namei.c

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/*
 *  linux/fs/namei.c
 *
 *  Copyright (C) 1991, 1992  Linus Torvalds
 */

/*
 * Some corrections by tytso.
 */

/* [Feb 1997 T. Schoebel-Theuer] Complete rewrite of the pathname
 * lookup logic.
 */
/* [Feb-Apr 2000, AV] Rewrite to the new namespace architecture.
 */

#include <linux/init.h>
#include <linux/export.h>
#include <linux/kernel.h>
#include <linux/slab.h>
#include <linux/fs.h>
#include <linux/namei.h>
#include <linux/pagemap.h>
#include <linux/fsnotify.h>
#include <linux/personality.h>
#include <linux/security.h>
#include <linux/ima.h>
#include <linux/syscalls.h>
#include <linux/mount.h>
#include <linux/audit.h>
#include <linux/capability.h>
#include <linux/file.h>
#include <linux/fcntl.h>
#include <linux/device_cgroup.h>
#include <linux/fs_struct.h>
#include <linux/posix_acl.h>
#include <asm/uaccess.h>

#include "internal.h"
#include "mount.h"

/* [Feb-1997 T. Schoebel-Theuer]
 * Fundamental changes in the pathname lookup mechanisms (namei)
 * were necessary because of omirr.  The reason is that omirr needs
 * to know the _real_ pathname, not the user-supplied one, in case
 * of symlinks (and also when transname replacements occur).
 *
 * The new code replaces the old recursive symlink resolution with
 * an iterative one (in case of non-nested symlink chains).  It does
 * this with calls to <fs>_follow_link().
 * As a side effect, dir_namei(), _namei() and follow_link() are now 
 * replaced with a single function lookup_dentry() that can handle all 
 * the special cases of the former code.
 *
 * With the new dcache, the pathname is stored at each inode, at least as
 * long as the refcount of the inode is positive.  As a side effect, the
 * size of the dcache depends on the inode cache and thus is dynamic.
 *
 * [29-Apr-1998 C. Scott Ananian] Updated above description of symlink
 * resolution to correspond with current state of the code.
 *
 * Note that the symlink resolution is not *completely* iterative.
 * There is still a significant amount of tail- and mid- recursion in
 * the algorithm.  Also, note that <fs>_readlink() is not used in
 * lookup_dentry(): lookup_dentry() on the result of <fs>_readlink()
 * may return different results than <fs>_follow_link().  Many virtual
 * filesystems (including /proc) exhibit this behavior.
 */

/* [24-Feb-97 T. Schoebel-Theuer] Side effects caused by new implementation:
 * New symlink semantics: when open() is called with flags O_CREAT | O_EXCL
 * and the name already exists in form of a symlink, try to create the new
 * name indicated by the symlink. The old code always complained that the
 * name already exists, due to not following the symlink even if its target
 * is nonexistent.  The new semantics affects also mknod() and link() when
 * the name is a symlink pointing to a non-existent name.
 *
 * I don't know which semantics is the right one, since I have no access
 * to standards. But I found by trial that HP-UX 9.0 has the full "new"
 * semantics implemented, while SunOS 4.1.1 and Solaris (SunOS 5.4) have the
 * "old" one. Personally, I think the new semantics is much more logical.
 * Note that "ln old new" where "new" is a symlink pointing to a non-existing
 * file does succeed in both HP-UX and SunOs, but not in Solaris
 * and in the old Linux semantics.
 */

/* [16-Dec-97 Kevin Buhr] For security reasons, we change some symlink
 * semantics.  See the comments in "open_namei" and "do_link" below.
 *
 * [10-Sep-98 Alan Modra] Another symlink change.
 */

/* [Feb-Apr 2000 AV] Complete rewrite. Rules for symlinks:
 *  inside the path - always follow.
 *  in the last component in creation/removal/renaming - never follow.
 *  if LOOKUP_FOLLOW passed - follow.
 *  if the pathname has trailing slashes - follow.
 *  otherwise - don't follow.
 * (applied in that order).
 *
 * [Jun 2000 AV] Inconsistent behaviour of open() in case if flags==O_CREAT
 * restored for 2.4. This is the last surviving part of old 4.2BSD bug.
 * During the 2.4 we need to fix the userland stuff depending on it -
 * hopefully we will be able to get rid of that wart in 2.5. So far only
 * XEmacs seems to be relying on it...
 */
/*
 * [Sep 2001 AV] Single-semaphore locking scheme (kudos to David Holland)
 * implemented.  Let's see if raised priority of ->s_vfs_rename_mutex gives
 * any extra contention...
 */

/* In order to reduce some races, while at the same time doing additional
 * checking and hopefully speeding things up, we copy filenames to the
 * kernel data space before using them..
 *
 * POSIX.1 2.4: an empty pathname is invalid (ENOENT).
 * PATH_MAX includes the nul terminator --RR.
 */
void final_putname(struct filename *name)
{
    if (name->separate) {
        __putname(name->name);
        kfree(name);
    } else {
        __putname(name);
    }
}

#define EMBEDDED_NAME_MAX   (PATH_MAX - sizeof(struct filename))

static struct filename *
getname_flags(const char __user *filename, int flags, int *empty)
{
    struct filename *result, *err;
    int len;
    long max;
    char *kname;

    result = audit_reusename(filename);
    if (result)
        return result;

    result = __getname();
    if (unlikely(!result))
        return ERR_PTR(-ENOMEM);

    /*
     * First, try to embed the struct filename inside the names_cache
     * allocation
     */
    kname = (char *)result + sizeof(*result);
    result->name = kname;
    result->separate = false;
    max = EMBEDDED_NAME_MAX;

recopy:
    len = strncpy_from_user(kname, filename, max);
    if (unlikely(len < 0)) {
        err = ERR_PTR(len);
        goto error;
    }

    /*
     * Uh-oh. We have a name that's approaching PATH_MAX. Allocate a
     * separate struct filename so we can dedicate the entire
     * names_cache allocation for the pathname, and re-do the copy from
     * userland.
     */
    if (len == EMBEDDED_NAME_MAX && max == EMBEDDED_NAME_MAX) {
        kname = (char *)result;

        result = kzalloc(sizeof(*result), GFP_KERNEL);
        if (!result) {
            err = ERR_PTR(-ENOMEM);
            result = (struct filename *)kname;
            goto error;
        }
        result->name = kname;
        result->separate = true;
        max = PATH_MAX;
        goto recopy;
    }

    /* The empty path is special. */
    if (unlikely(!len)) {
        if (empty)
            *empty = 1;
        err = ERR_PTR(-ENOENT);
        if (!(flags & LOOKUP_EMPTY))
            goto error;
    }

    err = ERR_PTR(-ENAMETOOLONG);
    if (unlikely(len >= PATH_MAX))
        goto error;

    result->uptr = filename;
    audit_getname(result);
    return result;

error:
    final_putname(result);
    return err;
}

struct filename *
getname(const char __user * filename)
{
    return getname_flags(filename, 0, NULL);
}
EXPORT_SYMBOL(getname);

#ifdef CONFIG_AUDITSYSCALL
void putname(struct filename *name)
{
    if (unlikely(!audit_dummy_context()))
        return audit_putname(name);
    final_putname(name);
}
#endif

static int check_acl(struct inode *inode, int mask)
{
#ifdef CONFIG_FS_POSIX_ACL
    struct posix_acl *acl;

    if (mask & MAY_NOT_BLOCK) {
        acl = get_cached_acl_rcu(inode, ACL_TYPE_ACCESS);
            if (!acl)
                    return -EAGAIN;
        /* no ->get_acl() calls in RCU mode... */
        if (acl == ACL_NOT_CACHED)
            return -ECHILD;
            return posix_acl_permission(inode, acl, mask & ~MAY_NOT_BLOCK);
    }

    acl = get_cached_acl(inode, ACL_TYPE_ACCESS);

    /*
     * A filesystem can force a ACL callback by just never filling the
     * ACL cache. But normally you'd fill the cache either at inode
     * instantiation time, or on the first ->get_acl call.
     *
     * If the filesystem doesn't have a get_acl() function at all, we'll
     * just create the negative cache entry.
     */
    if (acl == ACL_NOT_CACHED) {
            if (inode->i_op->get_acl) {
            acl = inode->i_op->get_acl(inode, ACL_TYPE_ACCESS);
            if (IS_ERR(acl))
                return PTR_ERR(acl);
        } else {
                set_cached_acl(inode, ACL_TYPE_ACCESS, NULL);
                return -EAGAIN;
        }
    }

    if (acl) {
            int error = posix_acl_permission(inode, acl, mask);
            posix_acl_release(acl);
            return error;
    }
#endif

    return -EAGAIN;
}

/*
 * This does the basic permission checking
 */
static int acl_permission_check(struct inode *inode, int mask)
{
    unsigned int mode = inode->i_mode;

    if (likely(uid_eq(current_fsuid(), inode->i_uid)))
        mode >>= 6;
    else {
        if (IS_POSIXACL(inode) && (mode & S_IRWXG)) {
            int error = check_acl(inode, mask);
            if (error != -EAGAIN)
                return error;
        }

        if (in_group_p(inode->i_gid))
            mode >>= 3;
    }

    /*
     * If the DACs are ok we don't need any capability check.
     */
    if ((mask & ~mode & (MAY_READ | MAY_WRITE | MAY_EXEC)) == 0)
        return 0;
    return -EACCES;
}

int generic_permission(struct inode *inode, int mask)
{
    int ret;

    /*
     * Do the basic permission checks.
     */
    ret = acl_permission_check(inode, mask);
    if (ret != -EACCES)
        return ret;

    if (S_ISDIR(inode->i_mode)) {
        /* DACs are overridable for directories */
        if (inode_capable(inode, CAP_DAC_OVERRIDE))
            return 0;
        if (!(mask & MAY_WRITE))
            if (inode_capable(inode, CAP_DAC_READ_SEARCH))
                return 0;
        return -EACCES;
    }
    /*
     * Read/write DACs are always overridable.
     * Executable DACs are overridable when there is
     * at least one exec bit set.
     */
    if (!(mask & MAY_EXEC) || (inode->i_mode & S_IXUGO))
        if (inode_capable(inode, CAP_DAC_OVERRIDE))
            return 0;

    /*
     * Searching includes executable on directories, else just read.
     */
    mask &= MAY_READ | MAY_WRITE | MAY_EXEC;
    if (mask == MAY_READ)
        if (inode_capable(inode, CAP_DAC_READ_SEARCH))
            return 0;

    return -EACCES;
}

/*
 * We _really_ want to just do "generic_permission()" without
 * even looking at the inode->i_op values. So we keep a cache
 * flag in inode->i_opflags, that says "this has not special
 * permission function, use the fast case".
 */
static inline int do_inode_permission(struct inode *inode, int mask)
{
    if (unlikely(!(inode->i_opflags & IOP_FASTPERM))) {
        if (likely(inode->i_op->permission))
            return inode->i_op->permission(inode, mask);

        /* This gets set once for the inode lifetime */
        spin_lock(&inode->i_lock);
        inode->i_opflags |= IOP_FASTPERM;
        spin_unlock(&inode->i_lock);
    }
    return generic_permission(inode, mask);
}

int __inode_permission(struct inode *inode, int mask)
{
    int retval;

    if (unlikely(mask & MAY_WRITE)) {
        /*
         * Nobody gets write access to an immutable file.
         */
        if (IS_IMMUTABLE(inode))
            return -EACCES;
    }

    retval = do_inode_permission(inode, mask);
    if (retval)
        return retval;

    retval = devcgroup_inode_permission(inode, mask);
    if (retval)
        return retval;

    return security_inode_permission(inode, mask);
}

static int sb_permission(struct super_block *sb, struct inode *inode, int mask)
{
    if (unlikely(mask & MAY_WRITE)) {
        umode_t mode = inode->i_mode;

        /* Nobody gets write access to a read-only fs. */
        if ((sb->s_flags & MS_RDONLY) &&
            (S_ISREG(mode) || S_ISDIR(mode) || S_ISLNK(mode)))
            return -EROFS;
    }
    return 0;
}

int inode_permission(struct inode *inode, int mask)
{
    int retval;

    retval = sb_permission(inode->i_sb, inode, mask);
    if (retval)
        return retval;
    return __inode_permission(inode, mask);
}

void path_get(struct path *path)
{
    mntget(path->mnt);
    dget(path->dentry);
}
EXPORT_SYMBOL(path_get);

void path_put(struct path *path)
{
    dput(path->dentry);
    mntput(path->mnt);
}
EXPORT_SYMBOL(path_put);

/*
 * Path walking has 2 modes, rcu-walk and ref-walk (see
 * Documentation/filesystems/path-lookup.txt).  In situations when we can't
 * continue in RCU mode, we attempt to drop out of rcu-walk mode and grab
 * normal reference counts on dentries and vfsmounts to transition to rcu-walk
 * mode.  Refcounts are grabbed at the last known good point before rcu-walk
 * got stuck, so ref-walk may continue from there. If this is not successful
 * (eg. a seqcount has changed), then failure is returned and it's up to caller
 * to restart the path walk from the beginning in ref-walk mode.
 */

static inline void lock_rcu_walk(void)
{
    br_read_lock(&vfsmount_lock);
    rcu_read_lock();
}

static inline void unlock_rcu_walk(void)
{
    rcu_read_unlock();
    br_read_unlock(&vfsmount_lock);
}

static int unlazy_walk(struct nameidata *nd, struct dentry *dentry)
{
    struct fs_struct *fs = current->fs;
    struct dentry *parent = nd->path.dentry;
    int want_root = 0;

