userns: rename is_owner_or_cap to inode_owner_or_capable

[GitHub/mt8127/android_kernel_alcatel_ttab.git] / fs / namei.c

/*
 *  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/module.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 <asm/uaccess.h>

#include "internal.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-existant 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.
 */
static int do_getname(const char __user *filename, char *page)
{
        int retval;
        unsigned long len = PATH_MAX;

        if (!segment_eq(get_fs(), KERNEL_DS)) {
                if ((unsigned long) filename >= TASK_SIZE)
                        return -EFAULT;
                if (TASK_SIZE - (unsigned long) filename < PATH_MAX)
                        len = TASK_SIZE - (unsigned long) filename;
        }

        retval = strncpy_from_user(page, filename, len);
        if (retval > 0) {
                if (retval < len)
                        return 0;
                return -ENAMETOOLONG;
        } else if (!retval)
                retval = -ENOENT;
        return retval;
}

static char *getname_flags(const char __user * filename, int flags)
{
        char *tmp, *result;

        result = ERR_PTR(-ENOMEM);
        tmp = __getname();
        if (tmp)  {
                int retval = do_getname(filename, tmp);

                result = tmp;
                if (retval < 0) {
                        if (retval != -ENOENT || !(flags & LOOKUP_EMPTY)) {
                                __putname(tmp);
                                result = ERR_PTR(retval);
                        }
                }
        }
        audit_getname(result);
        return result;
}

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

#ifdef CONFIG_AUDITSYSCALL
void putname(const char *name)
{
        if (unlikely(!audit_dummy_context()))
                audit_putname(name);
        else
                __putname(name);
}
EXPORT_SYMBOL(putname);
#endif

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

        mask &= MAY_READ | MAY_WRITE | MAY_EXEC;

        if (current_user_ns() != inode_userns(inode))
                goto other_perms;

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

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

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

/**
 * generic_permission -  check for access rights on a Posix-like filesystem
 * @inode:      inode to check access rights for
 * @mask:       right to check for (%MAY_READ, %MAY_WRITE, %MAY_EXEC)
 * @check_acl:  optional callback to check for Posix ACLs
 * @flags:      IPERM_FLAG_ flags.
 *
 * Used to check for read/write/execute permissions on a file.
 * We use "fsuid" for this, letting us set arbitrary permissions
 * for filesystem access without changing the "normal" uids which
 * are used for other things.
 *
 * generic_permission is rcu-walk aware. It returns -ECHILD in case an rcu-walk
 * request cannot be satisfied (eg. requires blocking or too much complexity).
 * It would then be called again in ref-walk mode.
 */
int generic_permission(struct inode *inode, int mask, unsigned int flags,
        int (*check_acl)(struct inode *inode, int mask, unsigned int flags))
{
        int ret;

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

        /*
         * Read/write DACs are always overridable.
         * Executable DACs are overridable if at least one exec bit is set.
         */
        if (!(mask & MAY_EXEC) || execute_ok(inode))
                if (ns_capable(inode_userns(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 || (S_ISDIR(inode->i_mode) && !(mask & MAY_WRITE)))
                if (ns_capable(inode_userns(inode), CAP_DAC_READ_SEARCH))
                        return 0;

        return -EACCES;
}

/**
 * inode_permission  -  check for access rights to a given inode
 * @inode:      inode to check permission on
 * @mask:       right to check for (%MAY_READ, %MAY_WRITE, %MAY_EXEC)
 *
 * Used to check for read/write/execute permissions on an inode.
 * We use "fsuid" for this, letting us set arbitrary permissions
 * for filesystem access without changing the "normal" uids which
 * are used for other things.
 */
int inode_permission(struct inode *inode, int mask)
{
        int retval;

        if (mask & MAY_WRITE) {
                umode_t mode = inode->i_mode;

                /*
                 * Nobody gets write access to a read-only fs.
                 */
                if (IS_RDONLY(inode) &&
                    (S_ISREG(mode) || S_ISDIR(mode) || S_ISLNK(mode)))
                        return -EROFS;

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

        if (inode->i_op->permission)
                retval = inode->i_op->permission(inode, mask, 0);
        else
                retval = generic_permission(inode, mask, 0,
                                inode->i_op->check_acl);

        if (retval)
                return retval;

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

        return security_inode_permission(inode, mask);
}

/**
 * file_permission  -  check for additional access rights to a given file
 * @file:       file to check access rights for
 * @mask:       right to check for (%MAY_READ, %MAY_WRITE, %MAY_EXEC)
 *
 * Used to check for read/write/execute permissions on an already opened
 * file.
 *
 * Note:
 *      Do not use this function in new code.  All access checks should
 *      be done using inode_permission().
 */
int file_permission(struct file *file, int mask)
{
        return inode_permission(file->f_path.dentry->d_inode, mask);
}

/*
 * get_write_access() gets write permission for a file.
 * put_write_access() releases this write permission.
 * This is used for regular files.
 * We cannot support write (and maybe mmap read-write shared) accesses and
 * MAP_DENYWRITE mmappings simultaneously. The i_writecount field of an inode
 * can have the following values:
 * 0: no writers, no VM_DENYWRITE mappings
 * < 0: (-i_writecount) vm_area_structs with VM_DENYWRITE set exist
 * > 0: (i_writecount) users are writing to the file.
 *
 * Normally we operate on that counter with atomic_{inc,dec} and it's safe
 * except for the cases where we don't hold i_writecount yet. Then we need to
 * use {get,deny}_write_access() - these functions check the sign and refuse
 * to do the change if sign is wrong. Exclusion between them is provided by
 * the inode->i_lock spinlock.
 */

int get_write_access(struct inode * inode)
{
        spin_lock(&inode->i_lock);
        if (atomic_read(&inode->i_writecount) < 0) {
                spin_unlock(&inode->i_lock);
                return -ETXTBSY;
        }
        atomic_inc(&inode->i_writecount);
        spin_unlock(&inode->i_lock);

        return 0;
}

int deny_write_access(struct file * file)
{
        struct inode *inode = file->f_path.dentry->d_inode;

        spin_lock(&inode->i_lock);
        if (atomic_read(&inode->i_writecount) > 0) {
                spin_unlock(&inode->i_lock);
                return -ETXTBSY;
        }
        atomic_dec(&inode->i_writecount);
        spin_unlock(&inode->i_lock);

        return 0;
}

/**
 * path_get - get a reference to a path
 * @path: path to get the reference to
 *
 * Given a path increment the reference count to the dentry and the vfsmount.
 */
void path_get(struct path *path)
{
        mntget(path->mnt);
        dget(path->dentry);
}
EXPORT_SYMBOL(path_get);

