
        dput(mnt->mnt_root);
        free_vfsmnt(mnt);
        deactivate_super(sb);
}

EXPORT_SYMBOL(__mntput);

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

        down_read(&n->sem);
        list_for_each(p, &n->list)
                if (!l--)
                        return list_entry(p, struct vfsmount, mnt_list);
        return NULL;
}

static void *m_next(struct seq_file *m, void *v, loff_t *pos)
{
        struct namespace *n = m->private;
        struct list_head *p = ((struct vfsmount *)v)->mnt_list.next;
        (*pos)++;
        return p==&n->list ? NULL : list_entry(p, struct vfsmount, mnt_list);
}

static void m_stop(struct seq_file *m, void *v)
{
        struct namespace *n = m->private;
        up_read(&n->sem);
}

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

static int show_vfsmnt(struct seq_file *m, void *v)
{
        struct vfsmount *mnt = v;
        int err = 0;
        static struct proc_fs_info {
                int flag;
                char *str;
        } fs_info[] = {
                { MS_SYNCHRONOUS, ",sync" },
                { MS_DIRSYNC, ",dirsync" },
                { MS_MANDLOCK, ",mand" },
                { MS_NOATIME, ",noatime" },
                { MS_NODIRATIME, ",nodiratime" },
                { 0, NULL }
        };
        static struct proc_fs_info mnt_info[] = {
                { MNT_NOSUID, ",nosuid" },
                { MNT_NODEV, ",nodev" },
                { MNT_NOEXEC, ",noexec" },
                { 0, NULL }
        };
        struct proc_fs_info *fs_infop;

        mangle(m, mnt->mnt_devname ? mnt->mnt_devname : "none");
        seq_putc(m, ' ');
        seq_path(m, mnt, mnt->mnt_root, " \t\n\\");
        seq_putc(m, ' ');
        mangle(m, mnt->mnt_sb->s_type->name);
        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
};

/**
 * 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)
{
        struct list_head *next;
        struct vfsmount *this_parent = mnt;
        int actual_refs;
        int minimum_refs;

        spin_lock(&vfsmount_lock);
        actual_refs = atomic_read(&mnt->mnt_count);
        minimum_refs = 2;
repeat:
        next = this_parent->mnt_mounts.next;
resume:
        while (next != &this_parent->mnt_mounts) {
                struct vfsmount *p = list_entry(next, struct vfsmount, mnt_child);

                next = next->next;

                actual_refs += atomic_read(&p->mnt_count);
                minimum_refs += 2;

                if (!list_empty(&p->mnt_mounts)) {
                        this_parent = p;
                        goto repeat;
                }
        }

        if (this_parent != mnt) {
                next = this_parent->mnt_child.next;
                this_parent = this_parent->mnt_parent;
                goto resume;
        }
        spin_unlock(&vfsmount_lock);

        if (actual_refs > minimum_refs)
                return -EBUSY;

        return 0;
}

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)
{
        if (atomic_read(&mnt->mnt_count) > 2)
                return -EBUSY;
        return 0;
}

EXPORT_SYMBOL(may_umount);

void umount_tree(struct vfsmount *mnt)
{
        struct vfsmount *p;
        LIST_HEAD(kill);

        for (p = mnt; p; p = next_mnt(p, mnt)) {
                list_del(&p->mnt_list);
                list_add(&p->mnt_list, &kill);
        }

        while (!list_empty(&kill)) {
                mnt = list_entry(kill.next, struct vfsmount, mnt_list);
                list_del_init(&mnt->mnt_list);
                if (mnt->mnt_parent == mnt) {
                        spin_unlock(&vfsmount_lock);
                } else {
                        struct nameidata old_nd;
                        detach_mnt(mnt, &old_nd);
                        spin_unlock(&vfsmount_lock);
                        path_release(&old_nd);
                }
                mntput(mnt);
                spin_lock(&vfsmount_lock);
        }
}

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

        retval = security_sb_umount(mnt, flags);
        if (retval)
                return retval;

