sys/kern/vfs_lookup.c

/*-
 * SPDX-License-Identifier: BSD-3-Clause
 *
 * Copyright (c) 1982, 1986, 1989, 1993
 *	The Regents of the University of California.  All rights reserved.
 * (c) UNIX System Laboratories, Inc.
 * All or some portions of this file are derived from material licensed
 * to the University of California by American Telephone and Telegraph
 * Co. or Unix System Laboratories, Inc. and are reproduced herein with
 * the permission of UNIX System Laboratories, Inc.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 3. Neither the name of the University nor the names of its contributors
 *    may be used to endorse or promote products derived from this software
 *    without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 */

#include <sys/cdefs.h>
#include "opt_capsicum.h"
#include "opt_ktrace.h"

#include <sys/param.h>
#include <sys/systm.h>
#include <sys/dirent.h>
#include <sys/kernel.h>
#include <sys/capsicum.h>
#include <sys/fcntl.h>
#include <sys/jail.h>
#include <sys/lock.h>
#include <sys/mutex.h>
#include <sys/namei.h>
#include <sys/vnode.h>
#include <sys/mount.h>
#include <sys/filedesc.h>
#include <sys/proc.h>
#include <sys/sdt.h>
#include <sys/syscallsubr.h>
#include <sys/sysctl.h>
#ifdef KTRACE
#include <sys/ktrace.h>
#endif
#ifdef INVARIANTS
#include <machine/_inttypes.h>
#endif

#include <security/audit/audit.h>
#include <security/mac/mac_framework.h>

#include <vm/uma.h>

#ifdef INVARIANTS
static void NDVALIDATE_impl(struct nameidata *, int);
#define NDVALIDATE(ndp) NDVALIDATE_impl(ndp, __LINE__)
#else
#define NDVALIDATE(ndp)
#endif

/*
 * Reset ndp to its original state.
 */
#define	NDRESET(ndp) do {						\
	NDREINIT_DBG(ndp);						\
	ndp->ni_resflags = 0;						\
	ndp->ni_cnd.cn_flags &= ~NAMEI_INTERNAL_FLAGS;			\
} while (0)
/*
 * Prepare namei() to restart. Reset components to its original state and set
 * ISRESTARTED flag which signals the underlying lookup code to change the root
 * from ABI root to actual root and prevents a further restarts.
 */
#define	NDRESTART(ndp) do {						\
	NDRESET(ndp);						\
	ndp->ni_cnd.cn_flags |= ISRESTARTED;				\
} while (0)

#ifdef KTRACE
#define	NIKTRCAPFAIL(path)	ktrcapfail(CAPFAIL_NAMEI, (path))
#else
#define	NIKTRCAPFAIL(path)
#endif

#define	NI_CAP_VIOLATION(ndp, path)	do {			\
	NIKTRCAPFAIL(path);					\
	(ndp)->ni_lcf &= ~NI_LCF_KTR_FLAGS;			\
} while (0)

SDT_PROVIDER_DEFINE(vfs);
SDT_PROBE_DEFINE4(vfs, namei, lookup, entry, "struct vnode *", "char *",
    "unsigned long", "bool");
SDT_PROBE_DEFINE4(vfs, namei, lookup, return, "int", "struct vnode *", "bool",
    "struct nameidata");

/* Allocation zone for namei. */
uma_zone_t namei_zone;

/* Placeholder vnode for mp traversal. */
static struct vnode *vp_crossmp;

static int
crossmp_vop_islocked(struct vop_islocked_args *ap)
{

	return (LK_SHARED);
}

static int
crossmp_vop_lock1(struct vop_lock1_args *ap)
{
	struct vnode *vp;
	struct lock *lk __diagused;
	int flags;

	vp = ap->a_vp;
	lk = vp->v_vnlock;
	flags = ap->a_flags;