/**
 * path_put - put a reference to a path
 * @path: path to put the reference to
 *
 * Given a path decrement the reference count to the dentry and the vfsmount.
 */
void path_put(struct path *path)
{
        dput(path->dentry);
        mntput(path->mnt);
}
EXPORT_SYMBOL(path_put);

/**
 * nameidata_drop_rcu - drop this nameidata out of rcu-walk
 * @nd: nameidata pathwalk data to drop
 * Returns: 0 on success, -ECHILD on failure
 *
 * Path walking has 2 modes, rcu-walk and ref-walk (see
 * Documentation/filesystems/path-lookup.txt). __drop_rcu* functions attempt
 * to drop out of rcu-walk mode and take normal reference counts on dentries
 * and vfsmounts to transition to rcu-walk mode. __drop_rcu* functions take
 * refcounts 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 path walk restarts from the
 * beginning in ref-walk mode.
 *
 * nameidata_drop_rcu attempts to drop the current nd->path and nd->root into
 * ref-walk. Must be called from rcu-walk context.
 */
static int nameidata_drop_rcu(struct nameidata *nd)
{
        struct fs_struct *fs = current->fs;
        struct dentry *dentry = 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(&dentry->d_lock);
        if (!__d_rcu_to_refcount(dentry, nd->seq))
                goto err;
        BUG_ON(nd->inode != dentry->d_inode);
        spin_unlock(&dentry->d_lock);
        if (want_root) {
                path_get(&nd->root);
                spin_unlock(&fs->lock);
        }
        mntget(nd->path.mnt);

        rcu_read_unlock();
        br_read_unlock(vfsmount_lock);
        nd->flags &= ~LOOKUP_RCU;
        return 0;
err:
        spin_unlock(&dentry->d_lock);
err_root:
        if (want_root)
                spin_unlock(&fs->lock);
        return -ECHILD;
}

/* Try to drop out of rcu-walk mode if we were in it, otherwise do nothing.  */
static inline int nameidata_drop_rcu_maybe(struct nameidata *nd)
{
        if (nd->flags & LOOKUP_RCU)
                return nameidata_drop_rcu(nd);
        return 0;
}

/**
 * nameidata_dentry_drop_rcu - drop nameidata and dentry out of rcu-walk
 * @nd: nameidata pathwalk data to drop
 * @dentry: dentry to drop
 * Returns: 0 on success, -ECHILD on failure
 *
 * nameidata_dentry_drop_rcu attempts to drop the current nd->path and nd->root,
 * and dentry into ref-walk. @dentry must be a path found by a do_lookup call on
 * @nd. Must be called from rcu-walk context.
 */
static int nameidata_dentry_drop_rcu(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);
        spin_lock_nested(&dentry->d_lock, DENTRY_D_LOCK_NESTED);
        if (!__d_rcu_to_refcount(dentry, nd->seq))
                goto err;
        /*
         * 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);

        rcu_read_unlock();
        br_read_unlock(vfsmount_lock);
        nd->flags &= ~LOOKUP_RCU;
        return 0;
err:
        spin_unlock(&dentry->d_lock);
        spin_unlock(&parent->d_lock);
err_root:
        if (want_root)
                spin_unlock(&fs->lock);
        return -ECHILD;
}

/* Try to drop out of rcu-walk mode if we were in it, otherwise do nothing.  */
static inline int nameidata_dentry_drop_rcu_maybe(struct nameidata *nd, struct dentry *dentry)
{
        if (nd->flags & LOOKUP_RCU) {
                if (unlikely(nameidata_dentry_drop_rcu(nd, dentry))) {
                        nd->flags &= ~LOOKUP_RCU;
                        if (!(nd->flags & LOOKUP_ROOT))
                                nd->root.mnt = NULL;
                        rcu_read_unlock();
                        br_read_unlock(vfsmount_lock);
                        return -ECHILD;
                }
        }
        return 0;
}

/**
 * nameidata_drop_rcu_last - drop nameidata ending path walk out of rcu-walk
 * @nd: nameidata pathwalk data to drop
 * Returns: 0 on success, -ECHILD on failure
 *
 * nameidata_drop_rcu_last attempts to drop the current nd->path into ref-walk.
 * nd->path should be the final element of the lookup, so nd->root is discarded.
 * Must be called from rcu-walk context.
 */
static int nameidata_drop_rcu_last(struct nameidata *nd)
{
        struct dentry *dentry = nd->path.dentry;

        BUG_ON(!(nd->flags & LOOKUP_RCU));
        nd->flags &= ~LOOKUP_RCU;
        if (!(nd->flags & LOOKUP_ROOT))
                nd->root.mnt = NULL;
        spin_lock(&dentry->d_lock);
        if (!__d_rcu_to_refcount(dentry, nd->seq))
                goto err_unlock;
        BUG_ON(nd->inode != dentry->d_inode);
        spin_unlock(&dentry->d_lock);

        mntget(nd->path.mnt);

        rcu_read_unlock();
        br_read_unlock(vfsmount_lock);

        return 0;

err_unlock:
        spin_unlock(&dentry->d_lock);
        rcu_read_unlock();
        br_read_unlock(vfsmount_lock);
        return -ECHILD;
}