    BUG_ON(!(nd->flags & LOOKUP_RCU));
    if (nd->root.mnt && !(nd->flags & LOOKUP_ROOT)) {
        want_root = 1;
        spin_lock(&fs->lock);
        if (nd->root.mnt != fs->root.mnt ||
                nd->root.dentry != fs->root.dentry)
            goto err_root;
    }
    spin_lock(&parent->d_lock);
    if (!dentry) {
        if (!__d_rcu_to_refcount(parent, nd->seq))
            goto err_parent;
        BUG_ON(nd->inode != parent->d_inode);
    } else {
        if (dentry->d_parent != parent)
            goto err_parent;
        spin_lock_nested(&dentry->d_lock, DENTRY_D_LOCK_NESTED);
        if (!__d_rcu_to_refcount(dentry, nd->seq))
            goto err_child;
        /*
         * If the sequence check on the child dentry passed, then
         * the child has not been removed from its parent. This
         * means the parent dentry must be valid and able to take
         * a reference at this point.
         */
        BUG_ON(!IS_ROOT(dentry) && dentry->d_parent != parent);
        BUG_ON(!parent->d_count);
        parent->d_count++;
        spin_unlock(&dentry->d_lock);
    }
    spin_unlock(&parent->d_lock);
    if (want_root) {
        path_get(&nd->root);
        spin_unlock(&fs->lock);
    }
    mntget(nd->path.mnt);

    unlock_rcu_walk();
    nd->flags &= ~LOOKUP_RCU;
    return 0;

err_child:
    spin_unlock(&dentry->d_lock);
err_parent:
    spin_unlock(&parent->d_lock);
err_root:
    if (want_root)
        spin_unlock(&fs->lock);
    return -ECHILD;
}

static inline int d_revalidate(struct dentry *dentry, unsigned int flags)
{
    return dentry->d_op->d_revalidate(dentry, flags);
}

static int complete_walk(struct nameidata *nd)
{
    struct dentry *dentry = nd->path.dentry;
    int status;

    if (nd->flags & LOOKUP_RCU) {
        nd->flags &= ~LOOKUP_RCU;
        if (!(nd->flags & LOOKUP_ROOT))
            nd->root.mnt = NULL;
        spin_lock(&dentry->d_lock);
        if (unlikely(!__d_rcu_to_refcount(dentry, nd->seq))) {
            spin_unlock(&dentry->d_lock);
            unlock_rcu_walk();
            return -ECHILD;
        }
        BUG_ON(nd->inode != dentry->d_inode);
        spin_unlock(&dentry->d_lock);
        mntget(nd->path.mnt);
        unlock_rcu_walk();
    }

    if (likely(!(nd->flags & LOOKUP_JUMPED)))
        return 0;

    if (likely(!(dentry->d_flags & DCACHE_OP_REVALIDATE)))
        return 0;

    if (likely(!(dentry->d_sb->s_type->fs_flags & FS_REVAL_DOT)))
        return 0;

    /* Note: we do not d_invalidate() */
    status = d_revalidate(dentry, nd->flags);
    if (status > 0)
        return 0;

    if (!status)
        status = -ESTALE;

    path_put(&nd->path);
    return status;
}

static __always_inline void set_root(struct nameidata *nd)
{
    if (!nd->root.mnt)
        get_fs_root(current->fs, &nd->root);
}

static int link_path_walk(const char *, struct nameidata *);

static __always_inline void set_root_rcu(struct nameidata *nd)
{
    if (!nd->root.mnt) {
        struct fs_struct *fs = current->fs;
        unsigned seq;

        do {
            seq = read_seqcount_begin(&fs->seq);
            nd->root = fs->root;
            nd->seq = __read_seqcount_begin(&nd->root.dentry->d_seq);
        } while (read_seqcount_retry(&fs->seq, seq));
    }
}

static __always_inline int __vfs_follow_link(struct nameidata *nd, const char *link)
{
    int ret;

    if (IS_ERR(link))
        goto fail;

    if (*link == '/') {
        set_root(nd);
        path_put(&nd->path);
        nd->path = nd->root;
        path_get(&nd->root);
        nd->flags |= LOOKUP_JUMPED;
    }
    nd->inode = nd->path.dentry->d_inode;

    ret = link_path_walk(link, nd);
    return ret;
fail:
    path_put(&nd->path);
    return PTR_ERR(link);
}

static void path_put_conditional(struct path *path, struct nameidata *nd)
{
    dput(path->dentry);
    if (path->mnt != nd->path.mnt)
        mntput(path->mnt);
}

static inline void path_to_nameidata(const struct path *path,
                    struct nameidata *nd)
{
    if (!(nd->flags & LOOKUP_RCU)) {
        dput(nd->path.dentry);
        if (nd->path.mnt != path->mnt)
            mntput(nd->path.mnt);
    }
    nd->path.mnt = path->mnt;
    nd->path.dentry = path->dentry;
}

/*
 * Helper to directly jump to a known parsed path from ->follow_link,
 * caller must have taken a reference to path beforehand.
 */
void nd_jump_link(struct nameidata *nd, struct path *path)
{
    path_put(&nd->path);

    nd->path = *path;
    nd->inode = nd->path.dentry->d_inode;
    nd->flags |= LOOKUP_JUMPED;

    BUG_ON(nd->inode->i_op->follow_link);
}

static inline void put_link(struct nameidata *nd, struct path *link, void *cookie)
{
    struct inode *inode = link->dentry->d_inode;
    if (inode->i_op->put_link)
        inode->i_op->put_link(link->dentry, nd, cookie);
    path_put(link);
}

int sysctl_protected_symlinks __read_mostly = 0;
int sysctl_protected_hardlinks __read_mostly = 0;

static inline int may_follow_link(struct path *link, struct nameidata *nd)
{
    const struct inode *inode;
    const struct inode *parent;

    if (!sysctl_protected_symlinks)
        return 0;

    /* Allowed if owner and follower match. */
    inode = link->dentry->d_inode;
    if (uid_eq(current_cred()->fsuid, inode->i_uid))
        return 0;

    /* Allowed if parent directory not sticky and world-writable. */
    parent = nd->path.dentry->d_inode;
    if ((parent->i_mode & (S_ISVTX|S_IWOTH)) != (S_ISVTX|S_IWOTH))
        return 0;

    /* Allowed if parent directory and link owner match. */
    if (uid_eq(parent->i_uid, inode->i_uid))
        return 0;

    audit_log_link_denied("follow_link", link);
    path_put_conditional(link, nd);
    path_put(&nd->path);
    return -EACCES;
}

static bool safe_hardlink_source(struct inode *inode)
{
    umode_t mode = inode->i_mode;

    /* Special files should not get pinned to the filesystem. */
    if (!S_ISREG(mode))
        return false;

    /* Setuid files should not get pinned to the filesystem. */
    if (mode & S_ISUID)
        return false;

    /* Executable setgid files should not get pinned to the filesystem. */
    if ((mode & (S_ISGID | S_IXGRP)) == (S_ISGID | S_IXGRP))
        return false;

    /* Hardlinking to unreadable or unwritable sources is dangerous. */
    if (inode_permission(inode, MAY_READ | MAY_WRITE))
        return false;

    return true;
}

static int may_linkat(struct path *link)
{
    const struct cred *cred;
    struct inode *inode;

    if (!sysctl_protected_hardlinks)
        return 0;

    cred = current_cred();
    inode = link->dentry->d_inode;

    /* Source inode owner (or CAP_FOWNER) can hardlink all they like,
     * otherwise, it must be a safe source.
     */
    if (uid_eq(cred->fsuid, inode->i_uid) || safe_hardlink_source(inode) ||
        capable(CAP_FOWNER))
        return 0;

    audit_log_link_denied("linkat", link);
    return -EPERM;
}

static __always_inline int
follow_link(struct path *link, struct nameidata *nd, void **p)
{
    struct dentry *dentry = link->dentry;
    int error;
    char *s;

    BUG_ON(nd->flags & LOOKUP_RCU);

    if (link->mnt == nd->path.mnt)
        mntget(link->mnt);

    error = -ELOOP;
    if (unlikely(current->total_link_count >= 40))
        goto out_put_nd_path;

    cond_resched();
    current->total_link_count++;

    touch_atime(link);
    nd_set_link(nd, NULL);

    error = security_inode_follow_link(link->dentry, nd);
    if (error)
        goto out_put_nd_path;

    nd->last_type = LAST_BIND;
    *p = dentry->d_inode->i_op->follow_link(dentry, nd);
    error = PTR_ERR(*p);
    if (IS_ERR(*p))
        goto out_put_nd_path;

    error = 0;
    s = nd_get_link(nd);
    if (s) {
        error = __vfs_follow_link(nd, s);
        if (unlikely(error))
            put_link(nd, link, *p);
    }

    return error;

out_put_nd_path:
    *p = NULL;
    path_put(&nd->path);
    path_put(link);
    return error;
}

static int follow_up_rcu(struct path *path)
{
    struct mount *mnt = real_mount(path->mnt);
    struct mount *parent;
    struct dentry *mountpoint;

    parent = mnt->mnt_parent;
    if (&parent->mnt == path->mnt)
        return 0;
    mountpoint = mnt->mnt_mountpoint;
    path->dentry = mountpoint;
    path->mnt = &parent->mnt;
    return 1;
}

/*
 * follow_up - Find the mountpoint of path's vfsmount
 *
 * Given a path, find the mountpoint of its source file system.
 * Replace @path with the path of the mountpoint in the parent mount.
 * Up is towards /.
 *
 * Return 1 if we went up a level and 0 if we were already at the
 * root.
 */
int follow_up(struct path *path)
{
    struct mount *mnt = real_mount(path->mnt);
    struct mount *parent;
    struct dentry *mountpoint;

    br_read_lock(&vfsmount_lock);
    parent = mnt->mnt_parent;
    if (parent == mnt) {
        br_read_unlock(&vfsmount_lock);
        return 0;
    }
    mntget(&parent->mnt);
    mountpoint = dget(mnt->mnt_mountpoint);
    br_read_unlock(&vfsmount_lock);
    dput(path->dentry);
    path->dentry = mountpoint;
    mntput(path->mnt);
    path->mnt = &parent->mnt;
    return 1;
}

/*
 * Perform an automount
 * - return -EISDIR to tell follow_managed() to stop and return the path we
 *   were called with.
 */
static int follow_automount(struct path *path, unsigned flags,
                bool *need_mntput)
{
    struct vfsmount *mnt;
    int err;

    if (!path->dentry->d_op || !path->dentry->d_op->d_automount)
        return -EREMOTE;

    /* We don't want to mount if someone's just doing a stat -
     * unless they're stat'ing a directory and appended a '/' to
     * the name.
     *
     * We do, however, want to mount if someone wants to open or
     * create a file of any type under the mountpoint, wants to
     * traverse through the mountpoint or wants to open the
     * mounted directory.  Also, autofs may mark negative dentries
     * as being automount points.  These will need the attentions
     * of the daemon to instantiate them before they can be used.
     */
    if (!(flags & (LOOKUP_PARENT | LOOKUP_DIRECTORY |
             LOOKUP_OPEN | LOOKUP_CREATE | LOOKUP_AUTOMOUNT)) &&
        path->dentry->d_inode)
        return -EISDIR;

    current->total_link_count++;
    if (current->total_link_count >= 40)
        return -ELOOP;

    mnt = path->dentry->d_op->d_automount(path);
    if (IS_ERR(mnt)) {
        /*
         * The filesystem is allowed to return -EISDIR here to indicate
         * it doesn't want to automount.  For instance, autofs would do
         * this so that its userspace daemon can mount on this dentry.
         *
         * However, we can only permit this if it's a terminal point in
         * the path being looked up; if it wasn't then the remainder of
         * the path is inaccessible and we should say so.
         */
        if (PTR_ERR(mnt) == -EISDIR && (flags & LOOKUP_PARENT))
            return -EREMOTE;
        return PTR_ERR(mnt);
    }

    if (!mnt) /* mount collision */
        return 0;

    if (!*need_mntput) {
        /* lock_mount() may release path->mnt on error */
        mntget(path->mnt);
        *need_mntput = true;
    }
    err = finish_automount(mnt, path);

    switch (err) {
    case -EBUSY:
        /* Someone else made a mount here whilst we were busy */
        return 0;
    case 0:
        path_put(path);
        path->mnt = mnt;
        path->dentry = dget(mnt->mnt_root);
        return 0;
    default:
        return err;
    }

}

/*
 * Handle a dentry that is managed in some way.
 * - Flagged for transit management (autofs)
 * - Flagged as mountpoint
 * - Flagged as automount point
 *
 * This may only be called in refwalk mode.
 *
 * Serialization is taken care of in namespace.c
 */
static int follow_managed(struct path *path, unsigned flags)
{
    struct vfsmount *mnt = path->mnt; /* held by caller, must be left alone */
    unsigned managed;
    bool need_mntput = false;
    int ret = 0;

    /* Given that we're not holding a lock here, we retain the value in a
     * local variable for each dentry as we look at it so that we don't see
     * the components of that value change under us */
    while (managed = ACCESS_ONCE(path->dentry->d_flags),
           managed &= DCACHE_MANAGED_DENTRY,
           unlikely(managed != 0)) {
        /* Allow the filesystem to manage the transit without i_mutex
         * being held. */
        if (managed & DCACHE_MANAGE_TRANSIT) {
            BUG_ON(!path->dentry->d_op);
            BUG_ON(!path->dentry->d_op->d_manage);
            ret = path->dentry->d_op->d_manage(path->dentry, false);
            if (ret < 0)
                break;
        }

        /* Transit to a mounted filesystem. */
        if (managed & DCACHE_MOUNTED) {
            struct vfsmount *mounted = lookup_mnt(path);
            if (mounted) {
                dput(path->dentry);
                if (need_mntput)
                    mntput(path->mnt);
                path->mnt = mounted;
                path->dentry = dget(mounted->mnt_root);
                need_mntput = true;
                continue;
            }

            /* Something is mounted on this dentry in another
             * namespace and/or whatever was mounted there in this
             * namespace got unmounted before we managed to get the
             * vfsmount_lock */
        }

        /* Handle an automount point */
        if (managed & DCACHE_NEED_AUTOMOUNT) {
            ret = follow_automount(path, flags, &need_mntput);
            if (ret < 0)
                break;
            continue;
        }

        /* We didn't change the current path point */
        break;
    }

    if (need_mntput && path->mnt == mnt)
        mntput(path->mnt);
    if (ret == -EISDIR)
        ret = 0;
    return ret < 0 ? ret : need_mntput;
}

int follow_down_one(struct path *path)
{
    struct vfsmount *mounted;

    mounted = lookup_mnt(path);
    if (mounted) {
        dput(path->dentry);
        mntput(path->mnt);
        path->mnt = mounted;
        path->dentry = dget(mounted->mnt_root);
        return 1;
    }
    return 0;
}

static inline bool managed_dentry_might_block(struct dentry *dentry)
{
    return (dentry->d_flags & DCACHE_MANAGE_TRANSIT &&
        dentry->d_op->d_manage(dentry, true) < 0);
}

