// SPDX-License-Identifier: GPL-2.0
/*
 *  linux/fs/namei.c
 *
 *  Copyright (C) 1991, 1992  Linus Torvalds
 */

/*
 * Some corrections by tytso.
 */

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

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

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

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

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

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

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

/* In order to reduce some races, while at the same time doing additional
 * checking and hopefully speeding things up, we copy filenames to the
 * kernel data space before using them..
 *
 * POSIX.1 2.4: an empty pathname is invalid (ENOENT).
 * PATH_MAX includes the nul terminator --RR.
 */

#define EMBEDDED_NAME_MAX       (PATH_MAX - offsetof(struct filename, iname))

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

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

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

        /*
         * First, try to embed the struct filename inside the names_cache
         * allocation
         */
        kname = (char *)result->iname;
        result->name = kname;

        len = strncpy_from_user(kname, filename, EMBEDDED_NAME_MAX);
        if (unlikely(len < 0)) {
                __putname(result);
                return ERR_PTR(len);
        }

        /*
         * Uh-oh. We have a name that's approaching PATH_MAX. Allocate a
         * separate struct filename so we can dedicate the entire
         * names_cache allocation for the pathname, and re-do the copy from
         * userland.
         */
        if (unlikely(len == EMBEDDED_NAME_MAX)) {
                const size_t size = offsetof(struct filename, iname[1]);
                kname = (char *)result;

                /*
                 * size is chosen that way we to guarantee that
                 * result->iname[0] is within the same object and that
                 * kname can't be equal to result->iname, no matter what.
                 */
                result = kzalloc(size, GFP_KERNEL);
                if (unlikely(!result)) {
                        __putname(kname);
                        return ERR_PTR(-ENOMEM);
                }
                result->name = kname;
                len = strncpy_from_user(kname, filename, PATH_MAX);
                if (unlikely(len < 0)) {
                        __putname(kname);
                        kfree(result);
                        return ERR_PTR(len);
                }
                if (unlikely(len == PATH_MAX)) {
                        __putname(kname);
                        kfree(result);
                        return ERR_PTR(-ENAMETOOLONG);
                }
        }

        result->refcnt = 1;
        /* The empty path is special. */
        if (unlikely(!len)) {
                if (empty)
                        *empty = 1;
                if (!(flags & LOOKUP_EMPTY)) {
                        putname(result);
                        return ERR_PTR(-ENOENT);
                }
        }

        result->uptr = filename;
        result->aname = NULL;
        audit_getname(result);
        return result;
}

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

struct filename *
getname_kernel(const char * filename)
{
        struct filename *result;
        int len = strlen(filename) + 1;

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

        if (len <= EMBEDDED_NAME_MAX) {
                result->name = (char *)result->iname;
        } else if (len <= PATH_MAX) {
                const size_t size = offsetof(struct filename, iname[1]);
                struct filename *tmp;

                tmp = kmalloc(size, GFP_KERNEL);
                if (unlikely(!tmp)) {
                        __putname(result);
                        return ERR_PTR(-ENOMEM);
                }
                tmp->name = (char *)result;
                result = tmp;
        } else {
                __putname(result);
                return ERR_PTR(-ENAMETOOLONG);
        }
        memcpy((char *)result->name, filename, len);
        result->uptr = NULL;
        result->aname = NULL;
        result->refcnt = 1;
        audit_getname(result);

        return result;
}

void putname(struct filename *name)
{
        BUG_ON(name->refcnt <= 0);

        if (--name->refcnt > 0)
                return;

        if (name->name != name->iname) {
                __putname(name->name);
                kfree(name);
        } else
                __putname(name);
}

/**
 * check_acl - perform ACL permission checking
 * @mnt_userns: user namespace of the mount the inode was found from
 * @inode:      inode to check permissions on
 * @mask:       right to check for (%MAY_READ, %MAY_WRITE, %MAY_EXEC ...)
 *
 * This function performs the ACL permission checking. Since this function
 * retrieve POSIX acls it needs to know whether it is called from a blocking or
 * non-blocking context and thus cares about the MAY_NOT_BLOCK bit.
 *
 * If the inode has been found through an idmapped mount the user namespace of
 * the vfsmount must be passed through @mnt_userns. This function will then take
 * care to map the inode according to @mnt_userns before checking permissions.
 * On non-idmapped mounts or if permission checking is to be performed on the
 * raw inode simply passs init_user_ns.
 */
static int check_acl(struct user_namespace *mnt_userns,
                     struct inode *inode, int mask)
{
#ifdef CONFIG_FS_POSIX_ACL
        struct posix_acl *acl;

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

        acl = get_acl(inode, ACL_TYPE_ACCESS);
        if (IS_ERR(acl))
                return PTR_ERR(acl);
        if (acl) {
                int error = posix_acl_permission(mnt_userns, inode, acl, mask);
                posix_acl_release(acl);
                return error;
        }
#endif

        return -EAGAIN;
}

/**
 * acl_permission_check - perform basic UNIX permission checking
 * @mnt_userns: user namespace of the mount the inode was found from
 * @inode:      inode to check permissions on
 * @mask:       right to check for (%MAY_READ, %MAY_WRITE, %MAY_EXEC ...)
 *
 * This function performs the basic UNIX permission checking. Since this
 * function may retrieve POSIX acls it needs to know whether it is called from a
 * blocking or non-blocking context and thus cares about the MAY_NOT_BLOCK bit.
 *
 * If the inode has been found through an idmapped mount the user namespace of
 * the vfsmount must be passed through @mnt_userns. This function will then take
 * care to map the inode according to @mnt_userns before checking permissions.
 * On non-idmapped mounts or if permission checking is to be performed on the
 * raw inode simply passs init_user_ns.
 */
static int acl_permission_check(struct user_namespace *mnt_userns,
                                struct inode *inode, int mask)
{
        unsigned int mode = inode->i_mode;
        kuid_t i_uid;

        /* Are we the owner? If so, ACL's don't matter */
        i_uid = i_uid_into_mnt(mnt_userns, inode);
        if (likely(uid_eq(current_fsuid(), i_uid))) {
                mask &= 7;
                mode >>= 6;
                return (mask & ~mode) ? -EACCES : 0;
        }

        /* Do we have ACL's? */
        if (IS_POSIXACL(inode) && (mode & S_IRWXG)) {
                int error = check_acl(mnt_userns, inode, mask);
                if (error != -EAGAIN)
                        return error;
        }

        /* Only RWX matters for group/other mode bits */
        mask &= 7;

        /*
         * Are the group permissions different from
         * the other permissions in the bits we care
         * about? Need to check group ownership if so.
         */
        if (mask & (mode ^ (mode >> 3))) {
                kgid_t kgid = i_gid_into_mnt(mnt_userns, inode);
                if (in_group_p(kgid))
                        mode >>= 3;
        }

        /* Bits in 'mode' clear that we require? */
        return (mask & ~mode) ? -EACCES : 0;
}

/**
 * generic_permission -  check for access rights on a Posix-like filesystem
 * @mnt_userns: user namespace of the mount the inode was found from
 * @inode:      inode to check access rights for
 * @mask:       right to check for (%MAY_READ, %MAY_WRITE, %MAY_EXEC,
 *              %MAY_NOT_BLOCK ...)
 *
 * 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.
 *
 * If the inode has been found through an idmapped mount the user namespace of
 * the vfsmount must be passed through @mnt_userns. This function will then take
 * care to map the inode according to @mnt_userns before checking permissions.
 * On non-idmapped mounts or if permission checking is to be performed on the
 * raw inode simply passs init_user_ns.
 */
int generic_permission(struct user_namespace *mnt_userns, struct inode *inode,
                       int mask)
{
        int ret;

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

        if (S_ISDIR(inode->i_mode)) {
                /* DACs are overridable for directories */
                if (!(mask & MAY_WRITE))
                        if (capable_wrt_inode_uidgid(mnt_userns, inode,
                                                     CAP_DAC_READ_SEARCH))
                                return 0;
                if (capable_wrt_inode_uidgid(mnt_userns, inode,
                                             CAP_DAC_OVERRIDE))
                        return 0;
                return -EACCES;
        }

        /*
         * Searching includes executable on directories, else just read.
         */
        mask &= MAY_READ | MAY_WRITE | MAY_EXEC;
        if (mask == MAY_READ)
                if (capable_wrt_inode_uidgid(mnt_userns, inode,
                                             CAP_DAC_READ_SEARCH))
                        return 0;
        /*
         * Read/write DACs are always overridable.
         * Executable DACs are overridable when there is
         * at least one exec bit set.
         */
        if (!(mask & MAY_EXEC) || (inode->i_mode & S_IXUGO))
                if (capable_wrt_inode_uidgid(mnt_userns, inode,
                                             CAP_DAC_OVERRIDE))
                        return 0;

        return -EACCES;
}
EXPORT_SYMBOL(generic_permission);

/**
 * do_inode_permission - UNIX permission checking
 * @mnt_userns: user namespace of the mount the inode was found from
 * @inode:      inode to check permissions on
 * @mask:       right to check for (%MAY_READ, %MAY_WRITE, %MAY_EXEC ...)
 *
 * We _really_ want to just do "generic_permission()" without
 * even looking at the inode->i_op values. So we keep a cache
 * flag in inode->i_opflags, that says "this has not special
 * permission function, use the fast case".
 */
static inline int do_inode_permission(struct user_namespace *mnt_userns,
                                      struct inode *inode, int mask)
{
        if (unlikely(!(inode->i_opflags & IOP_FASTPERM))) {
                if (likely(inode->i_op->permission))
                        return inode->i_op->permission(mnt_userns, inode, mask);

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

/**
 * sb_permission - Check superblock-level permissions
 * @sb: Superblock of inode to check permission on
 * @inode: Inode to check permission on
 * @mask: Right to check for (%MAY_READ, %MAY_WRITE, %MAY_EXEC)
 *
 * Separate out file-system wide checks from inode-specific permission checks.
 */
static int sb_permission(struct super_block *sb, struct inode *inode, int mask)
{
        if (unlikely(mask & MAY_WRITE)) {
                umode_t mode = inode->i_mode;