/**
 * release_open_intent - free up open intent resources
 * @nd: pointer to nameidata
 */
void release_open_intent(struct nameidata *nd)
{
        struct file *file = nd->intent.open.file;

        if (file && !IS_ERR(file)) {
                if (file->f_path.dentry == NULL)
                        put_filp(file);
                else
                        fput(file);
        }
}

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

static struct dentry *
do_revalidate(struct dentry *dentry, struct nameidata *nd)
{
        int status = d_revalidate(dentry, nd);
        if (unlikely(status <= 0)) {
                /*
                 * The dentry failed validation.
                 * If d_revalidate returned 0 attempt to invalidate
                 * the dentry otherwise d_revalidate is asking us
                 * to return a fail status.
                 */
                if (status < 0) {
                        dput(dentry);
                        dentry = ERR_PTR(status);
                } else if (!d_invalidate(dentry)) {
                        dput(dentry);
                        dentry = NULL;
                }
        }
        return dentry;
}

/*
 * handle_reval_path - force revalidation of a dentry
 *
 * In some situations the path walking code will trust dentries without
 * revalidating them. This causes problems for filesystems that depend on
 * d_revalidate to handle file opens (e.g. NFSv4). When FS_REVAL_DOT is set
 * (which indicates that it's possible for the dentry to go stale), force
 * a d_revalidate call before proceeding.
 *
 * Returns 0 if the revalidation was successful. If the revalidation fails,
 * either return the error returned by d_revalidate or -ESTALE if the
 * revalidation it just returned 0. If d_revalidate returns 0, we attempt to
 * invalidate the dentry. It's up to the caller to handle putting references
 * to the path if necessary.
 */
static inline int handle_reval_path(struct nameidata *nd)
{
        struct dentry *dentry = nd->path.dentry;
        int status;

        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);
        if (status > 0)
                return 0;

        if (!status)
                status = -ESTALE;

        return status;
}

/*
 * Short-cut version of permission(), for calling on directories
 * during pathname resolution.  Combines parts of permission()
 * and generic_permission(), and tests ONLY for MAY_EXEC permission.
 *
 * If appropriate, check DAC only.  If not appropriate, or
 * short-cut DAC fails, then call ->permission() to do more
 * complete permission check.
 */
static inline int exec_permission(struct inode *inode, unsigned int flags)
{
        int ret;
        struct user_namespace *ns = inode_userns(inode);

        if (inode->i_op->permission) {
                ret = inode->i_op->permission(inode, MAY_EXEC, flags);
        } else {
                ret = acl_permission_check(inode, MAY_EXEC, flags,
                                inode->i_op->check_acl);
        }
        if (likely(!ret))
                goto ok;
        if (ret == -ECHILD)
                return ret;

        if (ns_capable(ns, CAP_DAC_OVERRIDE) ||
                        ns_capable(ns, CAP_DAC_READ_SEARCH))
                goto ok;

        return ret;
ok:
        return security_inode_exec_permission(inode, flags);
}

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;
                } 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;
}

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

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

        BUG_ON(nd->flags & LOOKUP_RCU);

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

        if (unlikely(current->total_link_count >= 40)) {
                *p = ERR_PTR(-ELOOP); /* no ->put_link(), please */
                path_put(&nd->path);
                return -ELOOP;
        }
        cond_resched();
        current->total_link_count++;

        touch_atime(link->mnt, dentry);
        nd_set_link(nd, NULL);

        error = security_inode_follow_link(link->dentry, nd);
        if (error) {
                *p = ERR_PTR(error); /* no ->put_link(), please */
                path_put(&nd->path);
                return error;
        }

        nd->last_type = LAST_BIND;
        *p = dentry->d_inode->i_op->follow_link(dentry, nd);
        error = PTR_ERR(*p);
        if (!IS_ERR(*p)) {
                char *s = nd_get_link(nd);
                error = 0;
                if (s)
                        error = __vfs_follow_link(nd, s);
                else if (nd->last_type == LAST_BIND) {
                        nd->flags |= LOOKUP_JUMPED;
                        nd->inode = nd->path.dentry->d_inode;
                        if (nd->inode->i_op->follow_link) {
                                /* stepped on a _really_ weird one */
                                path_put(&nd->path);
                                error = -ELOOP;
                        }
                }
        }
        return error;
}

static int follow_up_rcu(struct path *path)
{
        struct vfsmount *parent;
        struct dentry *mountpoint;

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

int follow_up(struct path *path)
{
        struct vfsmount *parent;
        struct dentry *mountpoint;

        br_read_lock(vfsmount_lock);
        parent = path->mnt->mnt_parent;
        if (parent == path->mnt) {
                br_read_unlock(vfsmount_lock);
                return 0;
        }
        mntget(parent);
        mountpoint = dget(path->mnt->mnt_mountpoint);
        br_read_unlock(vfsmount_lock);
        dput(path->dentry);
        path->dentry = mountpoint;
        mntput(path->mnt);
        path->mnt = parent;
        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 supplied AT_NO_AUTOMOUNT
         * and this is the terminal part of the path.
         */
        if ((flags & LOOKUP_NO_AUTOMOUNT) && !(flags & LOOKUP_CONTINUE))
                return -EISDIR; /* we actually want to stop here */

        /* We want to mount if someone is trying to open/create a file of any
         * type under the mountpoint, wants to traverse through the mountpoint
         * or wants to open the mounted directory.
         *
         * We don't want to mount if someone's just doing a stat and they've
         * set AT_SYMLINK_NOFOLLOW - unless they're stat'ing a directory and
         * appended a '/' to the name.
         */
        if (!(flags & LOOKUP_FOLLOW) &&
            !(flags & (LOOKUP_CONTINUE | LOOKUP_DIRECTORY |
                       LOOKUP_OPEN | LOOKUP_CREATE)))
                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_CONTINUE))
                        return -EREMOTE;
                return PTR_ERR(mnt);
        }

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

        err = finish_automount(mnt, path);

        switch (err) {
        case -EBUSY:
                /* Someone else made a mount here whilst we were busy */
                return 0;
        case 0:
                dput(path->dentry);
                if (*need_mntput)
                        mntput(path->mnt);
                path->mnt = mnt;
                path->dentry = dget(mnt->mnt_root);
                *need_mntput = true;
                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)
{
        unsigned managed;
        bool need_mntput = false;
        int ret;