        /*
         * 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( (flags&MNT_FORCE) && sb->s_op->umount_begin)
                sb->s_op->umount_begin(sb);
        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->rootmnt && !(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();
                        retval = do_remount_sb(sb, MS_RDONLY, 0, 0);
                        unlock_kernel();
                }
                up_write(&sb->s_umount);
                return retval;
        }

        down_write(&current->namespace->sem);
        spin_lock(&vfsmount_lock);

        if (atomic_read(&sb->s_active) == 1) {
                /* last instance - try to be smart */
                spin_unlock(&vfsmount_lock);
                lock_kernel();
                DQUOT_OFF(sb);
                acct_auto_close(sb);
                unlock_kernel();
                security_sb_umount_close(mnt);
                spin_lock(&vfsmount_lock);
        }
        retval = -EBUSY;
        if (atomic_read(&mnt->mnt_count) == 2 || flags & MNT_DETACH) {
                if (!list_empty(&mnt->mnt_list))
                        umount_tree(mnt);
                retval = 0;
        }
        spin_unlock(&vfsmount_lock);
        if (retval)
                security_sb_umount_busy(mnt);
        up_write(&current->namespace->sem);
        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.dentry != nd.mnt->mnt_root)
                goto dput_and_out;
        if (!check_mnt(nd.mnt))
                goto dput_and_out;

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

        retval = do_umount(nd.mnt, flags);
dput_and_out:
        path_release(&nd);
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->dentry->d_inode->i_mode))
                return -EPERM;
        if (nd->dentry->d_inode->i_mode & S_ISVTX) {
                if (current->uid != nd->dentry->d_inode->i_uid)
                        return -EPERM;
        }
        if (permission(nd->dentry->d_inode, MAY_WRITE, nd))
                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;
        }
}

static struct vfsmount *copy_tree(struct vfsmount *mnt, struct dentry *dentry)
{
        struct vfsmount *res, *p, *q, *r, *s;
        struct list_head *h;
        struct nameidata nd;

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

        p = mnt;
        for (h = mnt->mnt_mounts.next; h != &mnt->mnt_mounts; h = h->next) {
                r = list_entry(h, struct vfsmount, mnt_child);
                if (!lives_below_in_same_fs(r->mnt_mountpoint, dentry))
                        continue;

                for (s = r; s; s = next_mnt(s, r)) {
                        while (p != s->mnt_parent) {
                                p = p->mnt_parent;
                                q = q->mnt_parent;
                        }
                        p = s;
                        nd.mnt = q;
                        nd.dentry = p->mnt_mountpoint;
                        q = clone_mnt(p, p->mnt_root);
                        if (!q)
                                goto Enomem;
                        spin_lock(&vfsmount_lock);
                        list_add_tail(&q->mnt_list, &res->mnt_list);
                        attach_mnt(q, &nd);
                        spin_unlock(&vfsmount_lock);
                }
        }
        return res;
 Enomem:
        if (res) {
                spin_lock(&vfsmount_lock);
                umount_tree(res);
                spin_unlock(&vfsmount_lock);
        }
        return NULL;
}

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

        err = -ENOENT;
        down(&nd->dentry->d_inode->i_sem);
        if (IS_DEADDIR(nd->dentry->d_inode))
                goto out_unlock;

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

        err = -ENOENT;
        spin_lock(&vfsmount_lock);
        if (IS_ROOT(nd->dentry) || !d_unhashed(nd->dentry)) {
                struct list_head head;

                attach_mnt(mnt, nd);
                list_add_tail(&head, &mnt->mnt_list);
                list_splice(&head, current->namespace->list.prev);
                mntget(mnt);
                err = 0;
        }
        spin_unlock(&vfsmount_lock);
out_unlock:
        up(&nd->dentry->d_inode->i_sem);
        if (!err)
                security_sb_post_addmount(mnt, nd);
        return err;
}