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

/**
 * inode_permission - Check for access rights to a given inode
 * @mnt_userns: User namespace of the mount the inode was found from
 * @inode:      Inode to check permission on
 * @mask:       Right to check for (%MAY_READ, %MAY_WRITE, %MAY_EXEC)
 *
 * Check for read/write/execute permissions on an inode.  We use fs[ug]id for
 * this, letting us set arbitrary permissions for filesystem access without
 * changing the "normal" UIDs which are used for other things.
 *
 * When checking for MAY_APPEND, MAY_WRITE must also be set in @mask.
 */
int inode_permission(struct user_namespace *mnt_userns,
                     struct inode *inode, int mask)
{
        int retval;

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

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

                /*
                 * Updating mtime will likely cause i_uid and i_gid to be
                 * written back improperly if their true value is unknown
                 * to the vfs.
                 */
                if (HAS_UNMAPPED_ID(mnt_userns, inode))
                        return -EACCES;
        }

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

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

        return security_inode_permission(inode, mask);
}
EXPORT_SYMBOL(inode_permission);

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

#define EMBEDDED_LEVELS 2
struct nameidata {
        struct path     path;
        struct qstr     last;
        struct path     root;
        struct inode    *inode; /* path.dentry.d_inode */
        unsigned int    flags, state;
        unsigned        seq, m_seq, r_seq;
        int             last_type;
        unsigned        depth;
        int             total_link_count;
        struct saved {
                struct path link;
                struct delayed_call done;
                const char *name;
                unsigned seq;
        } *stack, internal[EMBEDDED_LEVELS];
        struct filename *name;
        struct nameidata *saved;
        unsigned        root_seq;
        int             dfd;
        kuid_t          dir_uid;
        umode_t         dir_mode;
} __randomize_layout;

#define ND_ROOT_PRESET 1
#define ND_ROOT_GRABBED 2
#define ND_JUMPED 4

static void __set_nameidata(struct nameidata *p, int dfd, struct filename *name)
{
        struct nameidata *old = current->nameidata;
        p->stack = p->internal;
        p->depth = 0;
        p->dfd = dfd;
        p->name = name;
        p->path.mnt = NULL;
        p->path.dentry = NULL;
        p->total_link_count = old ? old->total_link_count : 0;
        p->saved = old;
        current->nameidata = p;
}

static inline void set_nameidata(struct nameidata *p, int dfd, struct filename *name,
                          const struct path *root)
{
        __set_nameidata(p, dfd, name);
        p->state = 0;
        if (unlikely(root)) {
                p->state = ND_ROOT_PRESET;
                p->root = *root;
        }
}

static void restore_nameidata(void)
{
        struct nameidata *now = current->nameidata, *old = now->saved;

        current->nameidata = old;
        if (old)
                old->total_link_count = now->total_link_count;
        if (now->stack != now->internal)
                kfree(now->stack);
}

static bool nd_alloc_stack(struct nameidata *nd)
{
        struct saved *p;

        p= kmalloc_array(MAXSYMLINKS, sizeof(struct saved),
                         nd->flags & LOOKUP_RCU ? GFP_ATOMIC : GFP_KERNEL);
        if (unlikely(!p))
                return false;
        memcpy(p, nd->internal, sizeof(nd->internal));
        nd->stack = p;
        return true;
}

/**
 * path_connected - Verify that a dentry is below mnt.mnt_root
 *
 * Rename can sometimes move a file or directory outside of a bind
 * mount, path_connected allows those cases to be detected.
 */
static bool path_connected(struct vfsmount *mnt, struct dentry *dentry)
{
        struct super_block *sb = mnt->mnt_sb;

        /* Bind mounts can have disconnected paths */
        if (mnt->mnt_root == sb->s_root)
                return true;

        return is_subdir(dentry, mnt->mnt_root);
}

static void drop_links(struct nameidata *nd)
{
        int i = nd->depth;
        while (i--) {
                struct saved *last = nd->stack + i;
                do_delayed_call(&last->done);
                clear_delayed_call(&last->done);
        }
}

static void terminate_walk(struct nameidata *nd)
{
        drop_links(nd);
        if (!(nd->flags & LOOKUP_RCU)) {
                int i;
                path_put(&nd->path);
                for (i = 0; i < nd->depth; i++)
                        path_put(&nd->stack[i].link);
                if (nd->state & ND_ROOT_GRABBED) {
                        path_put(&nd->root);
                        nd->state &= ~ND_ROOT_GRABBED;
                }
        } else {
                nd->flags &= ~LOOKUP_RCU;
                rcu_read_unlock();
        }
        nd->depth = 0;
        nd->path.mnt = NULL;
        nd->path.dentry = NULL;
}

/* path_put is needed afterwards regardless of success or failure */
static bool __legitimize_path(struct path *path, unsigned seq, unsigned mseq)
{
        int res = __legitimize_mnt(path->mnt, mseq);
        if (unlikely(res)) {
                if (res > 0)
                        path->mnt = NULL;
                path->dentry = NULL;
                return false;
        }
        if (unlikely(!lockref_get_not_dead(&path->dentry->d_lockref))) {
                path->dentry = NULL;
                return false;
        }
        return !read_seqcount_retry(&path->dentry->d_seq, seq);
}

static inline bool legitimize_path(struct nameidata *nd,
                            struct path *path, unsigned seq)
{
        return __legitimize_path(path, seq, nd->m_seq);
}

static bool legitimize_links(struct nameidata *nd)
{
        int i;
        if (unlikely(nd->flags & LOOKUP_CACHED)) {
                drop_links(nd);
                nd->depth = 0;
                return false;
        }
        for (i = 0; i < nd->depth; i++) {
                struct saved *last = nd->stack + i;
                if (unlikely(!legitimize_path(nd, &last->link, last->seq))) {
                        drop_links(nd);
                        nd->depth = i + 1;
                        return false;
                }
        }
        return true;
}

static bool legitimize_root(struct nameidata *nd)
{
        /*
         * For scoped-lookups (where nd->root has been zeroed), we need to
         * restart the whole lookup from scratch -- because set_root() is wrong
         * for these lookups (nd->dfd is the root, not the filesystem root).
         */
        if (!nd->root.mnt && (nd->flags & LOOKUP_IS_SCOPED))
                return false;
        /* Nothing to do if nd->root is zero or is managed by the VFS user. */
        if (!nd->root.mnt || (nd->state & ND_ROOT_PRESET))
                return true;
        nd->state |= ND_ROOT_GRABBED;
        return legitimize_path(nd, &nd->root, nd->root_seq);
}

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

/**
 * try_to_unlazy - try to switch to ref-walk mode.
 * @nd: nameidata pathwalk data
 * Returns: true on success, false on failure
 *
 * try_to_unlazy attempts to legitimize the current nd->path and nd->root
 * for ref-walk mode.
 * Must be called from rcu-walk context.
 * Nothing should touch nameidata between try_to_unlazy() failure and
 * terminate_walk().
 */
static bool try_to_unlazy(struct nameidata *nd)
{
        struct dentry *parent = nd->path.dentry;

        BUG_ON(!(nd->flags & LOOKUP_RCU));

        nd->flags &= ~LOOKUP_RCU;
        if (unlikely(!legitimize_links(nd)))
                goto out1;
        if (unlikely(!legitimize_path(nd, &nd->path, nd->seq)))
                goto out;
        if (unlikely(!legitimize_root(nd)))
                goto out;
        rcu_read_unlock();
        BUG_ON(nd->inode != parent->d_inode);
        return true;

out1:
        nd->path.mnt = NULL;
        nd->path.dentry = NULL;
out:
        rcu_read_unlock();
        return false;
}

/**
 * try_to_unlazy_next - try to switch to ref-walk mode.
 * @nd: nameidata pathwalk data
 * @dentry: next dentry to step into
 * @seq: seq number to check @dentry against
 * Returns: true on success, false on failure
 *
 * Similar to to try_to_unlazy(), but here we have the next dentry already
 * picked by rcu-walk and want to legitimize that in addition to the current
 * nd->path and nd->root for ref-walk mode.  Must be called from rcu-walk context.
 * Nothing should touch nameidata between try_to_unlazy_next() failure and
 * terminate_walk().
 */
static bool try_to_unlazy_next(struct nameidata *nd, struct dentry *dentry, unsigned seq)
{
        BUG_ON(!(nd->flags & LOOKUP_RCU));

        nd->flags &= ~LOOKUP_RCU;
        if (unlikely(!legitimize_links(nd)))
                goto out2;
        if (unlikely(!legitimize_mnt(nd->path.mnt, nd->m_seq)))
                goto out2;
        if (unlikely(!lockref_get_not_dead(&nd->path.dentry->d_lockref)))
                goto out1;

        /*
         * We need to move both the parent and the dentry from the RCU domain
         * to be properly refcounted. And the sequence number in the dentry
         * validates *both* dentry counters, since we checked the sequence
         * number of the parent after we got the child sequence number. So we
         * know the parent must still be valid if the child sequence number is
         */
        if (unlikely(!lockref_get_not_dead(&dentry->d_lockref)))
                goto out;
        if (unlikely(read_seqcount_retry(&dentry->d_seq, seq)))
                goto out_dput;
        /*
         * Sequence counts matched. Now make sure that the root is
         * still valid and get it if required.
         */
        if (unlikely(!legitimize_root(nd)))
                goto out_dput;
        rcu_read_unlock();
        return true;

out2:
        nd->path.mnt = NULL;
out1:
        nd->path.dentry = NULL;
out:
        rcu_read_unlock();
        return false;
out_dput:
        rcu_read_unlock();
        dput(dentry);
        return false;
}

static inline int d_revalidate(struct dentry *dentry, unsigned int flags)
{
        if (unlikely(dentry->d_flags & DCACHE_OP_REVALIDATE))
                return dentry->d_op->d_revalidate(dentry, flags);
        else
                return 1;
}