        /* 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)
                                return ret == -EISDIR ? 0 : ret;
                }

                /* 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)
                                return ret == -EISDIR ? 0 : ret;
                        continue;
                }

                /* We didn't change the current path point */
                break;
        }
        return 0;
}

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;
}

/*
 * Skip to top of mountpoint pile in rcuwalk mode.  We abort the rcu-walk if we
 * meet a managed dentry and we're not walking to "..".  True is returned to
 * continue, false to abort.
 */
static bool __follow_mount_rcu(struct nameidata *nd, struct path *path,
                               struct inode **inode, bool reverse_transit)
{
        while (d_mountpoint(path->dentry)) {
                struct vfsmount *mounted;
                if (unlikely(path->dentry->d_flags & DCACHE_MANAGE_TRANSIT) &&
                    !reverse_transit &&
                    path->dentry->d_op->d_manage(path->dentry, true) < 0)
                        return false;
                mounted = __lookup_mnt(path->mnt, path->dentry, 1);
                if (!mounted)
                        break;
                path->mnt = mounted;
                path->dentry = mounted->mnt_root;
                nd->seq = read_seqcount_begin(&path->dentry->d_seq);
                *inode = path->dentry->d_inode;
        }

        if (unlikely(path->dentry->d_flags & DCACHE_NEED_AUTOMOUNT))
                return reverse_transit;
        return true;
}

static int follow_dotdot_rcu(struct nameidata *nd)
{
        struct inode *inode = nd->inode;

        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;
                        inode = parent->d_inode;
                        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);
                inode = nd->path.dentry->d_inode;
        }
        __follow_mount_rcu(nd, &nd->path, &inode, true);
        nd->inode = inode;
        return 0;

failed:
        nd->flags &= ~LOOKUP_RCU;
        if (!(nd->flags & LOOKUP_ROOT))
                nd->root.mnt = NULL;
        rcu_read_unlock();
        br_read_unlock(vfsmount_lock);
        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.
 *
 * Care must be taken as namespace_sem may be held (indicated by mounting_here
 * being true).
 */
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;
}

/*
 * Allocate a dentry with name and parent, and perform a parent
 * directory ->lookup on it. Returns the new dentry, or ERR_PTR
 * on error. parent->d_inode->i_mutex must be held. d_lookup must
 * have verified that no child exists while under i_mutex.
 */
static struct dentry *d_alloc_and_lookup(struct dentry *parent,
                                struct qstr *name, struct nameidata *nd)
{
        struct inode *inode = parent->d_inode;
        struct dentry *dentry;
        struct dentry *old;

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

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

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

/*
 *  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 do_lookup(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;
                *inode = nd->inode;
                dentry = __d_lookup_rcu(parent, name, &seq, inode);
                if (!dentry)
                        goto unlazy;

                /* Memory barrier in read_seqcount_begin of child is enough */
                if (__read_seqcount_retry(&parent->d_seq, nd->seq))
                        return -ECHILD;
                nd->seq = seq;

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

retry:
        if (unlikely(!dentry)) {
                struct inode *dir = parent->d_inode;
                BUG_ON(nd->inode != dir);

                mutex_lock(&dir->i_mutex);
                dentry = d_lookup(parent, name);
                if (likely(!dentry)) {
                        dentry = d_alloc_and_lookup(parent, name, nd);
                        if (IS_ERR(dentry)) {
                                mutex_unlock(&dir->i_mutex);
                                return PTR_ERR(dentry);
                        }
                        /* known good */
                        need_reval = 0;
                        status = 1;
                }
                mutex_unlock(&dir->i_mutex);
        }
        if (unlikely(dentry->d_flags & DCACHE_OP_REVALIDATE) && need_reval)
                status = d_revalidate(dentry, nd);
        if (unlikely(status <= 0)) {
                if (status < 0) {
                        dput(dentry);
                        return status;
                }
                if (!d_invalidate(dentry)) {
                        dput(dentry);
                        dentry = NULL;
                        need_reval = 1;
                        goto retry;
                }
        }

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

static inline int may_lookup(struct nameidata *nd)
{
        if (nd->flags & LOOKUP_RCU) {
                int err = exec_permission(nd->inode, IPERM_FLAG_RCU);
                if (err != -ECHILD)
                        return err;
                if (nameidata_drop_rcu(nd))
                        return -ECHILD;
        }
        return exec_permission(nd->inode, 0);
}

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;
                rcu_read_unlock();
                br_read_unlock(vfsmount_lock);
        }
}

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 = do_lookup(nd, name, path, &inode);
        if (unlikely(err)) {
                terminate_walk(nd);
                return err;
        }
        if (!inode) {
                path_to_nameidata(path, nd);
                terminate_walk(nd);
                return -ENOENT;
        }
        if (unlikely(inode->i_op->follow_link) && follow) {
                if (nameidata_dentry_drop_rcu_maybe(nd, path->dentry))
                        return -ECHILD;
                BUG_ON(inode != path->dentry->d_inode);
                return 1;
        }
        path_to_nameidata(path, nd);
        nd->inode = inode;
        return 0;
}

/*
 * 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;

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

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

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

                res = follow_link(&link, nd, &cookie);
                if (!res)
                        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;
}

/*
 * 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;
        unsigned int lookup_flags = nd->flags;
        
        while (*name=='/')
                name++;
        if (!*name)
                return 0;

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

                nd->flags |= LOOKUP_CONTINUE;

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

                this.name = name;
                c = *(const unsigned char *)name;

                hash = init_name_hash();
                do {
                        name++;
                        hash = partial_name_hash(c, hash);
                        c = *(const unsigned char *)name;
                } while (c && (c != '/'));
                this.len = name - (const char *) this.name;
                this.hash = end_name_hash(hash);

                type = LAST_NORM;
                if (this.name[0] == '.') switch (this.len) {
                        case 2:
                                if (this.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;
                        }
                }