	KASSERT((flags & (LK_SHARED | LK_NOWAIT)) == (LK_SHARED | LK_NOWAIT),
	    ("%s: invalid lock request 0x%x for crossmp", __func__, flags));

	if ((flags & LK_INTERLOCK) != 0)
		VI_UNLOCK(vp);
	LOCK_LOG_LOCK("SLOCK", &lk->lock_object, 0, 0, ap->a_file, ap->a_line);
	return (0);
}

static int
crossmp_vop_unlock(struct vop_unlock_args *ap)
{
	struct vnode *vp;
	struct lock *lk __diagused;

	vp = ap->a_vp;
	lk = vp->v_vnlock;

	LOCK_LOG_LOCK("SUNLOCK", &lk->lock_object, 0, 0, LOCK_FILE,
	    LOCK_LINE);
	return (0);
}

static struct vop_vector crossmp_vnodeops = {
	.vop_default =		&default_vnodeops,
	.vop_islocked =		crossmp_vop_islocked,
	.vop_lock1 =		crossmp_vop_lock1,
	.vop_unlock =		crossmp_vop_unlock,
};
/*
 * VFS_VOP_VECTOR_REGISTER(crossmp_vnodeops) is not used here since the vnode
 * gets allocated early. See nameiinit for the direct call below.
 */

struct nameicap_tracker {
	TAILQ_ENTRY(nameicap_tracker) nm_link;
	struct mount *mp;
};

/* Zone for cap mode tracker elements used for dotdot capability checks. */
MALLOC_DEFINE(M_NAMEITRACKER, "namei_tracker", "namei tracking for dotdot");

static void
nameiinit(void *dummy __unused)
{

	namei_zone = uma_zcreate("NAMEI", MAXPATHLEN, NULL, NULL, NULL, NULL,
	    UMA_ALIGN_PTR, 0);
	vfs_vector_op_register(&crossmp_vnodeops);
	getnewvnode("crossmp", NULL, &crossmp_vnodeops, &vp_crossmp);
	vp_crossmp->v_state = VSTATE_CONSTRUCTED;
	vp_crossmp->v_irflag |= VIRF_CROSSMP;
}
SYSINIT(vfs, SI_SUB_VFS, SI_ORDER_SECOND, nameiinit, NULL);

static int lookup_cap_dotdot = 1;
SYSCTL_INT(_vfs, OID_AUTO, lookup_cap_dotdot, CTLFLAG_RWTUN,
    &lookup_cap_dotdot, 0,
    "enables \"..\" components in path lookup in capability mode");
static int lookup_cap_dotdot_nonlocal = 1;
SYSCTL_INT(_vfs, OID_AUTO, lookup_cap_dotdot_nonlocal, CTLFLAG_RWTUN,
    &lookup_cap_dotdot_nonlocal, 0,
    "enables \"..\" components in path lookup in capability mode "
    "on non-local mount");

static int
nameicap_tracker_add(struct nameidata *ndp, struct vnode *dp)
{
	struct nameicap_tracker *nt;
	struct mount *mp;
	int error;

	if ((ndp->ni_lcf & NI_LCF_CAP_DOTDOT) == 0 || dp->v_type != VDIR)
		return (0);
	mp = NULL;
	error = VOP_GETWRITEMOUNT(dp, &mp);
	if (error != 0)
		return (error);
	nt = TAILQ_LAST(&ndp->ni_cap_tracker, nameicap_tracker_head);
	if (nt != NULL && nt->mp == mp) {
		vfs_rel(mp);
		return (0);
	}
	nt = malloc(sizeof(*nt), M_NAMEITRACKER, M_WAITOK);
	nt->mp = mp;
	error = lockmgr(&mp->mnt_renamelock, LK_SHARED | LK_NOWAIT, 0);
	if (error != 0) {
		MPASS(ndp->ni_nctrack_mnt == NULL);
		ndp->ni_nctrack_mnt = mp;
		free(nt, M_NAMEITRACKER);
		error = ERESTART;
	} else {
		TAILQ_INSERT_TAIL(&ndp->ni_cap_tracker, nt, nm_link);
	}
	return (error);
}

static void
nameicap_cleanup(struct nameidata *ndp, int error)
{
	struct nameicap_tracker *nt, *nt1;
	struct mount *mp;

