FreeBSD/Linux Kernel Cross Reference
sys/fs/namespace.c

     /*
      *  linux/fs/namespace.c
      *
      * (C) Copyright Al Viro 2000, 2001
      *      Released under GPL v2.
      *
      * Based on code from fs/super.c, copyright Linus Torvalds and others.
      * Heavily rewritten.
      */
    
    #include <linux/syscalls.h>
    #include <linux/export.h>
    #include <linux/capability.h>
    #include <linux/mnt_namespace.h>
    #include <linux/user_namespace.h>
    #include <linux/namei.h>
    #include <linux/security.h>
    #include <linux/idr.h>
    #include <linux/acct.h>         /* acct_auto_close_mnt */
    #include <linux/ramfs.h>        /* init_rootfs */
    #include <linux/fs_struct.h>    /* get_fs_root et.al. */
    #include <linux/fsnotify.h>     /* fsnotify_vfsmount_delete */
    #include <linux/uaccess.h>
    #include <linux/proc_fs.h>
    #include "pnode.h"
    #include "internal.h"
    
    #define HASH_SHIFT ilog2(PAGE_SIZE / sizeof(struct list_head))
    #define HASH_SIZE (1UL << HASH_SHIFT)
    
    static int event;
    static DEFINE_IDA(mnt_id_ida);
    static DEFINE_IDA(mnt_group_ida);
    static DEFINE_SPINLOCK(mnt_id_lock);
    static int mnt_id_start = 0;
    static int mnt_group_start = 1;
    
    static struct list_head *mount_hashtable __read_mostly;
    static struct kmem_cache *mnt_cache __read_mostly;
    static struct rw_semaphore namespace_sem;
    
    /* /sys/fs */
    struct kobject *fs_kobj;
    EXPORT_SYMBOL_GPL(fs_kobj);
    
    /*
     * vfsmount lock may be taken for read to prevent changes to the
     * vfsmount hash, ie. during mountpoint lookups or walking back
     * up the tree.
     *
     * It should be taken for write in all cases where the vfsmount
     * tree or hash is modified or when a vfsmount structure is modified.
     */
    DEFINE_BRLOCK(vfsmount_lock);
    
    static inline unsigned long hash(struct vfsmount *mnt, struct dentry *dentry)
    {
            unsigned long tmp = ((unsigned long)mnt / L1_CACHE_BYTES);
            tmp += ((unsigned long)dentry / L1_CACHE_BYTES);
            tmp = tmp + (tmp >> HASH_SHIFT);
            return tmp & (HASH_SIZE - 1);
    }
    
    #define MNT_WRITER_UNDERFLOW_LIMIT -(1<<16)
    
    /*
     * allocation is serialized by namespace_sem, but we need the spinlock to
     * serialize with freeing.
     */
    static int mnt_alloc_id(struct mount *mnt)
    {
            int res;
    
    retry:
            ida_pre_get(&mnt_id_ida, GFP_KERNEL);
            spin_lock(&mnt_id_lock);
            res = ida_get_new_above(&mnt_id_ida, mnt_id_start, &mnt->mnt_id);
            if (!res)
                    mnt_id_start = mnt->mnt_id + 1;
            spin_unlock(&mnt_id_lock);
            if (res == -EAGAIN)
                    goto retry;
    
            return res;
    }
    
    static void mnt_free_id(struct mount *mnt)
    {
            int id = mnt->mnt_id;
            spin_lock(&mnt_id_lock);
            ida_remove(&mnt_id_ida, id);
            if (mnt_id_start > id)
                    mnt_id_start = id;
            spin_unlock(&mnt_id_lock);
    }
    
    /*
     * Allocate a new peer group ID
     *
    * mnt_group_ida is protected by namespace_sem
    */
   static int mnt_alloc_group_id(struct mount *mnt)
   {
           int res;
   
           if (!ida_pre_get(&mnt_group_ida, GFP_KERNEL))
                   return -ENOMEM;
   
           res = ida_get_new_above(&mnt_group_ida,
                                   mnt_group_start,
                                   &mnt->mnt_group_id);
           if (!res)
                   mnt_group_start = mnt->mnt_group_id + 1;
   
           return res;
   }
   
   /*
    * Release a peer group ID
    */
   void mnt_release_group_id(struct mount *mnt)
   {
           int id = mnt->mnt_group_id;
           ida_remove(&mnt_group_ida, id);
           if (mnt_group_start > id)
                   mnt_group_start = id;
           mnt->mnt_group_id = 0;
   }
   
   /*
    * vfsmount lock must be held for read
    */
   static inline void mnt_add_count(struct mount *mnt, int n)
   {
   #ifdef CONFIG_SMP
           this_cpu_add(mnt->mnt_pcp->mnt_count, n);
   #else
           preempt_disable();
           mnt->mnt_count += n;
           preempt_enable();
   #endif
   }
   
   /*
    * vfsmount lock must be held for write
    */
   unsigned int mnt_get_count(struct mount *mnt)
   {
   #ifdef CONFIG_SMP
           unsigned int count = 0;
           int cpu;
   
           for_each_possible_cpu(cpu) {
                   count += per_cpu_ptr(mnt->mnt_pcp, cpu)->mnt_count;
           }
   
           return count;
   #else
           return mnt->mnt_count;
   #endif
   }
   
   static struct mount *alloc_vfsmnt(const char *name)
   {
           struct mount *mnt = kmem_cache_zalloc(mnt_cache, GFP_KERNEL);
           if (mnt) {
                   int err;
   
                   err = mnt_alloc_id(mnt);
                   if (err)
                           goto out_free_cache;
   
                   if (name) {
                           mnt->mnt_devname = kstrdup(name, GFP_KERNEL);
                           if (!mnt->mnt_devname)
                                   goto out_free_id;
                   }
   
   #ifdef CONFIG_SMP
                   mnt->mnt_pcp = alloc_percpu(struct mnt_pcp);
                   if (!mnt->mnt_pcp)
                           goto out_free_devname;
   
                   this_cpu_add(mnt->mnt_pcp->mnt_count, 1);
   #else
                   mnt->mnt_count = 1;
                   mnt->mnt_writers = 0;
   #endif
   
                   INIT_LIST_HEAD(&mnt->mnt_hash);
                   INIT_LIST_HEAD(&mnt->mnt_child);
                   INIT_LIST_HEAD(&mnt->mnt_mounts);
                   INIT_LIST_HEAD(&mnt->mnt_list);
                   INIT_LIST_HEAD(&mnt->mnt_expire);
                   INIT_LIST_HEAD(&mnt->mnt_share);
                   INIT_LIST_HEAD(&mnt->mnt_slave_list);
                   INIT_LIST_HEAD(&mnt->mnt_slave);
   #ifdef CONFIG_FSNOTIFY
                   INIT_HLIST_HEAD(&mnt->mnt_fsnotify_marks);
   #endif
           }
           return mnt;
   
   #ifdef CONFIG_SMP
   out_free_devname:
           kfree(mnt->mnt_devname);
   #endif
   out_free_id:
           mnt_free_id(mnt);
   out_free_cache:
           kmem_cache_free(mnt_cache, mnt);
           return NULL;
   }
   
   /*
    * Most r/o checks on a fs are for operations that take
    * discrete amounts of time, like a write() or unlink().
    * We must keep track of when those operations start
    * (for permission checks) and when they end, so that
    * we can determine when writes are able to occur to
    * a filesystem.
    */
   /*
    * __mnt_is_readonly: check whether a mount is read-only
    * @mnt: the mount to check for its write status
    *
    * This shouldn't be used directly ouside of the VFS.
    * It does not guarantee that the filesystem will stay
    * r/w, just that it is right *now*.  This can not and
    * should not be used in place of IS_RDONLY(inode).
    * mnt_want/drop_write() will _keep_ the filesystem
    * r/w.
    */
   int __mnt_is_readonly(struct vfsmount *mnt)
   {
           if (mnt->mnt_flags & MNT_READONLY)
                   return 1;
           if (mnt->mnt_sb->s_flags & MS_RDONLY)
                   return 1;
           return 0;
   }
   EXPORT_SYMBOL_GPL(__mnt_is_readonly);
   
   static inline void mnt_inc_writers(struct mount *mnt)
   {
   #ifdef CONFIG_SMP
           this_cpu_inc(mnt->mnt_pcp->mnt_writers);
   #else
           mnt->mnt_writers++;
   #endif
   }
   
   static inline void mnt_dec_writers(struct mount *mnt)
   {
   #ifdef CONFIG_SMP
           this_cpu_dec(mnt->mnt_pcp->mnt_writers);
   #else
           mnt->mnt_writers--;
   #endif
   }
   
   static unsigned int mnt_get_writers(struct mount *mnt)
   {
   #ifdef CONFIG_SMP
           unsigned int count = 0;
           int cpu;
   
           for_each_possible_cpu(cpu) {
                   count += per_cpu_ptr(mnt->mnt_pcp, cpu)->mnt_writers;
           }
   
           return count;
   #else
           return mnt->mnt_writers;
   #endif
   }
   
   static int mnt_is_readonly(struct vfsmount *mnt)
   {
           if (mnt->mnt_sb->s_readonly_remount)
                   return 1;
           /* Order wrt setting s_flags/s_readonly_remount in do_remount() */
           smp_rmb();
           return __mnt_is_readonly(mnt);
   }
   
   /*
    * Most r/o & frozen 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 without freeze protection
    * @m: the mount on which to take a write
    *
    * This tells the low-level filesystem that a write is about to be performed to
    * it, and makes sure that writes are allowed (mnt it read-write) before
    * returning success. This operation does not protect against filesystem being
    * frozen. When the write operation is finished, __mnt_drop_write() must be
    * called. This is effectively a refcount.
    */
   int __mnt_want_write(struct vfsmount *m)
   {
           struct mount *mnt = real_mount(m);
           int ret = 0;
   
           preempt_disable();
           mnt_inc_writers(mnt);
           /*
            * The store to mnt_inc_writers must be visible before we pass
            * MNT_WRITE_HOLD loop below, so that the slowpath can see our
            * incremented count after it has set MNT_WRITE_HOLD.
            */
           smp_mb();
           while (ACCESS_ONCE(mnt->mnt.mnt_flags) & MNT_WRITE_HOLD)
                   cpu_relax();
           /*
            * After the slowpath clears MNT_WRITE_HOLD, mnt_is_readonly will
            * be set to match its requirements. So we must not load that until
            * MNT_WRITE_HOLD is cleared.
            */
           smp_rmb();
           if (mnt_is_readonly(m)) {
                   mnt_dec_writers(mnt);
                   ret = -EROFS;
           }
           preempt_enable();
   
           return ret;
   }
   
   /**
    * mnt_want_write - get write access to a mount
    * @m: the mount on which to take a write
    *
    * This tells the low-level filesystem that a write is about to be performed to
    * it, and makes sure that writes are allowed (mount is read-write, filesystem
    * is not frozen) before returning success.  When the write operation is
    * finished, mnt_drop_write() must be called.  This is effectively a refcount.
    */
   int mnt_want_write(struct vfsmount *m)
   {
           int ret;
   
           sb_start_write(m->mnt_sb);
           ret = __mnt_want_write(m);
           if (ret)
                   sb_end_write(m->mnt_sb);
           return ret;
   }
   EXPORT_SYMBOL_GPL(mnt_want_write);
   