                /* remove trailing slashes? */
                if (!c)
                        goto last_component;
                while (*++name == '/');
                if (!*name)
                        goto last_component;

                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;
                }
                err = -ENOTDIR; 
                if (!nd->inode->i_op->lookup)
                        break;
                continue;
                /* here ends the main loop */

last_component:
                /* Clear LOOKUP_CONTINUE iff it was previously unset */
                nd->flags &= lookup_flags | ~LOOKUP_CONTINUE;
                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;
        int fput_needed;
        struct file *file;

        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) {
                        br_read_lock(vfsmount_lock);
                        rcu_read_lock();
                        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) {
                        br_read_lock(vfsmount_lock);
                        rcu_read_lock();
                        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;

                        br_read_lock(vfsmount_lock);
                        rcu_read_lock();

                        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 dentry *dentry;

                file = fget_raw_light(dfd, &fput_needed);
                retval = -EBADF;
                if (!file)
                        goto out_fail;

                dentry = file->f_path.dentry;

                if (*name) {
                        retval = -ENOTDIR;
                        if (!S_ISDIR(dentry->d_inode->i_mode))
                                goto fput_fail;

                        retval = file_permission(file, MAY_EXEC);
                        if (retval)
                                goto fput_fail;
                }

                nd->path = file->f_path;
                if (flags & LOOKUP_RCU) {
                        if (fput_needed)
                                *fp = file;
                        nd->seq = __read_seqcount_begin(&nd->path.dentry->d_seq);
                        br_read_lock(vfsmount_lock);
                        rcu_read_lock();
                } else {
                        path_get(&file->f_path);
                        fput_light(file, fput_needed);
                }
        }

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

fput_fail:
        fput_light(file, fput_needed);
out_fail:
        return retval;
}

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;
                        nd->flags |= LOOKUP_PARENT;
                        err = follow_link(&link, nd, &cookie);
                        if (!err)
                                err = lookup_last(nd, &path);
                        put_link(nd, &link, cookie);
                }
        }

        if (nd->flags & LOOKUP_RCU) {
                /* went all way through without dropping RCU */
                BUG_ON(err);
                if (nameidata_drop_rcu_last(nd))
                        err = -ECHILD;
        }

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

        if (!err && nd->flags & LOOKUP_DIRECTORY) {
                if (!nd->inode->i_op->lookup) {
                        path_put(&nd->path);
                        return -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 do_path_lookup(int dfd, const char *name,
                                unsigned int flags, struct nameidata *nd)
{
        int retval = path_lookupat(dfd, name, flags | LOOKUP_RCU, nd);
        if (unlikely(retval == -ECHILD))
                retval = path_lookupat(dfd, name, flags, nd);
        if (unlikely(retval == -ESTALE))
                retval = path_lookupat(dfd, name, flags | LOOKUP_REVAL, nd);

        if (likely(!retval)) {
                if (unlikely(!audit_dummy_context())) {
                        if (nd->path.dentry && nd->inode)
                                audit_inode(name, nd->path.dentry);
                }
        }
        return retval;
}

int kern_path_parent(const char *name, struct nameidata *nd)
{
        return do_path_lookup(AT_FDCWD, name, LOOKUP_PARENT, nd);
}

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;
}

/**
 * vfs_path_lookup - lookup a file path relative to a dentry-vfsmount pair
 * @dentry:  pointer to dentry of the base directory
 * @mnt: pointer to vfs mount of the base directory
 * @name: pointer to file name
 * @flags: lookup flags
 * @nd: pointer to nameidata
 */
int vfs_path_lookup(struct dentry *dentry, struct vfsmount *mnt,
                    const char *name, unsigned int flags,
                    struct nameidata *nd)
{
        nd->root.dentry = dentry;
        nd->root.mnt = mnt;
        /* the first argument of do_path_lookup() is ignored with LOOKUP_ROOT */
        return do_path_lookup(AT_FDCWD, name, flags | LOOKUP_ROOT, nd);
}

static struct dentry *__lookup_hash(struct qstr *name,
                struct dentry *base, struct nameidata *nd)
{
        struct inode *inode = base->d_inode;
        struct dentry *dentry;
        int err;

        err = exec_permission(inode, 0);
        if (err)
                return ERR_PTR(err);

        /*
         * Don't bother with __d_lookup: callers are for creat as
         * well as unlink, so a lot of the time it would cost
         * a double lookup.
         */
        dentry = d_lookup(base, name);

        if (dentry && (dentry->d_flags & DCACHE_OP_REVALIDATE))
                dentry = do_revalidate(dentry, nd);

        if (!dentry)
                dentry = d_alloc_and_lookup(base, name, nd);

        return dentry;
}

/*
 * 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);
}

/**
 * lookup_one_len - filesystem helper to lookup single pathname component
 * @name:       pathname component to lookup
 * @base:       base directory to lookup from
 * @len:        maximum length @len should be interpreted to
 *
 * Note that this routine is purely a helper for filesystem usage and should
 * not be called by generic code.  Also note that by using this function the
 * nameidata argument is passed to the filesystem methods and a filesystem
 * using this helper needs to be prepared for that.
 */
struct dentry *lookup_one_len(const char *name, struct dentry *base, int len)
{
        struct qstr this;
        unsigned long hash;
        unsigned int c;

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

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

        hash = init_name_hash();
        while (len--) {
                c = *(const unsigned char *)name++;
                if (c == '/' || c == '\0')
                        return ERR_PTR(-EACCES);
                hash = partial_name_hash(c, hash);
        }
        this.hash = end_name_hash(hash);
        /*
         * 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);
        }

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

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

                BUG_ON(flags & LOOKUP_PARENT);