/*
 * do loopback mount.
 */
static 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(&current->namespace->sem);
        err = -EINVAL;
        if (check_mnt(nd->mnt) && (!recurse || check_mnt(old_nd.mnt))) {
                err = -ENOMEM;
                if (recurse)
                        mnt = copy_tree(old_nd.mnt, old_nd.dentry);
                else
                        mnt = clone_mnt(old_nd.mnt, old_nd.dentry);
        }

        if (mnt) {
                err = graft_tree(mnt, nd);
                if (err) {
                        spin_lock(&vfsmount_lock);
                        umount_tree(mnt);
                        spin_unlock(&vfsmount_lock);
                } else
                        mntput(mnt);
        }

        up_write(&current->namespace->sem);
        path_release(&old_nd);
        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.
 */

static int do_remount(struct nameidata *nd,int flags,int mnt_flags,void *data)
{
        int err;
        struct super_block * sb = nd->mnt->mnt_sb;

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

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

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

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

static int do_move_mount(struct nameidata *nd, char *old_name)
{
        struct nameidata old_nd, parent_nd;
        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(&current->namespace->sem);
        while(d_mountpoint(nd->dentry) && follow_down(&nd->mnt, &nd->dentry))
                ;
        err = -EINVAL;
        if (!check_mnt(nd->mnt) || !check_mnt(old_nd.mnt))
                goto out;

        err = -ENOENT;
        down(&nd->dentry->d_inode->i_sem);
        if (IS_DEADDIR(nd->dentry->d_inode))
                goto out1;

        spin_lock(&vfsmount_lock);
        if (!IS_ROOT(nd->dentry) && d_unhashed(nd->dentry))
                goto out2;

        err = -EINVAL;
        if (old_nd.dentry != old_nd.mnt->mnt_root)
                goto out2;

        if (old_nd.mnt == old_nd.mnt->mnt_parent)
                goto out2;

        if (S_ISDIR(nd->dentry->d_inode->i_mode) !=
              S_ISDIR(old_nd.dentry->d_inode->i_mode))
                goto out2;

        err = -ELOOP;
        for (p = nd->mnt; p->mnt_parent!=p; p = p->mnt_parent)
                if (p == old_nd.mnt)
                        goto out2;
        err = 0;

        detach_mnt(old_nd.mnt, &parent_nd);
        attach_mnt(old_nd.mnt, nd);
out2:
        spin_unlock(&vfsmount_lock);
out1:
        up(&nd->dentry->d_inode->i_sem);
out:
        up_write(&current->namespace->sem);
        if (!err)
                path_release(&parent_nd);
        path_release(&old_nd);
        return err;
}

static int do_add_mount(struct nameidata *nd, char *type, int flags,
                        int mnt_flags, char *name, void *data)
{
        struct vfsmount *mnt;
        int err;

        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);
        err = PTR_ERR(mnt);
        if (IS_ERR(mnt))
                goto out;

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

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

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

        mnt->mnt_flags = mnt_flags;
        err = graft_tree(mnt, nd);
unlock:
        up_write(&current->namespace->sem);
        mntput(mnt);
out:
        return err;
}

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 - 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;
        flags &= ~(MS_NOSUID|MS_NOEXEC|MS_NODEV|MS_ACTIVE);

        /* ... 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_MOVE)
                retval = do_move_mount(&nd, dev_name);
        else
                retval = do_add_mount(&nd, type_page, flags, mnt_flags,
                                      dev_name, data_page);
dput_out:
        path_release(&nd);
        return retval;
}

int copy_namespace(int flags, struct task_struct *tsk)
{
        struct namespace *namespace = tsk->namespace;
        struct namespace *new_ns;
        struct vfsmount *rootmnt = NULL, *pwdmnt = NULL, *altrootmnt = NULL;
        struct fs_struct *fs = tsk->fs;

        if (!namespace)
                return 0;

        get_namespace(namespace);

        if (!(flags & CLONE_NEWNS))
                return 0;

        if (!capable(CAP_SYS_ADMIN)) {
                put_namespace(namespace);
                return -EPERM;
        }

        new_ns = kmalloc(sizeof(struct namespace), GFP_KERNEL);
        if (!new_ns)
                goto out;

        atomic_set(&new_ns->count, 1);
        init_rwsem(&new_ns->sem);
        INIT_LIST_HEAD(&new_ns->list);