   /**
    * mnt_clone_write - get write access to a mount
    * @mnt: the mount on which to take a write
    *
    * This is effectively like mnt_want_write, except
    * it must only be used to take an extra write reference
    * on a mountpoint that we already know has a write reference
    * on it. This allows some optimisation.
    *
    * After finished, mnt_drop_write must be called as usual to
    * drop the reference.
    */
   int mnt_clone_write(struct vfsmount *mnt)
   {
           /* superblock may be r/o */
           if (__mnt_is_readonly(mnt))
                   return -EROFS;
           preempt_disable();
           mnt_inc_writers(real_mount(mnt));
           preempt_enable();
           return 0;
   }
   EXPORT_SYMBOL_GPL(mnt_clone_write);
   
   /**
    * __mnt_want_write_file - get write access to a file's mount
    * @file: the file who's mount on which to take a write
    *
    * This is like __mnt_want_write, but it takes a file and can
    * do some optimisations if the file is open for write already
    */
   int __mnt_want_write_file(struct file *file)
   {
           struct inode *inode = file->f_dentry->d_inode;
   
           if (!(file->f_mode & FMODE_WRITE) || special_file(inode->i_mode))
                   return __mnt_want_write(file->f_path.mnt);
           else
                   return mnt_clone_write(file->f_path.mnt);
   }
   
   /**
    * mnt_want_write_file - get write access to a file's mount
    * @file: the file who's mount on which to take a write
    *
    * This is like mnt_want_write, but it takes a file and can
    * do some optimisations if the file is open for write already
    */
   int mnt_want_write_file(struct file *file)
   {
           int ret;
   
           sb_start_write(file->f_path.mnt->mnt_sb);
           ret = __mnt_want_write_file(file);
           if (ret)
                   sb_end_write(file->f_path.mnt->mnt_sb);
           return ret;
   }
   EXPORT_SYMBOL_GPL(mnt_want_write_file);
   
   /**
    * __mnt_drop_write - give up write access to a mount
    * @mnt: the mount on which to give up write access
    *
    * Tells the low-level filesystem that we are done
    * performing writes to it.  Must be matched with
    * __mnt_want_write() call above.
    */
   void __mnt_drop_write(struct vfsmount *mnt)
   {
           preempt_disable();
           mnt_dec_writers(real_mount(mnt));
           preempt_enable();
   }
   
   /**
    * 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 and
    * also allows filesystem to be frozen again.  Must be matched with
    * mnt_want_write() call above.
    */
   void mnt_drop_write(struct vfsmount *mnt)
   {
           __mnt_drop_write(mnt);
           sb_end_write(mnt->mnt_sb);
   }
   EXPORT_SYMBOL_GPL(mnt_drop_write);
   
   void __mnt_drop_write_file(struct file *file)
   {
           __mnt_drop_write(file->f_path.mnt);
   }
   
   void mnt_drop_write_file(struct file *file)
   {
           mnt_drop_write(file->f_path.mnt);
   }
   EXPORT_SYMBOL(mnt_drop_write_file);
   
   static int mnt_make_readonly(struct mount *mnt)
   {
           int ret = 0;
   
           br_write_lock(&vfsmount_lock);
           mnt->mnt.mnt_flags |= MNT_WRITE_HOLD;
           /*
            * After storing MNT_WRITE_HOLD, we'll read the counters. This store
            * should be visible before we do.
            */
           smp_mb();
   
           /*
            * With writers on hold, if this value is zero, then there are
            * definitely no active writers (although held writers may subsequently
            * increment the count, they'll have to wait, and decrement it after
            * seeing MNT_READONLY).
            *
            * It is OK to have counter incremented on one CPU and decremented on
            * another: the sum will add up correctly. The danger would be when we
            * sum up each counter, if we read a counter before it is incremented,
            * but then read another CPU's count which it has been subsequently
            * decremented from -- we would see more decrements than we should.
            * MNT_WRITE_HOLD protects against this scenario, because
            * mnt_want_write first increments count, then smp_mb, then spins on
            * MNT_WRITE_HOLD, so it can't be decremented by another CPU while
            * we're counting up here.
            */
           if (mnt_get_writers(mnt) > 0)
                   ret = -EBUSY;
           else
                   mnt->mnt.mnt_flags |= MNT_READONLY;
           /*
            * MNT_READONLY must become visible before ~MNT_WRITE_HOLD, so writers
            * that become unheld will see MNT_READONLY.
            */
           smp_wmb();
           mnt->mnt.mnt_flags &= ~MNT_WRITE_HOLD;
           br_write_unlock(&vfsmount_lock);
           return ret;
   }
   
   static void __mnt_unmake_readonly(struct mount *mnt)
   {
           br_write_lock(&vfsmount_lock);
           mnt->mnt.mnt_flags &= ~MNT_READONLY;
           br_write_unlock(&vfsmount_lock);
   }
   
   int sb_prepare_remount_readonly(struct super_block *sb)
   {
           struct mount *mnt;
           int err = 0;
   
           /* Racy optimization.  Recheck the counter under MNT_WRITE_HOLD */
           if (atomic_long_read(&sb->s_remove_count))
                   return -EBUSY;
   
           br_write_lock(&vfsmount_lock);
           list_for_each_entry(mnt, &sb->s_mounts, mnt_instance) {
                   if (!(mnt->mnt.mnt_flags & MNT_READONLY)) {
                           mnt->mnt.mnt_flags |= MNT_WRITE_HOLD;
                           smp_mb();
                           if (mnt_get_writers(mnt) > 0) {
                                   err = -EBUSY;
                                   break;
                           }
                   }
           }
           if (!err && atomic_long_read(&sb->s_remove_count))
                   err = -EBUSY;
   
           if (!err) {
                   sb->s_readonly_remount = 1;
                   smp_wmb();
           }
           list_for_each_entry(mnt, &sb->s_mounts, mnt_instance) {
                   if (mnt->mnt.mnt_flags & MNT_WRITE_HOLD)
                           mnt->mnt.mnt_flags &= ~MNT_WRITE_HOLD;
           }
           br_write_unlock(&vfsmount_lock);
   
           return err;
   }
   
   static void free_vfsmnt(struct mount *mnt)
   {
           kfree(mnt->mnt_devname);
           mnt_free_id(mnt);
   #ifdef CONFIG_SMP
           free_percpu(mnt->mnt_pcp);
   #endif
           kmem_cache_free(mnt_cache, mnt);
   }
   
   /*
    * find the first or last mount at @dentry on vfsmount @mnt depending on
    * @dir. If @dir is set return the first mount else return the last mount.
    * vfsmount_lock must be held for read or write.
    */
   struct mount *__lookup_mnt(struct vfsmount *mnt, struct dentry *dentry,
                                 int dir)
   {
           struct list_head *head = mount_hashtable + hash(mnt, dentry);
           struct list_head *tmp = head;
           struct mount *p, *found = NULL;
   
           for (;;) {
                   tmp = dir ? tmp->next : tmp->prev;
                   p = NULL;
                   if (tmp == head)
                           break;
                   p = list_entry(tmp, struct mount, mnt_hash);
                   if (&p->mnt_parent->mnt == mnt && p->mnt_mountpoint == dentry) {
                           found = p;
                           break;
                   }
           }
           return found;
   }
   
   /*
    * lookup_mnt - Return the first child mount mounted at path
    *
    * "First" means first mounted chronologically.  If you create the
    * following mounts:
    *
    * mount /dev/sda1 /mnt
    * mount /dev/sda2 /mnt
    * mount /dev/sda3 /mnt
    *
    * Then lookup_mnt() on the base /mnt dentry in the root mount will
    * return successively the root dentry and vfsmount of /dev/sda1, then
    * /dev/sda2, then /dev/sda3, then NULL.
    *
    * lookup_mnt takes a reference to the found vfsmount.
    */
   struct vfsmount *lookup_mnt(struct path *path)
   {
           struct mount *child_mnt;
   
           br_read_lock(&vfsmount_lock);
           child_mnt = __lookup_mnt(path->mnt, path->dentry, 1);
           if (child_mnt) {
                   mnt_add_count(child_mnt, 1);
                   br_read_unlock(&vfsmount_lock);
                   return &child_mnt->mnt;
           } else {
                   br_read_unlock(&vfsmount_lock);
                   return NULL;
           }
   }
   
   static inline int check_mnt(struct mount *mnt)
   {
           return mnt->mnt_ns == current->nsproxy->mnt_ns;
   }
   
   /*
    * vfsmount lock must be held for write
    */
   static void touch_mnt_namespace(struct mnt_namespace *ns)
   {
           if (ns) {
                   ns->event = ++event;
                   wake_up_interruptible(&ns->poll);
           }
   }
   
   /*
    * vfsmount lock must be held for write
    */
   static void __touch_mnt_namespace(struct mnt_namespace *ns)
   {
           if (ns && ns->event != event) {
                   ns->event = event;
                   wake_up_interruptible(&ns->poll);
           }
   }
   
   /*
    * Clear dentry's mounted state if it has no remaining mounts.
    * vfsmount_lock must be held for write.
    */
   static void dentry_reset_mounted(struct dentry *dentry)
   {
           unsigned u;
   
           for (u = 0; u < HASH_SIZE; u++) {
                   struct mount *p;
   
                   list_for_each_entry(p, &mount_hashtable[u], mnt_hash) {
                           if (p->mnt_mountpoint == dentry)
                                   return;
                   }
           }
           spin_lock(&dentry->d_lock);
           dentry->d_flags &= ~DCACHE_MOUNTED;
           spin_unlock(&dentry->d_lock);
   }
   
   /*
    * vfsmount lock must be held for write
    */
   static void detach_mnt(struct mount *mnt, struct path *old_path)
   {
           old_path->dentry = mnt->mnt_mountpoint;
           old_path->mnt = &mnt->mnt_parent->mnt;
           mnt->mnt_parent = mnt;
           mnt->mnt_mountpoint = mnt->mnt.mnt_root;
           list_del_init(&mnt->mnt_child);
           list_del_init(&mnt->mnt_hash);
           dentry_reset_mounted(old_path->dentry);
   }
   
   /*
    * vfsmount lock must be held for write
    */
   void mnt_set_mountpoint(struct mount *mnt, struct dentry *dentry,
                           struct mount *child_mnt)
   {
           mnt_add_count(mnt, 1);  /* essentially, that's mntget */
           child_mnt->mnt_mountpoint = dget(dentry);
           child_mnt->mnt_parent = mnt;
           spin_lock(&dentry->d_lock);
           dentry->d_flags |= DCACHE_MOUNTED;
           spin_unlock(&dentry->d_lock);
   }
   
   /*
    * vfsmount lock must be held for write
    */
   static void attach_mnt(struct mount *mnt, struct path *path)
   {
           mnt_set_mountpoint(real_mount(path->mnt), path->dentry, mnt);
           list_add_tail(&mnt->mnt_hash, mount_hashtable +
                           hash(path->mnt, path->dentry));
           list_add_tail(&mnt->mnt_child, &real_mount(path->mnt)->mnt_mounts);
   }
   
   /*
    * vfsmount lock must be held for write
    */
   static void commit_tree(struct mount *mnt)
   {
           struct mount *parent = mnt->mnt_parent;
           struct mount *m;
           LIST_HEAD(head);
           struct mnt_namespace *n = parent->mnt_ns;
   