/*
 * Try to skip to top of mountpoint pile in rcuwalk mode.  Fail if
 * we meet a managed dentry that would need blocking.
 */
static bool __follow_mount_rcu(struct nameidata *nd, struct path *path,
                   struct inode **inode)
{
    for (;;) {
        struct mount *mounted;
        /*
         * Don't forget we might have a non-mountpoint managed dentry
         * that wants to block transit.
         */
        if (unlikely(managed_dentry_might_block(path->dentry)))
            return false;

        if (!d_mountpoint(path->dentry))
            break;

        mounted = __lookup_mnt(path->mnt, path->dentry, 1);
        if (!mounted)
            break;
        path->mnt = &mounted->mnt;
        path->dentry = mounted->mnt.mnt_root;
        nd->flags |= LOOKUP_JUMPED;
        nd->seq = read_seqcount_begin(&path->dentry->d_seq);
        /*
         * Update the inode too. We don't need to re-check the
         * dentry sequence number here after this d_inode read,
         * because a mount-point is always pinned.
         */
        *inode = path->dentry->d_inode;
    }
    return true;
}

static void follow_mount_rcu(struct nameidata *nd)
{
    while (d_mountpoint(nd->path.dentry)) {
        struct mount *mounted;
        mounted = __lookup_mnt(nd->path.mnt, nd->path.dentry, 1);
        if (!mounted)
            break;
        nd->path.mnt = &mounted->mnt;
        nd->path.dentry = mounted->mnt.mnt_root;
        nd->seq = read_seqcount_begin(&nd->path.dentry->d_seq);
    }
}

static int follow_dotdot_rcu(struct nameidata *nd)
{
    set_root_rcu(nd);

    while (1) {
        if (nd->path.dentry == nd->root.dentry &&
            nd->path.mnt == nd->root.mnt) {
            break;
        }
        if (nd->path.dentry != nd->path.mnt->mnt_root) {
            struct dentry *old = nd->path.dentry;
            struct dentry *parent = old->d_parent;
            unsigned seq;

            seq = read_seqcount_begin(&parent->d_seq);
            if (read_seqcount_retry(&old->d_seq, nd->seq))
                goto failed;
            nd->path.dentry = parent;
            nd->seq = seq;
            break;
        }
        if (!follow_up_rcu(&nd->path))
            break;
        nd->seq = read_seqcount_begin(&nd->path.dentry->d_seq);
    }
    follow_mount_rcu(nd);
    nd->inode = nd->path.dentry->d_inode;
    return 0;

failed:
    nd->flags &= ~LOOKUP_RCU;
    if (!(nd->flags & LOOKUP_ROOT))
        nd->root.mnt = NULL;
    unlock_rcu_walk();
    return -ECHILD;
}

/*
 * Follow down to the covering mount currently visible to userspace.  At each
 * point, the filesystem owning that dentry may be queried as to whether the
 * caller is permitted to proceed or not.
 */
int follow_down(struct path *path)
{
    unsigned managed;
    int ret;

    while (managed = ACCESS_ONCE(path->dentry->d_flags),
           unlikely(managed & DCACHE_MANAGED_DENTRY)) {
        /* Allow the filesystem to manage the transit without i_mutex
         * being held.
         *
         * We indicate to the filesystem if someone is trying to mount
         * something here.  This gives autofs the chance to deny anyone
         * other than its daemon the right to mount on its
         * superstructure.
         *
         * The filesystem may sleep at this point.
         */
        if (managed & DCACHE_MANAGE_TRANSIT) {
            BUG_ON(!path->dentry->d_op);
            BUG_ON(!path->dentry->d_op->d_manage);
            ret = path->dentry->d_op->d_manage(
                path->dentry, false);
            if (ret < 0)
                return ret == -EISDIR ? 0 : ret;
        }

        /* Transit to a mounted filesystem. */
        if (managed & DCACHE_MOUNTED) {
            struct vfsmount *mounted = lookup_mnt(path);
            if (!mounted)
                break;
            dput(path->dentry);
            mntput(path->mnt);
            path->mnt = mounted;
            path->dentry = dget(mounted->mnt_root);
            continue;
        }

        /* Don't handle automount points here */
        break;
    }
    return 0;
}

/*
 * Skip to top of mountpoint pile in refwalk mode for follow_dotdot()
 */
static void follow_mount(struct path *path)
{
    while (d_mountpoint(path->dentry)) {
        struct vfsmount *mounted = lookup_mnt(path);
        if (!mounted)
            break;
        dput(path->dentry);
        mntput(path->mnt);
        path->mnt = mounted;
        path->dentry = dget(mounted->mnt_root);
    }
}

static void follow_dotdot(struct nameidata *nd)
{
    set_root(nd);

    while(1) {
        struct dentry *old = nd->path.dentry;

        if (nd->path.dentry == nd->root.dentry &&
            nd->path.mnt == nd->root.mnt) {
            break;
        }
        if (nd->path.dentry != nd->path.mnt->mnt_root) {
            /* rare case of legitimate dget_parent()... */
            nd->path.dentry = dget_parent(nd->path.dentry);
            dput(old);
            break;
        }
        if (!follow_up(&nd->path))
            break;
    }
    follow_mount(&nd->path);
    nd->inode = nd->path.dentry->d_inode;
}

/*
 * This looks up the name in dcache, possibly revalidates the old dentry and
 * allocates a new one if not found or not valid.  In the need_lookup argument
 * returns whether i_op->lookup is necessary.
 *
 * dir->d_inode->i_mutex must be held
 */
static struct dentry *lookup_dcache(struct qstr *name, struct dentry *dir,
                    unsigned int flags, bool *need_lookup)
{
    struct dentry *dentry;
    int error;

    *need_lookup = false;
    dentry = d_lookup(dir, name);
    if (dentry) {
        if (d_need_lookup(dentry)) {
            *need_lookup = true;
        } else if (dentry->d_flags & DCACHE_OP_REVALIDATE) {
            error = d_revalidate(dentry, flags);
            if (unlikely(error <= 0)) {
                if (error < 0) {
                    dput(dentry);
                    return ERR_PTR(error);
                } else if (!d_invalidate(dentry)) {
                    dput(dentry);
                    dentry = NULL;
                }
            }
        }
    }

    if (!dentry) {
        dentry = d_alloc(dir, name);
        if (unlikely(!dentry))
            return ERR_PTR(-ENOMEM);

        *need_lookup = true;
    }
    return dentry;
}

/*
 * Call i_op->lookup on the dentry.  The dentry must be negative but may be
 * hashed if it was pouplated with DCACHE_NEED_LOOKUP.
 *
 * dir->d_inode->i_mutex must be held
 */
static struct dentry *lookup_real(struct inode *dir, struct dentry *dentry,
                  unsigned int flags)
{
    struct dentry *old;

    /* Don't create child dentry for a dead directory. */
    if (unlikely(IS_DEADDIR(dir))) {
        dput(dentry);
        return ERR_PTR(-ENOENT);
    }

    old = dir->i_op->lookup(dir, dentry, flags);
    if (unlikely(old)) {
        dput(dentry);
        dentry = old;
    }
    return dentry;
}

static struct dentry *__lookup_hash(struct qstr *name,
        struct dentry *base, unsigned int flags)
{
    bool need_lookup;
    struct dentry *dentry;

    dentry = lookup_dcache(name, base, flags, &need_lookup);
    if (!need_lookup)
        return dentry;

    return lookup_real(base->d_inode, dentry, flags);
}

/*
 *  It's more convoluted than I'd like it to be, but... it's still fairly
 *  small and for now I'd prefer to have fast path as straight as possible.
 *  It _is_ time-critical.
 */
static int lookup_fast(struct nameidata *nd, struct qstr *name,
               struct path *path, struct inode **inode)
{
    struct vfsmount *mnt = nd->path.mnt;
    struct dentry *dentry, *parent = nd->path.dentry;
    int need_reval = 1;
    int status = 1;
    int err;

    /*
     * Rename seqlock is not required here because in the off chance
     * of a false negative due to a concurrent rename, we're going to
     * do the non-racy lookup, below.
     */
    if (nd->flags & LOOKUP_RCU) {
        unsigned seq;
        dentry = __d_lookup_rcu(parent, name, &seq, nd->inode);
        if (!dentry)
            goto unlazy;

        /*
         * This sequence count validates that the inode matches
         * the dentry name information from lookup.
         */
        *inode = dentry->d_inode;
        if (read_seqcount_retry(&dentry->d_seq, seq))
            return -ECHILD;

        /*
         * This sequence count validates that the parent had no
         * changes while we did the lookup of the dentry above.
         *
         * The memory barrier in read_seqcount_begin of child is
         *  enough, we can use __read_seqcount_retry here.
         */
        if (__read_seqcount_retry(&parent->d_seq, nd->seq))
            return -ECHILD;
        nd->seq = seq;

        if (unlikely(d_need_lookup(dentry)))
            goto unlazy;
        if (unlikely(dentry->d_flags & DCACHE_OP_REVALIDATE)) {
            status = d_revalidate(dentry, nd->flags);
            if (unlikely(status <= 0)) {
                if (status != -ECHILD)
                    need_reval = 0;
                goto unlazy;
            }
        }
        path->mnt = mnt;
        path->dentry = dentry;
        if (unlikely(!__follow_mount_rcu(nd, path, inode)))
            goto unlazy;
        if (unlikely(path->dentry->d_flags & DCACHE_NEED_AUTOMOUNT))
            goto unlazy;
        return 0;
unlazy:
        if (unlazy_walk(nd, dentry))
            return -ECHILD;
    } else {
        dentry = __d_lookup(parent, name);
    }

    if (unlikely(!dentry))
        goto need_lookup;

    if (unlikely(d_need_lookup(dentry))) {
        dput(dentry);
        goto need_lookup;
    }

    if (unlikely(dentry->d_flags & DCACHE_OP_REVALIDATE) && need_reval)
        status = d_revalidate(dentry, nd->flags);
    if (unlikely(status <= 0)) {
        if (status < 0) {
            dput(dentry);
            return status;
        }
        if (!d_invalidate(dentry)) {
            dput(dentry);
            goto need_lookup;
        }
    }

    path->mnt = mnt;
    path->dentry = dentry;
    err = follow_managed(path, nd->flags);
    if (unlikely(err < 0)) {
        path_put_conditional(path, nd);
        return err;
    }
    if (err)
        nd->flags |= LOOKUP_JUMPED;
    *inode = path->dentry->d_inode;
    return 0;

need_lookup:
    return 1;
}

/* Fast lookup failed, do it the slow way */
static int lookup_slow(struct nameidata *nd, struct qstr *name,
               struct path *path)
{
    struct dentry *dentry, *parent;
    int err;

    parent = nd->path.dentry;
    BUG_ON(nd->inode != parent->d_inode);

    mutex_lock(&parent->d_inode->i_mutex);
    dentry = __lookup_hash(name, parent, nd->flags);
    mutex_unlock(&parent->d_inode->i_mutex);
    if (IS_ERR(dentry))
        return PTR_ERR(dentry);
    path->mnt = nd->path.mnt;
    path->dentry = dentry;
    err = follow_managed(path, nd->flags);
    if (unlikely(err < 0)) {
        path_put_conditional(path, nd);
        return err;
    }
    if (err)
        nd->flags |= LOOKUP_JUMPED;
    return 0;
}

static inline int may_lookup(struct nameidata *nd)
{
    if (nd->flags & LOOKUP_RCU) {
        int err = inode_permission(nd->inode, MAY_EXEC|MAY_NOT_BLOCK);
        if (err != -ECHILD)
            return err;
        if (unlazy_walk(nd, NULL))
            return -ECHILD;
    }
    return inode_permission(nd->inode, MAY_EXEC);
}

static inline int handle_dots(struct nameidata *nd, int type)
{
    if (type == LAST_DOTDOT) {
        if (nd->flags & LOOKUP_RCU) {
            if (follow_dotdot_rcu(nd))
                return -ECHILD;
        } else
            follow_dotdot(nd);
    }
    return 0;
}

static void terminate_walk(struct nameidata *nd)
{
    if (!(nd->flags & LOOKUP_RCU)) {
        path_put(&nd->path);
    } else {
        nd->flags &= ~LOOKUP_RCU;
        if (!(nd->flags & LOOKUP_ROOT))
            nd->root.mnt = NULL;
        unlock_rcu_walk();
    }
}

/*
 * Do we need to follow links? We _really_ want to be able
 * to do this check without having to look at inode->i_op,
 * so we keep a cache of "no, this doesn't need follow_link"
 * for the common case.
 */
static inline int should_follow_link(struct inode *inode, int follow)
{
    if (unlikely(!(inode->i_opflags & IOP_NOFOLLOW))) {
        if (likely(inode->i_op->follow_link))
            return follow;

        /* This gets set once for the inode lifetime */
        spin_lock(&inode->i_lock);
        inode->i_opflags |= IOP_NOFOLLOW;
        spin_unlock(&inode->i_lock);
    }
    return 0;
}

static inline int walk_component(struct nameidata *nd, struct path *path,
        struct qstr *name, int type, int follow)
{
    struct inode *inode;
    int err;
    /*
     * "." and ".." are special - ".." especially so because it has
     * to be able to know about the current root directory and
     * parent relationships.
     */
    if (unlikely(type != LAST_NORM))
        return handle_dots(nd, type);
    err = lookup_fast(nd, name, path, &inode);
    if (unlikely(err)) {
        if (err < 0)
            goto out_err;

        err = lookup_slow(nd, name, path);
        if (err < 0)
            goto out_err;

        inode = path->dentry->d_inode;
    }
    err = -ENOENT;
    if (!inode)
        goto out_path_put;

    if (should_follow_link(inode, follow)) {
        if (nd->flags & LOOKUP_RCU) {
            if (unlikely(unlazy_walk(nd, path->dentry))) {
                err = -ECHILD;
                goto out_err;
            }
        }
        BUG_ON(inode != path->dentry->d_inode);
        return 1;
    }
    path_to_nameidata(path, nd);
    nd->inode = inode;
    return 0;

out_path_put:
    path_to_nameidata(path, nd);
out_err:
    terminate_walk(nd);
    return err;
}

/*
 * This limits recursive symlink follows to 8, while
 * limiting consecutive symlinks to 40.
 *
 * Without that kind of total limit, nasty chains of consecutive
 * symlinks can cause almost arbitrarily long lookups.
 */
static inline int nested_symlink(struct path *path, struct nameidata *nd)
{
    int res;

    if (unlikely(current->link_count >= MAX_NESTED_LINKS)) {
        path_put_conditional(path, nd);
        path_put(&nd->path);
        return -ELOOP;
    }
    BUG_ON(nd->depth >= MAX_NESTED_LINKS);

    nd->depth++;
    current->link_count++;

    do {
        struct path link = *path;
        void *cookie;

        res = follow_link(&link, nd, &cookie);
        if (res)
            break;
        res = walk_component(nd, path, &nd->last,
                     nd->last_type, LOOKUP_FOLLOW);
        put_link(nd, &link, cookie);
    } while (res > 0);

    current->link_count--;
    nd->depth--;
    return res;
}

/*
 * We really don't want to look at inode->i_op->lookup
 * when we don't have to. So we keep a cache bit in
 * the inode ->i_opflags field that says "yes, we can
 * do lookup on this inode".
 */
static inline int can_lookup(struct inode *inode)
{
    if (likely(inode->i_opflags & IOP_LOOKUP))
        return 1;
    if (likely(!inode->i_op->lookup))
        return 0;