/**
 * complete_walk - successful completion of path walk
 * @nd:  pointer nameidata
 *
 * If we had been in RCU mode, drop out of it and legitimize nd->path.
 * Revalidate the final result, unless we'd already done that during
 * the path walk or the filesystem doesn't ask for it.  Return 0 on
 * success, -error on failure.  In case of failure caller does not
 * need to drop nd->path.
 */
static int complete_walk(struct nameidata *nd)
{
        struct dentry *dentry = nd->path.dentry;
        int status;

        if (nd->flags & LOOKUP_RCU) {
                /*
                 * We don't want to zero nd->root for scoped-lookups or
                 * externally-managed nd->root.
                 */
                if (!(nd->state & ND_ROOT_PRESET))
                        if (!(nd->flags & LOOKUP_IS_SCOPED))
                                nd->root.mnt = NULL;
                nd->flags &= ~LOOKUP_CACHED;
                if (!try_to_unlazy(nd))
                        return -ECHILD;
        }

        if (unlikely(nd->flags & LOOKUP_IS_SCOPED)) {
                /*
                 * While the guarantee of LOOKUP_IS_SCOPED is (roughly) "don't
                 * ever step outside the root during lookup" and should already
                 * be guaranteed by the rest of namei, we want to avoid a namei
                 * BUG resulting in userspace being given a path that was not
                 * scoped within the root at some point during the lookup.
                 *
                 * So, do a final sanity-check to make sure that in the
                 * worst-case scenario (a complete bypass of LOOKUP_IS_SCOPED)
                 * we won't silently return an fd completely outside of the
                 * requested root to userspace.
                 *
                 * Userspace could move the path outside the root after this
                 * check, but as discussed elsewhere this is not a concern (the
                 * resolved file was inside the root at some point).
                 */
                if (!path_is_under(&nd->path, &nd->root))
                        return -EXDEV;
        }

        if (likely(!(nd->state & ND_JUMPED)))
                return 0;

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

        status = dentry->d_op->d_weak_revalidate(dentry, nd->flags);
        if (status > 0)
                return 0;

        if (!status)
                status = -ESTALE;

        return status;
}

static int set_root(struct nameidata *nd)
{
        struct fs_struct *fs = current->fs;

        /*
         * Jumping to the real root in a scoped-lookup is a BUG in namei, but we
         * still have to ensure it doesn't happen because it will cause a breakout
         * from the dirfd.
         */
        if (WARN_ON(nd->flags & LOOKUP_IS_SCOPED))
                return -ENOTRECOVERABLE;

        if (nd->flags & LOOKUP_RCU) {
                unsigned seq;

                do {
                        seq = read_seqcount_begin(&fs->seq);
                        nd->root = fs->root;
                        nd->root_seq = __read_seqcount_begin(&nd->root.dentry->d_seq);
                } while (read_seqcount_retry(&fs->seq, seq));
        } else {
                get_fs_root(fs, &nd->root);
                nd->state |= ND_ROOT_GRABBED;
        }
        return 0;
}

static int nd_jump_root(struct nameidata *nd)
{
        if (unlikely(nd->flags & LOOKUP_BENEATH))
                return -EXDEV;
        if (unlikely(nd->flags & LOOKUP_NO_XDEV)) {
                /* Absolute path arguments to path_init() are allowed. */
                if (nd->path.mnt != NULL && nd->path.mnt != nd->root.mnt)
                        return -EXDEV;
        }
        if (!nd->root.mnt) {
                int error = set_root(nd);
                if (error)
                        return error;
        }
        if (nd->flags & LOOKUP_RCU) {
                struct dentry *d;
                nd->path = nd->root;
                d = nd->path.dentry;
                nd->inode = d->d_inode;
                nd->seq = nd->root_seq;
                if (unlikely(read_seqcount_retry(&d->d_seq, nd->seq)))
                        return -ECHILD;
        } else {
                path_put(&nd->path);
                nd->path = nd->root;
                path_get(&nd->path);
                nd->inode = nd->path.dentry->d_inode;
        }
        nd->state |= ND_JUMPED;
        return 0;
}

/*
 * Helper to directly jump to a known parsed path from ->get_link,
 * caller must have taken a reference to path beforehand.
 */
int nd_jump_link(struct path *path)
{
        int error = -ELOOP;
        struct nameidata *nd = current->nameidata;

        if (unlikely(nd->flags & LOOKUP_NO_MAGICLINKS))
                goto err;

        error = -EXDEV;
        if (unlikely(nd->flags & LOOKUP_NO_XDEV)) {
                if (nd->path.mnt != path->mnt)
                        goto err;
        }
        /* Not currently safe for scoped-lookups. */
        if (unlikely(nd->flags & LOOKUP_IS_SCOPED))
                goto err;

        path_put(&nd->path);
        nd->path = *path;
        nd->inode = nd->path.dentry->d_inode;
        nd->state |= ND_JUMPED;
        return 0;

err:
        path_put(path);

        return error;
}

static inline void put_link(struct nameidata *nd)
{
        struct saved *last = nd->stack + --nd->depth;
        do_delayed_call(&last->done);
        if (!(nd->flags & LOOKUP_RCU))
                path_put(&last->link);
}

int sysctl_protected_symlinks __read_mostly = 0;
int sysctl_protected_hardlinks __read_mostly = 0;
int sysctl_protected_fifos __read_mostly;
int sysctl_protected_regular __read_mostly;

/**
 * may_follow_link - Check symlink following for unsafe situations
 * @nd: nameidata pathwalk data
 *
 * In the case of the sysctl_protected_symlinks sysctl being enabled,
 * CAP_DAC_OVERRIDE needs to be specifically ignored if the symlink is
 * in a sticky world-writable directory. This is to protect privileged
 * processes from failing races against path names that may change out
 * from under them by way of other users creating malicious symlinks.
 * It will permit symlinks to be followed only when outside a sticky
 * world-writable directory, or when the uid of the symlink and follower
 * match, or when the directory owner matches the symlink's owner.
 *
 * Returns 0 if following the symlink is allowed, -ve on error.
 */
static inline int may_follow_link(struct nameidata *nd, const struct inode *inode)
{
        struct user_namespace *mnt_userns;
        kuid_t i_uid;

        if (!sysctl_protected_symlinks)
                return 0;

        mnt_userns = mnt_user_ns(nd->path.mnt);
        i_uid = i_uid_into_mnt(mnt_userns, inode);
        /* Allowed if owner and follower match. */
        if (uid_eq(current_cred()->fsuid, i_uid))
                return 0;

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

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

        if (nd->flags & LOOKUP_RCU)
                return -ECHILD;

        audit_inode(nd->name, nd->stack[0].link.dentry, 0);
        audit_log_path_denied(AUDIT_ANOM_LINK, "follow_link");
        return -EACCES;
}

/**
 * safe_hardlink_source - Check for safe hardlink conditions
 * @mnt_userns: user namespace of the mount the inode was found from
 * @inode: the source inode to hardlink from
 *
 * Return false if at least one of the following conditions:
 *    - inode is not a regular file
 *    - inode is setuid
 *    - inode is setgid and group-exec
 *    - access failure for read and write
 *
 * Otherwise returns true.
 */
static bool safe_hardlink_source(struct user_namespace *mnt_userns,
                                 struct inode *inode)
{
        umode_t mode = inode->i_mode;

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

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

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

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

        return true;
}

/**
 * may_linkat - Check permissions for creating a hardlink
 * @mnt_userns: user namespace of the mount the inode was found from
 * @link: the source to hardlink from
 *
 * Block hardlink when all of:
 *  - sysctl_protected_hardlinks enabled
 *  - fsuid does not match inode
 *  - hardlink source is unsafe (see safe_hardlink_source() above)
 *  - not CAP_FOWNER in a namespace with the inode owner uid mapped
 *
 * If the inode has been found through an idmapped mount the user namespace of
 * the vfsmount must be passed through @mnt_userns. This function will then take
 * care to map the inode according to @mnt_userns before checking permissions.
 * On non-idmapped mounts or if permission checking is to be performed on the
 * raw inode simply passs init_user_ns.
 *
 * Returns 0 if successful, -ve on error.
 */
int may_linkat(struct user_namespace *mnt_userns, struct path *link)
{
        struct inode *inode = link->dentry->d_inode;

        /* Inode writeback is not safe when the uid or gid are invalid. */
        if (!uid_valid(i_uid_into_mnt(mnt_userns, inode)) ||
            !gid_valid(i_gid_into_mnt(mnt_userns, inode)))
                return -EOVERFLOW;

        if (!sysctl_protected_hardlinks)
                return 0;

        /* Source inode owner (or CAP_FOWNER) can hardlink all they like,
         * otherwise, it must be a safe source.
         */
        if (safe_hardlink_source(mnt_userns, inode) ||
            inode_owner_or_capable(mnt_userns, inode))
                return 0;

        audit_log_path_denied(AUDIT_ANOM_LINK, "linkat");
        return -EPERM;
}

/**
 * may_create_in_sticky - Check whether an O_CREAT open in a sticky directory
 *                        should be allowed, or not, on files that already
 *                        exist.
 * @mnt_userns: user namespace of the mount the inode was found from
 * @nd: nameidata pathwalk data
 * @inode: the inode of the file to open
 *
 * Block an O_CREAT open of a FIFO (or a regular file) when:
 *   - sysctl_protected_fifos (or sysctl_protected_regular) is enabled
 *   - the file already exists
 *   - we are in a sticky directory
 *   - we don't own the file
 *   - the owner of the directory doesn't own the file
 *   - the directory is world writable
 * If the sysctl_protected_fifos (or sysctl_protected_regular) is set to 2
 * the directory doesn't have to be world writable: being group writable will
 * be enough.
 *
 * If the inode has been found through an idmapped mount the user namespace of
 * the vfsmount must be passed through @mnt_userns. This function will then take
 * care to map the inode according to @mnt_userns before checking permissions.
 * On non-idmapped mounts or if permission checking is to be performed on the
 * raw inode simply passs init_user_ns.
 *
 * Returns 0 if the open is allowed, -ve on error.
 */
static int may_create_in_sticky(struct user_namespace *mnt_userns,
                                struct nameidata *nd, struct inode *const inode)
{
        umode_t dir_mode = nd->dir_mode;
        kuid_t dir_uid = nd->dir_uid;

        if ((!sysctl_protected_fifos && S_ISFIFO(inode->i_mode)) ||
            (!sysctl_protected_regular && S_ISREG(inode->i_mode)) ||
            likely(!(dir_mode & S_ISVTX)) ||
            uid_eq(i_uid_into_mnt(mnt_userns, inode), dir_uid) ||
            uid_eq(current_fsuid(), i_uid_into_mnt(mnt_userns, inode)))
                return 0;

        if (likely(dir_mode & 0002) ||
            (dir_mode & 0020 &&
             ((sysctl_protected_fifos >= 2 && S_ISFIFO(inode->i_mode)) ||
              (sysctl_protected_regular >= 2 && S_ISREG(inode->i_mode))))) {
                const char *operation = S_ISFIFO(inode->i_mode) ?
                                        "sticky_create_fifo" :
                                        "sticky_create_regular";
                audit_log_path_denied(AUDIT_ANOM_CREAT, operation);
                return -EACCES;
        }
        return 0;
}