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

static int user_path_parent(int dfd, const char __user *path,
                        struct nameidata *nd, char **name)
{
        char *s = getname(path);
        int error;

        if (IS_ERR(s))
                return PTR_ERR(s);

        error = do_path_lookup(dfd, s, LOOKUP_PARENT, nd);
        if (error)
                putname(s);
        else
                *name = s;

        return error;
}

/*
 * 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)
{
        uid_t fsuid = current_fsuid();

        if (!(dir->i_mode & S_ISVTX))
                return 0;
        if (current_user_ns() != inode_userns(inode))
                goto other_userns;
        if (inode->i_uid == fsuid)
                return 0;
        if (dir->i_uid == fsuid)
                return 0;

other_userns:
        return !ns_capable(inode_userns(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(victim, dir);

        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, int mode,
                struct nameidata *nd)
{
        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, nd);
        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;

        /*
         * Ensure there are no outstanding leases on the file.
         */
        return break_lease(inode, flag);
}

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;
}

/*
 * Note that while the flag value (low two bits) for sys_open means:
 *      00 - read-only
 *      01 - write-only
 *      10 - read-write
 *      11 - special
 * it is changed into
 *      00 - no permissions needed
 *      01 - read-permission
 *      10 - write-permission
 *      11 - read-write
 * for the internal routines (ie open_namei()/follow_link() etc)
 * This is more logical, and also allows the 00 "no perm needed"
 * to be used for symlinks (where the permissions are checked
 * later).
 *
*/
static inline int open_to_namei_flags(int flag)
{
        if ((flag+1) & O_ACCMODE)
                flag++;
        return flag;
}

/*
 * Handle the last step of open()
 */
static struct file *do_last(struct nameidata *nd, struct path *path,
                            const struct open_flags *op, const char *pathname)
{
        struct dentry *dir = nd->path.dentry;
        struct dentry *dentry;
        int open_flag = op->open_flag;
        int will_truncate = open_flag & O_TRUNC;
        int want_write = 0;
        int acc_mode = op->acc_mode;
        struct file *filp;
        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 ERR_PTR(error);
                /* fallthrough */
        case LAST_ROOT:
                if (nd->flags & LOOKUP_RCU) {
                        if (nameidata_drop_rcu_last(nd))
                                return ERR_PTR(-ECHILD);
                }
                error = handle_reval_path(nd);
                if (error)
                        goto exit;
                audit_inode(pathname, nd->path.dentry);
                if (open_flag & O_CREAT) {
                        error = -EISDIR;
                        goto exit;
                }
                goto ok;
        case LAST_BIND:
                /* can't be RCU mode here */
                error = handle_reval_path(nd);
                if (error)
                        goto exit;
                audit_inode(pathname, dir);
                goto ok;
        }

        if (!(open_flag & O_CREAT)) {
                int symlink_ok = 0;
                if (nd->last.name[nd->last.len])
                        nd->flags |= LOOKUP_FOLLOW | LOOKUP_DIRECTORY;
                if (open_flag & O_PATH && !(nd->flags & LOOKUP_FOLLOW))
                        symlink_ok = 1;
                /* we _can_ be in RCU mode here */
                error = walk_component(nd, path, &nd->last, LAST_NORM,
                                        !symlink_ok);
                if (error < 0)
                        return ERR_PTR(error);
                if (error) /* symlink */
                        return NULL;
                /* sayonara */
                if (nd->flags & LOOKUP_RCU) {
                        if (nameidata_drop_rcu_last(nd))
                                return ERR_PTR(-ECHILD);
                }

                error = -ENOTDIR;
                if (nd->flags & LOOKUP_DIRECTORY) {
                        if (!nd->inode->i_op->lookup)
                                goto exit;
                }
                audit_inode(pathname, nd->path.dentry);
                goto ok;
        }

        /* create side of things */

        if (nd->flags & LOOKUP_RCU) {
                if (nameidata_drop_rcu_last(nd))
                        return ERR_PTR(-ECHILD);
        }

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

        mutex_lock(&dir->d_inode->i_mutex);

        dentry = lookup_hash(nd);
        error = PTR_ERR(dentry);
        if (IS_ERR(dentry)) {
                mutex_unlock(&dir->d_inode->i_mutex);
                goto exit;
        }

        path->dentry = dentry;
        path->mnt = nd->path.mnt;

        /* Negative dentry, just create the file */
        if (!dentry->d_inode) {
                int 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 nameidata_to_filp().
                 */
                error = mnt_want_write(nd->path.mnt);
                if (error)
                        goto exit_mutex_unlock;
                want_write = 1;
                /* Don't check for write permission, don't truncate */
                open_flag &= ~O_TRUNC;
                will_truncate = 0;
                acc_mode = MAY_OPEN;
                error = security_path_mknod(&nd->path, dentry, mode, 0);
                if (error)
                        goto exit_mutex_unlock;
                error = vfs_create(dir->d_inode, dentry, mode, nd);
                if (error)
                        goto exit_mutex_unlock;
                mutex_unlock(&dir->d_inode->i_mutex);
                dput(nd->path.dentry);
                nd->path.dentry = dentry;
                goto common;
        }

        /*
         * It already exists.
         */
        mutex_unlock(&dir->d_inode->i_mutex);
        audit_inode(pathname, path->dentry);

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

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

        error = -ENOENT;
        if (!path->dentry->d_inode)
                goto exit_dput;

        if (path->dentry->d_inode->i_op->follow_link)
                return NULL;

        path_to_nameidata(path, nd);
        nd->inode = path->dentry->d_inode;
        error = -EISDIR;
        if (S_ISDIR(nd->inode->i_mode))
                goto exit;
ok:
        if (!S_ISREG(nd->inode->i_mode))
                will_truncate = 0;

        if (will_truncate) {
                error = mnt_want_write(nd->path.mnt);
                if (error)
                        goto exit;
                want_write = 1;
        }
common:
        error = may_open(&nd->path, acc_mode, open_flag);
        if (error)
                goto exit;
        filp = nameidata_to_filp(nd);
        if (!IS_ERR(filp)) {
                error = ima_file_check(filp, op->acc_mode);
                if (error) {
                        fput(filp);
                        filp = ERR_PTR(error);
                }
        }
        if (!IS_ERR(filp)) {
                if (will_truncate) {
                        error = handle_truncate(filp);
                        if (error) {
                                fput(filp);
                                filp = ERR_PTR(error);
                        }
                }
        }
out:
        if (want_write)
                mnt_drop_write(nd->path.mnt);
        path_put(&nd->path);
        return filp;

exit_mutex_unlock:
        mutex_unlock(&dir->d_inode->i_mutex);
exit_dput:
        path_put_conditional(path, nd);
exit:
        filp = ERR_PTR(error);
        goto out;
}