    /* We do this once for the lifetime of the inode */
    spin_lock(&inode->i_lock);
    inode->i_opflags |= IOP_LOOKUP;
    spin_unlock(&inode->i_lock);
    return 1;
}

/*
 * We can do the critical dentry name comparison and hashing
 * operations one word at a time, but we are limited to:
 *
 * - Architectures with fast unaligned word accesses. We could
 *   do a "get_unaligned()" if this helps and is sufficiently
 *   fast.
 *
 * - Little-endian machines (so that we can generate the mask
 *   of low bytes efficiently). Again, we *could* do a byte
 *   swapping load on big-endian architectures if that is not
 *   expensive enough to make the optimization worthless.
 *
 * - non-CONFIG_DEBUG_PAGEALLOC configurations (so that we
 *   do not trap on the (extremely unlikely) case of a page
 *   crossing operation.
 *
 * - Furthermore, we need an efficient 64-bit compile for the
 *   64-bit case in order to generate the "number of bytes in
 *   the final mask". Again, that could be replaced with a
 *   efficient population count instruction or similar.
 */
#ifdef CONFIG_DCACHE_WORD_ACCESS

#include <asm/word-at-a-time.h>

#ifdef CONFIG_64BIT

static inline unsigned int fold_hash(unsigned long hash)
{
    hash += hash >> (8*sizeof(int));
    return hash;
}

#else   /* 32-bit case */

#define fold_hash(x) (x)

#endif

unsigned int full_name_hash(const unsigned char *name, unsigned int len)
{
    unsigned long a, mask;
    unsigned long hash = 0;

    for (;;) {
        a = load_unaligned_zeropad(name);
        if (len < sizeof(unsigned long))
            break;
        hash += a;
        hash *= 9;
        name += sizeof(unsigned long);
        len -= sizeof(unsigned long);
        if (!len)
            goto done;
    }
    mask = ~(~0ul << len*8);
    hash += mask & a;
done:
    return fold_hash(hash);
}
EXPORT_SYMBOL(full_name_hash);

/*
 * Calculate the length and hash of the path component, and
 * return the length of the component;
 */
static inline unsigned long hash_name(const char *name, unsigned int *hashp)
{
    unsigned long a, b, adata, bdata, mask, hash, len;
    const struct word_at_a_time constants = WORD_AT_A_TIME_CONSTANTS;

    hash = a = 0;
    len = -sizeof(unsigned long);
    do {
        hash = (hash + a) * 9;
        len += sizeof(unsigned long);
        a = load_unaligned_zeropad(name+len);
        b = a ^ REPEAT_BYTE('/');
    } while (!(has_zero(a, &adata, &constants) | has_zero(b, &bdata, &constants)));

    adata = prep_zero_mask(a, adata, &constants);
    bdata = prep_zero_mask(b, bdata, &constants);

    mask = create_zero_mask(adata | bdata);

    hash += a & zero_bytemask(mask);
    *hashp = fold_hash(hash);

    return len + find_zero(mask);
}

#else

unsigned int full_name_hash(const unsigned char *name, unsigned int len)
{
    unsigned long hash = init_name_hash();
    while (len--)
        hash = partial_name_hash(*name++, hash);
    return end_name_hash(hash);
}
EXPORT_SYMBOL(full_name_hash);

/*
 * We know there's a real path component here of at least
 * one character.
 */
static inline unsigned long hash_name(const char *name, unsigned int *hashp)
{
    unsigned long hash = init_name_hash();
    unsigned long len = 0, c;

    c = (unsigned char)*name;
    do {
        len++;
        hash = partial_name_hash(c, hash);
        c = (unsigned char)name[len];
    } while (c && c != '/');
    *hashp = end_name_hash(hash);
    return len;
}

#endif

/*
 * Name resolution.
 * This is the basic name resolution function, turning a pathname into
 * the final dentry. We expect 'base' to be positive and a directory.
 *
 * Returns 0 and nd will have valid dentry and mnt on success.
 * Returns error and drops reference to input namei data on failure.
 */
static int link_path_walk(const char *name, struct nameidata *nd)
{
    struct path next;
    int err;
    
    while (*name=='/')
        name++;
    if (!*name)
        return 0;

    /* At this point we know we have a real path component. */
    for(;;) {
        struct qstr this;
        long len;
        int type;

        err = may_lookup(nd);
        if (err)
            break;

        len = hash_name(name, &this.hash);
        this.name = name;
        this.len = len;

        type = LAST_NORM;
        if (name[0] == '.') switch (len) {
            case 2:
                if (name[1] == '.') {
                    type = LAST_DOTDOT;
                    nd->flags |= LOOKUP_JUMPED;
                }
                break;
            case 1:
                type = LAST_DOT;
        }
        if (likely(type == LAST_NORM)) {
            struct dentry *parent = nd->path.dentry;
            nd->flags &= ~LOOKUP_JUMPED;
            if (unlikely(parent->d_flags & DCACHE_OP_HASH)) {
                err = parent->d_op->d_hash(parent, nd->inode,
                               &this);
                if (err < 0)
                    break;
            }
        }

        if (!name[len])
            goto last_component;
        /*
         * If it wasn't NUL, we know it was '/'. Skip that
         * slash, and continue until no more slashes.
         */
        do {
            len++;
        } while (unlikely(name[len] == '/'));
        if (!name[len])
            goto last_component;
        name += len;

        err = walk_component(nd, &next, &this, type, LOOKUP_FOLLOW);
        if (err < 0)
            return err;

        if (err) {
            err = nested_symlink(&next, nd);
            if (err)
                return err;
        }
        if (can_lookup(nd->inode))
            continue;
        err = -ENOTDIR; 
        break;
        /* here ends the main loop */

last_component:
        nd->last = this;
        nd->last_type = type;
        return 0;
    }
    terminate_walk(nd);
    return err;
}

static int path_init(int dfd, const char *name, unsigned int flags,
             struct nameidata *nd, struct file **fp)
{
    int retval = 0;

    nd->last_type = LAST_ROOT; /* if there are only slashes... */
    nd->flags = flags | LOOKUP_JUMPED;
    nd->depth = 0;
    if (flags & LOOKUP_ROOT) {
        struct inode *inode = nd->root.dentry->d_inode;
        if (*name) {
            if (!inode->i_op->lookup)
                return -ENOTDIR;
            retval = inode_permission(inode, MAY_EXEC);
            if (retval)
                return retval;
        }
        nd->path = nd->root;
        nd->inode = inode;
        if (flags & LOOKUP_RCU) {
            lock_rcu_walk();
            nd->seq = __read_seqcount_begin(&nd->path.dentry->d_seq);
        } else {
            path_get(&nd->path);
        }
        return 0;
    }

    nd->root.mnt = NULL;

    if (*name=='/') {
        if (flags & LOOKUP_RCU) {
            lock_rcu_walk();
            set_root_rcu(nd);
        } else {
            set_root(nd);
            path_get(&nd->root);
        }
        nd->path = nd->root;
    } else if (dfd == AT_FDCWD) {
        if (flags & LOOKUP_RCU) {
            struct fs_struct *fs = current->fs;
            unsigned seq;

            lock_rcu_walk();

            do {
                seq = read_seqcount_begin(&fs->seq);
                nd->path = fs->pwd;
                nd->seq = __read_seqcount_begin(&nd->path.dentry->d_seq);
            } while (read_seqcount_retry(&fs->seq, seq));
        } else {
            get_fs_pwd(current->fs, &nd->path);
        }
    } else {
        struct fd f = fdget_raw(dfd);
        struct dentry *dentry;

        if (!f.file)
            return -EBADF;

        dentry = f.file->f_path.dentry;

        if (*name) {
            if (!S_ISDIR(dentry->d_inode->i_mode)) {
                fdput(f);
                return -ENOTDIR;
            }

            retval = inode_permission(dentry->d_inode, MAY_EXEC);
            if (retval) {
                fdput(f);
                return retval;
            }
        }

        nd->path = f.file->f_path;
        if (flags & LOOKUP_RCU) {
            if (f.need_put)
                *fp = f.file;
            nd->seq = __read_seqcount_begin(&nd->path.dentry->d_seq);
            lock_rcu_walk();
        } else {
            path_get(&nd->path);
            fdput(f);
        }
    }

    nd->inode = nd->path.dentry->d_inode;
    return 0;
}

static inline int lookup_last(struct nameidata *nd, struct path *path)
{
    if (nd->last_type == LAST_NORM && nd->last.name[nd->last.len])
        nd->flags |= LOOKUP_FOLLOW | LOOKUP_DIRECTORY;

    nd->flags &= ~LOOKUP_PARENT;
    return walk_component(nd, path, &nd->last, nd->last_type,
                    nd->flags & LOOKUP_FOLLOW);
}

/* Returns 0 and nd will be valid on success; Retuns error, otherwise. */
static int path_lookupat(int dfd, const char *name,
                unsigned int flags, struct nameidata *nd)
{
    struct file *base = NULL;
    struct path path;
    int err;

    /*
     * Path walking is largely split up into 2 different synchronisation
     * schemes, rcu-walk and ref-walk (explained in
     * Documentation/filesystems/path-lookup.txt). These share much of the
     * path walk code, but some things particularly setup, cleanup, and
     * following mounts are sufficiently divergent that functions are
     * duplicated. Typically there is a function foo(), and its RCU
     * analogue, foo_rcu().
     *
     * -ECHILD is the error number of choice (just to avoid clashes) that
     * is returned if some aspect of an rcu-walk fails. Such an error must
     * be handled by restarting a traditional ref-walk (which will always
     * be able to complete).
     */
    err = path_init(dfd, name, flags | LOOKUP_PARENT, nd, &base);

    if (unlikely(err))
        return err;

    current->total_link_count = 0;
    err = link_path_walk(name, nd);

    if (!err && !(flags & LOOKUP_PARENT)) {
        err = lookup_last(nd, &path);
        while (err > 0) {
            void *cookie;
            struct path link = path;
            err = may_follow_link(&link, nd);
            if (unlikely(err))
                break;
            nd->flags |= LOOKUP_PARENT;
            err = follow_link(&link, nd, &cookie);
            if (err)
                break;
            err = lookup_last(nd, &path);
            put_link(nd, &link, cookie);
        }
    }

    if (!err)
        err = complete_walk(nd);

    if (!err && nd->flags & LOOKUP_DIRECTORY) {
        if (!nd->inode->i_op->lookup) {
            path_put(&nd->path);
            err = -ENOTDIR;
        }
    }

    if (base)
        fput(base);

    if (nd->root.mnt && !(nd->flags & LOOKUP_ROOT)) {
        path_put(&nd->root);
        nd->root.mnt = NULL;
    }
    return err;
}

static int filename_lookup(int dfd, struct filename *name,
                unsigned int flags, struct nameidata *nd)
{
    int retval = path_lookupat(dfd, name->name, flags | LOOKUP_RCU, nd);
    if (unlikely(retval == -ECHILD))
        retval = path_lookupat(dfd, name->name, flags, nd);
    if (unlikely(retval == -ESTALE))
        retval = path_lookupat(dfd, name->name,
                        flags | LOOKUP_REVAL, nd);

    if (likely(!retval))
        audit_inode(name, nd->path.dentry, flags & LOOKUP_PARENT);
    return retval;
}

static int do_path_lookup(int dfd, const char *name,
                unsigned int flags, struct nameidata *nd)
{
    struct filename filename = { .name = name };

    return filename_lookup(dfd, &filename, flags, nd);
}

/* does lookup, returns the object with parent locked */
struct dentry *kern_path_locked(const char *name, struct path *path)
{
    struct nameidata nd;
    struct dentry *d;
    int err = do_path_lookup(AT_FDCWD, name, LOOKUP_PARENT, &nd);
    if (err)
        return ERR_PTR(err);
    if (nd.last_type != LAST_NORM) {
        path_put(&nd.path);
        return ERR_PTR(-EINVAL);
    }
    mutex_lock_nested(&nd.path.dentry->d_inode->i_mutex, I_MUTEX_PARENT);
    d = __lookup_hash(&nd.last, nd.path.dentry, 0);
    if (IS_ERR(d)) {
        mutex_unlock(&nd.path.dentry->d_inode->i_mutex);
        path_put(&nd.path);
        return d;
    }
    *path = nd.path;
    return d;
}

int kern_path(const char *name, unsigned int flags, struct path *path)
{
    struct nameidata nd;
    int res = do_path_lookup(AT_FDCWD, name, flags, &nd);
    if (!res)
        *path = nd.path;
    return res;
}

int vfs_path_lookup(struct dentry *dentry, struct vfsmount *mnt,
            const char *name, unsigned int flags,
            struct path *path)
{
    struct nameidata nd;
    int err;
    nd.root.dentry = dentry;
    nd.root.mnt = mnt;
    BUG_ON(flags & LOOKUP_PARENT);
    /* the first argument of do_path_lookup() is ignored with LOOKUP_ROOT */
    err = do_path_lookup(AT_FDCWD, name, flags | LOOKUP_ROOT, &nd);
    if (!err)
        *path = nd.path;
    return err;
}

/*
 * Restricted form of lookup. Doesn't follow links, single-component only,
 * needs parent already locked. Doesn't follow mounts.
 * SMP-safe.
 */
static struct dentry *lookup_hash(struct nameidata *nd)
{
    return __lookup_hash(&nd->last, nd->path.dentry, nd->flags);
}

struct dentry *lookup_one_len(const char *name, struct dentry *base, int len)
{
    struct qstr this;
    unsigned int c;
    int err;