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

        read_seqlock_excl(&mount_lock);
        parent = mnt->mnt_parent;
        if (parent == mnt) {
                read_sequnlock_excl(&mount_lock);
                return 0;
        }
        mntget(&parent->mnt);
        mountpoint = dget(mnt->mnt_mountpoint);
        read_sequnlock_excl(&mount_lock);
        dput(path->dentry);
        path->dentry = mountpoint;
        mntput(path->mnt);
        path->mnt = &parent->mnt;
        return 1;
}
EXPORT_SYMBOL(follow_up);

static bool choose_mountpoint_rcu(struct mount *m, const struct path *root,
                                  struct path *path, unsigned *seqp)
{
        while (mnt_has_parent(m)) {
                struct dentry *mountpoint = m->mnt_mountpoint;

                m = m->mnt_parent;
                if (unlikely(root->dentry == mountpoint &&
                             root->mnt == &m->mnt))
                        break;
                if (mountpoint != m->mnt.mnt_root) {
                        path->mnt = &m->mnt;
                        path->dentry = mountpoint;
                        *seqp = read_seqcount_begin(&mountpoint->d_seq);
                        return true;
                }
        }
        return false;
}

static bool choose_mountpoint(struct mount *m, const struct path *root,
                              struct path *path)
{
        bool found;

        rcu_read_lock();
        while (1) {
                unsigned seq, mseq = read_seqbegin(&mount_lock);

                found = choose_mountpoint_rcu(m, root, path, &seq);
                if (unlikely(!found)) {
                        if (!read_seqretry(&mount_lock, mseq))
                                break;
                } else {
                        if (likely(__legitimize_path(path, seq, mseq)))
                                break;
                        rcu_read_unlock();
                        path_put(path);
                        rcu_read_lock();
                }
        }
        rcu_read_unlock();
        return found;
}

/*
 * 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, int *count, unsigned lookup_flags)
{
        struct dentry *dentry = path->dentry;

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

        if (count && (*count)++ >= MAXSYMLINKS)
                return -ELOOP;

        return finish_automount(dentry->d_op->d_automount(path), path);
}

/*
 * mount traversal - out-of-line part.  One note on ->d_flags accesses -
 * dentries are pinned but not locked here, so negative dentry can go
 * positive right under us.  Use of smp_load_acquire() provides a barrier
 * sufficient for ->d_inode and ->d_flags consistency.
 */
static int __traverse_mounts(struct path *path, unsigned flags, bool *jumped,
                             int *count, unsigned lookup_flags)
{
        struct vfsmount *mnt = path->mnt;
        bool need_mntput = false;
        int ret = 0;

        while (flags & DCACHE_MANAGED_DENTRY) {
                /* Allow the filesystem to manage the transit without i_mutex
                 * being held. */
                if (flags & DCACHE_MANAGE_TRANSIT) {
                        ret = path->dentry->d_op->d_manage(path, false);
                        flags = smp_load_acquire(&path->dentry->d_flags);
                        if (ret < 0)
                                break;
                }

                if (flags & DCACHE_MOUNTED) {   // something's mounted on it..
                        struct vfsmount *mounted = lookup_mnt(path);
                        if (mounted) {          // ... in our namespace
                                dput(path->dentry);
                                if (need_mntput)
                                        mntput(path->mnt);
                                path->mnt = mounted;
                                path->dentry = dget(mounted->mnt_root);
                                // here we know it's positive
                                flags = path->dentry->d_flags;
                                need_mntput = true;
                                continue;
                        }
                }

                if (!(flags & DCACHE_NEED_AUTOMOUNT))
                        break;

                // uncovered automount point
                ret = follow_automount(path, count, lookup_flags);
                flags = smp_load_acquire(&path->dentry->d_flags);
                if (ret < 0)
                        break;
        }

        if (ret == -EISDIR)
                ret = 0;
        // possible if you race with several mount --move
        if (need_mntput && path->mnt == mnt)
                mntput(path->mnt);
        if (!ret && unlikely(d_flags_negative(flags)))
                ret = -ENOENT;
        *jumped = need_mntput;
        return ret;
}

static inline int traverse_mounts(struct path *path, bool *jumped,
                                  int *count, unsigned lookup_flags)
{
        unsigned flags = smp_load_acquire(&path->dentry->d_flags);

        /* fastpath */
        if (likely(!(flags & DCACHE_MANAGED_DENTRY))) {
                *jumped = false;
                if (unlikely(d_flags_negative(flags)))
                        return -ENOENT;
                return 0;
        }
        return __traverse_mounts(path, flags, jumped, count, lookup_flags);
}

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;
}
EXPORT_SYMBOL(follow_down_one);

/*
 * Follow down to the covering mount currently visible to userspace.  At each
 * point, the filesystem owning that dentry may be queried as to whether the
 * caller is permitted to proceed or not.
 */
int follow_down(struct path *path)
{
        struct vfsmount *mnt = path->mnt;
        bool jumped;
        int ret = traverse_mounts(path, &jumped, NULL, 0);

        if (path->mnt != mnt)
                mntput(mnt);
        return ret;
}
EXPORT_SYMBOL(follow_down);

/*
 * Try to skip to top of mountpoint pile in rcuwalk mode.  Fail if
 * we meet a managed dentry that would need blocking.
 */
static bool __follow_mount_rcu(struct nameidata *nd, struct path *path,
                               struct inode **inode, unsigned *seqp)
{
        struct dentry *dentry = path->dentry;
        unsigned int flags = dentry->d_flags;

        if (likely(!(flags & DCACHE_MANAGED_DENTRY)))
                return true;

        if (unlikely(nd->flags & LOOKUP_NO_XDEV))
                return false;

        for (;;) {
                /*
                 * Don't forget we might have a non-mountpoint managed dentry
                 * that wants to block transit.
                 */
                if (unlikely(flags & DCACHE_MANAGE_TRANSIT)) {
                        int res = dentry->d_op->d_manage(path, true);
                        if (res)
                                return res == -EISDIR;
                        flags = dentry->d_flags;
                }

                if (flags & DCACHE_MOUNTED) {
                        struct mount *mounted = __lookup_mnt(path->mnt, dentry);
                        if (mounted) {
                                path->mnt = &mounted->mnt;
                                dentry = path->dentry = mounted->mnt.mnt_root;
                                nd->state |= ND_JUMPED;
                                *seqp = read_seqcount_begin(&dentry->d_seq);
                                *inode = dentry->d_inode;
                                /*
                                 * We don't need to re-check ->d_seq after this
                                 * ->d_inode read - there will be an RCU delay
                                 * between mount hash removal and ->mnt_root
                                 * becoming unpinned.
                                 */
                                flags = dentry->d_flags;
                                continue;
                        }
                        if (read_seqretry(&mount_lock, nd->m_seq))
                                return false;
                }
                return !(flags & DCACHE_NEED_AUTOMOUNT);
        }
}

static inline int handle_mounts(struct nameidata *nd, struct dentry *dentry,
                          struct path *path, struct inode **inode,
                          unsigned int *seqp)
{
        bool jumped;
        int ret;

        path->mnt = nd->path.mnt;
        path->dentry = dentry;
        if (nd->flags & LOOKUP_RCU) {
                unsigned int seq = *seqp;
                if (unlikely(!*inode))
                        return -ENOENT;
                if (likely(__follow_mount_rcu(nd, path, inode, seqp)))
                        return 0;
                if (!try_to_unlazy_next(nd, dentry, seq))
                        return -ECHILD;
                // *path might've been clobbered by __follow_mount_rcu()
                path->mnt = nd->path.mnt;
                path->dentry = dentry;
        }
        ret = traverse_mounts(path, &jumped, &nd->total_link_count, nd->flags);
        if (jumped) {
                if (unlikely(nd->flags & LOOKUP_NO_XDEV))
                        ret = -EXDEV;
                else
                        nd->state |= ND_JUMPED;
        }
        if (unlikely(ret)) {
                dput(path->dentry);
                if (path->mnt != nd->path.mnt)
                        mntput(path->mnt);
        } else {
                *inode = d_backing_inode(path->dentry);
                *seqp = 0; /* out of RCU mode, so the value doesn't matter */
        }
        return ret;
}

/*
 * This looks up the name in dcache and possibly revalidates the found dentry.
 * NULL is returned if the dentry does not exist in the cache.
 */
static struct dentry *lookup_dcache(const struct qstr *name,
                                    struct dentry *dir,
                                    unsigned int flags)
{
        struct dentry *dentry = d_lookup(dir, name);
        if (dentry) {
                int error = d_revalidate(dentry, flags);
                if (unlikely(error <= 0)) {
                        if (!error)
                                d_invalidate(dentry);
                        dput(dentry);
                        return ERR_PTR(error);
                }
        }
        return dentry;
}