        down_write(&tsk->namespace->sem);
        /* First pass: copy the tree topology */
        new_ns->root = copy_tree(namespace->root, namespace->root->mnt_root);
        if (!new_ns->root) {
                up_write(&tsk->namespace->sem);
                kfree(new_ns);
                goto out;
        }
        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 */
        if (fs) {
                struct vfsmount *p, *q;
                write_lock(&fs->lock);

                p = namespace->root;
                q = new_ns->root;
                while (p) {
                        if (p == fs->rootmnt) {
                                rootmnt = p;
                                fs->rootmnt = mntget(q);
                        }
                        if (p == fs->pwdmnt) {
                                pwdmnt = p;
                                fs->pwdmnt = mntget(q);
                        }
                        if (p == fs->altrootmnt) {
                                altrootmnt = p;
                                fs->altrootmnt = mntget(q);
                        }
                        p = next_mnt(p, namespace->root);
                        q = next_mnt(q, new_ns->root);
                }
                write_unlock(&fs->lock);
        }
        up_write(&tsk->namespace->sem);

        tsk->namespace = new_ns;

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

        put_namespace(namespace);
        return 0;

out:
        put_namespace(namespace);
        return -ENOMEM;
}

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 vfsmount *mnt,
                 struct dentry *dentry)
{
        struct dentry *old_root;
        struct vfsmount *old_rootmnt;
        write_lock(&fs->lock);
        old_root = fs->root;
        old_rootmnt = fs->rootmnt;
        fs->rootmnt = mntget(mnt);
        fs->root = dget(dentry);
        write_unlock(&fs->lock);
        if (old_root) {
                dput(old_root);
                mntput(old_rootmnt);
        }
}

EXPORT_SYMBOL(set_fs_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 vfsmount *mnt,
                struct dentry *dentry)
{
        struct dentry *old_pwd;
        struct vfsmount *old_pwdmnt;

        write_lock(&fs->lock);
        old_pwd = fs->pwd;
        old_pwdmnt = fs->pwdmnt;
        fs->pwdmnt = mntget(mnt);
        fs->pwd = dget(dentry);
        write_unlock(&fs->lock);

        if (old_pwd) {
                dput(old_pwd);
                mntput(old_pwdmnt);
        }
}

EXPORT_SYMBOL(set_fs_pwd);

static void chroot_fs_refs(struct nameidata *old_nd, struct nameidata *new_nd)
{
        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==old_nd->dentry&&fs->rootmnt==old_nd->mnt)
                                set_fs_root(fs, new_nd->mnt, new_nd->dentry);
                        if (fs->pwd==old_nd->dentry&&fs->pwdmnt==old_nd->mnt)
                                set_fs_pwd(fs, new_nd->mnt, new_nd->dentry);
                        put_fs_struct(fs);
                } else
                        task_unlock(p);
        } while_each_thread(g, p);
        read_unlock(&tasklist_lock);
}

/*
 * Moves the current root to put_root, and sets root/cwd of all processes
 * which had them on the old root to new_root.
 *
 * Note:
 *  - 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, parent_nd, root_parent, user_nd;
        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.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_release(&old_nd);
                goto out1;
        }