	KASSERT((ndp->ni_nctrack_mnt == NULL &&
	    TAILQ_EMPTY(&ndp->ni_cap_tracker)) ||
	    (ndp->ni_lcf & NI_LCF_CAP_DOTDOT) != 0,
	    ("tracker active and not strictrelative"));

	TAILQ_FOREACH_SAFE(nt, &ndp->ni_cap_tracker, nm_link, nt1) {
		mp = nt->mp;
		lockmgr(&mp->mnt_renamelock, LK_RELEASE, 0);
		vfs_rel(mp);
		TAILQ_REMOVE(&ndp->ni_cap_tracker, nt, nm_link);
		free(nt, M_NAMEITRACKER);
	}

	mp = ndp->ni_nctrack_mnt;
	if (mp != NULL) {
		if (error == ERESTART) {
			lockmgr(&mp->mnt_renamelock, LK_EXCLUSIVE, 0);
			lockmgr(&mp->mnt_renamelock, LK_RELEASE, 0);
		}
		vfs_rel(mp);
		ndp->ni_nctrack_mnt = NULL;
	}
}

/*
 * For dotdot lookups in capability mode, disallow walking over the
 * directory no_rbeneath_dpp that was used as the starting point of
 * the lookup.  Since we take the mnt_renamelocks of all mounts we
 * ever walked over during lookup, parallel renames are disabled.
 * This prevents the situation where we circumvent walk over
 * ni_rbeneath_dpp following dotdots.
 *
 * Also allow to force failure of dotdot lookups for non-local
 * filesystems, where external agents might assist local lookups to
 * escape the compartment.
 */
static int
nameicap_check_dotdot(struct nameidata *ndp, struct vnode *dp)
{
	struct mount *mp;

	if (dp == NULL || dp->v_type != VDIR || (ndp->ni_lcf &
	    NI_LCF_STRICTREL) == 0)
		return (0);
	if (__predict_false((ndp->ni_lcf & (NI_LCF_STRICTREL_KTR |
	    NI_LCF_CAP_DOTDOT_KTR)) == NI_LCF_STRICTREL_KTR))
		NI_CAP_VIOLATION(ndp, ndp->ni_cnd.cn_pnbuf);
	if ((ndp->ni_lcf & NI_LCF_CAP_DOTDOT) == 0)
		goto violation;
	if (dp == ndp->ni_rbeneath_dpp)
		goto violation;
	mp = dp->v_mount;
	if (lookup_cap_dotdot_nonlocal == 0 && mp != NULL &&
	    (mp->mnt_flag & MNT_LOCAL) == 0)
		goto violation;
	return (0);

violation:
	if (__predict_false((ndp->ni_lcf & NI_LCF_STRICTREL_KTR) != 0))
		NI_CAP_VIOLATION(ndp, ndp->ni_cnd.cn_pnbuf);
	return (ENOTCAPABLE);
}

static void
namei_cleanup_cnp(struct componentname *cnp)
{

	uma_zfree(namei_zone, cnp->cn_pnbuf);
	cnp->cn_pnbuf = NULL;
	cnp->cn_nameptr = NULL;
}

static int
namei_handle_root(struct nameidata *ndp, struct vnode **dpp)
{
	struct componentname *cnp;

	cnp = &ndp->ni_cnd;
	if (__predict_false((ndp->ni_lcf & (NI_LCF_STRICTREL |
	    NI_LCF_STRICTREL_KTR)) != 0)) {
		if ((ndp->ni_lcf & NI_LCF_STRICTREL_KTR) != 0)
			NI_CAP_VIOLATION(ndp, cnp->cn_pnbuf);
		if ((ndp->ni_lcf & NI_LCF_STRICTREL) != 0)
			return (ENOTCAPABLE);
	}
	while (*(cnp->cn_nameptr) == '/') {
		cnp->cn_nameptr++;
		ndp->ni_pathlen--;
	}
	*dpp = ndp->ni_rootdir;
	vrefact(*dpp);
	return (0);
}

static int
namei_setup(struct nameidata *ndp, struct vnode **dpp, struct pwd **pwdp)
{
	struct componentname *cnp;
	struct thread *td;
	struct pwd *pwd;
	int error;
	bool startdir_used;

	cnp = &ndp->ni_cnd;
	td = curthread;

	startdir_used = false;
	*pwdp = NULL;
	*dpp = NULL;