/*
 * Parent directory has inode locked exclusive.  This is one
 * and only case when ->lookup() gets called on non in-lookup
 * dentries - as the matter of fact, this only gets called
 * when directory is guaranteed to have no in-lookup children
 * at all.
 */
static struct dentry *__lookup_hash(const struct qstr *name,
                struct dentry *base, unsigned int flags)
{
        struct dentry *dentry = lookup_dcache(name, base, flags);
        struct dentry *old;
        struct inode *dir = base->d_inode;

        if (dentry)
                return dentry;

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

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

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

static struct dentry *lookup_fast(struct nameidata *nd,
                                  struct inode **inode,
                                  unsigned *seqp)
{
        struct dentry *dentry, *parent = nd->path.dentry;
        int status = 1;

        /*
         * Rename seqlock is not required here because in the off chance
         * of a false negative due to a concurrent rename, the caller is
         * going to fall back to non-racy lookup.
         */
        if (nd->flags & LOOKUP_RCU) {
                unsigned seq;
                dentry = __d_lookup_rcu(parent, &nd->last, &seq);
                if (unlikely(!dentry)) {
                        if (!try_to_unlazy(nd))
                                return ERR_PTR(-ECHILD);
                        return NULL;
                }

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

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

                *seqp = seq;
                status = d_revalidate(dentry, nd->flags);
                if (likely(status > 0))
                        return dentry;
                if (!try_to_unlazy_next(nd, dentry, seq))
                        return ERR_PTR(-ECHILD);
                if (status == -ECHILD)
                        /* we'd been told to redo it in non-rcu mode */
                        status = d_revalidate(dentry, nd->flags);
        } else {
                dentry = __d_lookup(parent, &nd->last);
                if (unlikely(!dentry))
                        return NULL;
                status = d_revalidate(dentry, nd->flags);
        }
        if (unlikely(status <= 0)) {
                if (!status)
                        d_invalidate(dentry);
                dput(dentry);
                return ERR_PTR(status);
        }
        return dentry;
}

/* Fast lookup failed, do it the slow way */
static struct dentry *__lookup_slow(const struct qstr *name,
                                    struct dentry *dir,
                                    unsigned int flags)
{
        struct dentry *dentry, *old;
        struct inode *inode = dir->d_inode;
        DECLARE_WAIT_QUEUE_HEAD_ONSTACK(wq);

        /* Don't go there if it's already dead */
        if (unlikely(IS_DEADDIR(inode)))
                return ERR_PTR(-ENOENT);
again:
        dentry = d_alloc_parallel(dir, name, &wq);
        if (IS_ERR(dentry))
                return dentry;
        if (unlikely(!d_in_lookup(dentry))) {
                int error = d_revalidate(dentry, flags);
                if (unlikely(error <= 0)) {
                        if (!error) {
                                d_invalidate(dentry);
                                dput(dentry);
                                goto again;
                        }
                        dput(dentry);
                        dentry = ERR_PTR(error);
                }
        } else {
                old = inode->i_op->lookup(inode, dentry, flags);
                d_lookup_done(dentry);
                if (unlikely(old)) {
                        dput(dentry);
                        dentry = old;
                }
        }
        return dentry;
}

static struct dentry *lookup_slow(const struct qstr *name,
                                  struct dentry *dir,
                                  unsigned int flags)
{
        struct inode *inode = dir->d_inode;
        struct dentry *res;
        inode_lock_shared(inode);
        res = __lookup_slow(name, dir, flags);
        inode_unlock_shared(inode);
        return res;
}

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

static int reserve_stack(struct nameidata *nd, struct path *link, unsigned seq)
{
        if (unlikely(nd->total_link_count++ >= MAXSYMLINKS))
                return -ELOOP;

        if (likely(nd->depth != EMBEDDED_LEVELS))
                return 0;
        if (likely(nd->stack != nd->internal))
                return 0;
        if (likely(nd_alloc_stack(nd)))
                return 0;

        if (nd->flags & LOOKUP_RCU) {
                // we need to grab link before we do unlazy.  And we can't skip
                // unlazy even if we fail to grab the link - cleanup needs it
                bool grabbed_link = legitimize_path(nd, link, seq);

                if (!try_to_unlazy(nd) != 0 || !grabbed_link)
                        return -ECHILD;

                if (nd_alloc_stack(nd))
                        return 0;
        }
        return -ENOMEM;
}

enum {WALK_TRAILING = 1, WALK_MORE = 2, WALK_NOFOLLOW = 4};

static const char *pick_link(struct nameidata *nd, struct path *link,
                     struct inode *inode, unsigned seq, int flags)
{
        struct saved *last;
        const char *res;
        int error = reserve_stack(nd, link, seq);

        if (unlikely(error)) {
                if (!(nd->flags & LOOKUP_RCU))
                        path_put(link);
                return ERR_PTR(error);
        }
        last = nd->stack + nd->depth++;
        last->link = *link;
        clear_delayed_call(&last->done);
        last->seq = seq;

        if (flags & WALK_TRAILING) {
                error = may_follow_link(nd, inode);
                if (unlikely(error))
                        return ERR_PTR(error);
        }

        if (unlikely(nd->flags & LOOKUP_NO_SYMLINKS) ||
                        unlikely(link->mnt->mnt_flags & MNT_NOSYMFOLLOW))
                return ERR_PTR(-ELOOP);

        if (!(nd->flags & LOOKUP_RCU)) {
                touch_atime(&last->link);
                cond_resched();
        } else if (atime_needs_update(&last->link, inode)) {
                if (!try_to_unlazy(nd))
                        return ERR_PTR(-ECHILD);
                touch_atime(&last->link);
        }

        error = security_inode_follow_link(link->dentry, inode,
                                           nd->flags & LOOKUP_RCU);
        if (unlikely(error))
                return ERR_PTR(error);

        res = READ_ONCE(inode->i_link);
        if (!res) {
                const char * (*get)(struct dentry *, struct inode *,
                                struct delayed_call *);
                get = inode->i_op->get_link;
                if (nd->flags & LOOKUP_RCU) {
                        res = get(NULL, inode, &last->done);
                        if (res == ERR_PTR(-ECHILD) && try_to_unlazy(nd))
                                res = get(link->dentry, inode, &last->done);
                } else {
                        res = get(link->dentry, inode, &last->done);
                }
                if (!res)
                        goto all_done;
                if (IS_ERR(res))
                        return res;
        }
        if (*res == '/') {
                error = nd_jump_root(nd);
                if (unlikely(error))
                        return ERR_PTR(error);
                while (unlikely(*++res == '/'))
                        ;
        }
        if (*res)
                return res;
all_done: // pure jump
        put_link(nd);
        return NULL;
}

/*
 * Do we need to follow links? We _really_ want to be able
 * to do this check without having to look at inode->i_op,
 * so we keep a cache of "no, this doesn't need follow_link"
 * for the common case.
 */
static const char *step_into(struct nameidata *nd, int flags,
                     struct dentry *dentry, struct inode *inode, unsigned seq)
{
        struct path path;
        int err = handle_mounts(nd, dentry, &path, &inode, &seq);

        if (err < 0)
                return ERR_PTR(err);
        if (likely(!d_is_symlink(path.dentry)) ||
           ((flags & WALK_TRAILING) && !(nd->flags & LOOKUP_FOLLOW)) ||
           (flags & WALK_NOFOLLOW)) {
                /* not a symlink or should not follow */
                if (!(nd->flags & LOOKUP_RCU)) {
                        dput(nd->path.dentry);
                        if (nd->path.mnt != path.mnt)
                                mntput(nd->path.mnt);
                }
                nd->path = path;
                nd->inode = inode;
                nd->seq = seq;
                return NULL;
        }
        if (nd->flags & LOOKUP_RCU) {
                /* make sure that d_is_symlink above matches inode */
                if (read_seqcount_retry(&path.dentry->d_seq, seq))
                        return ERR_PTR(-ECHILD);
        } else {
                if (path.mnt == nd->path.mnt)
                        mntget(path.mnt);
        }
        return pick_link(nd, &path, inode, seq, flags);
}

static struct dentry *follow_dotdot_rcu(struct nameidata *nd,
                                        struct inode **inodep,
                                        unsigned *seqp)
{
        struct dentry *parent, *old;

        if (path_equal(&nd->path, &nd->root))
                goto in_root;
        if (unlikely(nd->path.dentry == nd->path.mnt->mnt_root)) {
                struct path path;
                unsigned seq;
                if (!choose_mountpoint_rcu(real_mount(nd->path.mnt),
                                           &nd->root, &path, &seq))
                        goto in_root;
                if (unlikely(nd->flags & LOOKUP_NO_XDEV))
                        return ERR_PTR(-ECHILD);
                nd->path = path;
                nd->inode = path.dentry->d_inode;
                nd->seq = seq;
                if (unlikely(read_seqretry(&mount_lock, nd->m_seq)))
                        return ERR_PTR(-ECHILD);
                /* we know that mountpoint was pinned */
        }
        old = nd->path.dentry;
        parent = old->d_parent;
        *inodep = parent->d_inode;
        *seqp = read_seqcount_begin(&parent->d_seq);
        if (unlikely(read_seqcount_retry(&old->d_seq, nd->seq)))
                return ERR_PTR(-ECHILD);
        if (unlikely(!path_connected(nd->path.mnt, parent)))
                return ERR_PTR(-ECHILD);
        return parent;
in_root:
        if (unlikely(read_seqretry(&mount_lock, nd->m_seq)))
                return ERR_PTR(-ECHILD);
        if (unlikely(nd->flags & LOOKUP_BENEATH))
                return ERR_PTR(-ECHILD);
        return NULL;
}

static struct dentry *follow_dotdot(struct nameidata *nd,
                                 struct inode **inodep,
                                 unsigned *seqp)
{
        struct dentry *parent;

        if (path_equal(&nd->path, &nd->root))
                goto in_root;
        if (unlikely(nd->path.dentry == nd->path.mnt->mnt_root)) {
                struct path path;