    WARN_ON_ONCE(!mutex_is_locked(&base->d_inode->i_mutex));

    this.name = name;
    this.len = len;
    this.hash = full_name_hash(name, len);
    if (!len)
        return ERR_PTR(-EACCES);

    if (unlikely(name[0] == '.')) {
        if (len < 2 || (len == 2 && name[1] == '.'))
            return ERR_PTR(-EACCES);
    }

    while (len--) {
        c = *(const unsigned char *)name++;
        if (c == '/' || c == '\0')
            return ERR_PTR(-EACCES);
    }
    /*
     * See if the low-level filesystem might want
     * to use its own hash..
     */
    if (base->d_flags & DCACHE_OP_HASH) {
        int err = base->d_op->d_hash(base, base->d_inode, &this);
        if (err < 0)
            return ERR_PTR(err);
    }

    err = inode_permission(base->d_inode, MAY_EXEC);
    if (err)
        return ERR_PTR(err);

    return __lookup_hash(&this, base, 0);
}

int user_path_at_empty(int dfd, const char __user *name, unsigned flags,
         struct path *path, int *empty)
{
    struct nameidata nd;
    struct filename *tmp = getname_flags(name, flags, empty);
    int err = PTR_ERR(tmp);
    if (!IS_ERR(tmp)) {

        BUG_ON(flags & LOOKUP_PARENT);

        err = filename_lookup(dfd, tmp, flags, &nd);
        putname(tmp);
        if (!err)
            *path = nd.path;
    }
    return err;
}

int user_path_at(int dfd, const char __user *name, unsigned flags,
         struct path *path)
{
    return user_path_at_empty(dfd, name, flags, path, NULL);
}

/*
 * NB: most callers don't do anything directly with the reference to the
 *     to struct filename, but the nd->last pointer points into the name string
 *     allocated by getname. So we must hold the reference to it until all
 *     path-walking is complete.
 */
static struct filename *
user_path_parent(int dfd, const char __user *path, struct nameidata *nd)
{
    struct filename *s = getname(path);
    int error;

    if (IS_ERR(s))
        return s;

    error = filename_lookup(dfd, s, LOOKUP_PARENT, nd);
    if (error) {
        putname(s);
        return ERR_PTR(error);
    }

    return s;
}

/*
 * It's inline, so penalty for filesystems that don't use sticky bit is
 * minimal.
 */
static inline int check_sticky(struct inode *dir, struct inode *inode)
{
    kuid_t fsuid = current_fsuid();

    if (!(dir->i_mode & S_ISVTX))
        return 0;
    if (uid_eq(inode->i_uid, fsuid))
        return 0;
    if (uid_eq(dir->i_uid, fsuid))
        return 0;
    return !inode_capable(inode, CAP_FOWNER);
}

/*
 *  Check whether we can remove a link victim from directory dir, check
 *  whether the type of victim is right.
 *  1. We can't do it if dir is read-only (done in permission())
 *  2. We should have write and exec permissions on dir
 *  3. We can't remove anything from append-only dir
 *  4. We can't do anything with immutable dir (done in permission())
 *  5. If the sticky bit on dir is set we should either
 *  a. be owner of dir, or
 *  b. be owner of victim, or
 *  c. have CAP_FOWNER capability
 *  6. If the victim is append-only or immutable we can't do antyhing with
 *     links pointing to it.
 *  7. If we were asked to remove a directory and victim isn't one - ENOTDIR.
 *  8. If we were asked to remove a non-directory and victim isn't one - EISDIR.
 *  9. We can't remove a root or mountpoint.
 * 10. We don't allow removal of NFS sillyrenamed files; it's handled by
 *     nfs_async_unlink().
 */
static int may_delete(struct inode *dir,struct dentry *victim,int isdir)
{
    int error;

    if (!victim->d_inode)
        return -ENOENT;

    BUG_ON(victim->d_parent->d_inode != dir);
    audit_inode_child(dir, victim, AUDIT_TYPE_CHILD_DELETE);

    error = inode_permission(dir, MAY_WRITE | MAY_EXEC);
    if (error)
        return error;
    if (IS_APPEND(dir))
        return -EPERM;
    if (check_sticky(dir, victim->d_inode)||IS_APPEND(victim->d_inode)||
        IS_IMMUTABLE(victim->d_inode) || IS_SWAPFILE(victim->d_inode))
        return -EPERM;
    if (isdir) {
        if (!S_ISDIR(victim->d_inode->i_mode))
            return -ENOTDIR;
        if (IS_ROOT(victim))
            return -EBUSY;
    } else if (S_ISDIR(victim->d_inode->i_mode))
        return -EISDIR;
    if (IS_DEADDIR(dir))
        return -ENOENT;
    if (victim->d_flags & DCACHE_NFSFS_RENAMED)
        return -EBUSY;
    return 0;
}

/*  Check whether we can create an object with dentry child in directory
 *  dir.
 *  1. We can't do it if child already exists (open has special treatment for
 *     this case, but since we are inlined it's OK)
 *  2. We can't do it if dir is read-only (done in permission())
 *  3. We should have write and exec permissions on dir
 *  4. We can't do it if dir is immutable (done in permission())
 */
static inline int may_create(struct inode *dir, struct dentry *child)
{
    if (child->d_inode)
        return -EEXIST;
    if (IS_DEADDIR(dir))
        return -ENOENT;
    return inode_permission(dir, MAY_WRITE | MAY_EXEC);
}

/*
 * p1 and p2 should be directories on the same fs.
 */
struct dentry *lock_rename(struct dentry *p1, struct dentry *p2)
{
    struct dentry *p;

    if (p1 == p2) {
        mutex_lock_nested(&p1->d_inode->i_mutex, I_MUTEX_PARENT);
        return NULL;
    }

    mutex_lock(&p1->d_inode->i_sb->s_vfs_rename_mutex);

    p = d_ancestor(p2, p1);
    if (p) {
        mutex_lock_nested(&p2->d_inode->i_mutex, I_MUTEX_PARENT);
        mutex_lock_nested(&p1->d_inode->i_mutex, I_MUTEX_CHILD);
        return p;
    }

    p = d_ancestor(p1, p2);
    if (p) {
        mutex_lock_nested(&p1->d_inode->i_mutex, I_MUTEX_PARENT);
        mutex_lock_nested(&p2->d_inode->i_mutex, I_MUTEX_CHILD);
        return p;
    }

    mutex_lock_nested(&p1->d_inode->i_mutex, I_MUTEX_PARENT);
    mutex_lock_nested(&p2->d_inode->i_mutex, I_MUTEX_CHILD);
    return NULL;
}

void unlock_rename(struct dentry *p1, struct dentry *p2)
{
    mutex_unlock(&p1->d_inode->i_mutex);
    if (p1 != p2) {
        mutex_unlock(&p2->d_inode->i_mutex);
        mutex_unlock(&p1->d_inode->i_sb->s_vfs_rename_mutex);
    }
}

int vfs_create(struct inode *dir, struct dentry *dentry, umode_t mode,
        bool want_excl)
{
    int error = may_create(dir, dentry);
    if (error)
        return error;

    if (!dir->i_op->create)
        return -EACCES; /* shouldn't it be ENOSYS? */
    mode &= S_IALLUGO;
    mode |= S_IFREG;
    error = security_inode_create(dir, dentry, mode);
    if (error)
        return error;
    error = dir->i_op->create(dir, dentry, mode, want_excl);
    if (!error)
        fsnotify_create(dir, dentry);
    return error;
}

static int may_open(struct path *path, int acc_mode, int flag)
{
    struct dentry *dentry = path->dentry;
    struct inode *inode = dentry->d_inode;
    int error;

    /* O_PATH? */
    if (!acc_mode)
        return 0;

    if (!inode)
        return -ENOENT;

    switch (inode->i_mode & S_IFMT) {
    case S_IFLNK:
        return -ELOOP;
    case S_IFDIR:
        if (acc_mode & MAY_WRITE)
            return -EISDIR;
        break;
    case S_IFBLK:
    case S_IFCHR:
        if (path->mnt->mnt_flags & MNT_NODEV)
            return -EACCES;
        /*FALLTHRU*/
    case S_IFIFO:
    case S_IFSOCK:
        flag &= ~O_TRUNC;
        break;
    }

    error = inode_permission(inode, acc_mode);
    if (error)
        return error;

    /*
     * An append-only file must be opened in append mode for writing.
     */
    if (IS_APPEND(inode)) {
        if  ((flag & O_ACCMODE) != O_RDONLY && !(flag & O_APPEND))
            return -EPERM;
        if (flag & O_TRUNC)
            return -EPERM;
    }

    /* O_NOATIME can only be set by the owner or superuser */
    if (flag & O_NOATIME && !inode_owner_or_capable(inode))
        return -EPERM;

    return 0;
}

static int handle_truncate(struct file *filp)
{
    struct path *path = &filp->f_path;
    struct inode *inode = path->dentry->d_inode;
    int error = get_write_access(inode);
    if (error)
        return error;
    /*
     * Refuse to truncate files with mandatory locks held on them.
     */
    error = locks_verify_locked(inode);
    if (!error)
        error = security_path_truncate(path);
    if (!error) {
        error = do_truncate(path->dentry, 0,
                    ATTR_MTIME|ATTR_CTIME|ATTR_OPEN,
                    filp);
    }
    put_write_access(inode);
    return error;
}

static inline int open_to_namei_flags(int flag)
{
    if ((flag & O_ACCMODE) == 3)
        flag--;
    return flag;
}

static int may_o_create(struct path *dir, struct dentry *dentry, umode_t mode)
{
    int error = security_path_mknod(dir, dentry, mode, 0);
    if (error)
        return error;

    error = inode_permission(dir->dentry->d_inode, MAY_WRITE | MAY_EXEC);
    if (error)
        return error;

    return security_inode_create(dir->dentry->d_inode, dentry, mode);
}

/*
 * Attempt to atomically look up, create and open a file from a negative
 * dentry.
 *
 * Returns 0 if successful.  The file will have been created and attached to
 * @file by the filesystem calling finish_open().
 *
 * Returns 1 if the file was looked up only or didn't need creating.  The
 * caller will need to perform the open themselves.  @path will have been
 * updated to point to the new dentry.  This may be negative.
 *
 * Returns an error code otherwise.
 */
static int atomic_open(struct nameidata *nd, struct dentry *dentry,
            struct path *path, struct file *file,
            const struct open_flags *op,
            bool got_write, bool need_lookup,
            int *opened)
{
    struct inode *dir =  nd->path.dentry->d_inode;
    unsigned open_flag = open_to_namei_flags(op->open_flag);
    umode_t mode;
    int error;
    int acc_mode;
    int create_error = 0;
    struct dentry *const DENTRY_NOT_SET = (void *) -1UL;

    BUG_ON(dentry->d_inode);

    /* Don't create child dentry for a dead directory. */
    if (unlikely(IS_DEADDIR(dir))) {
        error = -ENOENT;
        goto out;
    }

    mode = op->mode;
    if ((open_flag & O_CREAT) && !IS_POSIXACL(dir))
        mode &= ~current_umask();

    if ((open_flag & (O_EXCL | O_CREAT)) == (O_EXCL | O_CREAT)) {
        open_flag &= ~O_TRUNC;
        *opened |= FILE_CREATED;
    }

    /*
     * Checking write permission is tricky, bacuse we don't know if we are
     * going to actually need it: O_CREAT opens should work as long as the
     * file exists.  But checking existence breaks atomicity.  The trick is
     * to check access and if not granted clear O_CREAT from the flags.
     *
     * Another problem is returing the "right" error value (e.g. for an
     * O_EXCL open we want to return EEXIST not EROFS).
     */
    if (((open_flag & (O_CREAT | O_TRUNC)) ||
        (open_flag & O_ACCMODE) != O_RDONLY) && unlikely(!got_write)) {
        if (!(open_flag & O_CREAT)) {
            /*
             * No O_CREATE -> atomicity not a requirement -> fall
             * back to lookup + open
             */
            goto no_open;
        } else if (open_flag & (O_EXCL | O_TRUNC)) {
            /* Fall back and fail with the right error */
            create_error = -EROFS;
            goto no_open;
        } else {
            /* No side effects, safe to clear O_CREAT */
            create_error = -EROFS;
            open_flag &= ~O_CREAT;
        }
    }

    if (open_flag & O_CREAT) {
        error = may_o_create(&nd->path, dentry, mode);
        if (error) {
            create_error = error;
            if (open_flag & O_EXCL)
                goto no_open;
            open_flag &= ~O_CREAT;
        }
    }

    if (nd->flags & LOOKUP_DIRECTORY)
        open_flag |= O_DIRECTORY;

    file->f_path.dentry = DENTRY_NOT_SET;
    file->f_path.mnt = nd->path.mnt;
    error = dir->i_op->atomic_open(dir, dentry, file, open_flag, mode,
                      opened);
    if (error < 0) {
        if (create_error && error == -ENOENT)
            error = create_error;
        goto out;
    }

    acc_mode = op->acc_mode;
    if (*opened & FILE_CREATED) {
        fsnotify_create(dir, dentry);
        acc_mode = MAY_OPEN;
    }

    if (error) {    /* returned 1, that is */
        if (WARN_ON(file->f_path.dentry == DENTRY_NOT_SET)) {
            error = -EIO;
            goto out;
        }
        if (file->f_path.dentry) {
            dput(dentry);
            dentry = file->f_path.dentry;
        }
        if (create_error && dentry->d_inode == NULL) {
            error = create_error;
            goto out;
        }
        goto looked_up;
    }

    /*
     * We didn't have the inode before the open, so check open permission
     * here.
     */
    error = may_open(&file->f_path, acc_mode, open_flag);
    if (error)
        fput(file);

out:
    dput(dentry);
    return error;

no_open:
    if (need_lookup) {
        dentry = lookup_real(dir, dentry, nd->flags);
        if (IS_ERR(dentry))
            return PTR_ERR(dentry);

        if (create_error) {
            int open_flag = op->open_flag;

            error = create_error;
            if ((open_flag & O_EXCL)) {
                if (!dentry->d_inode)
                    goto out;
            } else if (!dentry->d_inode) {
                goto out;
            } else if ((open_flag & O_TRUNC) &&
                   S_ISREG(dentry->d_inode->i_mode)) {
                goto out;
            }
            /* will fail later, go on to get the right error */
        }
    }
looked_up:
    path->dentry = dentry;
    path->mnt = nd->path.mnt;
    return 1;
}

/*
 * Look up and maybe create and open the last component.
 *
 * Must be called with i_mutex held on parent.
 *
 * Returns 0 if the file was successfully atomically created (if necessary) and
 * opened.  In this case the file will be returned attached to @file.
 *
 * Returns 1 if the file was not completely opened at this time, though lookups
 * and creations will have been performed and the dentry returned in @path will
 * be positive upon return if O_CREAT was specified.  If O_CREAT wasn't
 * specified then a negative dentry may be returned.
 *
 * An error code is returned otherwise.
 *
 * FILE_CREATE will be set in @*opened if the dentry was created and will be
 * cleared otherwise prior to returning.
 */
static int lookup_open(struct nameidata *nd, struct path *path,
            struct file *file,
            const struct open_flags *op,
            bool got_write, int *opened)
{
    struct dentry *dir = nd->path.dentry;
    struct inode *dir_inode = dir->d_inode;
    struct dentry *dentry;
    int error;
    bool need_lookup;