#ifdef CAPABILITY_MODE
	/*
	 * In capability mode, lookups must be restricted to happen in
	 * the subtree with the root specified by the file descriptor:
	 * - The root must be real file descriptor, not the pseudo-descriptor
	 *   AT_FDCWD.
	 * - The passed path must be relative and not absolute.
	 * - If lookup_cap_dotdot is disabled, path must not contain the
	 *   '..' components.
	 * - If lookup_cap_dotdot is enabled, we verify that all '..'
	 *   components lookups result in the directories which were
	 *   previously walked by us, which prevents an escape from
	 *   the relative root.
	 */
	if ((cnp->cn_flags & NOCAPCHECK) == 0) {
		if (CAP_TRACING(td)) {
			ndp->ni_lcf |= NI_LCF_STRICTREL_KTR;
			if (ndp->ni_dirfd == AT_FDCWD)
				NI_CAP_VIOLATION(ndp, "AT_FDCWD");
		}
		if (IN_CAPABILITY_MODE(td)) {
			ndp->ni_lcf |= NI_LCF_STRICTREL;
			ndp->ni_resflags |= NIRES_STRICTREL;
			if (ndp->ni_dirfd == AT_FDCWD)
				return (ECAPMODE);
		}
	}
#endif
	error = 0;

	/*
	 * Get starting point for the translation.
	 */
	pwd = pwd_hold(td);
	/*
	 * The reference on ni_rootdir is acquired in the block below to avoid
	 * back-to-back atomics for absolute lookups.
	 */
	namei_setup_rootdir(ndp, cnp, pwd);
	ndp->ni_topdir = pwd->pwd_jdir;

	if (cnp->cn_pnbuf[0] == '/') {
		ndp->ni_resflags |= NIRES_ABS;
		error = namei_handle_root(ndp, dpp);
	} else {
		if (ndp->ni_startdir != NULL) {
			*dpp = ndp->ni_startdir;
			startdir_used = true;
		} else if (ndp->ni_dirfd == AT_FDCWD) {
			*dpp = pwd->pwd_cdir;
			vrefact(*dpp);
		} else {
			if (cnp->cn_flags & AUDITVNODE1)
				AUDIT_ARG_ATFD1(ndp->ni_dirfd);
			if (cnp->cn_flags & AUDITVNODE2)
				AUDIT_ARG_ATFD2(ndp->ni_dirfd);

			error = fgetvp_lookup(ndp, dpp);
		}
		if (error == 0 && (*dpp)->v_type != VDIR &&
		    (cnp->cn_flags & OPENNAMED) == 0 &&
		    (cnp->cn_pnbuf[0] != '\0' ||
		    (cnp->cn_flags & EMPTYPATH) == 0))
			error = ENOTDIR;
	}
	if (error == 0 && (cnp->cn_flags & RBENEATH) != 0) {
		if (cnp->cn_pnbuf[0] == '/') {
			error = ENOTCAPABLE;
		} else if ((ndp->ni_lcf & NI_LCF_STRICTREL) == 0) {
			ndp->ni_lcf |= NI_LCF_STRICTREL |
			    NI_LCF_CAP_DOTDOT;
		}
	}
	if (error == 0 && (ndp->ni_lcf & NI_LCF_STRICTREL) != 0)
		ndp->ni_rbeneath_dpp = *dpp;