           BUG_ON(parent == mnt);
   
           list_add_tail(&head, &mnt->mnt_list);
           list_for_each_entry(m, &head, mnt_list)
                   m->mnt_ns = n;
   
           list_splice(&head, n->list.prev);
   
           list_add_tail(&mnt->mnt_hash, mount_hashtable +
                                   hash(&parent->mnt, mnt->mnt_mountpoint));
           list_add_tail(&mnt->mnt_child, &parent->mnt_mounts);
           touch_mnt_namespace(n);
   }
   
   static struct mount *next_mnt(struct mount *p, struct mount *root)
   {
           struct list_head *next = p->mnt_mounts.next;
           if (next == &p->mnt_mounts) {
                   while (1) {
                           if (p == root)
                                   return NULL;
                           next = p->mnt_child.next;
                           if (next != &p->mnt_parent->mnt_mounts)
                                   break;
                           p = p->mnt_parent;
                   }
           }
           return list_entry(next, struct mount, mnt_child);
   }
   
   static struct mount *skip_mnt_tree(struct mount *p)
   {
           struct list_head *prev = p->mnt_mounts.prev;
           while (prev != &p->mnt_mounts) {
                   p = list_entry(prev, struct mount, mnt_child);
                   prev = p->mnt_mounts.prev;
           }
           return p;
   }
   
   struct vfsmount *
   vfs_kern_mount(struct file_system_type *type, int flags, const char *name, void *data)
   {
           struct mount *mnt;
           struct dentry *root;
   
           if (!type)
                   return ERR_PTR(-ENODEV);
   
           mnt = alloc_vfsmnt(name);
           if (!mnt)
                   return ERR_PTR(-ENOMEM);
   
           if (flags & MS_KERNMOUNT)
                   mnt->mnt.mnt_flags = MNT_INTERNAL;
   
           root = mount_fs(type, flags, name, data);
           if (IS_ERR(root)) {
                   free_vfsmnt(mnt);
                   return ERR_CAST(root);
           }
   
           mnt->mnt.mnt_root = root;
           mnt->mnt.mnt_sb = root->d_sb;
           mnt->mnt_mountpoint = mnt->mnt.mnt_root;
           mnt->mnt_parent = mnt;
           br_write_lock(&vfsmount_lock);
           list_add_tail(&mnt->mnt_instance, &root->d_sb->s_mounts);
           br_write_unlock(&vfsmount_lock);
           return &mnt->mnt;
   }
   EXPORT_SYMBOL_GPL(vfs_kern_mount);
   
   static struct mount *clone_mnt(struct mount *old, struct dentry *root,
                                           int flag)
   {
           struct super_block *sb = old->mnt.mnt_sb;
           struct mount *mnt;
           int err;
   
           mnt = alloc_vfsmnt(old->mnt_devname);
           if (!mnt)
                   return ERR_PTR(-ENOMEM);
   
           if (flag & (CL_SLAVE | CL_PRIVATE | CL_SHARED_TO_SLAVE))
                   mnt->mnt_group_id = 0; /* not a peer of original */
           else
                   mnt->mnt_group_id = old->mnt_group_id;
   
           if ((flag & CL_MAKE_SHARED) && !mnt->mnt_group_id) {
                   err = mnt_alloc_group_id(mnt);
                   if (err)
                           goto out_free;
           }
   
           mnt->mnt.mnt_flags = old->mnt.mnt_flags & ~MNT_WRITE_HOLD;
           atomic_inc(&sb->s_active);
           mnt->mnt.mnt_sb = sb;
           mnt->mnt.mnt_root = dget(root);
           mnt->mnt_mountpoint = mnt->mnt.mnt_root;
           mnt->mnt_parent = mnt;
           br_write_lock(&vfsmount_lock);
           list_add_tail(&mnt->mnt_instance, &sb->s_mounts);
           br_write_unlock(&vfsmount_lock);
   
           if ((flag & CL_SLAVE) ||
               ((flag & CL_SHARED_TO_SLAVE) && IS_MNT_SHARED(old))) {
                   list_add(&mnt->mnt_slave, &old->mnt_slave_list);
                   mnt->mnt_master = old;
                   CLEAR_MNT_SHARED(mnt);
           } else if (!(flag & CL_PRIVATE)) {
                   if ((flag & CL_MAKE_SHARED) || IS_MNT_SHARED(old))
                           list_add(&mnt->mnt_share, &old->mnt_share);
                   if (IS_MNT_SLAVE(old))
                           list_add(&mnt->mnt_slave, &old->mnt_slave);
                   mnt->mnt_master = old->mnt_master;
           }
           if (flag & CL_MAKE_SHARED)
                   set_mnt_shared(mnt);
   
           /* stick the duplicate mount on the same expiry list
            * as the original if that was on one */
           if (flag & CL_EXPIRE) {
                   if (!list_empty(&old->mnt_expire))
                           list_add(&mnt->mnt_expire, &old->mnt_expire);
           }
   
           return mnt;
   
    out_free:
           free_vfsmnt(mnt);
           return ERR_PTR(err);
   }
   
   static inline void mntfree(struct mount *mnt)
   {
           struct vfsmount *m = &mnt->mnt;
           struct super_block *sb = m->mnt_sb;
   
           /*
            * This probably indicates that somebody messed
            * up a mnt_want/drop_write() pair.  If this
            * happens, the filesystem was probably unable
            * to make r/w->r/o transitions.
            */
           /*
            * The locking used to deal with mnt_count decrement provides barriers,
            * so mnt_get_writers() below is safe.
            */
           WARN_ON(mnt_get_writers(mnt));
           fsnotify_vfsmount_delete(m);
           dput(m->mnt_root);
           free_vfsmnt(mnt);
           deactivate_super(sb);
   }
   
   static void mntput_no_expire(struct mount *mnt)
   {
   put_again:
   #ifdef CONFIG_SMP
           br_read_lock(&vfsmount_lock);
           if (likely(mnt->mnt_ns)) {
                   /* shouldn't be the last one */
                   mnt_add_count(mnt, -1);
                   br_read_unlock(&vfsmount_lock);
                   return;
           }
           br_read_unlock(&vfsmount_lock);
   
           br_write_lock(&vfsmount_lock);
           mnt_add_count(mnt, -1);
           if (mnt_get_count(mnt)) {
                   br_write_unlock(&vfsmount_lock);
                   return;
           }
   #else
           mnt_add_count(mnt, -1);
           if (likely(mnt_get_count(mnt)))
                   return;
           br_write_lock(&vfsmount_lock);
   #endif
           if (unlikely(mnt->mnt_pinned)) {
                   mnt_add_count(mnt, mnt->mnt_pinned + 1);
                   mnt->mnt_pinned = 0;
                   br_write_unlock(&vfsmount_lock);
                   acct_auto_close_mnt(&mnt->mnt);
                   goto put_again;
           }
   
           list_del(&mnt->mnt_instance);
           br_write_unlock(&vfsmount_lock);
           mntfree(mnt);
   }
   
   void mntput(struct vfsmount *mnt)
   {
           if (mnt) {
                   struct mount *m = real_mount(mnt);
                   /* avoid cacheline pingpong, hope gcc doesn't get "smart" */
                   if (unlikely(m->mnt_expiry_mark))
                           m->mnt_expiry_mark = 0;
                   mntput_no_expire(m);
           }
   }
   EXPORT_SYMBOL(mntput);
   
   struct vfsmount *mntget(struct vfsmount *mnt)
   {
           if (mnt)
                   mnt_add_count(real_mount(mnt), 1);
           return mnt;
   }
   EXPORT_SYMBOL(mntget);
   
   void mnt_pin(struct vfsmount *mnt)
   {
           br_write_lock(&vfsmount_lock);
           real_mount(mnt)->mnt_pinned++;
           br_write_unlock(&vfsmount_lock);
   }
   EXPORT_SYMBOL(mnt_pin);
   
   void mnt_unpin(struct vfsmount *m)
   {
           struct mount *mnt = real_mount(m);
           br_write_lock(&vfsmount_lock);
           if (mnt->mnt_pinned) {
                   mnt_add_count(mnt, 1);
                   mnt->mnt_pinned--;
           }
           br_write_unlock(&vfsmount_lock);
   }
   EXPORT_SYMBOL(mnt_unpin);
   
   static inline void mangle(struct seq_file *m, const char *s)
   {
           seq_escape(m, s, " \t\n\\");
   }
   
   /*
    * Simple .show_options callback for filesystems which don't want to
    * implement more complex mount option showing.
    *
    * See also save_mount_options().
    */
   int generic_show_options(struct seq_file *m, struct dentry *root)
   {
           const char *options;
   
           rcu_read_lock();
           options = rcu_dereference(root->d_sb->s_options);
   
           if (options != NULL && options[0]) {
                   seq_putc(m, ',');
                   mangle(m, options);
           }
           rcu_read_unlock();
   
           return 0;
   }
   EXPORT_SYMBOL(generic_show_options);
   
   /*
    * If filesystem uses generic_show_options(), this function should be
    * called from the fill_super() callback.
    *
    * The .remount_fs callback usually needs to be handled in a special
    * way, to make sure, that previous options are not overwritten if the
    * remount fails.
    *
    * Also note, that if the filesystem's .remount_fs function doesn't
    * reset all options to their default value, but changes only newly
    * given options, then the displayed options will not reflect reality
    * any more.
    */
   void save_mount_options(struct super_block *sb, char *options)
   {
           BUG_ON(sb->s_options);
           rcu_assign_pointer(sb->s_options, kstrdup(options, GFP_KERNEL));
   }
   EXPORT_SYMBOL(save_mount_options);
   
   void replace_mount_options(struct super_block *sb, char *options)
   {
           char *old = sb->s_options;
           rcu_assign_pointer(sb->s_options, options);
           if (old) {
                   synchronize_rcu();
                   kfree(old);
           }
   }
   EXPORT_SYMBOL(replace_mount_options);
   
   #ifdef CONFIG_PROC_FS
   /* iterator; we want it to have access to namespace_sem, thus here... */
  static void *m_start(struct seq_file *m, loff_t *pos)
  {
          struct proc_mounts *p = proc_mounts(m);
  
          down_read(&namespace_sem);
          return seq_list_start(&p->ns->list, *pos);
  }
  
  static void *m_next(struct seq_file *m, void *v, loff_t *pos)
  {
          struct proc_mounts *p = proc_mounts(m);
  
          return seq_list_next(v, &p->ns->list, pos);
  }
  
  static void m_stop(struct seq_file *m, void *v)
  {
          up_read(&namespace_sem);
  }
  
  static int m_show(struct seq_file *m, void *v)
  {
          struct proc_mounts *p = proc_mounts(m);
          struct mount *r = list_entry(v, struct mount, mnt_list);
          return p->show(m, &r->mnt);
  }
  
  const struct seq_operations mounts_op = {
          .start  = m_start,
          .next   = m_next,
          .stop   = m_stop,
          .show   = m_show,
  };
  #endif  /* CONFIG_PROC_FS */
  
  /**
   * may_umount_tree - check if a mount tree is busy
   * @mnt: root of mount tree
   *
   * This is called to check if a tree of mounts has any
   * open files, pwds, chroots or sub mounts that are
   * busy.
   */
  int may_umount_tree(struct vfsmount *m)
  {
          struct mount *mnt = real_mount(m);
          int actual_refs = 0;
          int minimum_refs = 0;
          struct mount *p;
          BUG_ON(!m);
  