    *opened &= ~FILE_CREATED;
    dentry = lookup_dcache(&nd->last, dir, nd->flags, &need_lookup);
    if (IS_ERR(dentry))
        return PTR_ERR(dentry);

    /* Cached positive dentry: will open in f_op->open */
    if (!need_lookup && dentry->d_inode)
        goto out_no_open;

    if ((nd->flags & LOOKUP_OPEN) && dir_inode->i_op->atomic_open) {
        return atomic_open(nd, dentry, path, file, op, got_write,
                   need_lookup, opened);
    }

    if (need_lookup) {
        BUG_ON(dentry->d_inode);

        dentry = lookup_real(dir_inode, dentry, nd->flags);
        if (IS_ERR(dentry))
            return PTR_ERR(dentry);
    }

    /* Negative dentry, just create the file */
    if (!dentry->d_inode && (op->open_flag & O_CREAT)) {
        umode_t mode = op->mode;
        if (!IS_POSIXACL(dir->d_inode))
            mode &= ~current_umask();
        /*
         * This write is needed to ensure that a
         * rw->ro transition does not occur between
         * the time when the file is created and when
         * a permanent write count is taken through
         * the 'struct file' in finish_open().
         */
        if (!got_write) {
            error = -EROFS;
            goto out_dput;
        }
        *opened |= FILE_CREATED;
        error = security_path_mknod(&nd->path, dentry, mode, 0);
        if (error)
            goto out_dput;
        error = vfs_create(dir->d_inode, dentry, mode,
                   nd->flags & LOOKUP_EXCL);
        if (error)
            goto out_dput;
    }
out_no_open:
    path->dentry = dentry;
    path->mnt = nd->path.mnt;
    return 1;

out_dput:
    dput(dentry);
    return error;
}

/*
 * Handle the last step of open()
 */
static int do_last(struct nameidata *nd, struct path *path,
           struct file *file, const struct open_flags *op,
           int *opened, struct filename *name)
{
    struct dentry *dir = nd->path.dentry;
    int open_flag = op->open_flag;
    bool will_truncate = (open_flag & O_TRUNC) != 0;
    bool got_write = false;
    int acc_mode = op->acc_mode;
    struct inode *inode;
    bool symlink_ok = false;
    struct path save_parent = { .dentry = NULL, .mnt = NULL };
    bool retried = false;
    int error;

    nd->flags &= ~LOOKUP_PARENT;
    nd->flags |= op->intent;

    switch (nd->last_type) {
    case LAST_DOTDOT:
    case LAST_DOT:
        error = handle_dots(nd, nd->last_type);
        if (error)
            return error;
        /* fallthrough */
    case LAST_ROOT:
        error = complete_walk(nd);
        if (error)
            return error;
        audit_inode(name, nd->path.dentry, 0);
        if (open_flag & O_CREAT) {
            error = -EISDIR;
            goto out;
        }
        goto finish_open;
    case LAST_BIND:
        error = complete_walk(nd);
        if (error)
            return error;
        audit_inode(name, dir, 0);
        goto finish_open;
    }

    if (!(open_flag & O_CREAT)) {
        if (nd->last.name[nd->last.len])
            nd->flags |= LOOKUP_FOLLOW | LOOKUP_DIRECTORY;
        if (open_flag & O_PATH && !(nd->flags & LOOKUP_FOLLOW))
            symlink_ok = true;
        /* we _can_ be in RCU mode here */
        error = lookup_fast(nd, &nd->last, path, &inode);
        if (likely(!error))
            goto finish_lookup;

        if (error < 0)
            goto out;

        BUG_ON(nd->inode != dir->d_inode);
    } else {
        /* create side of things */
        /*
         * This will *only* deal with leaving RCU mode - LOOKUP_JUMPED
         * has been cleared when we got to the last component we are
         * about to look up
         */
        error = complete_walk(nd);
        if (error)
            return error;

        audit_inode(name, dir, 0);
        error = -EISDIR;
        /* trailing slashes? */
        if (nd->last.name[nd->last.len])
            goto out;
    }

retry_lookup:
    if (op->open_flag & (O_CREAT | O_TRUNC | O_WRONLY | O_RDWR)) {
        error = mnt_want_write(nd->path.mnt);
        if (!error)
            got_write = true;
        /*
         * do _not_ fail yet - we might not need that or fail with
         * a different error; let lookup_open() decide; we'll be
         * dropping this one anyway.
         */
    }
    mutex_lock(&dir->d_inode->i_mutex);
    error = lookup_open(nd, path, file, op, got_write, opened);
    mutex_unlock(&dir->d_inode->i_mutex);

    if (error <= 0) {
        if (error)
            goto out;

        if ((*opened & FILE_CREATED) ||
            !S_ISREG(file->f_path.dentry->d_inode->i_mode))
            will_truncate = false;

        audit_inode(name, file->f_path.dentry, 0);
        goto opened;
    }

    if (*opened & FILE_CREATED) {
        /* Don't check for write permission, don't truncate */
        open_flag &= ~O_TRUNC;
        will_truncate = false;
        acc_mode = MAY_OPEN;
        path_to_nameidata(path, nd);
        goto finish_open_created;
    }

    /*
     * create/update audit record if it already exists.
     */
    if (path->dentry->d_inode)
        audit_inode(name, path->dentry, 0);

    /*
     * If atomic_open() acquired write access it is dropped now due to
     * possible mount and symlink following (this might be optimized away if
     * necessary...)
     */
    if (got_write) {
        mnt_drop_write(nd->path.mnt);
        got_write = false;
    }

    error = -EEXIST;
    if ((open_flag & (O_EXCL | O_CREAT)) == (O_EXCL | O_CREAT))
        goto exit_dput;

    error = follow_managed(path, nd->flags);
    if (error < 0)
        goto exit_dput;

    if (error)
        nd->flags |= LOOKUP_JUMPED;

    BUG_ON(nd->flags & LOOKUP_RCU);
    inode = path->dentry->d_inode;
finish_lookup:
    /* we _can_ be in RCU mode here */
    error = -ENOENT;
    if (!inode) {
        path_to_nameidata(path, nd);
        goto out;
    }

    if (should_follow_link(inode, !symlink_ok)) {
        if (nd->flags & LOOKUP_RCU) {
            if (unlikely(unlazy_walk(nd, path->dentry))) {
                error = -ECHILD;
                goto out;
            }
        }
        BUG_ON(inode != path->dentry->d_inode);
        return 1;
    }

    if ((nd->flags & LOOKUP_RCU) || nd->path.mnt != path->mnt) {
        path_to_nameidata(path, nd);
    } else {
        save_parent.dentry = nd->path.dentry;
        save_parent.mnt = mntget(path->mnt);
        nd->path.dentry = path->dentry;

    }
    nd->inode = inode;
    /* Why this, you ask?  _Now_ we might have grown LOOKUP_JUMPED... */
    error = complete_walk(nd);
    if (error) {
        path_put(&save_parent);
        return error;
    }
    error = -EISDIR;
    if ((open_flag & O_CREAT) && S_ISDIR(nd->inode->i_mode))
        goto out;
    error = -ENOTDIR;
    if ((nd->flags & LOOKUP_DIRECTORY) && !nd->inode->i_op->lookup)
        goto out;
    audit_inode(name, nd->path.dentry, 0);
finish_open:
    if (!S_ISREG(nd->inode->i_mode))
        will_truncate = false;

    if (will_truncate) {
        error = mnt_want_write(nd->path.mnt);
        if (error)
            goto out;
        got_write = true;
    }
finish_open_created:
    error = may_open(&nd->path, acc_mode, open_flag);
    if (error)
        goto out;
    file->f_path.mnt = nd->path.mnt;
    error = finish_open(file, nd->path.dentry, NULL, opened);
    if (error) {
        if (error == -EOPENSTALE)
            goto stale_open;
        goto out;
    }
opened:
    error = open_check_o_direct(file);
    if (error)
        goto exit_fput;
    error = ima_file_check(file, op->acc_mode);
    if (error)
        goto exit_fput;

    if (will_truncate) {
        error = handle_truncate(file);
        if (error)
            goto exit_fput;
    }
out:
    if (got_write)
        mnt_drop_write(nd->path.mnt);
    path_put(&save_parent);
    terminate_walk(nd);
    return error;

exit_dput:
    path_put_conditional(path, nd);
    goto out;
exit_fput:
    fput(file);
    goto out;

stale_open:
    /* If no saved parent or already retried then can't retry */
    if (!save_parent.dentry || retried)
        goto out;

    BUG_ON(save_parent.dentry != dir);
    path_put(&nd->path);
    nd->path = save_parent;
    nd->inode = dir->d_inode;
    save_parent.mnt = NULL;
    save_parent.dentry = NULL;
    if (got_write) {
        mnt_drop_write(nd->path.mnt);
        got_write = false;
    }
    retried = true;
    goto retry_lookup;
}

static struct file *path_openat(int dfd, struct filename *pathname,
        struct nameidata *nd, const struct open_flags *op, int flags)
{
    struct file *base = NULL;
    struct file *file;
    struct path path;
    int opened = 0;
    int error;

    file = get_empty_filp();
    if (!file)
        return ERR_PTR(-ENFILE);

    file->f_flags = op->open_flag;

    error = path_init(dfd, pathname->name, flags | LOOKUP_PARENT, nd, &base);
    if (unlikely(error))
        goto out;

    current->total_link_count = 0;
    error = link_path_walk(pathname->name, nd);
    if (unlikely(error))
        goto out;

    error = do_last(nd, &path, file, op, &opened, pathname);
    while (unlikely(error > 0)) { /* trailing symlink */
        struct path link = path;
        void *cookie;
        if (!(nd->flags & LOOKUP_FOLLOW)) {
            path_put_conditional(&path, nd);
            path_put(&nd->path);
            error = -ELOOP;
            break;
        }
        error = may_follow_link(&link, nd);
        if (unlikely(error))
            break;
        nd->flags |= LOOKUP_PARENT;
        nd->flags &= ~(LOOKUP_OPEN|LOOKUP_CREATE|LOOKUP_EXCL);
        error = follow_link(&link, nd, &cookie);
        if (unlikely(error))
            break;
        error = do_last(nd, &path, file, op, &opened, pathname);
        put_link(nd, &link, cookie);
    }
out:
    if (nd->root.mnt && !(nd->flags & LOOKUP_ROOT))
        path_put(&nd->root);
    if (base)
        fput(base);
    if (!(opened & FILE_OPENED)) {
        BUG_ON(!error);
        put_filp(file);
    }
    if (unlikely(error)) {
        if (error == -EOPENSTALE) {
            if (flags & LOOKUP_RCU)
                error = -ECHILD;
            else
                error = -ESTALE;
        }
        file = ERR_PTR(error);
    }
    return file;
}

struct file *do_filp_open(int dfd, struct filename *pathname,
        const struct open_flags *op, int flags)
{
    struct nameidata nd;
    struct file *filp;

    filp = path_openat(dfd, pathname, &nd, op, flags | LOOKUP_RCU);
    if (unlikely(filp == ERR_PTR(-ECHILD)))
        filp = path_openat(dfd, pathname, &nd, op, flags);
    if (unlikely(filp == ERR_PTR(-ESTALE)))
        filp = path_openat(dfd, pathname, &nd, op, flags | LOOKUP_REVAL);
    return filp;
}

struct file *do_file_open_root(struct dentry *dentry, struct vfsmount *mnt,
        const char *name, const struct open_flags *op, int flags)
{
    struct nameidata nd;
    struct file *file;
    struct filename filename = { .name = name };

    nd.root.mnt = mnt;
    nd.root.dentry = dentry;

    flags |= LOOKUP_ROOT;

    if (dentry->d_inode->i_op->follow_link && op->intent & LOOKUP_OPEN)
        return ERR_PTR(-ELOOP);

    file = path_openat(-1, &filename, &nd, op, flags | LOOKUP_RCU);
    if (unlikely(file == ERR_PTR(-ECHILD)))
        file = path_openat(-1, &filename, &nd, op, flags);
    if (unlikely(file == ERR_PTR(-ESTALE)))
        file = path_openat(-1, &filename, &nd, op, flags | LOOKUP_REVAL);
    return file;
}

struct dentry *kern_path_create(int dfd, const char *pathname, struct path *path, int is_dir)
{
    struct dentry *dentry = ERR_PTR(-EEXIST);
    struct nameidata nd;
    int err2;
    int error = do_path_lookup(dfd, pathname, LOOKUP_PARENT, &nd);
    if (error)
        return ERR_PTR(error);

    /*
     * Yucky last component or no last component at all?
     * (foo/., foo/.., /////)
     */
    if (nd.last_type != LAST_NORM)
        goto out;
    nd.flags &= ~LOOKUP_PARENT;
    nd.flags |= LOOKUP_CREATE | LOOKUP_EXCL;