	/*
	 * If we are auditing the kernel pathname, save the user pathname.
	 */
	if (AUDITING_TD(td)) {
		if (cnp->cn_flags & AUDITVNODE1)
			AUDIT_ARG_UPATH1_VP(td, ndp->ni_rootdir, *dpp, cnp->cn_pnbuf);
		if (cnp->cn_flags & AUDITVNODE2)
			AUDIT_ARG_UPATH2_VP(td, ndp->ni_rootdir, *dpp, cnp->cn_pnbuf);
	}
	if (ndp->ni_startdir != NULL && !startdir_used)
		vrele(ndp->ni_startdir);
	if (error != 0) {
		if (*dpp != NULL)
			vrele(*dpp);
		pwd_drop(pwd);
		return (error);
	}
	if (lookup_cap_dotdot != 0) {
		if ((ndp->ni_lcf & NI_LCF_STRICTREL_KTR) != 0)
			ndp->ni_lcf |= NI_LCF_CAP_DOTDOT_KTR;
		if ((ndp->ni_lcf & NI_LCF_STRICTREL) != 0)
			ndp->ni_lcf |= NI_LCF_CAP_DOTDOT;
	}
	SDT_PROBE4(vfs, namei, lookup, entry, *dpp, cnp->cn_pnbuf,
	    cnp->cn_flags, false);
	*pwdp = pwd;
	return (0);
}

static int
namei_getpath(struct nameidata *ndp)
{
	struct componentname *cnp;
	int error;

	cnp = &ndp->ni_cnd;

	/*
	 * Get a buffer for the name to be translated, and copy the
	 * name into the buffer.
	 */
	cnp->cn_pnbuf = uma_zalloc(namei_zone, M_WAITOK);
	if (ndp->ni_segflg == UIO_SYSSPACE) {
		error = copystr(ndp->ni_dirp, cnp->cn_pnbuf, MAXPATHLEN,
		    &ndp->ni_pathlen);
	} else {
		error = copyinstr(ndp->ni_dirp, cnp->cn_pnbuf, MAXPATHLEN,
		    &ndp->ni_pathlen);
	}

	return (error);
}

static int
namei_emptypath(struct nameidata *ndp)
{
	struct componentname *cnp;
	struct pwd *pwd;
	struct vnode *dp;
	int error;

	cnp = &ndp->ni_cnd;
	MPASS(*cnp->cn_pnbuf == '\0');
	MPASS((cnp->cn_flags & EMPTYPATH) != 0);
	MPASS((cnp->cn_flags & (LOCKPARENT | WANTPARENT)) == 0);

	ndp->ni_resflags |= NIRES_EMPTYPATH;
	error = namei_setup(ndp, &dp, &pwd);
	if (error != 0) {
		goto errout;
	}

	/*
	 * Usecount on dp already provided by namei_setup.
	 */
	ndp->ni_vp = dp;
	pwd_drop(pwd);
	NDVALIDATE(ndp);
	if ((cnp->cn_flags & LOCKLEAF) != 0) {
		VOP_LOCK(dp, (cnp->cn_flags & LOCKSHARED) != 0 ?
		    LK_SHARED : LK_EXCLUSIVE);
		if (VN_IS_DOOMED(dp)) {
			vput(dp);
			error = ENOENT;
			goto errout;
		}
	}
	SDT_PROBE4(vfs, namei, lookup, return, 0, ndp->ni_vp, false, ndp);
	return (0);

errout:
	SDT_PROBE4(vfs, namei, lookup, return, error, NULL, false, ndp);
	namei_cleanup_cnp(cnp);
	return (error);
}

static int __noinline
namei_follow_link(struct nameidata *ndp)
{
	char *cp;
	struct iovec aiov;
	struct uio auio;
	struct componentname *cnp;
	struct thread *td;
	int error, linklen;

	error = 0;
	cnp = &ndp->ni_cnd;
	td = curthread;