          /* write lock needed for mnt_get_count */
          br_write_lock(&vfsmount_lock);
          for (p = mnt; p; p = next_mnt(p, mnt)) {
                  actual_refs += mnt_get_count(p);
                  minimum_refs += 2;
          }
          br_write_unlock(&vfsmount_lock);
  
          if (actual_refs > minimum_refs)
                  return 0;
  
          return 1;
  }
  
  EXPORT_SYMBOL(may_umount_tree);
  
  /**
   * may_umount - check if a mount point is busy
   * @mnt: root of mount
   *
   * This is called to check if a mount point has any
   * open files, pwds, chroots or sub mounts. If the
   * mount has sub mounts this will return busy
   * regardless of whether the sub mounts are busy.
   *
   * Doesn't take quota and stuff into account. IOW, in some cases it will
   * give false negatives. The main reason why it's here is that we need
   * a non-destructive way to look for easily umountable filesystems.
   */
  int may_umount(struct vfsmount *mnt)
  {
          int ret = 1;
          down_read(&namespace_sem);
          br_write_lock(&vfsmount_lock);
          if (propagate_mount_busy(real_mount(mnt), 2))
                  ret = 0;
          br_write_unlock(&vfsmount_lock);
          up_read(&namespace_sem);
          return ret;
  }
  
  EXPORT_SYMBOL(may_umount);
  
  void release_mounts(struct list_head *head)
  {
          struct mount *mnt;
          while (!list_empty(head)) {
                  mnt = list_first_entry(head, struct mount, mnt_hash);
                  list_del_init(&mnt->mnt_hash);
                  if (mnt_has_parent(mnt)) {
                          struct dentry *dentry;
                          struct mount *m;
  
                          br_write_lock(&vfsmount_lock);
                          dentry = mnt->mnt_mountpoint;
                          m = mnt->mnt_parent;
                          mnt->mnt_mountpoint = mnt->mnt.mnt_root;
                          mnt->mnt_parent = mnt;
                          m->mnt_ghosts--;
                          br_write_unlock(&vfsmount_lock);
                          dput(dentry);
                          mntput(&m->mnt);
                  }
                  mntput(&mnt->mnt);
          }
  }
  
  /*
   * vfsmount lock must be held for write
   * namespace_sem must be held for write
   */
  void umount_tree(struct mount *mnt, int propagate, struct list_head *kill)
  {
          LIST_HEAD(tmp_list);
          struct mount *p;
  
          for (p = mnt; p; p = next_mnt(p, mnt))
                  list_move(&p->mnt_hash, &tmp_list);
  
          if (propagate)
                  propagate_umount(&tmp_list);
  
          list_for_each_entry(p, &tmp_list, mnt_hash) {
                  list_del_init(&p->mnt_expire);
                  list_del_init(&p->mnt_list);
                  __touch_mnt_namespace(p->mnt_ns);
                  p->mnt_ns = NULL;
                  list_del_init(&p->mnt_child);
                  if (mnt_has_parent(p)) {
                          p->mnt_parent->mnt_ghosts++;
                          dentry_reset_mounted(p->mnt_mountpoint);
                  }
                  change_mnt_propagation(p, MS_PRIVATE);
          }
          list_splice(&tmp_list, kill);
  }
  
  static void shrink_submounts(struct mount *mnt, struct list_head *umounts);
  
  static int do_umount(struct mount *mnt, int flags)
  {
          struct super_block *sb = mnt->mnt.mnt_sb;
          int retval;
          LIST_HEAD(umount_list);
  
          retval = security_sb_umount(&mnt->mnt, flags);
          if (retval)
                  return retval;
  
          /*
           * Allow userspace to request a mountpoint be expired rather than
           * unmounting unconditionally. Unmount only happens if:
           *  (1) the mark is already set (the mark is cleared by mntput())
           *  (2) the usage count == 1 [parent vfsmount] + 1 [sys_umount]
           */
          if (flags & MNT_EXPIRE) {
                  if (&mnt->mnt == current->fs->root.mnt ||
                      flags & (MNT_FORCE | MNT_DETACH))
                          return -EINVAL;
  
                  /*
                   * probably don't strictly need the lock here if we examined
                   * all race cases, but it's a slowpath.
                   */
                  br_write_lock(&vfsmount_lock);
                  if (mnt_get_count(mnt) != 2) {
                          br_write_unlock(&vfsmount_lock);
                          return -EBUSY;
                  }
                  br_write_unlock(&vfsmount_lock);
  
                  if (!xchg(&mnt->mnt_expiry_mark, 1))
                          return -EAGAIN;
          }
  
          /*
           * If we may have to abort operations to get out of this
           * mount, and they will themselves hold resources we must
           * allow the fs to do things. In the Unix tradition of
           * 'Gee thats tricky lets do it in userspace' the umount_begin
           * might fail to complete on the first run through as other tasks
           * must return, and the like. Thats for the mount program to worry
           * about for the moment.
           */
  
          if (flags & MNT_FORCE && sb->s_op->umount_begin) {
                  sb->s_op->umount_begin(sb);
          }
  
          /*
           * No sense to grab the lock for this test, but test itself looks
           * somewhat bogus. Suggestions for better replacement?
           * Ho-hum... In principle, we might treat that as umount + switch
           * to rootfs. GC would eventually take care of the old vfsmount.
           * Actually it makes sense, especially if rootfs would contain a
           * /reboot - static binary that would close all descriptors and
           * call reboot(9). Then init(8) could umount root and exec /reboot.
           */
          if (&mnt->mnt == current->fs->root.mnt && !(flags & MNT_DETACH)) {
                  /*
                   * Special case for "unmounting" root ...
                   * we just try to remount it readonly.
                   */
                  down_write(&sb->s_umount);
                  if (!(sb->s_flags & MS_RDONLY))
                          retval = do_remount_sb(sb, MS_RDONLY, NULL, 0);
                  up_write(&sb->s_umount);
                  return retval;
          }
  
          down_write(&namespace_sem);
          br_write_lock(&vfsmount_lock);
          event++;
  
          if (!(flags & MNT_DETACH))
                  shrink_submounts(mnt, &umount_list);
  
          retval = -EBUSY;
          if (flags & MNT_DETACH || !propagate_mount_busy(mnt, 2)) {
                  if (!list_empty(&mnt->mnt_list))
                          umount_tree(mnt, 1, &umount_list);
                  retval = 0;
          }
          br_write_unlock(&vfsmount_lock);
          up_write(&namespace_sem);
          release_mounts(&umount_list);
          return retval;
  }
  
  /*
   * Now umount can handle mount points as well as block devices.
   * This is important for filesystems which use unnamed block devices.
   *
   * We now support a flag for forced unmount like the other 'big iron'
   * unixes. Our API is identical to OSF/1 to avoid making a mess of AMD
   */
  
  SYSCALL_DEFINE2(umount, char __user *, name, int, flags)
  {
          struct path path;
          struct mount *mnt;
          int retval;
          int lookup_flags = 0;
  
          if (flags & ~(MNT_FORCE | MNT_DETACH | MNT_EXPIRE | UMOUNT_NOFOLLOW))
                  return -EINVAL;
  
          if (!(flags & UMOUNT_NOFOLLOW))
                  lookup_flags |= LOOKUP_FOLLOW;
  
          retval = user_path_at(AT_FDCWD, name, lookup_flags, &path);
          if (retval)
                  goto out;
          mnt = real_mount(path.mnt);
          retval = -EINVAL;
          if (path.dentry != path.mnt->mnt_root)
                  goto dput_and_out;
          if (!check_mnt(mnt))
                  goto dput_and_out;
  
          retval = -EPERM;
          if (!ns_capable(mnt->mnt_ns->user_ns, CAP_SYS_ADMIN))
                  goto dput_and_out;
  
          retval = do_umount(mnt, flags);
  dput_and_out:
          /* we mustn't call path_put() as that would clear mnt_expiry_mark */
          dput(path.dentry);
          mntput_no_expire(mnt);
  out:
          return retval;
  }
  
  #ifdef __ARCH_WANT_SYS_OLDUMOUNT
  
  /*
   *      The 2.0 compatible umount. No flags.
   */
  SYSCALL_DEFINE1(oldumount, char __user *, name)
  {
          return sys_umount(name, 0);
  }
  
  #endif
  
  static int mount_is_safe(struct path *path)
  {
          if (ns_capable(real_mount(path->mnt)->mnt_ns->user_ns, CAP_SYS_ADMIN))
                  return 0;
          return -EPERM;
  #ifdef notyet
          if (S_ISLNK(path->dentry->d_inode->i_mode))
                  return -EPERM;
          if (path->dentry->d_inode->i_mode & S_ISVTX) {
                  if (current_uid() != path->dentry->d_inode->i_uid)
                          return -EPERM;
          }
          if (inode_permission(path->dentry->d_inode, MAY_WRITE))
                  return -EPERM;
          return 0;
  #endif
  }
  
  static bool mnt_ns_loop(struct path *path)
  {
          /* Could bind mounting the mount namespace inode cause a
           * mount namespace loop?
           */
          struct inode *inode = path->dentry->d_inode;
          struct proc_inode *ei;
          struct mnt_namespace *mnt_ns;
  
          if (!proc_ns_inode(inode))
                  return false;
  
          ei = PROC_I(inode);
          if (ei->ns_ops != &mntns_operations)
                  return false;
  
          mnt_ns = ei->ns;
          return current->nsproxy->mnt_ns->seq >= mnt_ns->seq;
  }
  
  struct mount *copy_tree(struct mount *mnt, struct dentry *dentry,
                                          int flag)
  {
          struct mount *res, *p, *q, *r;
          struct path path;
  
          if (!(flag & CL_COPY_ALL) && IS_MNT_UNBINDABLE(mnt))
                  return ERR_PTR(-EINVAL);
  
          res = q = clone_mnt(mnt, dentry, flag);
          if (IS_ERR(q))
                  return q;
  
          q->mnt_mountpoint = mnt->mnt_mountpoint;
  
          p = mnt;
          list_for_each_entry(r, &mnt->mnt_mounts, mnt_child) {
                  struct mount *s;
                  if (!is_subdir(r->mnt_mountpoint, dentry))
                          continue;
  
                  for (s = r; s; s = next_mnt(s, r)) {
                          if (!(flag & CL_COPY_ALL) && IS_MNT_UNBINDABLE(s)) {
                                  s = skip_mnt_tree(s);
                                  continue;
                          }
                          while (p != s->mnt_parent) {
                                  p = p->mnt_parent;
                                  q = q->mnt_parent;
                          }
                          p = s;
                          path.mnt = &q->mnt;
                          path.dentry = p->mnt_mountpoint;
                          q = clone_mnt(p, p->mnt.mnt_root, flag);
                          if (IS_ERR(q))
                                  goto out;
                          br_write_lock(&vfsmount_lock);
                          list_add_tail(&q->mnt_list, &res->mnt_list);
                          attach_mnt(q, &path);
                          br_write_unlock(&vfsmount_lock);
                  }
          }
          return res;
  out:
          if (res) {
                  LIST_HEAD(umount_list);
                  br_write_lock(&vfsmount_lock);
                  umount_tree(res, 0, &umount_list);
                  br_write_unlock(&vfsmount_lock);
                  release_mounts(&umount_list);
          }
          return q;
  }
  