    /* don't fail immediately if it's r/o, at least try to report other errors */
    err2 = mnt_want_write(nd.path.mnt);
    /*
     * Do the final lookup.
     */
    mutex_lock_nested(&nd.path.dentry->d_inode->i_mutex, I_MUTEX_PARENT);
    dentry = lookup_hash(&nd);
    if (IS_ERR(dentry))
        goto unlock;

    error = -EEXIST;
    if (dentry->d_inode)
        goto fail;
    /*
     * Special case - lookup gave negative, but... we had foo/bar/
     * From the vfs_mknod() POV we just have a negative dentry -
     * all is fine. Let's be bastards - you had / on the end, you've
     * been asking for (non-existent) directory. -ENOENT for you.
     */
    if (unlikely(!is_dir && nd.last.name[nd.last.len])) {
        error = -ENOENT;
        goto fail;
    }
    if (unlikely(err2)) {
        error = err2;
        goto fail;
    }
    *path = nd.path;
    return dentry;
fail:
    dput(dentry);
    dentry = ERR_PTR(error);
unlock:
    mutex_unlock(&nd.path.dentry->d_inode->i_mutex);
    if (!err2)
        mnt_drop_write(nd.path.mnt);
out:
    path_put(&nd.path);
    return dentry;
}
EXPORT_SYMBOL(kern_path_create);

void done_path_create(struct path *path, struct dentry *dentry)
{
    dput(dentry);
    mutex_unlock(&path->dentry->d_inode->i_mutex);
    mnt_drop_write(path->mnt);
    path_put(path);
}
EXPORT_SYMBOL(done_path_create);

struct dentry *user_path_create(int dfd, const char __user *pathname, struct path *path, int is_dir)
{
    struct filename *tmp = getname(pathname);
    struct dentry *res;
    if (IS_ERR(tmp))
        return ERR_CAST(tmp);
    res = kern_path_create(dfd, tmp->name, path, is_dir);
    putname(tmp);
    return res;
}
EXPORT_SYMBOL(user_path_create);

int vfs_mknod(struct inode *dir, struct dentry *dentry, umode_t mode, dev_t dev)
{
    int error = may_create(dir, dentry);

    if (error)
        return error;

    if ((S_ISCHR(mode) || S_ISBLK(mode)) && !capable(CAP_MKNOD))
        return -EPERM;

    if (!dir->i_op->mknod)
        return -EPERM;

    error = devcgroup_inode_mknod(mode, dev);
    if (error)
        return error;

    error = security_inode_mknod(dir, dentry, mode, dev);
    if (error)
        return error;

    error = dir->i_op->mknod(dir, dentry, mode, dev);
    if (!error)
        fsnotify_create(dir, dentry);
    return error;
}

static int may_mknod(umode_t mode)
{
    switch (mode & S_IFMT) {
    case S_IFREG:
    case S_IFCHR:
    case S_IFBLK:
    case S_IFIFO:
    case S_IFSOCK:
    case 0: /* zero mode translates to S_IFREG */
        return 0;
    case S_IFDIR:
        return -EPERM;
    default:
        return -EINVAL;
    }
}

SYSCALL_DEFINE4(mknodat, int, dfd, const char __user *, filename, umode_t, mode,
        unsigned, dev)
{
    struct dentry *dentry;
    struct path path;
    int error;

    error = may_mknod(mode);
    if (error)
        return error;

    dentry = user_path_create(dfd, filename, &path, 0);
    if (IS_ERR(dentry))
        return PTR_ERR(dentry);

    if (!IS_POSIXACL(path.dentry->d_inode))
        mode &= ~current_umask();
    error = security_path_mknod(&path, dentry, mode, dev);
    if (error)
        goto out;
    switch (mode & S_IFMT) {
        case 0: case S_IFREG:
            error = vfs_create(path.dentry->d_inode,dentry,mode,true);
            break;
        case S_IFCHR: case S_IFBLK:
            error = vfs_mknod(path.dentry->d_inode,dentry,mode,
                    new_decode_dev(dev));
            break;
        case S_IFIFO: case S_IFSOCK:
            error = vfs_mknod(path.dentry->d_inode,dentry,mode,0);
            break;
    }
out:
    done_path_create(&path, dentry);
    return error;
}

SYSCALL_DEFINE3(mknod, const char __user *, filename, umode_t, mode, unsigned, dev)
{
    return sys_mknodat(AT_FDCWD, filename, mode, dev);
}

int vfs_mkdir(struct inode *dir, struct dentry *dentry, umode_t mode)
{
    int error = may_create(dir, dentry);
    unsigned max_links = dir->i_sb->s_max_links;

    if (error)
        return error;

    if (!dir->i_op->mkdir)
        return -EPERM;

    mode &= (S_IRWXUGO|S_ISVTX);
    error = security_inode_mkdir(dir, dentry, mode);
    if (error)
        return error;

    if (max_links && dir->i_nlink >= max_links)
        return -EMLINK;

    error = dir->i_op->mkdir(dir, dentry, mode);
    if (!error)
        fsnotify_mkdir(dir, dentry);
    return error;
}

SYSCALL_DEFINE3(mkdirat, int, dfd, const char __user *, pathname, umode_t, mode)
{
    struct dentry *dentry;
    struct path path;
    int error;

    dentry = user_path_create(dfd, pathname, &path, 1);
    if (IS_ERR(dentry))
        return PTR_ERR(dentry);

    if (!IS_POSIXACL(path.dentry->d_inode))
        mode &= ~current_umask();
    error = security_path_mkdir(&path, dentry, mode);
    if (!error)
        error = vfs_mkdir(path.dentry->d_inode, dentry, mode);
    done_path_create(&path, dentry);
    return error;
}

SYSCALL_DEFINE2(mkdir, const char __user *, pathname, umode_t, mode)
{
    return sys_mkdirat(AT_FDCWD, pathname, mode);
}

/*
 * The dentry_unhash() helper will try to drop the dentry early: we
 * should have a usage count of 1 if we're the only user of this
 * dentry, and if that is true (possibly after pruning the dcache),
 * then we drop the dentry now.
 *
 * A low-level filesystem can, if it choses, legally
 * do a
 *
 *  if (!d_unhashed(dentry))
 *      return -EBUSY;
 *
 * if it cannot handle the case of removing a directory
 * that is still in use by something else..
 */
void dentry_unhash(struct dentry *dentry)
{
    shrink_dcache_parent(dentry);
    spin_lock(&dentry->d_lock);
    if (dentry->d_count == 1)
        __d_drop(dentry);
    spin_unlock(&dentry->d_lock);
}

int vfs_rmdir(struct inode *dir, struct dentry *dentry)
{
    int error = may_delete(dir, dentry, 1);

    if (error)
        return error;

    if (!dir->i_op->rmdir)
        return -EPERM;

    dget(dentry);
    mutex_lock(&dentry->d_inode->i_mutex);

    error = -EBUSY;
    if (d_mountpoint(dentry))
        goto out;

    error = security_inode_rmdir(dir, dentry);
    if (error)
        goto out;

    shrink_dcache_parent(dentry);
    error = dir->i_op->rmdir(dir, dentry);
    if (error)
        goto out;

    dentry->d_inode->i_flags |= S_DEAD;
    dont_mount(dentry);

out:
    mutex_unlock(&dentry->d_inode->i_mutex);
    dput(dentry);
    if (!error)
        d_delete(dentry);
    return error;
}

static long do_rmdir(int dfd, const char __user *pathname)
{
    int error = 0;
    struct filename *name;
    struct dentry *dentry;
    struct nameidata nd;

    name = user_path_parent(dfd, pathname, &nd);
    if (IS_ERR(name))
        return PTR_ERR(name);

    switch(nd.last_type) {
    case LAST_DOTDOT:
        error = -ENOTEMPTY;
        goto exit1;
    case LAST_DOT:
        error = -EINVAL;
        goto exit1;
    case LAST_ROOT:
        error = -EBUSY;
        goto exit1;
    }

    nd.flags &= ~LOOKUP_PARENT;
    error = mnt_want_write(nd.path.mnt);
    if (error)
        goto exit1;

    mutex_lock_nested(&nd.path.dentry->d_inode->i_mutex, I_MUTEX_PARENT);
    dentry = lookup_hash(&nd);
    error = PTR_ERR(dentry);
    if (IS_ERR(dentry))
        goto exit2;
    if (!dentry->d_inode) {
        error = -ENOENT;
        goto exit3;
    }
    error = security_path_rmdir(&nd.path, dentry);
    if (error)
        goto exit3;
    error = vfs_rmdir(nd.path.dentry->d_inode, dentry);
exit3:
    dput(dentry);
exit2:
    mutex_unlock(&nd.path.dentry->d_inode->i_mutex);
    mnt_drop_write(nd.path.mnt);
exit1:
    path_put(&nd.path);
    putname(name);
    return error;
}

SYSCALL_DEFINE1(rmdir, const char __user *, pathname)
{
    return do_rmdir(AT_FDCWD, pathname);
}

int vfs_unlink(struct inode *dir, struct dentry *dentry)
{
    int error = may_delete(dir, dentry, 0);

    if (error)
        return error;

    if (!dir->i_op->unlink)
        return -EPERM;

    mutex_lock(&dentry->d_inode->i_mutex);
    if (d_mountpoint(dentry))
        error = -EBUSY;
    else {
        error = security_inode_unlink(dir, dentry);
        if (!error) {
            error = dir->i_op->unlink(dir, dentry);
            if (!error)
                dont_mount(dentry);
        }
    }
    mutex_unlock(&dentry->d_inode->i_mutex);

    /* We don't d_delete() NFS sillyrenamed files--they still exist. */
    if (!error && !(dentry->d_flags & DCACHE_NFSFS_RENAMED)) {
        fsnotify_link_count(dentry->d_inode);
        d_delete(dentry);
    }

    return error;
}

/*
 * Make sure that the actual truncation of the file will occur outside its
 * directory's i_mutex.  Truncate can take a long time if there is a lot of
 * writeout happening, and we don't want to prevent access to the directory
 * while waiting on the I/O.
 */
static long do_unlinkat(int dfd, const char __user *pathname)
{
    int error;
    struct filename *name;
    struct dentry *dentry;
    struct nameidata nd;
    struct inode *inode = NULL;

    name = user_path_parent(dfd, pathname, &nd);
    if (IS_ERR(name))
        return PTR_ERR(name);

    error = -EISDIR;
    if (nd.last_type != LAST_NORM)
        goto exit1;

    nd.flags &= ~LOOKUP_PARENT;
    error = mnt_want_write(nd.path.mnt);
    if (error)
        goto exit1;

    mutex_lock_nested(&nd.path.dentry->d_inode->i_mutex, I_MUTEX_PARENT);
    dentry = lookup_hash(&nd);
    error = PTR_ERR(dentry);
    if (!IS_ERR(dentry)) {
        /* Why not before? Because we want correct error value */
        if (nd.last.name[nd.last.len])
            goto slashes;
        inode = dentry->d_inode;
        if (!inode)
            goto slashes;
        ihold(inode);
        error = security_path_unlink(&nd.path, dentry);
        if (error)
            goto exit2;
        error = vfs_unlink(nd.path.dentry->d_inode, dentry);
exit2:
        dput(dentry);
    }
    mutex_unlock(&nd.path.dentry->d_inode->i_mutex);
    if (inode)
        iput(inode);    /* truncate the inode here */
    mnt_drop_write(nd.path.mnt);
exit1:
    path_put(&nd.path);
    putname(name);
    return error;

slashes:
    error = !dentry->d_inode ? -ENOENT :
        S_ISDIR(dentry->d_inode->i_mode) ? -EISDIR : -ENOTDIR;
    goto exit2;
}

SYSCALL_DEFINE3(unlinkat, int, dfd, const char __user *, pathname, int, flag)
{
    if ((flag & ~AT_REMOVEDIR) != 0)
        return -EINVAL;

    if (flag & AT_REMOVEDIR)
        return do_rmdir(dfd, pathname);

    return do_unlinkat(dfd, pathname);
}

SYSCALL_DEFINE1(unlink, const char __user *, pathname)
{
    return do_unlinkat(AT_FDCWD, pathname);
}

int vfs_symlink(struct inode *dir, struct dentry *dentry, const char *oldname)
{
    int error = may_create(dir, dentry);

    if (error)
        return error;

    if (!dir->i_op->symlink)
        return -EPERM;

    error = security_inode_symlink(dir, dentry, oldname);
    if (error)
        return error;

    error = dir->i_op->symlink(dir, dentry, oldname);
    if (!error)
        fsnotify_create(dir, dentry);
    return error;
}

SYSCALL_DEFINE3(symlinkat, const char __user *, oldname,
        int, newdfd, const char __user *, newname)
{
    int error;
    struct filename *from;
    struct dentry *dentry;
    struct path path;

    from = getname(oldname);
    if (IS_ERR(from))
        return PTR_ERR(from);

    dentry = user_path_create(newdfd, newname, &path, 0);
    error = PTR_ERR(dentry);
    if (IS_ERR(dentry))
        goto out_putname;

    error = security_path_symlink(&path, dentry, from->name);
    if (!error)
        error = vfs_symlink(path.dentry->d_inode, dentry, from->name);
    done_path_create(&path, dentry);
out_putname:
    putname(from);
    return error;
}

SYSCALL_DEFINE2(symlink, const char __user *, oldname, const char __user *, newname)
{
    return sys_symlinkat(oldname, AT_FDCWD, newname);
}

int vfs_link(struct dentry *old_dentry, struct inode *dir, struct dentry *new_dentry)
{
    struct inode *inode = old_dentry->d_inode;
    unsigned max_links = dir->i_sb->s_max_links;
    int error;

    if (!inode)
        return -ENOENT;

    error = may_create(dir, new_dentry);
    if (error)
        return error;

    if (dir->i_sb != inode->i_sb)
        return -EXDEV;

    /*
     * A link to an append-only or immutable file cannot be created.
     */
    if (IS_APPEND(inode) || IS_IMMUTABLE(inode))
        return -EPERM;
    if (!dir->i_op->link)
        return -EPERM;
    if (S_ISDIR(inode->i_mode))
        return -EPERM;

    error = security_inode_link(old_dentry, dir, new_dentry);
    if (error)
        return error;

    mutex_lock(&inode->i_mutex);
    /* Make sure we don't allow creating hardlink to an unlinked file */
    if (inode->i_nlink == 0)
        error =  -ENOENT;
    else if (max_links && inode->i_nlink >= max_links)
        error = -EMLINK;
    else
        error = dir->i_op->link(old_dentry, dir, new_dentry);
    mutex_unlock(&inode->i_mutex);
    if (!error)
        fsnotify_link(dir, inode, new_dentry);
    return error;
}

/*
 * Hardlinks are often used in delicate situations.  We avoid
 * security-related surprises by not following symlinks on the
 * newname.  --KAB
 *
 * We don't follow them on the oldname either to be compatible
 * with linux 2.0, and to avoid hard-linking to directories
 * and other special files.  --ADM
 */
SYSCALL_DEFINE5(linkat, int, olddfd, const char __user *, oldname,
        int, newdfd, const char __user *, newname, int, flags)
{
    struct dentry *new_dentry;
    struct path old_path, new_path;
    int how = 0;
    int error;

    if ((flags & ~(AT_SYMLINK_FOLLOW | AT_EMPTY_PATH)) != 0)
        return -EINVAL;
    /*
     * To use null names we require CAP_DAC_READ_SEARCH
     * This ensures that not everyone will be able to create
     * handlink using the passed filedescriptor.
     */
    if (flags & AT_EMPTY_PATH) {
        if (!capable(CAP_DAC_READ_SEARCH))
            return -ENOENT;
        how = LOOKUP_EMPTY;
    }

    if (flags & AT_SYMLINK_FOLLOW)
        how |= LOOKUP_FOLLOW;

    error = user_path_at(olddfd, oldname, how, &old_path);
    if (error)
        return error;

    new_dentry = user_path_create(newdfd, newname, &new_path, 0);
    error = PTR_ERR(new_dentry);
    if (IS_ERR(new_dentry))
        goto out;

    error = -EXDEV;
    if (old_path.mnt != new_path.mnt)
        goto out_dput;
    error = may_linkat(&old_path);
    if (unlikely(error))
        goto out_dput;
    error = security_path_link(old_path.dentry, &new_path, new_dentry);
    if (error)
        goto out_dput;
    error = vfs_link(old_path.dentry, new_path.dentry->d_inode, new_dentry);
out_dput:
    done_path_create(&new_path, new_dentry);
out:
    path_put(&old_path);

    return error;
}

SYSCALL_DEFINE2(link, const char __user *, oldname, const char __user *, newname)
{
    return sys_linkat(AT_FDCWD, oldname, AT_FDCWD, newname, 0);
}