                if (!choose_mountpoint(real_mount(nd->path.mnt),
                                       &nd->root, &path))
                        goto in_root;
                path_put(&nd->path);
                nd->path = path;
                nd->inode = path.dentry->d_inode;
                if (unlikely(nd->flags & LOOKUP_NO_XDEV))
                        return ERR_PTR(-EXDEV);
        }
        /* rare case of legitimate dget_parent()... */
        parent = dget_parent(nd->path.dentry);
        if (unlikely(!path_connected(nd->path.mnt, parent))) {
                dput(parent);
                return ERR_PTR(-ENOENT);
        }
        *seqp = 0;
        *inodep = parent->d_inode;
        return parent;

in_root:
        if (unlikely(nd->flags & LOOKUP_BENEATH))
                return ERR_PTR(-EXDEV);
        dget(nd->path.dentry);
        return NULL;
}

static const char *handle_dots(struct nameidata *nd, int type)
{
        if (type == LAST_DOTDOT) {
                const char *error = NULL;
                struct dentry *parent;
                struct inode *inode;
                unsigned seq;

                if (!nd->root.mnt) {
                        error = ERR_PTR(set_root(nd));
                        if (error)
                                return error;
                }
                if (nd->flags & LOOKUP_RCU)
                        parent = follow_dotdot_rcu(nd, &inode, &seq);
                else
                        parent = follow_dotdot(nd, &inode, &seq);
                if (IS_ERR(parent))
                        return ERR_CAST(parent);
                if (unlikely(!parent))
                        error = step_into(nd, WALK_NOFOLLOW,
                                         nd->path.dentry, nd->inode, nd->seq);
                else
                        error = step_into(nd, WALK_NOFOLLOW,
                                         parent, inode, seq);
                if (unlikely(error))
                        return error;

                if (unlikely(nd->flags & LOOKUP_IS_SCOPED)) {
                        /*
                         * If there was a racing rename or mount along our
                         * path, then we can't be sure that ".." hasn't jumped
                         * above nd->root (and so userspace should retry or use
                         * some fallback).
                         */
                        smp_rmb();
                        if (unlikely(__read_seqcount_retry(&mount_lock.seqcount, nd->m_seq)))
                                return ERR_PTR(-EAGAIN);
                        if (unlikely(__read_seqcount_retry(&rename_lock.seqcount, nd->r_seq)))
                                return ERR_PTR(-EAGAIN);
                }
        }
        return NULL;
}

static const char *walk_component(struct nameidata *nd, int flags)
{
        struct dentry *dentry;
        struct inode *inode;
        unsigned seq;
        /*
         * "." and ".." are special - ".." especially so because it has
         * to be able to know about the current root directory and
         * parent relationships.
         */
        if (unlikely(nd->last_type != LAST_NORM)) {
                if (!(flags & WALK_MORE) && nd->depth)
                        put_link(nd);
                return handle_dots(nd, nd->last_type);
        }
        dentry = lookup_fast(nd, &inode, &seq);
        if (IS_ERR(dentry))
                return ERR_CAST(dentry);
        if (unlikely(!dentry)) {
                dentry = lookup_slow(&nd->last, nd->path.dentry, nd->flags);
                if (IS_ERR(dentry))
                        return ERR_CAST(dentry);
        }
        if (!(flags & WALK_MORE) && nd->depth)
                put_link(nd);
        return step_into(nd, flags, dentry, inode, seq);
}

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

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

#ifdef HASH_MIX

/* Architecture provides HASH_MIX and fold_hash() in <asm/hash.h> */

#elif defined(CONFIG_64BIT)
/*
 * Register pressure in the mixing function is an issue, particularly
 * on 32-bit x86, but almost any function requires one state value and
 * one temporary.  Instead, use a function designed for two state values
 * and no temporaries.
 *
 * This function cannot create a collision in only two iterations, so

 * we have two iterations to achieve avalanche.  In those two iterations,
 * we have six layers of mixing, which is enough to spread one bit's
 * influence out to 2^6 = 64 state bits.
 *
 * Rotate constants are scored by considering either 64 one-bit input
 * deltas or 64*63/2 = 2016 two-bit input deltas, and finding the
 * probability of that delta causing a change to each of the 128 output
 * bits, using a sample of random initial states.
 *
 * The Shannon entropy of the computed probabilities is then summed
 * to produce a score.  Ideally, any input change has a 50% chance of
 * toggling any given output bit.
 *
 * Mixing scores (in bits) for (12,45):
 * Input delta: 1-bit      2-bit
 * 1 round:     713.3    42542.6
 * 2 rounds:   2753.7   140389.8
 * 3 rounds:   5954.1   233458.2
 * 4 rounds:   7862.6   256672.2
 * Perfect:    8192     258048
 *            (64*128) (64*63/2 * 128)
 */
#define HASH_MIX(x, y, a)       \
        (       x ^= (a),       \
        y ^= x, x = rol64(x,12),\
        x += y, y = rol64(y,45),\
        y *= 9                  )

/*
 * Fold two longs into one 32-bit hash value.  This must be fast, but
 * latency isn't quite as critical, as there is a fair bit of additional
 * work done before the hash value is used.
 */
static inline unsigned int fold_hash(unsigned long x, unsigned long y)
{
        y ^= x * GOLDEN_RATIO_64;
        y *= GOLDEN_RATIO_64;
        return y >> 32;
}

#else   /* 32-bit case */

/*
 * Mixing scores (in bits) for (7,20):
 * Input delta: 1-bit      2-bit
 * 1 round:     330.3     9201.6
 * 2 rounds:   1246.4    25475.4
 * 3 rounds:   1907.1    31295.1
 * 4 rounds:   2042.3    31718.6
 * Perfect:    2048      31744
 *            (32*64)   (32*31/2 * 64)
 */
#define HASH_MIX(x, y, a)       \
        (       x ^= (a),       \
        y ^= x, x = rol32(x, 7),\
        x += y, y = rol32(y,20),\
        y *= 9                  )

static inline unsigned int fold_hash(unsigned long x, unsigned long y)
{
        /* Use arch-optimized multiply if one exists */
        return __hash_32(y ^ __hash_32(x));
}

#endif

/*
 * Return the hash of a string of known length.  This is carfully
 * designed to match hash_name(), which is the more critical function.
 * In particular, we must end by hashing a final word containing 0..7
 * payload bytes, to match the way that hash_name() iterates until it
 * finds the delimiter after the name.
 */
unsigned int full_name_hash(const void *salt, const char *name, unsigned int len)
{
        unsigned long a, x = 0, y = (unsigned long)salt;

        for (;;) {
                if (!len)
                        goto done;
                a = load_unaligned_zeropad(name);
                if (len < sizeof(unsigned long))
                        break;
                HASH_MIX(x, y, a);
                name += sizeof(unsigned long);
                len -= sizeof(unsigned long);
        }
        x ^= a & bytemask_from_count(len);
done:
        return fold_hash(x, y);
}
EXPORT_SYMBOL(full_name_hash);

/* Return the "hash_len" (hash and length) of a null-terminated string */
u64 hashlen_string(const void *salt, const char *name)
{
        unsigned long a = 0, x = 0, y = (unsigned long)salt;
        unsigned long adata, mask, len;
        const struct word_at_a_time constants = WORD_AT_A_TIME_CONSTANTS;

        len = 0;
        goto inside;

        do {
                HASH_MIX(x, y, a);
                len += sizeof(unsigned long);
inside:
                a = load_unaligned_zeropad(name+len);
        } while (!has_zero(a, &adata, &constants));

        adata = prep_zero_mask(a, adata, &constants);
        mask = create_zero_mask(adata);
        x ^= a & zero_bytemask(mask);

        return hashlen_create(fold_hash(x, y), len + find_zero(mask));
}
EXPORT_SYMBOL(hashlen_string);

/*
 * Calculate the length and hash of the path component, and
 * return the "hash_len" as the result.
 */
static inline u64 hash_name(const void *salt, const char *name)
{
        unsigned long a = 0, b, x = 0, y = (unsigned long)salt;
        unsigned long adata, bdata, mask, len;
        const struct word_at_a_time constants = WORD_AT_A_TIME_CONSTANTS;

        len = 0;
        goto inside;

        do {
                HASH_MIX(x, y, a);
                len += sizeof(unsigned long);
inside:
                a = load_unaligned_zeropad(name+len);
                b = a ^ REPEAT_BYTE('/');
        } while (!(has_zero(a, &adata, &constants) | has_zero(b, &bdata, &constants)));

        adata = prep_zero_mask(a, adata, &constants);
        bdata = prep_zero_mask(b, bdata, &constants);
        mask = create_zero_mask(adata | bdata);
        x ^= a & zero_bytemask(mask);

        return hashlen_create(fold_hash(x, y), len + find_zero(mask));
}

#else   /* !CONFIG_DCACHE_WORD_ACCESS: Slow, byte-at-a-time version */

/* Return the hash of a string of known length */
unsigned int full_name_hash(const void *salt, const char *name, unsigned int len)
{
        unsigned long hash = init_name_hash(salt);
        while (len--)
                hash = partial_name_hash((unsigned char)*name++, hash);
        return end_name_hash(hash);
}
EXPORT_SYMBOL(full_name_hash);

/* Return the "hash_len" (hash and length) of a null-terminated string */
u64 hashlen_string(const void *salt, const char *name)
{
        unsigned long hash = init_name_hash(salt);
        unsigned long len = 0, c;

        c = (unsigned char)*name;
        while (c) {
                len++;
                hash = partial_name_hash(c, hash);
                c = (unsigned char)name[len];
        }
        return hashlen_create(end_name_hash(hash), len);
}
EXPORT_SYMBOL(hashlen_string);

/*
 * We know there's a real path component here of at least
 * one character.
 */
static inline u64 hash_name(const void *salt, const char *name)
{
        unsigned long hash = init_name_hash(salt);
        unsigned long len = 0, c;