        read_lock(&current->fs->lock);
        user_nd.mnt = mntget(current->fs->rootmnt);
        user_nd.dentry = dget(current->fs->root);
        read_unlock(&current->fs->lock);
        down_write(&current->namespace->sem);
        down(&old_nd.dentry->d_inode->i_sem);
        error = -EINVAL;
        if (!check_mnt(user_nd.mnt))
                goto out2;
        error = -ENOENT;
        if (IS_DEADDIR(new_nd.dentry->d_inode))
                goto out2;
        if (d_unhashed(new_nd.dentry) && !IS_ROOT(new_nd.dentry))
                goto out2;
        if (d_unhashed(old_nd.dentry) && !IS_ROOT(old_nd.dentry))
                goto out2;
        error = -EBUSY;
        if (new_nd.mnt == user_nd.mnt || old_nd.mnt == user_nd.mnt)
                goto out2; /* loop */
        error = -EINVAL;
        if (user_nd.mnt->mnt_root != user_nd.dentry)
                goto out2;
        if (new_nd.mnt->mnt_root != new_nd.dentry)
                goto out2; /* not a mountpoint */
        tmp = old_nd.mnt; /* make sure we can reach put_old from new_root */
        spin_lock(&vfsmount_lock);
        if (tmp != new_nd.mnt) {
                for (;;) {
                        if (tmp->mnt_parent == tmp)
                                goto out3;
                        if (tmp->mnt_parent == new_nd.mnt)
                                break;
                        tmp = tmp->mnt_parent;
                }
                if (!is_subdir(tmp->mnt_mountpoint, new_nd.dentry))
                        goto out3;
        } else if (!is_subdir(old_nd.dentry, new_nd.dentry))
                goto out3;
        detach_mnt(new_nd.mnt, &parent_nd);
        detach_mnt(user_nd.mnt, &root_parent);
        attach_mnt(user_nd.mnt, &old_nd);
        attach_mnt(new_nd.mnt, &root_parent);
        spin_unlock(&vfsmount_lock);
        chroot_fs_refs(&user_nd, &new_nd);
        security_sb_post_pivotroot(&user_nd, &new_nd);
        error = 0;
        path_release(&root_parent);
        path_release(&parent_nd);
out2:
        up(&old_nd.dentry->d_inode->i_sem);
        up_write(&current->namespace->sem);
        path_release(&user_nd);
        path_release(&old_nd);
out1:
        path_release(&new_nd);
out0:
        unlock_kernel();
        return error;
out3:
        spin_unlock(&vfsmount_lock);
        goto out2;
}

static void __init init_mount_tree(void)
{
        struct vfsmount *mnt;
        struct namespace *namespace;
        struct task_struct *g, *p;

        mnt = do_kern_mount("rootfs", 0, "rootfs", NULL);
        if (IS_ERR(mnt))
                panic("Can't create rootfs");
        namespace = kmalloc(sizeof(*namespace), GFP_KERNEL);
        if (!namespace)
                panic("Can't allocate initial namespace");
        atomic_set(&namespace->count, 1);
        INIT_LIST_HEAD(&namespace->list);
        init_rwsem(&namespace->sem);
        list_add(&mnt->mnt_list, &namespace->list);
        namespace->root = mnt;

        init_task.namespace = namespace;
        read_lock(&tasklist_lock);
        do_each_thread(g, p) {
                get_namespace(namespace);
                p->namespace = namespace;
        } while_each_thread(g, p);
        read_unlock(&tasklist_lock);

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

void __init mnt_init(unsigned long mempages)
{
        struct list_head *d;
        unsigned long order;
        unsigned int nr_hash;
        int i;

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

        order = 0; 
        mount_hashtable = (struct list_head *)
                __get_free_pages(GFP_ATOMIC, order);

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

        /*
         * Find the power-of-two list-heads that can fit into the allocation..
         * We don't guarantee that "sizeof(struct list_head)" is necessarily
         * a power-of-two.
         */
        nr_hash = (1UL << order) * PAGE_SIZE / sizeof(struct list_head);
        hash_bits = 0;
        do {
                hash_bits++;
        } while ((nr_hash >> hash_bits) != 0);
        hash_bits--;

        /*
         * Re-calculate the actual number of entries and the mask
         * from the number of bits we can fit.
         */
        nr_hash = 1UL << hash_bits;
        hash_mask = nr_hash-1;

        printk("Mount-cache hash table entries: %d (order: %ld, %ld bytes)\n",
                        nr_hash, order, (PAGE_SIZE << order));

        /* And initialize the newly allocated array */
        d = mount_hashtable;
        i = nr_hash;
        do {
                INIT_LIST_HEAD(d);
                d++;
                i--;
        } while (i);
        sysfs_init();
        init_rootfs();
        init_mount_tree();
}

void __put_namespace(struct namespace *namespace)
{
        down_write(&namespace->sem);
        spin_lock(&vfsmount_lock);
        umount_tree(namespace->root);
        spin_unlock(&vfsmount_lock);
        up_write(&namespace->sem);
        kfree(namespace);
}