/*
 * The worst of all namespace operations - renaming directory. "Perverted"
 * doesn't even start to describe it. Somebody in UCB had a heck of a trip...
 * Problems:
 *  a) we can get into loop creation. Check is done in is_subdir().
 *  b) race potential - two innocent renames can create a loop together.
 *     That's where 4.4 screws up. Current fix: serialization on
 *     sb->s_vfs_rename_mutex. We might be more accurate, but that's another
 *     story.
 *  c) we have to lock _three_ objects - parents and victim (if it exists).
 *     And that - after we got ->i_mutex on parents (until then we don't know
 *     whether the target exists).  Solution: try to be smart with locking
 *     order for inodes.  We rely on the fact that tree topology may change
 *     only under ->s_vfs_rename_mutex _and_ that parent of the object we
 *     move will be locked.  Thus we can rank directories by the tree
 *     (ancestors first) and rank all non-directories after them.
 *     That works since everybody except rename does "lock parent, lookup,
 *     lock child" and rename is under ->s_vfs_rename_mutex.
 *     HOWEVER, it relies on the assumption that any object with ->lookup()
 *     has no more than 1 dentry.  If "hybrid" objects will ever appear,
 *     we'd better make sure that there's no link(2) for them.
 *  d) conversion from fhandle to dentry may come in the wrong moment - when
 *     we are removing the target. Solution: we will have to grab ->i_mutex
 *     in the fhandle_to_dentry code. [FIXME - current nfsfh.c relies on
 *     ->i_mutex on parents, which works but leads to some truly excessive
 *     locking].
 */
static int vfs_rename_dir(struct inode *old_dir, struct dentry *old_dentry,
              struct inode *new_dir, struct dentry *new_dentry)
{
    int error = 0;
    struct inode *target = new_dentry->d_inode;
    unsigned max_links = new_dir->i_sb->s_max_links;

    /*
     * If we are going to change the parent - check write permissions,
     * we'll need to flip '..'.
     */
    if (new_dir != old_dir) {
        error = inode_permission(old_dentry->d_inode, MAY_WRITE);
        if (error)
            return error;
    }

    error = security_inode_rename(old_dir, old_dentry, new_dir, new_dentry);
    if (error)
        return error;

    dget(new_dentry);
    if (target)
        mutex_lock(&target->i_mutex);

    error = -EBUSY;
    if (d_mountpoint(old_dentry) || d_mountpoint(new_dentry))
        goto out;

    error = -EMLINK;
    if (max_links && !target && new_dir != old_dir &&
        new_dir->i_nlink >= max_links)
        goto out;

    if (target)
        shrink_dcache_parent(new_dentry);
    error = old_dir->i_op->rename(old_dir, old_dentry, new_dir, new_dentry);
    if (error)
        goto out;

    if (target) {
        target->i_flags |= S_DEAD;
        dont_mount(new_dentry);
    }
out:
    if (target)
        mutex_unlock(&target->i_mutex);
    dput(new_dentry);
    if (!error)
        if (!(old_dir->i_sb->s_type->fs_flags & FS_RENAME_DOES_D_MOVE))
            d_move(old_dentry,new_dentry);
    return error;
}

static int vfs_rename_other(struct inode *old_dir, struct dentry *old_dentry,
                struct inode *new_dir, struct dentry *new_dentry)
{
    struct inode *target = new_dentry->d_inode;
    int error;

    error = security_inode_rename(old_dir, old_dentry, new_dir, new_dentry);
    if (error)
        return error;

    dget(new_dentry);
    if (target)
        mutex_lock(&target->i_mutex);

    error = -EBUSY;
    if (d_mountpoint(old_dentry)||d_mountpoint(new_dentry))
        goto out;

    error = old_dir->i_op->rename(old_dir, old_dentry, new_dir, new_dentry);
    if (error)
        goto out;

    if (target)
        dont_mount(new_dentry);
    if (!(old_dir->i_sb->s_type->fs_flags & FS_RENAME_DOES_D_MOVE))
        d_move(old_dentry, new_dentry);
out:
    if (target)
        mutex_unlock(&target->i_mutex);
    dput(new_dentry);
    return error;
}

int vfs_rename(struct inode *old_dir, struct dentry *old_dentry,
           struct inode *new_dir, struct dentry *new_dentry)
{
    int error;
    int is_dir = S_ISDIR(old_dentry->d_inode->i_mode);
    const unsigned char *old_name;

    if (old_dentry->d_inode == new_dentry->d_inode)
        return 0;
 
    error = may_delete(old_dir, old_dentry, is_dir);
    if (error)
        return error;

    if (!new_dentry->d_inode)
        error = may_create(new_dir, new_dentry);
    else
        error = may_delete(new_dir, new_dentry, is_dir);
    if (error)
        return error;

    if (!old_dir->i_op->rename)
        return -EPERM;

    old_name = fsnotify_oldname_init(old_dentry->d_name.name);

    if (is_dir)
        error = vfs_rename_dir(old_dir,old_dentry,new_dir,new_dentry);
    else
        error = vfs_rename_other(old_dir,old_dentry,new_dir,new_dentry);
    if (!error)
        fsnotify_move(old_dir, new_dir, old_name, is_dir,
                  new_dentry->d_inode, old_dentry);
    fsnotify_oldname_free(old_name);

    return error;
}

SYSCALL_DEFINE4(renameat, int, olddfd, const char __user *, oldname,
        int, newdfd, const char __user *, newname)
{
    struct dentry *old_dir, *new_dir;
    struct dentry *old_dentry, *new_dentry;
    struct dentry *trap;
    struct nameidata oldnd, newnd;
    struct filename *from;
    struct filename *to;
    int error;

    from = user_path_parent(olddfd, oldname, &oldnd);
    if (IS_ERR(from)) {
        error = PTR_ERR(from);
        goto exit;
    }

    to = user_path_parent(newdfd, newname, &newnd);
    if (IS_ERR(to)) {
        error = PTR_ERR(to);
        goto exit1;
    }

    error = -EXDEV;
    if (oldnd.path.mnt != newnd.path.mnt)
        goto exit2;

    old_dir = oldnd.path.dentry;
    error = -EBUSY;
    if (oldnd.last_type != LAST_NORM)
        goto exit2;

    new_dir = newnd.path.dentry;
    if (newnd.last_type != LAST_NORM)
        goto exit2;

    error = mnt_want_write(oldnd.path.mnt);
    if (error)
        goto exit2;

    oldnd.flags &= ~LOOKUP_PARENT;
    newnd.flags &= ~LOOKUP_PARENT;
    newnd.flags |= LOOKUP_RENAME_TARGET;

    trap = lock_rename(new_dir, old_dir);

    old_dentry = lookup_hash(&oldnd);
    error = PTR_ERR(old_dentry);
    if (IS_ERR(old_dentry))
        goto exit3;
    /* source must exist */
    error = -ENOENT;
    if (!old_dentry->d_inode)
        goto exit4;
    /* unless the source is a directory trailing slashes give -ENOTDIR */
    if (!S_ISDIR(old_dentry->d_inode->i_mode)) {
        error = -ENOTDIR;
        if (oldnd.last.name[oldnd.last.len])
            goto exit4;
        if (newnd.last.name[newnd.last.len])
            goto exit4;
    }
    /* source should not be ancestor of target */
    error = -EINVAL;
    if (old_dentry == trap)
        goto exit4;
    new_dentry = lookup_hash(&newnd);
    error = PTR_ERR(new_dentry);
    if (IS_ERR(new_dentry))
        goto exit4;
    /* target should not be an ancestor of source */
    error = -ENOTEMPTY;
    if (new_dentry == trap)
        goto exit5;

    error = security_path_rename(&oldnd.path, old_dentry,
                     &newnd.path, new_dentry);
    if (error)
        goto exit5;
    error = vfs_rename(old_dir->d_inode, old_dentry,
                   new_dir->d_inode, new_dentry);
exit5:
    dput(new_dentry);
exit4:
    dput(old_dentry);
exit3:
    unlock_rename(new_dir, old_dir);
    mnt_drop_write(oldnd.path.mnt);
exit2:
    path_put(&newnd.path);
    putname(to);
exit1:
    path_put(&oldnd.path);
    putname(from);
exit:
    return error;
}

SYSCALL_DEFINE2(rename, const char __user *, oldname, const char __user *, newname)
{
    return sys_renameat(AT_FDCWD, oldname, AT_FDCWD, newname);
}

int vfs_readlink(struct dentry *dentry, char __user *buffer, int buflen, const char *link)
{
    int len;

    len = PTR_ERR(link);
    if (IS_ERR(link))
        goto out;

    len = strlen(link);
    if (len > (unsigned) buflen)
        len = buflen;
    if (copy_to_user(buffer, link, len))
        len = -EFAULT;
out:
    return len;
}

/*
 * A helper for ->readlink().  This should be used *ONLY* for symlinks that
 * have ->follow_link() touching nd only in nd_set_link().  Using (or not
 * using) it for any given inode is up to filesystem.
 */
int generic_readlink(struct dentry *dentry, char __user *buffer, int buflen)
{
    struct nameidata nd;
    void *cookie;
    int res;

    nd.depth = 0;
    cookie = dentry->d_inode->i_op->follow_link(dentry, &nd);
    if (IS_ERR(cookie))
        return PTR_ERR(cookie);

    res = vfs_readlink(dentry, buffer, buflen, nd_get_link(&nd));
    if (dentry->d_inode->i_op->put_link)
        dentry->d_inode->i_op->put_link(dentry, &nd, cookie);
    return res;
}

int vfs_follow_link(struct nameidata *nd, const char *link)
{
    return __vfs_follow_link(nd, link);
}

/* get the link contents into pagecache */
static char *page_getlink(struct dentry * dentry, struct page **ppage)
{
    char *kaddr;
    struct page *page;
    struct address_space *mapping = dentry->d_inode->i_mapping;
    page = read_mapping_page(mapping, 0, NULL);
    if (IS_ERR(page))
        return (char*)page;
    *ppage = page;
    kaddr = kmap(page);
    nd_terminate_link(kaddr, dentry->d_inode->i_size, PAGE_SIZE - 1);
    return kaddr;
}

int page_readlink(struct dentry *dentry, char __user *buffer, int buflen)
{
    struct page *page = NULL;
    char *s = page_getlink(dentry, &page);
    int res = vfs_readlink(dentry,buffer,buflen,s);
    if (page) {
        kunmap(page);
        page_cache_release(page);
    }
    return res;
}

void *page_follow_link_light(struct dentry *dentry, struct nameidata *nd)
{
    struct page *page = NULL;
    nd_set_link(nd, page_getlink(dentry, &page));
    return page;
}

void page_put_link(struct dentry *dentry, struct nameidata *nd, void *cookie)
{
    struct page *page = cookie;

    if (page) {
        kunmap(page);
        page_cache_release(page);
    }
}

/*
 * The nofs argument instructs pagecache_write_begin to pass AOP_FLAG_NOFS
 */
int __page_symlink(struct inode *inode, const char *symname, int len, int nofs)
{
    struct address_space *mapping = inode->i_mapping;
    struct page *page;
    void *fsdata;
    int err;
    char *kaddr;
    unsigned int flags = AOP_FLAG_UNINTERRUPTIBLE;
    if (nofs)
        flags |= AOP_FLAG_NOFS;

retry:
    err = pagecache_write_begin(NULL, mapping, 0, len-1,
                flags, &page, &fsdata);
    if (err)
        goto fail;

    kaddr = kmap_atomic(page);
    memcpy(kaddr, symname, len-1);
    kunmap_atomic(kaddr);

    err = pagecache_write_end(NULL, mapping, 0, len-1, len-1,
                            page, fsdata);
    if (err < 0)
        goto fail;
    if (err < len-1)
        goto retry;

    mark_inode_dirty(inode);
    return 0;
fail:
    return err;
}

int page_symlink(struct inode *inode, const char *symname, int len)
{
    return __page_symlink(inode, symname, len,
            !(mapping_gfp_mask(inode->i_mapping) & __GFP_FS));
}

const struct inode_operations page_symlink_inode_operations = {
    .readlink   = generic_readlink,
    .follow_link    = page_follow_link_light,
    .put_link   = page_put_link,
};

EXPORT_SYMBOL(user_path_at);
EXPORT_SYMBOL(follow_down_one);
EXPORT_SYMBOL(follow_down);
EXPORT_SYMBOL(follow_up);
EXPORT_SYMBOL(get_write_access); /* nfsd */
EXPORT_SYMBOL(lock_rename);
EXPORT_SYMBOL(lookup_one_len);
EXPORT_SYMBOL(page_follow_link_light);
EXPORT_SYMBOL(page_put_link);
EXPORT_SYMBOL(page_readlink);
EXPORT_SYMBOL(__page_symlink);
EXPORT_SYMBOL(page_symlink);
EXPORT_SYMBOL(page_symlink_inode_operations);
EXPORT_SYMBOL(kern_path);
EXPORT_SYMBOL(vfs_path_lookup);
EXPORT_SYMBOL(inode_permission);
EXPORT_SYMBOL(unlock_rename);
EXPORT_SYMBOL(vfs_create);
EXPORT_SYMBOL(vfs_follow_link);
EXPORT_SYMBOL(vfs_link);
EXPORT_SYMBOL(vfs_mkdir);
EXPORT_SYMBOL(vfs_mknod);
EXPORT_SYMBOL(generic_permission);
EXPORT_SYMBOL(vfs_readlink);
EXPORT_SYMBOL(vfs_rename);
EXPORT_SYMBOL(vfs_rmdir);
EXPORT_SYMBOL(vfs_symlink);
EXPORT_SYMBOL(vfs_unlink);
EXPORT_SYMBOL(dentry_unhash);
EXPORT_SYMBOL(generic_readlink);