  /* Caller should check returned pointer for errors */
  
  struct vfsmount *collect_mounts(struct path *path)
  {
          struct mount *tree;
          down_write(&namespace_sem);
          tree = copy_tree(real_mount(path->mnt), path->dentry,
                           CL_COPY_ALL | CL_PRIVATE);
          up_write(&namespace_sem);
          if (IS_ERR(tree))
                  return NULL;
          return &tree->mnt;
  }
  
  void drop_collected_mounts(struct vfsmount *mnt)
  {
          LIST_HEAD(umount_list);
          down_write(&namespace_sem);
          br_write_lock(&vfsmount_lock);
          umount_tree(real_mount(mnt), 0, &umount_list);
          br_write_unlock(&vfsmount_lock);
          up_write(&namespace_sem);
          release_mounts(&umount_list);
  }
  
  int iterate_mounts(int (*f)(struct vfsmount *, void *), void *arg,
                     struct vfsmount *root)
  {
          struct mount *mnt;
          int res = f(root, arg);
          if (res)
                  return res;
          list_for_each_entry(mnt, &real_mount(root)->mnt_list, mnt_list) {
                  res = f(&mnt->mnt, arg);
                  if (res)
                          return res;
          }
          return 0;
  }
  
  static void cleanup_group_ids(struct mount *mnt, struct mount *end)
  {
          struct mount *p;
  
          for (p = mnt; p != end; p = next_mnt(p, mnt)) {
                  if (p->mnt_group_id && !IS_MNT_SHARED(p))
                          mnt_release_group_id(p);
          }
  }
  
  static int invent_group_ids(struct mount *mnt, bool recurse)
  {
          struct mount *p;
  
          for (p = mnt; p; p = recurse ? next_mnt(p, mnt) : NULL) {
                  if (!p->mnt_group_id && !IS_MNT_SHARED(p)) {
                          int err = mnt_alloc_group_id(p);
                          if (err) {
                                  cleanup_group_ids(mnt, p);
                                  return err;
                          }
                  }
          }
  
          return 0;
  }
  
  /*
   *  @source_mnt : mount tree to be attached
   *  @nd         : place the mount tree @source_mnt is attached
   *  @parent_nd  : if non-null, detach the source_mnt from its parent and
   *                 store the parent mount and mountpoint dentry.
   *                 (done when source_mnt is moved)
   *
   *  NOTE: in the table below explains the semantics when a source mount
   *  of a given type is attached to a destination mount of a given type.
   * ---------------------------------------------------------------------------
   * |         BIND MOUNT OPERATION                                            |
   * |**************************************************************************
   * | source-->| shared        |       private  |       slave    | unbindable |
   * | dest     |               |                |                |            |
   * |   |      |               |                |                |            |
   * |   v      |               |                |                |            |
   * |**************************************************************************
   * |  shared  | shared (++)   |     shared (+) |     shared(+++)|  invalid   |
   * |          |               |                |                |            |
   * |non-shared| shared (+)    |      private   |      slave (*) |  invalid   |
   * ***************************************************************************
   * A bind operation clones the source mount and mounts the clone on the
   * destination mount.
   *
   * (++)  the cloned mount is propagated to all the mounts in the propagation
   *       tree of the destination mount and the cloned mount is added to
   *       the peer group of the source mount.
   * (+)   the cloned mount is created under the destination mount and is marked
   *       as shared. The cloned mount is added to the peer group of the source
   *       mount.
   * (+++) the mount is propagated to all the mounts in the propagation tree
   *       of the destination mount and the cloned mount is made slave
   *       of the same master as that of the source mount. The cloned mount
   *       is marked as 'shared and slave'.
   * (*)   the cloned mount is made a slave of the same master as that of the
   *       source mount.
   *
   * ---------------------------------------------------------------------------
   * |                    MOVE MOUNT OPERATION                                 |
   * |**************************************************************************
   * | source-->| shared        |       private  |       slave    | unbindable |
   * | dest     |               |                |                |            |
   * |   |      |               |                |                |            |
   * |   v      |               |                |                |            |
   * |**************************************************************************
   * |  shared  | shared (+)    |     shared (+) |    shared(+++) |  invalid   |
   * |          |               |                |                |            |
   * |non-shared| shared (+*)   |      private   |    slave (*)   | unbindable |
   * ***************************************************************************
   *
   * (+)  the mount is moved to the destination. And is then propagated to
   *      all the mounts in the propagation tree of the destination mount.
   * (+*)  the mount is moved to the destination.
   * (+++)  the mount is moved to the destination and is then propagated to
   *      all the mounts belonging to the destination mount's propagation tree.
   *      the mount is marked as 'shared and slave'.
   * (*)  the mount continues to be a slave at the new location.
   *
   * if the source mount is a tree, the operations explained above is
   * applied to each mount in the tree.
   * Must be called without spinlocks held, since this function can sleep
   * in allocations.
   */
  static int attach_recursive_mnt(struct mount *source_mnt,
                          struct path *path, struct path *parent_path)
  {
          LIST_HEAD(tree_list);
          struct mount *dest_mnt = real_mount(path->mnt);
          struct dentry *dest_dentry = path->dentry;
          struct mount *child, *p;
          int err;
  
          if (IS_MNT_SHARED(dest_mnt)) {
                  err = invent_group_ids(source_mnt, true);
                  if (err)
                          goto out;
          }
          err = propagate_mnt(dest_mnt, dest_dentry, source_mnt, &tree_list);
          if (err)
                  goto out_cleanup_ids;
  
          br_write_lock(&vfsmount_lock);
  
          if (IS_MNT_SHARED(dest_mnt)) {
                  for (p = source_mnt; p; p = next_mnt(p, source_mnt))
                          set_mnt_shared(p);
          }
          if (parent_path) {
                  detach_mnt(source_mnt, parent_path);
                  attach_mnt(source_mnt, path);
                  touch_mnt_namespace(source_mnt->mnt_ns);
          } else {
                  mnt_set_mountpoint(dest_mnt, dest_dentry, source_mnt);
                  commit_tree(source_mnt);
          }
  
          list_for_each_entry_safe(child, p, &tree_list, mnt_hash) {
                  list_del_init(&child->mnt_hash);
                  commit_tree(child);
          }
          br_write_unlock(&vfsmount_lock);
  
          return 0;
  
   out_cleanup_ids:
          if (IS_MNT_SHARED(dest_mnt))
                  cleanup_group_ids(source_mnt, NULL);
   out:
          return err;
  }
  
  static int lock_mount(struct path *path)
  {
          struct vfsmount *mnt;
  retry:
          mutex_lock(&path->dentry->d_inode->i_mutex);
          if (unlikely(cant_mount(path->dentry))) {
                  mutex_unlock(&path->dentry->d_inode->i_mutex);
                  return -ENOENT;
          }
          down_write(&namespace_sem);
          mnt = lookup_mnt(path);
          if (likely(!mnt))
                  return 0;
          up_write(&namespace_sem);
          mutex_unlock(&path->dentry->d_inode->i_mutex);
          path_put(path);
          path->mnt = mnt;
          path->dentry = dget(mnt->mnt_root);
          goto retry;
  }
  
  static void unlock_mount(struct path *path)
  {
          up_write(&namespace_sem);
          mutex_unlock(&path->dentry->d_inode->i_mutex);
  }
  
  static int graft_tree(struct mount *mnt, struct path *path)
  {
          if (mnt->mnt.mnt_sb->s_flags & MS_NOUSER)
                  return -EINVAL;
  
          if (S_ISDIR(path->dentry->d_inode->i_mode) !=
                S_ISDIR(mnt->mnt.mnt_root->d_inode->i_mode))
                  return -ENOTDIR;
  
          if (d_unlinked(path->dentry))
                  return -ENOENT;
  
          return attach_recursive_mnt(mnt, path, NULL);
  }
  
  /*
   * Sanity check the flags to change_mnt_propagation.
   */
  
  static int flags_to_propagation_type(int flags)
  {
          int type = flags & ~(MS_REC | MS_SILENT);
  
          /* Fail if any non-propagation flags are set */
          if (type & ~(MS_SHARED | MS_PRIVATE | MS_SLAVE | MS_UNBINDABLE))
                  return 0;
          /* Only one propagation flag should be set */
          if (!is_power_of_2(type))
                  return 0;
          return type;
  }
  
  /*
   * recursively change the type of the mountpoint.
   */
  static int do_change_type(struct path *path, int flag)
  {
          struct mount *m;
          struct mount *mnt = real_mount(path->mnt);
          int recurse = flag & MS_REC;
          int type;
          int err = 0;
  
          if (!ns_capable(mnt->mnt_ns->user_ns, CAP_SYS_ADMIN))
                  return -EPERM;
  
          if (path->dentry != path->mnt->mnt_root)
                  return -EINVAL;
  
          type = flags_to_propagation_type(flag);
          if (!type)
                  return -EINVAL;
  
          down_write(&namespace_sem);
          if (type == MS_SHARED) {
                  err = invent_group_ids(mnt, recurse);
                  if (err)
                          goto out_unlock;
          }
  
          br_write_lock(&vfsmount_lock);
          for (m = mnt; m; m = (recurse ? next_mnt(m, mnt) : NULL))
                  change_mnt_propagation(m, type);
          br_write_unlock(&vfsmount_lock);
  
   out_unlock:
          up_write(&namespace_sem);
          return err;
  }
  
  /*
   * do loopback mount.
   */
  static int do_loopback(struct path *path, const char *old_name,
                                  int recurse)
  {
          LIST_HEAD(umount_list);
          struct path old_path;
          struct mount *mnt = NULL, *old;
          int err = mount_is_safe(path);
          if (err)
                  return err;
          if (!old_name || !*old_name)
                  return -EINVAL;
          err = kern_path(old_name, LOOKUP_FOLLOW|LOOKUP_AUTOMOUNT, &old_path);
          if (err)
                  return err;
  
          err = -EINVAL;
          if (mnt_ns_loop(&old_path))
                  goto out; 
  
          err = lock_mount(path);
          if (err)
                  goto out;
  
          old = real_mount(old_path.mnt);
  
          err = -EINVAL;
          if (IS_MNT_UNBINDABLE(old))
                  goto out2;
  
          if (!check_mnt(real_mount(path->mnt)) || !check_mnt(old))
                  goto out2;
  
          if (recurse)
                  mnt = copy_tree(old, old_path.dentry, 0);
          else
                  mnt = clone_mnt(old, old_path.dentry, 0);
  
          if (IS_ERR(mnt)) {
                  err = PTR_ERR(mnt);
                  goto out;
          }
  
          err = graft_tree(mnt, path);
          if (err) {
                  br_write_lock(&vfsmount_lock);
                  umount_tree(mnt, 0, &umount_list);
                  br_write_unlock(&vfsmount_lock);
          }
  out2:
          unlock_mount(path);
          release_mounts(&umount_list);
  out:
          path_put(&old_path);
          return err;
  }
  
  static int change_mount_flags(struct vfsmount *mnt, int ms_flags)
  {
          int error = 0;
          int readonly_request = 0;
  
          if (ms_flags & MS_RDONLY)
                  readonly_request = 1;
          if (readonly_request == __mnt_is_readonly(mnt))
                  return 0;
  
          if (readonly_request)
                  error = mnt_make_readonly(real_mount(mnt));
          else
                  __mnt_unmake_readonly(real_mount(mnt));
          return error;
  }
  