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

#endif

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

        nd->last_type = LAST_ROOT;
        nd->flags |= LOOKUP_PARENT;
        if (IS_ERR(name))
                return PTR_ERR(name);
        while (*name=='/')
                name++;
        if (!*name) {
                nd->dir_mode = 0; // short-circuit the 'hardening' idiocy
                return 0;
        }

        /* At this point we know we have a real path component. */
        for(;;) {
                struct user_namespace *mnt_userns;
                const char *link;
                u64 hash_len;
                int type;

                mnt_userns = mnt_user_ns(nd->path.mnt);
                err = may_lookup(mnt_userns, nd);
                if (err)
                        return err;

                hash_len = hash_name(nd->path.dentry, name);

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

                nd->last.hash_len = hash_len;
                nd->last.name = name;
                nd->last_type = type;

                name += hashlen_len(hash_len);
                if (!*name)
                        goto OK;
                /*
                 * If it wasn't NUL, we know it was '/'. Skip that
                 * slash, and continue until no more slashes.
                 */
                do {
                        name++;
                } while (unlikely(*name == '/'));
                if (unlikely(!*name)) {
OK:
                        /* pathname or trailing symlink, done */
                        if (!depth) {
                                nd->dir_uid = i_uid_into_mnt(mnt_userns, nd->inode);
                                nd->dir_mode = nd->inode->i_mode;
                                nd->flags &= ~LOOKUP_PARENT;
                                return 0;
                        }
                        /* last component of nested symlink */
                        name = nd->stack[--depth].name;
                        link = walk_component(nd, 0);
                } else {
                        /* not the last component */
                        link = walk_component(nd, WALK_MORE);
                }
                if (unlikely(link)) {
                        if (IS_ERR(link))
                                return PTR_ERR(link);
                        /* a symlink to follow */
                        nd->stack[depth++].name = name;
                        name = link;
                        continue;
                }
                if (unlikely(!d_can_lookup(nd->path.dentry))) {
                        if (nd->flags & LOOKUP_RCU) {
                                if (!try_to_unlazy(nd))
                                        return -ECHILD;
                        }
                        return -ENOTDIR;
                }
        }
}

/* must be paired with terminate_walk() */
static const char *path_init(struct nameidata *nd, unsigned flags)
{
        int error;
        const char *s = nd->name->name;

        /* LOOKUP_CACHED requires RCU, ask caller to retry */
        if ((flags & (LOOKUP_RCU | LOOKUP_CACHED)) == LOOKUP_CACHED)
                return ERR_PTR(-EAGAIN);

        if (!*s)
                flags &= ~LOOKUP_RCU;
        if (flags & LOOKUP_RCU)
                rcu_read_lock();

        nd->flags = flags;
        nd->state |= ND_JUMPED;

        nd->m_seq = __read_seqcount_begin(&mount_lock.seqcount);
        nd->r_seq = __read_seqcount_begin(&rename_lock.seqcount);
        smp_rmb();

        if (nd->state & ND_ROOT_PRESET) {
                struct dentry *root = nd->root.dentry;
                struct inode *inode = root->d_inode;
                if (*s && unlikely(!d_can_lookup(root)))
                        return ERR_PTR(-ENOTDIR);
                nd->path = nd->root;
                nd->inode = inode;
                if (flags & LOOKUP_RCU) {
                        nd->seq = read_seqcount_begin(&nd->path.dentry->d_seq);
                        nd->root_seq = nd->seq;
                } else {
                        path_get(&nd->path);
                }
                return s;
        }

        nd->root.mnt = NULL;

        /* Absolute pathname -- fetch the root (LOOKUP_IN_ROOT uses nd->dfd). */
        if (*s == '/' && !(flags & LOOKUP_IN_ROOT)) {
                error = nd_jump_root(nd);
                if (unlikely(error))
                        return ERR_PTR(error);
                return s;
        }

        /* Relative pathname -- get the starting-point it is relative to. */
        if (nd->dfd == AT_FDCWD) {
                if (flags & LOOKUP_RCU) {
                        struct fs_struct *fs = current->fs;
                        unsigned seq;

                        do {
                                seq = read_seqcount_begin(&fs->seq);
                                nd->path = fs->pwd;
                                nd->inode = nd->path.dentry->d_inode;
                                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);
                        nd->inode = nd->path.dentry->d_inode;
                }
        } else {
                /* Caller must check execute permissions on the starting path component */
                struct fd f = fdget_raw(nd->dfd);
                struct dentry *dentry;

                if (!f.file)
                        return ERR_PTR(-EBADF);

                dentry = f.file->f_path.dentry;

                if (*s && unlikely(!d_can_lookup(dentry))) {
                        fdput(f);
                        return ERR_PTR(-ENOTDIR);
                }

                nd->path = f.file->f_path;
                if (flags & LOOKUP_RCU) {
                        nd->inode = nd->path.dentry->d_inode;
                        nd->seq = read_seqcount_begin(&nd->path.dentry->d_seq);
                } else {
                        path_get(&nd->path);
                        nd->inode = nd->path.dentry->d_inode;
                }
                fdput(f);
        }

        /* For scoped-lookups we need to set the root to the dirfd as well. */
        if (flags & LOOKUP_IS_SCOPED) {
                nd->root = nd->path;
                if (flags & LOOKUP_RCU) {
                        nd->root_seq = nd->seq;
                } else {
                        path_get(&nd->root);
                        nd->state |= ND_ROOT_GRABBED;
                }
        }
        return s;
}

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

        return walk_component(nd, WALK_TRAILING);
}

static int handle_lookup_down(struct nameidata *nd)
{
        if (!(nd->flags & LOOKUP_RCU))
                dget(nd->path.dentry);
        return PTR_ERR(step_into(nd, WALK_NOFOLLOW,
                        nd->path.dentry, nd->inode, nd->seq));
}

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

        if (unlikely(flags & LOOKUP_DOWN) && !IS_ERR(s)) {
                err = handle_lookup_down(nd);
                if (unlikely(err < 0))
                        s = ERR_PTR(err);
        }

        while (!(err = link_path_walk(s, nd)) &&
               (s = lookup_last(nd)) != NULL)
                ;
        if (!err && unlikely(nd->flags & LOOKUP_MOUNTPOINT)) {
                err = handle_lookup_down(nd);
                nd->state &= ~ND_JUMPED; // no d_weak_revalidate(), please...
        }
        if (!err)
                err = complete_walk(nd);

        if (!err && nd->flags & LOOKUP_DIRECTORY)
                if (!d_can_lookup(nd->path.dentry))
                        err = -ENOTDIR;
        if (!err) {
                *path = nd->path;
                nd->path.mnt = NULL;
                nd->path.dentry = NULL;
        }
        terminate_walk(nd);
        return err;
}

int filename_lookup(int dfd, struct filename *name, unsigned flags,
                    struct path *path, struct path *root)
{
        int retval;
        struct nameidata nd;
        if (IS_ERR(name))
                return PTR_ERR(name);
        set_nameidata(&nd, dfd, name, root);
        retval = path_lookupat(&nd, flags | LOOKUP_RCU, path);
        if (unlikely(retval == -ECHILD))
                retval = path_lookupat(&nd, flags, path);
        if (unlikely(retval == -ESTALE))
                retval = path_lookupat(&nd, flags | LOOKUP_REVAL, path);

        if (likely(!retval))
                audit_inode(name, path->dentry,
                            flags & LOOKUP_MOUNTPOINT ? AUDIT_INODE_NOEVAL : 0);
        restore_nameidata();
        putname(name);
        return retval;
}

/* Returns 0 and nd will be valid on success; Retuns error, otherwise. */
static int path_parentat(struct nameidata *nd, unsigned flags,
                                struct path *parent)
{
        const char *s = path_init(nd, flags);
        int err = link_path_walk(s, nd);
        if (!err)
                err = complete_walk(nd);
        if (!err) {
                *parent = nd->path;
                nd->path.mnt = NULL;
                nd->path.dentry = NULL;
        }
        terminate_walk(nd);
        return err;
}

static struct filename *filename_parentat(int dfd, struct filename *name,
                                unsigned int flags, struct path *parent,
                                struct qstr *last, int *type)
{
        int retval;
        struct nameidata nd;

        if (IS_ERR(name))
                return name;
        set_nameidata(&nd, dfd, name, NULL);
        retval = path_parentat(&nd, flags | LOOKUP_RCU, parent);
        if (unlikely(retval == -ECHILD))
                retval = path_parentat(&nd, flags, parent);
        if (unlikely(retval == -ESTALE))
                retval = path_parentat(&nd, flags | LOOKUP_REVAL, parent);
        if (likely(!retval)) {
                *last = nd.last;
                *type = nd.last_type;
                audit_inode(name, parent->dentry, AUDIT_INODE_PARENT);
        } else {
                putname(name);
                name = ERR_PTR(retval);
        }
        restore_nameidata();
        return name;
}

/* does lookup, returns the object with parent locked */
struct dentry *kern_path_locked(const char *name, struct path *path)
{
        struct filename *filename;
        struct dentry *d;
        struct qstr last;
        int type;

        filename = filename_parentat(AT_FDCWD, getname_kernel(name), 0, path,
                                    &last, &type);
        if (IS_ERR(filename))
                return ERR_CAST(filename);
        if (unlikely(type != LAST_NORM)) {
                path_put(path);
                putname(filename);
                return ERR_PTR(-EINVAL);
        }
        inode_lock_nested(path->dentry->d_inode, I_MUTEX_PARENT);
        d = __lookup_hash(&last, path->dentry, 0);
        if (IS_ERR(d)) {
                inode_unlock(path->dentry->d_inode);
                path_put(path);
        }
        putname(filename);
        return d;
}

int kern_path(const char *name, unsigned int flags, struct path *path)
{
        return filename_lookup(AT_FDCWD, getname_kernel(name),
                               flags, path, NULL);
}
EXPORT_SYMBOL(kern_path);