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

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

        filp->f_flags = op->open_flag;
        nd->intent.open.file = filp;
        nd->intent.open.flags = open_to_namei_flags(op->open_flag);
        nd->intent.open.create_mode = op->mode;

        error = path_init(dfd, pathname, flags | LOOKUP_PARENT, nd, &base);
        if (unlikely(error))
                goto out_filp;

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

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

out_filp:
        filp = ERR_PTR(error);
        goto out;
}

struct file *do_filp_open(int dfd, const char *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;

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

/**
 * lookup_create - lookup a dentry, creating it if it doesn't exist
 * @nd: nameidata info
 * @is_dir: directory flag
 *
 * Simple function to lookup and return a dentry and create it
 * if it doesn't exist.  Is SMP-safe.
 *
 * Returns with nd->path.dentry->d_inode->i_mutex locked.
 */
struct dentry *lookup_create(struct nameidata *nd, int is_dir)
{
        struct dentry *dentry = ERR_PTR(-EEXIST);

        mutex_lock_nested(&nd->path.dentry->d_inode->i_mutex, I_MUTEX_PARENT);
        /*
         * Yucky last component or no last component at all?
         * (foo/., foo/.., /////)
         */
        if (nd->last_type != LAST_NORM)
                goto fail;
        nd->flags &= ~LOOKUP_PARENT;
        nd->flags |= LOOKUP_CREATE | LOOKUP_EXCL;
        nd->intent.open.flags = O_EXCL;

        /*
         * Do the final lookup.
         */
        dentry = lookup_hash(nd);
        if (IS_ERR(dentry))
                goto fail;

        if (dentry->d_inode)
                goto eexist;
        /*
         * 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])) {
                dput(dentry);
                dentry = ERR_PTR(-ENOENT);
        }
        return dentry;
eexist:
        dput(dentry);
        dentry = ERR_PTR(-EEXIST);
fail:
        return dentry;
}
EXPORT_SYMBOL_GPL(lookup_create);

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

        if (error)
                return error;

        if ((S_ISCHR(mode) || S_ISBLK(mode)) &&
            !ns_capable(inode_userns(dir), 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(mode_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, int, mode,
                unsigned, dev)
{
        int error;
        char *tmp;
        struct dentry *dentry;
        struct nameidata nd;

        if (S_ISDIR(mode))
                return -EPERM;

        error = user_path_parent(dfd, filename, &nd, &tmp);
        if (error)
                return error;

        dentry = lookup_create(&nd, 0);
        if (IS_ERR(dentry)) {
                error = PTR_ERR(dentry);
                goto out_unlock;
        }
        if (!IS_POSIXACL(nd.path.dentry->d_inode))
                mode &= ~current_umask();
        error = may_mknod(mode);
        if (error)
                goto out_dput;
        error = mnt_want_write(nd.path.mnt);
        if (error)
                goto out_dput;
        error = security_path_mknod(&nd.path, dentry, mode, dev);
        if (error)
                goto out_drop_write;
        switch (mode & S_IFMT) {
                case 0: case S_IFREG:
                        error = vfs_create(nd.path.dentry->d_inode,dentry,mode,&nd);
                        break;
                case S_IFCHR: case S_IFBLK:
                        error = vfs_mknod(nd.path.dentry->d_inode,dentry,mode,
                                        new_decode_dev(dev));
                        break;
                case S_IFIFO: case S_IFSOCK:
                        error = vfs_mknod(nd.path.dentry->d_inode,dentry,mode,0);
                        break;
        }
out_drop_write:
        mnt_drop_write(nd.path.mnt);
out_dput:
        dput(dentry);
out_unlock:
        mutex_unlock(&nd.path.dentry->d_inode->i_mutex);
        path_put(&nd.path);
        putname(tmp);

        return error;
}

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

int vfs_mkdir(struct inode *dir, struct dentry *dentry, int mode)
{
        int error = may_create(dir, dentry);

        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;

        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, int, mode)
{
        int error = 0;
        char * tmp;
        struct dentry *dentry;
        struct nameidata nd;

        error = user_path_parent(dfd, pathname, &nd, &tmp);
        if (error)
                goto out_err;

        dentry = lookup_create(&nd, 1);
        error = PTR_ERR(dentry);
        if (IS_ERR(dentry))
                goto out_unlock;

        if (!IS_POSIXACL(nd.path.dentry->d_inode))
                mode &= ~current_umask();
        error = mnt_want_write(nd.path.mnt);
        if (error)
                goto out_dput;
        error = security_path_mkdir(&nd.path, dentry, mode);
        if (error)
                goto out_drop_write;
        error = vfs_mkdir(nd.path.dentry->d_inode, dentry, mode);
out_drop_write:
        mnt_drop_write(nd.path.mnt);
out_dput:
        dput(dentry);
out_unlock:
        mutex_unlock(&nd.path.dentry->d_inode->i_mutex);
        path_put(&nd.path);
        putname(tmp);
out_err:
        return error;
}

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

/*
 * We try to drop the dentry early: we should have
 * a usage count of 2 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)
{
        dget(dentry);
        shrink_dcache_parent(dentry);
        spin_lock(&dentry->d_lock);
        if (dentry->d_count == 2)
                __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;

        mutex_lock(&dentry->d_inode->i_mutex);
        dentry_unhash(dentry);
        if (d_mountpoint(dentry))
                error = -EBUSY;
        else {
                error = security_inode_rmdir(dir, dentry);
                if (!error) {
                        error = dir->i_op->rmdir(dir, dentry);
                        if (!error) {
                                dentry->d_inode->i_flags |= S_DEAD;
                                dont_mount(dentry);
                        }
                }
        }
        mutex_unlock(&dentry->d_inode->i_mutex);
        if (!error) {
                d_delete(dentry);
        }
        dput(dentry);

        return error;
}

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

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

        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;