  /*
   * change filesystem flags. dir should be a physical root of filesystem.
   * If you've mounted a non-root directory somewhere and want to do remount
   * on it - tough luck.
   */
  static int do_remount(struct path *path, int flags, int mnt_flags,
                        void *data)
  {
          int err;
          struct super_block *sb = path->mnt->mnt_sb;
          struct mount *mnt = real_mount(path->mnt);
  
          if (!capable(CAP_SYS_ADMIN))
                  return -EPERM;
  
          if (!check_mnt(mnt))
                  return -EINVAL;
  
          if (path->dentry != path->mnt->mnt_root)
                  return -EINVAL;
  
          err = security_sb_remount(sb, data);
          if (err)
                  return err;
  
          down_write(&sb->s_umount);
          if (flags & MS_BIND)
                  err = change_mount_flags(path->mnt, flags);
          else
                  err = do_remount_sb(sb, flags, data, 0);
          if (!err) {
                  br_write_lock(&vfsmount_lock);
                  mnt_flags |= mnt->mnt.mnt_flags & MNT_PROPAGATION_MASK;
                  mnt->mnt.mnt_flags = mnt_flags;
                  br_write_unlock(&vfsmount_lock);
          }
          up_write(&sb->s_umount);
          if (!err) {
                  br_write_lock(&vfsmount_lock);
                  touch_mnt_namespace(mnt->mnt_ns);
                  br_write_unlock(&vfsmount_lock);
          }
          return err;
  }
  
  static inline int tree_contains_unbindable(struct mount *mnt)
  {
          struct mount *p;
          for (p = mnt; p; p = next_mnt(p, mnt)) {
                  if (IS_MNT_UNBINDABLE(p))
                          return 1;
          }
          return 0;
  }
  
  static int do_move_mount(struct path *path, const char *old_name)
  {
          struct path old_path, parent_path;
          struct mount *p;
          struct mount *old;
          int err = 0;
          if (!ns_capable(real_mount(path->mnt)->mnt_ns->user_ns, CAP_SYS_ADMIN))
                  return -EPERM;
          if (!old_name || !*old_name)
                  return -EINVAL;
          err = kern_path(old_name, LOOKUP_FOLLOW, &old_path);
          if (err)
                  return err;
  
          err = lock_mount(path);
          if (err < 0)
                  goto out;
  
          old = real_mount(old_path.mnt);
          p = real_mount(path->mnt);
  
          err = -EINVAL;
          if (!check_mnt(p) || !check_mnt(old))
                  goto out1;
  
          if (d_unlinked(path->dentry))
                  goto out1;
  
          err = -EINVAL;
          if (old_path.dentry != old_path.mnt->mnt_root)
                  goto out1;
  
          if (!mnt_has_parent(old))
                  goto out1;
  
          if (S_ISDIR(path->dentry->d_inode->i_mode) !=
                S_ISDIR(old_path.dentry->d_inode->i_mode))
                  goto out1;
          /*
           * Don't move a mount residing in a shared parent.
           */
          if (IS_MNT_SHARED(old->mnt_parent))
                  goto out1;
          /*
           * Don't move a mount tree containing unbindable mounts to a destination
           * mount which is shared.
           */
          if (IS_MNT_SHARED(p) && tree_contains_unbindable(old))
                  goto out1;
          err = -ELOOP;
          for (; mnt_has_parent(p); p = p->mnt_parent)
                  if (p == old)
                          goto out1;
  
          err = attach_recursive_mnt(old, path, &parent_path);
          if (err)
                  goto out1;
  
          /* if the mount is moved, it should no longer be expire
           * automatically */
          list_del_init(&old->mnt_expire);
  out1:
          unlock_mount(path);
  out:
          if (!err)
                  path_put(&parent_path);
          path_put(&old_path);
          return err;
  }
  
  static struct vfsmount *fs_set_subtype(struct vfsmount *mnt, const char *fstype)
  {
          int err;
          const char *subtype = strchr(fstype, '.');
          if (subtype) {
                  subtype++;
                  err = -EINVAL;
                  if (!subtype[0])
                          goto err;
          } else
                  subtype = "";
  
          mnt->mnt_sb->s_subtype = kstrdup(subtype, GFP_KERNEL);
          err = -ENOMEM;
          if (!mnt->mnt_sb->s_subtype)
                  goto err;
          return mnt;
  
   err:
          mntput(mnt);
          return ERR_PTR(err);
  }
  
  /*
   * add a mount into a namespace's mount tree
   */
  static int do_add_mount(struct mount *newmnt, struct path *path, int mnt_flags)
  {
          int err;
  
          mnt_flags &= ~(MNT_SHARED | MNT_WRITE_HOLD | MNT_INTERNAL);
  
          err = lock_mount(path);
          if (err)
                  return err;
  
          err = -EINVAL;
          if (unlikely(!check_mnt(real_mount(path->mnt)))) {
                  /* that's acceptable only for automounts done in private ns */
                  if (!(mnt_flags & MNT_SHRINKABLE))
                          goto unlock;
                  /* ... and for those we'd better have mountpoint still alive */
                  if (!real_mount(path->mnt)->mnt_ns)
                          goto unlock;
          }
  
          /* Refuse the same filesystem on the same mount point */
          err = -EBUSY;
          if (path->mnt->mnt_sb == newmnt->mnt.mnt_sb &&
              path->mnt->mnt_root == path->dentry)
                  goto unlock;
  
          err = -EINVAL;
          if (S_ISLNK(newmnt->mnt.mnt_root->d_inode->i_mode))
                  goto unlock;
  
          newmnt->mnt.mnt_flags = mnt_flags;
          err = graft_tree(newmnt, path);
  
  unlock:
          unlock_mount(path);
          return err;
  }
  
  /*
   * create a new mount for userspace and request it to be added into the
   * namespace's tree
   */
  static int do_new_mount(struct path *path, const char *fstype, int flags,
                          int mnt_flags, const char *name, void *data)
  {
          struct file_system_type *type;
          struct user_namespace *user_ns;
          struct vfsmount *mnt;
          int err;
  
          if (!fstype)
                  return -EINVAL;
  
          /* we need capabilities... */
          user_ns = real_mount(path->mnt)->mnt_ns->user_ns;
          if (!ns_capable(user_ns, CAP_SYS_ADMIN))
                  return -EPERM;
  
          type = get_fs_type(fstype);
          if (!type)
                  return -ENODEV;
  
          if (user_ns != &init_user_ns) {
                  if (!(type->fs_flags & FS_USERNS_MOUNT)) {
                          put_filesystem(type);
                          return -EPERM;
                  }
                  /* Only in special cases allow devices from mounts
                   * created outside the initial user namespace.
                   */
                  if (!(type->fs_flags & FS_USERNS_DEV_MOUNT)) {
                          flags |= MS_NODEV;
                          mnt_flags |= MNT_NODEV;
                  }
          }
  
          mnt = vfs_kern_mount(type, flags, name, data);
          if (!IS_ERR(mnt) && (type->fs_flags & FS_HAS_SUBTYPE) &&
              !mnt->mnt_sb->s_subtype)
                  mnt = fs_set_subtype(mnt, fstype);
  
          put_filesystem(type);
          if (IS_ERR(mnt))
                  return PTR_ERR(mnt);
  
          err = do_add_mount(real_mount(mnt), path, mnt_flags);
          if (err)
                  mntput(mnt);
          return err;
  }
  
  int finish_automount(struct vfsmount *m, struct path *path)
  {
          struct mount *mnt = real_mount(m);
          int err;
          /* The new mount record should have at least 2 refs to prevent it being
           * expired before we get a chance to add it
           */
          BUG_ON(mnt_get_count(mnt) < 2);
  
          if (m->mnt_sb == path->mnt->mnt_sb &&
              m->mnt_root == path->dentry) {
                  err = -ELOOP;
                  goto fail;
          }
  
          err = do_add_mount(mnt, path, path->mnt->mnt_flags | MNT_SHRINKABLE);
          if (!err)
                  return 0;
  fail:
          /* remove m from any expiration list it may be on */
          if (!list_empty(&mnt->mnt_expire)) {
                  down_write(&namespace_sem);
                  br_write_lock(&vfsmount_lock);
                  list_del_init(&mnt->mnt_expire);
                  br_write_unlock(&vfsmount_lock);
                  up_write(&namespace_sem);
          }
          mntput(m);
          mntput(m);
          return err;
  }
  
  /**
   * mnt_set_expiry - Put a mount on an expiration list
   * @mnt: The mount to list.
   * @expiry_list: The list to add the mount to.
   */
  void mnt_set_expiry(struct vfsmount *mnt, struct list_head *expiry_list)
  {
          down_write(&namespace_sem);
          br_write_lock(&vfsmount_lock);
  
          list_add_tail(&real_mount(mnt)->mnt_expire, expiry_list);
  
          br_write_unlock(&vfsmount_lock);
          up_write(&namespace_sem);
  }
  EXPORT_SYMBOL(mnt_set_expiry);
  
  /*
   * process a list of expirable mountpoints with the intent of discarding any
   * mountpoints that aren't in use and haven't been touched since last we came
   * here
   */
  void mark_mounts_for_expiry(struct list_head *mounts)
  {
          struct mount *mnt, *next;
          LIST_HEAD(graveyard);
          LIST_HEAD(umounts);
  
          if (list_empty(mounts))
                  return;
  
          down_write(&namespace_sem);
          br_write_lock(&vfsmount_lock);
  
          /* extract from the expiration list every vfsmount that matches the
           * following criteria:
           * - only referenced by its parent vfsmount
           * - still marked for expiry (marked on the last call here; marks are
           *   cleared by mntput())
           */
          list_for_each_entry_safe(mnt, next, mounts, mnt_expire) {
                  if (!xchg(&mnt->mnt_expiry_mark, 1) ||
                          propagate_mount_busy(mnt, 1))
                          continue;
                  list_move(&mnt->mnt_expire, &graveyard);
          }
          while (!list_empty(&graveyard)) {
                  mnt = list_first_entry(&graveyard, struct mount, mnt_expire);
                  touch_mnt_namespace(mnt->mnt_ns);
                  umount_tree(mnt, 1, &umounts);
          }
          br_write_unlock(&vfsmount_lock);
          up_write(&namespace_sem);
  
          release_mounts(&umounts);
  }
  
  EXPORT_SYMBOL_GPL(mark_mounts_for_expiry);
  
  /*
   * Ripoff of 'select_parent()'
   *
   * search the list of submounts for a given mountpoint, and move any
   * shrinkable submounts to the 'graveyard' list.
   */
  static int select_submounts(struct mount *parent, struct list_head *graveyard)
  {
          struct mount *this_parent = parent;
          struct list_head *next;
          int found = 0;
  
  repeat:
          next = this_parent->mnt_mounts.next;
  resume:
          while (next != &this_parent->mnt_mounts) {
                  struct list_head *tmp = next;
                  struct mount *mnt = list_entry(tmp, struct mount, mnt_child);
  
                  next = tmp->next;
                  if (!(mnt->mnt.mnt_flags & MNT_SHRINKABLE))
                          continue;
                  /*
                   * Descend a level if the d_mounts list is non-empty.
                   */
                  if (!list_empty(&mnt->mnt_mounts)) {
                          this_parent = mnt;
                          goto repeat;
                  }
  
                  if (!propagate_mount_busy(mnt, 1)) {
                          list_move_tail(&mnt->mnt_expire, graveyard);
                          found++;
                  }
          }
          /*
           * All done at this level ... ascend and resume the search
           */
          if (this_parent != parent) {
                  next = this_parent->mnt_child.next;
                  this_parent = this_parent->mnt_parent;
                  goto resume;
          }
          return found;
  }
  