/**
 * 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
 * @path: pointer to struct path to fill
 */
int vfs_path_lookup(struct dentry *dentry, struct vfsmount *mnt,
                    const char *name, unsigned int flags,
                    struct path *path)
{
        struct path root = {.mnt = mnt, .dentry = dentry};
        /* the first argument of filename_lookup() is ignored with root */
        return filename_lookup(AT_FDCWD, getname_kernel(name),
                               flags , path, &root);
}
EXPORT_SYMBOL(vfs_path_lookup);

static int lookup_one_len_common(const char *name, struct dentry *base,
                                 int len, struct qstr *this)
{
        this->name = name;
        this->len = len;
        this->hash = full_name_hash(base, name, len);
        if (!len)
                return -EACCES;

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

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

        return inode_permission(&init_user_ns, base->d_inode, MAY_EXEC);
}

/**
 * try_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
 *
 * Look up a dentry by name in the dcache, returning NULL if it does not
 * currently exist.  The function does not try to create a dentry.
 *
 * Note that this routine is purely a helper for filesystem usage and should
 * not be called by generic code.
 *
 * The caller must hold base->i_mutex.
 */
struct dentry *try_lookup_one_len(const char *name, struct dentry *base, int len)
{
        struct qstr this;
        int err;

        WARN_ON_ONCE(!inode_is_locked(base->d_inode));

        err = lookup_one_len_common(name, base, len, &this);
        if (err)
                return ERR_PTR(err);

        return lookup_dcache(&this, base, 0);
}
EXPORT_SYMBOL(try_lookup_one_len);

/**
 * 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.
 *
 * The caller must hold base->i_mutex.
 */
struct dentry *lookup_one_len(const char *name, struct dentry *base, int len)
{
        struct dentry *dentry;
        struct qstr this;
        int err;

        WARN_ON_ONCE(!inode_is_locked(base->d_inode));

        err = lookup_one_len_common(name, base, len, &this);
        if (err)
                return ERR_PTR(err);

        dentry = lookup_dcache(&this, base, 0);
        return dentry ? dentry : __lookup_slow(&this, base, 0);
}
EXPORT_SYMBOL(lookup_one_len);

/**
 * lookup_one_len_unlocked - 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.
 *
 * Unlike lookup_one_len, it should be called without the parent
 * i_mutex held, and will take the i_mutex itself if necessary.
 */
struct dentry *lookup_one_len_unlocked(const char *name,
                                       struct dentry *base, int len)
{
        struct qstr this;
        int err;
        struct dentry *ret;

        err = lookup_one_len_common(name, base, len, &this);
        if (err)
                return ERR_PTR(err);

        ret = lookup_dcache(&this, base, 0);
        if (!ret)
                ret = lookup_slow(&this, base, 0);
        return ret;
}
EXPORT_SYMBOL(lookup_one_len_unlocked);

/*
 * Like lookup_one_len_unlocked(), except that it yields ERR_PTR(-ENOENT)
 * on negatives.  Returns known positive or ERR_PTR(); that's what
 * most of the users want.  Note that pinned negative with unlocked parent
 * _can_ become positive at any time, so callers of lookup_one_len_unlocked()
 * need to be very careful; pinned positives have ->d_inode stable, so
 * this one avoids such problems.
 */
struct dentry *lookup_positive_unlocked(const char *name,
                                       struct dentry *base, int len)
{
        struct dentry *ret = lookup_one_len_unlocked(name, base, len);
        if (!IS_ERR(ret) && d_flags_negative(smp_load_acquire(&ret->d_flags))) {
                dput(ret);
                ret = ERR_PTR(-ENOENT);
        }
        return ret;
}
EXPORT_SYMBOL(lookup_positive_unlocked);

#ifdef CONFIG_UNIX98_PTYS
int path_pts(struct path *path)
{
        /* Find something mounted on "pts" in the same directory as
         * the input path.
         */
        struct dentry *parent = dget_parent(path->dentry);
        struct dentry *child;
        struct qstr this = QSTR_INIT("pts", 3);

        if (unlikely(!path_connected(path->mnt, parent))) {
                dput(parent);
                return -ENOENT;
        }
        dput(path->dentry);
        path->dentry = parent;
        child = d_hash_and_lookup(parent, &this);
        if (!child)
                return -ENOENT;

        path->dentry = child;
        dput(parent);
        follow_down(path);
        return 0;
}
#endif

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

int __check_sticky(struct user_namespace *mnt_userns, struct inode *dir,
                   struct inode *inode)
{
        kuid_t fsuid = current_fsuid();

        if (uid_eq(i_uid_into_mnt(mnt_userns, inode), fsuid))
                return 0;
        if (uid_eq(i_uid_into_mnt(mnt_userns, dir), fsuid))
                return 0;
        return !capable_wrt_inode_uidgid(mnt_userns, inode, CAP_FOWNER);
}
EXPORT_SYMBOL(__check_sticky);

/*
 *      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 the victim has an unknown uid or gid we can't change the inode.
 *  8. If we were asked to remove a directory and victim isn't one - ENOTDIR.
 *  9. If we were asked to remove a non-directory and victim isn't one - EISDIR.
 * 10. We can't remove a root or mountpoint.
 * 11. We don't allow removal of NFS sillyrenamed files; it's handled by
 *     nfs_async_unlink().
 */
static int may_delete(struct user_namespace *mnt_userns, struct inode *dir,
                      struct dentry *victim, bool isdir)
{
        struct inode *inode = d_backing_inode(victim);
        int error;

        if (d_is_negative(victim))
                return -ENOENT;
        BUG_ON(!inode);

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

        /* Inode writeback is not safe when the uid or gid are invalid. */
        if (!uid_valid(i_uid_into_mnt(mnt_userns, inode)) ||
            !gid_valid(i_gid_into_mnt(mnt_userns, inode)))
                return -EOVERFLOW;

        audit_inode_child(dir, victim, AUDIT_TYPE_CHILD_DELETE);

        error = inode_permission(mnt_userns, dir, MAY_WRITE | MAY_EXEC);
        if (error)
                return error;
        if (IS_APPEND(dir))
                return -EPERM;

        if (check_sticky(mnt_userns, dir, inode) || IS_APPEND(inode) ||
            IS_IMMUTABLE(inode) || IS_SWAPFILE(inode) ||
            HAS_UNMAPPED_ID(mnt_userns, inode))
                return -EPERM;
        if (isdir) {
                if (!d_is_dir(victim))
                        return -ENOTDIR;
                if (IS_ROOT(victim))
                        return -EBUSY;
        } else if (d_is_dir(victim))
                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 can't do it if the fs can't represent the fsuid or fsgid.
 *  4. We should have write and exec permissions on dir
 *  5. We can't do it if dir is immutable (done in permission())
 */
static inline int may_create(struct user_namespace *mnt_userns,
                             struct inode *dir, struct dentry *child)
{
        audit_inode_child(dir, child, AUDIT_TYPE_CHILD_CREATE);
        if (child->d_inode)
                return -EEXIST;
        if (IS_DEADDIR(dir))
                return -ENOENT;
        if (!fsuidgid_has_mapping(dir->i_sb, mnt_userns))
                return -EOVERFLOW;

        return inode_permission(mnt_userns, 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) {
                inode_lock_nested(p1->d_inode, I_MUTEX_PARENT);
                return NULL;
        }

        mutex_lock(&p1->d_sb->s_vfs_rename_mutex);

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

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

        inode_lock_nested(p1->d_inode, I_MUTEX_PARENT);
        inode_lock_nested(p2->d_inode, I_MUTEX_PARENT2);
        return NULL;
}
EXPORT_SYMBOL(lock_rename);

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

/**
 * vfs_create - create new file
 * @mnt_userns: user namespace of the mount the inode was found from
 * @dir:        inode of @dentry
 * @dentry:     pointer to dentry of the base directory
 * @mode:       mode of the new file
 * @want_excl:  whether the file must not yet exist
 *
 * Create a new file.
 *
 * If the inode has been found through an idmapped mount the user namespace of
 * the vfsmount must be passed through @mnt_userns. This function will then take
 * care to map the inode according to @mnt_userns before checking permissions.
 * On non-idmapped mounts or if permission checking is to be performed on the
 * raw inode simply passs init_user_ns.
 */
int vfs_create(struct user_namespace *mnt_userns, struct inode *dir,
               struct dentry *dentry, umode_t mode, bool want_excl)
{
        int error = may_create(mnt_userns, 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(mnt_userns, dir, dentry, mode, want_excl);
        if (!error)
                fsnotify_create(dir, dentry);
        return error;
}
EXPORT_SYMBOL(vfs_create);

int vfs_mkobj(struct dentry *dentry, umode_t mode,
                int (*f)(struct dentry *, umode_t, void *),
                void *arg)
{
        struct inode *dir = dentry->d_parent->d_inode;
        int error = may_create(&init_user_ns, dir, dentry);
        if (error)
                return error;

        mode &= S_IALLUGO;
        mode |= S_IFREG;
        error = security_inode_create(dir, dentry, mode);
        if (error)
                return error;
        error = f(dentry, mode, arg);
        if (!error)
                fsnotify_create(dir, dentry);
        return error;
}
EXPORT_SYMBOL(vfs_mkobj);

bool may_open_dev(const struct path *path)
{
        return !(path->mnt->mnt_flags & MNT_NODEV) &&
                !(path->mnt->mnt_sb->s_iflags & SB_I_NODEV);
}

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

        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;
                if (acc_mode & MAY_EXEC)
                        return -EACCES;
                break;
        case S_IFBLK:
        case S_IFCHR:
                if (!may_open_dev(path))
                        return -EACCES;
                fallthrough;
        case S_IFIFO:
        case S_IFSOCK:
                if (acc_mode & MAY_EXEC)
                        return -EACCES;
                flag &= ~O_TRUNC;
                break;
        case S_IFREG:
                if ((acc_mode & MAY_EXEC) && path_noexec(path))
                        return -EACCES;
                break;
        }

        error = inode_permission(mnt_userns, inode, MAY_OPEN | acc_mode);
        if (error)
                return error;

        /*
         * An append-only file must be opened in append mode for writing.