        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;
        error = mnt_want_write(nd.path.mnt);
        if (error)
                goto exit3;
        error = security_path_rmdir(&nd.path, dentry);
        if (error)
                goto exit4;
        error = vfs_rmdir(nd.path.dentry->d_inode, dentry);
exit4:
        mnt_drop_write(nd.path.mnt);
exit3:
        dput(dentry);
exit2:
        mutex_unlock(&nd.path.dentry->d_inode->i_mutex);
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;
        char *name;
        struct dentry *dentry;
        struct nameidata nd;
        struct inode *inode = NULL;

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

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

        nd.flags &= ~LOOKUP_PARENT;

        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)
                        ihold(inode);
                error = mnt_want_write(nd.path.mnt);
                if (error)
                        goto exit2;
                error = security_path_unlink(&nd.path, dentry);
                if (error)
                        goto exit3;
                error = vfs_unlink(nd.path.dentry->d_inode, dentry);
exit3:
                mnt_drop_write(nd.path.mnt);
        exit2:
                dput(dentry);
        }
        mutex_unlock(&nd.path.dentry->d_inode->i_mutex);
        if (inode)
                iput(inode);    /* truncate the inode here */
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;
        char *from;
        char *to;
        struct dentry *dentry;
        struct nameidata nd;

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

        error = user_path_parent(newdfd, newname, &nd, &to);
        if (error)
                goto out_putname;

        dentry = lookup_create(&nd, 0);
        error = PTR_ERR(dentry);
        if (IS_ERR(dentry))
                goto out_unlock;

        error = mnt_want_write(nd.path.mnt);
        if (error)
                goto out_dput;
        error = security_path_symlink(&nd.path, dentry, from);
        if (error)
                goto out_drop_write;
        error = vfs_symlink(nd.path.dentry->d_inode, dentry, from);
out_drop_write:
        mnt_drop_write(nd.path.mnt);
out_dput:
        dput(dentry);
out_unlock:
        mutex_unlock(&nd.path.dentry->d_inode->i_mutex);
        path_put(&nd.path);
        putname(to);
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;
        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
                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 nameidata nd;
        struct path old_path;
        int how = 0;
        int error;
        char *to;

        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;

        error = user_path_parent(newdfd, newname, &nd, &to);
        if (error)
                goto out;
        error = -EXDEV;
        if (old_path.mnt != nd.path.mnt)
                goto out_release;
        new_dentry = lookup_create(&nd, 0);
        error = PTR_ERR(new_dentry);
        if (IS_ERR(new_dentry))
                goto out_unlock;
        error = mnt_want_write(nd.path.mnt);
        if (error)
                goto out_dput;
        error = security_path_link(old_path.dentry, &nd.path, new_dentry);
        if (error)
                goto out_drop_write;
        error = vfs_link(old_path.dentry, nd.path.dentry->d_inode, new_dentry);
out_drop_write:
        mnt_drop_write(nd.path.mnt);
out_dput:
        dput(new_dentry);
out_unlock:
        mutex_unlock(&nd.path.dentry->d_inode->i_mutex);
out_release:
        path_put(&nd.path);
        putname(to);
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) some filesystems don't support opened-but-unlinked directories,
 *         either because of layout or because they are not ready to deal with
 *         all cases correctly. The latter will be fixed (taking this sort of
 *         stuff into VFS), but the former is not going away. Solution: the same
 *         trick as in rmdir().
 *      e) 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;

        /*
         * 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;

        target = new_dentry->d_inode;
        if (target)
                mutex_lock(&target->i_mutex);
        if (d_mountpoint(old_dentry)||d_mountpoint(new_dentry))
                error = -EBUSY;
        else {
                if (target)
                        dentry_unhash(new_dentry);
                error = old_dir->i_op->rename(old_dir, old_dentry, new_dir, new_dentry);
        }
        if (target) {
                if (!error) {
                        target->i_flags |= S_DEAD;
                        dont_mount(new_dentry);
                }
                mutex_unlock(&target->i_mutex);
                if (d_unhashed(new_dentry))
                        d_rehash(new_dentry);
                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;
        int error;

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

        dget(new_dentry);
        target = new_dentry->d_inode;
        if (target)
                mutex_lock(&target->i_mutex);
        if (d_mountpoint(old_dentry)||d_mountpoint(new_dentry))
                error = -EBUSY;
        else
                error = old_dir->i_op->rename(old_dir, old_dentry, new_dir, new_dentry);
        if (!error) {
                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);
        }
        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;
        char *from;
        char *to;
        int error;

        error = user_path_parent(olddfd, oldname, &oldnd, &from);
        if (error)
                goto exit;

        error = user_path_parent(newdfd, newname, &newnd, &to);
        if (error)
                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;

        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 = mnt_want_write(oldnd.path.mnt);
        if (error)
                goto exit5;
        error = security_path_rename(&oldnd.path, old_dentry,
                                     &newnd.path, new_dentry);
        if (error)
                goto exit6;
        error = vfs_rename(old_dir->d_inode, old_dentry,
                                   new_dir->d_inode, new_dentry);
exit6:
        mnt_drop_write(oldnd.path.mnt);
exit5:
        dput(new_dentry);
exit4:
        dput(old_dentry);
exit3:
        unlock_rename(new_dir, old_dir);
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, KM_USER0);
        memcpy(kaddr, symname, len-1);
        kunmap_atomic(kaddr, KM_USER0);

        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); /* binfmt_aout */
EXPORT_SYMBOL(getname);
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_parent);
EXPORT_SYMBOL(kern_path);
EXPORT_SYMBOL(vfs_path_lookup);
EXPORT_SYMBOL(inode_permission);
EXPORT_SYMBOL(file_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);