  /*
   * process a list of expirable mountpoints with the intent of discarding any
   * submounts of a specific parent mountpoint
   *
   * vfsmount_lock must be held for write
   */
  static void shrink_submounts(struct mount *mnt, struct list_head *umounts)
  {
          LIST_HEAD(graveyard);
          struct mount *m;
  
          /* extract submounts of 'mountpoint' from the expiration list */
          while (select_submounts(mnt, &graveyard)) {
                  while (!list_empty(&graveyard)) {
                          m = list_first_entry(&graveyard, struct mount,
                                                  mnt_expire);
                          touch_mnt_namespace(m->mnt_ns);
                          umount_tree(m, 1, umounts);
                  }
          }
  }
  
  /*
   * Some copy_from_user() implementations do not return the exact number of
   * bytes remaining to copy on a fault.  But copy_mount_options() requires that.
   * Note that this function differs from copy_from_user() in that it will oops
   * on bad values of `to', rather than returning a short copy.
   */
  static long exact_copy_from_user(void *to, const void __user * from,
                                   unsigned long n)
  {
          char *t = to;
          const char __user *f = from;
          char c;
  
          if (!access_ok(VERIFY_READ, from, n))
                  return n;
  
          while (n) {
                  if (__get_user(c, f)) {
                          memset(t, 0, n);
                          break;
                  }
                  *t++ = c;
                  f++;
                  n--;
          }
          return n;
  }
  
  int copy_mount_options(const void __user * data, unsigned long *where)
  {
          int i;
          unsigned long page;
          unsigned long size;
  
          *where = 0;
          if (!data)
                  return 0;
  
          if (!(page = __get_free_page(GFP_KERNEL)))
                  return -ENOMEM;
  
          /* We only care that *some* data at the address the user
           * gave us is valid.  Just in case, we'll zero
           * the remainder of the page.
           */
          /* copy_from_user cannot cross TASK_SIZE ! */
          size = TASK_SIZE - (unsigned long)data;
          if (size > PAGE_SIZE)
                  size = PAGE_SIZE;
  
          i = size - exact_copy_from_user((void *)page, data, size);
          if (!i) {
                  free_page(page);
                  return -EFAULT;
          }
          if (i != PAGE_SIZE)
                  memset((char *)page + i, 0, PAGE_SIZE - i);
          *where = page;
          return 0;
  }
  
  int copy_mount_string(const void __user *data, char **where)
  {
          char *tmp;
  
          if (!data) {
                  *where = NULL;
                  return 0;
          }
  
          tmp = strndup_user(data, PAGE_SIZE);
          if (IS_ERR(tmp))
                  return PTR_ERR(tmp);
  
          *where = tmp;
          return 0;
  }
  
  /*
   * Flags is a 32-bit value that allows up to 31 non-fs dependent flags to
   * be given to the mount() call (ie: read-only, no-dev, no-suid etc).
   *
   * data is a (void *) that can point to any structure up to
   * PAGE_SIZE-1 bytes, which can contain arbitrary fs-dependent
   * information (or be NULL).
   *
   * Pre-0.97 versions of mount() didn't have a flags word.
   * When the flags word was introduced its top half was required
   * to have the magic value 0xC0ED, and this remained so until 2.4.0-test9.
   * Therefore, if this magic number is present, it carries no information
   * and must be discarded.
   */
  long do_mount(const char *dev_name, const char *dir_name,
                  const char *type_page, unsigned long flags, void *data_page)
  {
          struct path path;
          int retval = 0;
          int mnt_flags = 0;
  
          /* Discard magic */
          if ((flags & MS_MGC_MSK) == MS_MGC_VAL)
                  flags &= ~MS_MGC_MSK;
  
          /* Basic sanity checks */
  
          if (!dir_name || !*dir_name || !memchr(dir_name, 0, PAGE_SIZE))
                  return -EINVAL;
  
          if (data_page)
                  ((char *)data_page)[PAGE_SIZE - 1] = 0;
  
          /* ... and get the mountpoint */
          retval = kern_path(dir_name, LOOKUP_FOLLOW, &path);
          if (retval)
                  return retval;
  
          retval = security_sb_mount(dev_name, &path,
                                     type_page, flags, data_page);
          if (retval)
                  goto dput_out;
  
          /* Default to relatime unless overriden */
          if (!(flags & MS_NOATIME))
                  mnt_flags |= MNT_RELATIME;
  
          /* Separate the per-mountpoint flags */
          if (flags & MS_NOSUID)
                  mnt_flags |= MNT_NOSUID;
          if (flags & MS_NODEV)
                  mnt_flags |= MNT_NODEV;
          if (flags & MS_NOEXEC)
                  mnt_flags |= MNT_NOEXEC;
          if (flags & MS_NOATIME)
                  mnt_flags |= MNT_NOATIME;
          if (flags & MS_NODIRATIME)
                  mnt_flags |= MNT_NODIRATIME;
          if (flags & MS_STRICTATIME)
                  mnt_flags &= ~(MNT_RELATIME | MNT_NOATIME);
          if (flags & MS_RDONLY)
                  mnt_flags |= MNT_READONLY;
  
          flags &= ~(MS_NOSUID | MS_NOEXEC | MS_NODEV | MS_ACTIVE | MS_BORN |
                     MS_NOATIME | MS_NODIRATIME | MS_RELATIME| MS_KERNMOUNT |
                     MS_STRICTATIME);
  
          if (flags & MS_REMOUNT)
                  retval = do_remount(&path, flags & ~MS_REMOUNT, mnt_flags,
                                      data_page);
          else if (flags & MS_BIND)
                  retval = do_loopback(&path, dev_name, flags & MS_REC);
          else if (flags & (MS_SHARED | MS_PRIVATE | MS_SLAVE | MS_UNBINDABLE))
                  retval = do_change_type(&path, flags);
          else if (flags & MS_MOVE)
                  retval = do_move_mount(&path, dev_name);
          else
                  retval = do_new_mount(&path, type_page, flags, mnt_flags,
                                        dev_name, data_page);
  dput_out:
          path_put(&path);
          return retval;
  }
  
  static void free_mnt_ns(struct mnt_namespace *ns)
  {
          proc_free_inum(ns->proc_inum);
          put_user_ns(ns->user_ns);
          kfree(ns);
  }
  
  /*
   * Assign a sequence number so we can detect when we attempt to bind
   * mount a reference to an older mount namespace into the current
   * mount namespace, preventing reference counting loops.  A 64bit
   * number incrementing at 10Ghz will take 12,427 years to wrap which
   * is effectively never, so we can ignore the possibility.
   */
  static atomic64_t mnt_ns_seq = ATOMIC64_INIT(1);
  
  static struct mnt_namespace *alloc_mnt_ns(struct user_namespace *user_ns)
  {
          struct mnt_namespace *new_ns;
          int ret;
  
          new_ns = kmalloc(sizeof(struct mnt_namespace), GFP_KERNEL);
          if (!new_ns)
                  return ERR_PTR(-ENOMEM);
          ret = proc_alloc_inum(&new_ns->proc_inum);
          if (ret) {
                  kfree(new_ns);
                  return ERR_PTR(ret);
          }
          new_ns->seq = atomic64_add_return(1, &mnt_ns_seq);
          atomic_set(&new_ns->count, 1);
          new_ns->root = NULL;
          INIT_LIST_HEAD(&new_ns->list);
          init_waitqueue_head(&new_ns->poll);
          new_ns->event = 0;
          new_ns->user_ns = get_user_ns(user_ns);
          return new_ns;
  }
  
  /*
   * 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 user_namespace *user_ns, struct fs_struct *fs)
  {
          struct mnt_namespace *new_ns;
          struct vfsmount *rootmnt = NULL, *pwdmnt = NULL;
          struct mount *p, *q;
          struct mount *old = mnt_ns->root;
          struct mount *new;
          int copy_flags;
  
          new_ns = alloc_mnt_ns(user_ns);
          if (IS_ERR(new_ns))
                  return new_ns;
  
          down_write(&namespace_sem);
          /* First pass: copy the tree topology */
          copy_flags = CL_COPY_ALL | CL_EXPIRE;
          if (user_ns != mnt_ns->user_ns)
                  copy_flags |= CL_SHARED_TO_SLAVE;
          new = copy_tree(old, old->mnt.mnt_root, copy_flags);
          if (IS_ERR(new)) {
                  up_write(&namespace_sem);
                  free_mnt_ns(new_ns);
                  return ERR_CAST(new);
          }
          new_ns->root = new;
          br_write_lock(&vfsmount_lock);
          list_add_tail(&new_ns->list, &new->mnt_list);
          br_write_unlock(&vfsmount_lock);
  
          /*
           * Second pass: switch the tsk->fs->* elements and mark new vfsmounts
           * as belonging to new namespace.  We have already acquired a private
           * fs_struct, so tsk->fs->lock is not needed.
           */
          p = old;
          q = new;
          while (p) {
                  q->mnt_ns = new_ns;
                  if (fs) {
                          if (&p->mnt == fs->root.mnt) {
                                  fs->root.mnt = mntget(&q->mnt);
                                  rootmnt = &p->mnt;
                          }
                          if (&p->mnt == fs->pwd.mnt) {
                                  fs->pwd.mnt = mntget(&q->mnt);
                                  pwdmnt = &p->mnt;
                          }
                  }
                  p = next_mnt(p, old);
                  q = next_mnt(q, new);
          }
          up_write(&namespace_sem);
  
          if (rootmnt)
                  mntput(rootmnt);
          if (pwdmnt)
                  mntput(pwdmnt);
  
          return new_ns;
  }
  
  struct mnt_namespace *copy_mnt_ns(unsigned long flags, struct mnt_namespace *ns,
                  struct user_namespace *user_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, user_ns, new_fs);
  
          put_mnt_ns(ns);
          return new_ns;
  }
  
  /**
   * create_mnt_ns - creates a private namespace and adds a root filesystem
   * @mnt: pointer to the new root filesystem mountpoint
   */
  static struct mnt_namespace *create_mnt_ns(struct vfsmount *m)
  {
          struct mnt_namespace *new_ns = alloc_mnt_ns(&init_user_ns);
          if (!IS_ERR(new_ns)) {
                  struct mount *mnt = real_mount(m);
                  mnt->mnt_ns = new_ns;
                  new_ns->root = mnt;
                  list_add(&new_ns->list, &mnt->mnt_list);
          } else {
                  mntput(m);
          }
          return new_ns;
  }
  
  struct dentry *mount_subtree(struct vfsmount *mnt, const char *name)
  {
          struct mnt_namespace *ns;
          struct super_block *s;
          struct path path;
          int err;
  
          ns = create_mnt_ns(mnt);
          if (IS_ERR(ns))
                  return ERR_CAST(ns);
  
          err = vfs_path_lookup(mnt->mnt_root, mnt,
                          name, LOOKUP_FOLLOW|LOOKUP_AUTOMOUNT, &path);
  
          put_mnt_ns(ns);
  
          if (err)
                  return ERR_PTR(err);
  
          /* trade a vfsmount reference for active sb one */
          s = path.mnt->mnt_sb;
          atomic_inc(&s->s_active);
          mntput(path.mnt);
          /* lock the sucker */
          down_write(&s->s_umount);
          /* ... and return the root of (sub)tree on it */
          return path.dentry;
  }
  EXPORT_SYMBOL(mount_subtree);
  
  SYSCALL_DEFINE5(mount, char __user *, dev_name, char __user *, dir_name,
                  char __user *, type, unsigned long, flags, void __user *, data)
  {
          int ret;
          char *kernel_type;
          struct filename *kernel_dir;
          char *kernel_dev;
          unsigned long data_page;
  
          ret = copy_mount_string(type, &kernel_type);
          if (ret < 0)
                  goto out_type;
  
          kernel_dir = getname(dir_name);
          if (IS_ERR(kernel_dir)) {
                  ret = PTR_ERR(kernel_dir);
                  goto out_dir;
          }
  
          ret = copy_mount_string(dev_name, &kernel_dev);
          if (ret < 0)
                  goto out_dev;
  
          ret = copy_mount_options(data, &data_page);
          if (ret < 0)
                  goto out_data;
  
          ret = do_mount(kernel_dev, kernel_dir->name, kernel_type, flags,
                  (void *) data_page);
  
          free_page(data_page);
  out_data:
          kfree(kernel_dev);
  out_dev:
          putname(kernel_dir);
  out_dir:
          kfree(kernel_type);
  out_type:
          return ret;
  }
  
  /*
   * Return true if path is reachable from root
   *
   * namespace_sem or vfsmount_lock is held
   */
  bool is_path_reachable(struct mount *mnt, struct dentry *dentry,
                           const struct path *root)
  {
          while (&mnt->mnt != root->mnt && mnt_has_parent(mnt)) {
                  dentry = mnt->mnt_mountpoint;
                  mnt = mnt->mnt_parent;
          }
          return &mnt->mnt == root->mnt && is_subdir(dentry, root->dentry);
  }
  
  int path_is_under(struct path *path1, struct path *path2)
  {
          int res;
          br_read_lock(&vfsmount_lock);
          res = is_path_reachable(real_mount(path1->mnt), path1->dentry, path2);
          br_read_unlock(&vfsmount_lock);
          return res;
  }
  EXPORT_SYMBOL(path_is_under);
  
  /*
   * pivot_root Semantics:
   * Moves the root file system of the current process to the directory put_old,
   * makes new_root as the new root file system of the current process, and sets
   * root/cwd of all processes which had them on the current root to new_root.
   *
   * Restrictions:
   * The new_root and put_old must be directories, and  must not be on the
   * same file  system as the current process root. The put_old  must  be
   * underneath new_root,  i.e. adding a non-zero number of /.. to the string
   * pointed to by put_old must yield the same directory as new_root. No other
   * file system may be mounted on put_old. After all, new_root is a mountpoint.
   *
   * Also, the current root cannot be on the 'rootfs' (initial ramfs) filesystem.
   * See Documentation/filesystems/ramfs-rootfs-initramfs.txt for alternatives
   * in this situation.
   *
   * Notes:
   *  - we don't move root/cwd if they are not at the root (reason: if something
   *    cared enough to change them, it's probably wrong to force them elsewhere)
   *  - it's okay to pick a root that isn't the root of a file system, e.g.
   *    /nfs/my_root where /nfs is the mount point. It must be a mountpoint,
   *    though, so you may need to say mount --bind /nfs/my_root /nfs/my_root
   *    first.
   */
  SYSCALL_DEFINE2(pivot_root, const char __user *, new_root,
                  const char __user *, put_old)
  {
          struct path new, old, parent_path, root_parent, root;
          struct mount *new_mnt, *root_mnt;
          int error;
  
          if (!ns_capable(current->nsproxy->mnt_ns->user_ns, CAP_SYS_ADMIN))
                  return -EPERM;
  
          error = user_path_dir(new_root, &new);
          if (error)
                  goto out0;
  
          error = user_path_dir(put_old, &old);
          if (error)
                  goto out1;
  
          error = security_sb_pivotroot(&old, &new);
          if (error)
                  goto out2;
  
          get_fs_root(current->fs, &root);
          error = lock_mount(&old);
          if (error)
                  goto out3;
  
          error = -EINVAL;
          new_mnt = real_mount(new.mnt);
          root_mnt = real_mount(root.mnt);
          if (IS_MNT_SHARED(real_mount(old.mnt)) ||
                  IS_MNT_SHARED(new_mnt->mnt_parent) ||
                  IS_MNT_SHARED(root_mnt->mnt_parent))
                  goto out4;
          if (!check_mnt(root_mnt) || !check_mnt(new_mnt))
                  goto out4;
          error = -ENOENT;
          if (d_unlinked(new.dentry))
                  goto out4;
          if (d_unlinked(old.dentry))
                  goto out4;
          error = -EBUSY;
          if (new.mnt == root.mnt ||
              old.mnt == root.mnt)
                  goto out4; /* loop, on the same file system  */
          error = -EINVAL;
          if (root.mnt->mnt_root != root.dentry)
                  goto out4; /* not a mountpoint */
          if (!mnt_has_parent(root_mnt))
                  goto out4; /* not attached */
          if (new.mnt->mnt_root != new.dentry)
                  goto out4; /* not a mountpoint */
          if (!mnt_has_parent(new_mnt))
                  goto out4; /* not attached */
          /* make sure we can reach put_old from new_root */
          if (!is_path_reachable(real_mount(old.mnt), old.dentry, &new))
                  goto out4;
          br_write_lock(&vfsmount_lock);
          detach_mnt(new_mnt, &parent_path);
          detach_mnt(root_mnt, &root_parent);
          /* mount old root on put_old */
          attach_mnt(root_mnt, &old);
          /* mount new_root on / */
          attach_mnt(new_mnt, &root_parent);
          touch_mnt_namespace(current->nsproxy->mnt_ns);
          br_write_unlock(&vfsmount_lock);
          chroot_fs_refs(&root, &new);
          error = 0;
  out4:
          unlock_mount(&old);
          if (!error) {
                  path_put(&root_parent);
                  path_put(&parent_path);
          }
  out3:
          path_put(&root);
  out2:
          path_put(&old);
  out1:
          path_put(&new);
  out0:
          return error;
  }
  
  static void __init init_mount_tree(void)
  {
          struct vfsmount *mnt;
          struct mnt_namespace *ns;
          struct path root;
          struct file_system_type *type;
  
          type = get_fs_type("rootfs");
          if (!type)
                  panic("Can't find rootfs type");
          mnt = vfs_kern_mount(type, 0, "rootfs", NULL);
          put_filesystem(type);
          if (IS_ERR(mnt))
                  panic("Can't create rootfs");
  
          ns = create_mnt_ns(mnt);
          if (IS_ERR(ns))
                  panic("Can't allocate initial namespace");
  
          init_task.nsproxy->mnt_ns = ns;
          get_mnt_ns(ns);
  
          root.mnt = mnt;
          root.dentry = mnt->mnt_root;
  
          set_fs_pwd(current->fs, &root);
          set_fs_root(current->fs, &root);
  }
  
  void __init mnt_init(void)
  {
          unsigned u;
          int err;
  
          init_rwsem(&namespace_sem);
  
          mnt_cache = kmem_cache_create("mnt_cache", sizeof(struct mount),
                          0, SLAB_HWCACHE_ALIGN | SLAB_PANIC, NULL);
  
          mount_hashtable = (struct list_head *)__get_free_page(GFP_ATOMIC);
  
          if (!mount_hashtable)
                  panic("Failed to allocate mount hash table\n");
  
          printk(KERN_INFO "Mount-cache hash table entries: %lu\n", HASH_SIZE);
  
          for (u = 0; u < HASH_SIZE; u++)
                  INIT_LIST_HEAD(&mount_hashtable[u]);
  
          br_lock_init(&vfsmount_lock);
  
          err = sysfs_init();
          if (err)
                  printk(KERN_WARNING "%s: sysfs_init error: %d\n",
                          __func__, err);
          fs_kobj = kobject_create_and_add("fs", NULL);
          if (!fs_kobj)
                  printk(KERN_WARNING "%s: kobj create error\n", __func__);
          init_rootfs();
          init_mount_tree();
  }
  
  void put_mnt_ns(struct mnt_namespace *ns)
  {
          LIST_HEAD(umount_list);
  
          if (!atomic_dec_and_test(&ns->count))
                  return;
          down_write(&namespace_sem);
          br_write_lock(&vfsmount_lock);
          umount_tree(ns->root, 0, &umount_list);
          br_write_unlock(&vfsmount_lock);
          up_write(&namespace_sem);
          release_mounts(&umount_list);
          free_mnt_ns(ns);
  }
  
  struct vfsmount *kern_mount_data(struct file_system_type *type, void *data)
  {
          struct vfsmount *mnt;
          mnt = vfs_kern_mount(type, MS_KERNMOUNT, type->name, data);
          if (!IS_ERR(mnt)) {
                  /*
                   * it is a longterm mount, don't release mnt until
                   * we unmount before file sys is unregistered
                  */
                  real_mount(mnt)->mnt_ns = MNT_NS_INTERNAL;
          }
          return mnt;
  }
  EXPORT_SYMBOL_GPL(kern_mount_data);
  
  void kern_unmount(struct vfsmount *mnt)
  {
          /* release long term mount so mount point can be released */
          if (!IS_ERR_OR_NULL(mnt)) {
                  br_write_lock(&vfsmount_lock);
                  real_mount(mnt)->mnt_ns = NULL;
                  br_write_unlock(&vfsmount_lock);
                  mntput(mnt);
          }
  }
  EXPORT_SYMBOL(kern_unmount);
  
  bool our_mnt(struct vfsmount *mnt)
  {
          return check_mnt(real_mount(mnt));
  }
  
  static void *mntns_get(struct task_struct *task)
  {
          struct mnt_namespace *ns = NULL;
          struct nsproxy *nsproxy;
  
          rcu_read_lock();
          nsproxy = task_nsproxy(task);
          if (nsproxy) {
                  ns = nsproxy->mnt_ns;
                  get_mnt_ns(ns);
          }
          rcu_read_unlock();
  
          return ns;
  }
  
  static void mntns_put(void *ns)
  {
          put_mnt_ns(ns);
  }
  
  static int mntns_install(struct nsproxy *nsproxy, void *ns)
  {
          struct fs_struct *fs = current->fs;
          struct mnt_namespace *mnt_ns = ns;
          struct path root;
  
          if (!ns_capable(mnt_ns->user_ns, CAP_SYS_ADMIN) ||
              !nsown_capable(CAP_SYS_CHROOT) ||
              !nsown_capable(CAP_SYS_ADMIN))
                  return -EPERM;
  
          if (fs->users != 1)
                  return -EINVAL;
  
          get_mnt_ns(mnt_ns);
          put_mnt_ns(nsproxy->mnt_ns);
          nsproxy->mnt_ns = mnt_ns;
  
          /* Find the root */
          root.mnt    = &mnt_ns->root->mnt;
          root.dentry = mnt_ns->root->mnt.mnt_root;
          path_get(&root);
          while(d_mountpoint(root.dentry) && follow_down_one(&root))
                  ;
  
          /* Update the pwd and root */
          set_fs_pwd(fs, &root);
          set_fs_root(fs, &root);
  
          path_put(&root);
          return 0;
  }
  
  static unsigned int mntns_inum(void *ns)
  {
          struct mnt_namespace *mnt_ns = ns;
          return mnt_ns->proc_inum;
  }
  
  const struct proc_ns_operations mntns_operations = {
          .name           = "mnt",
          .type           = CLONE_NEWNS,
          .get            = mntns_get,
          .put            = mntns_put,
          .install        = mntns_install,
          .inum           = mntns_inum,
  };

Cache object: 68308fbb56f3d6442a3d55c126eea86b