	if (ndp->ni_loopcnt++ >= MAXSYMLINKS) {
		error = ELOOP;
		goto out;
	}
#ifdef MAC
	if ((cnp->cn_flags & NOMACCHECK) == 0) {
		error = mac_vnode_check_readlink(td->td_ucred, ndp->ni_vp);
		if (error != 0)
			goto out;
	}
#endif
	if (ndp->ni_pathlen > 1)
		cp = uma_zalloc(namei_zone, M_WAITOK);
	else
		cp = cnp->cn_pnbuf;
	aiov.iov_base = cp;
	aiov.iov_len = MAXPATHLEN;
	auio.uio_iov = &aiov;
	auio.uio_iovcnt = 1;
	auio.uio_offset = 0;
	auio.uio_rw = UIO_READ;
	auio.uio_segflg = UIO_SYSSPACE;
	auio.uio_td = td;
	auio.uio_resid = MAXPATHLEN;
	error = VOP_READLINK(ndp->ni_vp, &auio, cnp->cn_cred);
	if (error != 0) {
		if (ndp->ni_pathlen > 1)
			uma_zfree(namei_zone, cp);
		goto out;
	}
	linklen = MAXPATHLEN - auio.uio_resid;
	if (linklen == 0) {
		if (ndp->ni_pathlen > 1)
			uma_zfree(namei_zone, cp);
		error = ENOENT;
		goto out;
	}
	if (linklen + ndp->ni_pathlen > MAXPATHLEN) {
		if (ndp->ni_pathlen > 1)
			uma_zfree(namei_zone, cp);
		error = ENAMETOOLONG;
		goto out;
	}
	if (ndp->ni_pathlen > 1) {
		bcopy(ndp->ni_next, cp + linklen, ndp->ni_pathlen);
		uma_zfree(namei_zone, cnp->cn_pnbuf);
		cnp->cn_pnbuf = cp;
	} else
		cnp->cn_pnbuf[linklen] = '\0';
	ndp->ni_pathlen += linklen;
out:
	return (error);
}

/*
 * Convert a pathname into a pointer to a locked vnode.
 *
 * The FOLLOW flag is set when symbolic links are to be followed
 * when they occur at the end of the name translation process.
 * Symbolic links are always followed for all other pathname
 * components other than the last.
 *
 * The segflg defines whether the name is to be copied from user
 * space or kernel space.
 *
 * Overall outline of namei:
 *
 *	copy in name
 *	get starting directory
 *	while (!done && !error) {
 *		call lookup to search path.
 *		if symbolic link, massage name in buffer and continue
 *	}
 */
int
namei(struct nameidata *ndp)
{
	struct vnode *dp;	/* the directory we are searching */
	struct componentname *cnp;
	struct thread *td;
	struct pwd *pwd;
	int error;
	enum cache_fpl_status status;

	cnp = &ndp->ni_cnd;
	td = curthread;
#ifdef INVARIANTS
	KASSERT((ndp->ni_debugflags & NAMEI_DBG_CALLED) == 0,
	    ("%s: repeated call to namei without NDREINIT", __func__));
	KASSERT(ndp->ni_debugflags == NAMEI_DBG_INITED,
	    ("%s: bad debugflags %d", __func__, ndp->ni_debugflags));
	ndp->ni_debugflags |= NAMEI_DBG_CALLED;
	if (ndp->ni_startdir != NULL)
		ndp->ni_debugflags |= NAMEI_DBG_HADSTARTDIR;
	if (cnp->cn_flags & FAILIFEXISTS) {
		KASSERT(cnp->cn_nameiop == CREATE,
		    ("%s: FAILIFEXISTS passed for op %d", __func__, cnp->cn_nameiop));
		/*
		 * The limitation below is to restrict hairy corner cases.
		 */
		KASSERT((cnp->cn_flags & (LOCKPARENT | LOCKLEAF)) == LOCKPARENT,
		    ("%s: FAILIFEXISTS must be passed with LOCKPARENT and without LOCKLEAF",
		    __func__));
	}
#endif
	ndp->ni_cnd.cn_cred = td->td_ucred;
	KASSERT(ndp->ni_resflags == 0, ("%s: garbage in ni_resflags: %x",
	    __func__, ndp->ni_resflags));
	KASSERT(cnp->cn_cred && td->td_proc, ("namei: bad cred/proc"));
	KASSERT((cnp->cn_flags & NAMEI_INTERNAL_FLAGS) == 0,
	    ("namei: unexpected flags: %#jx",
	    (uintmax_t)(cnp->cn_flags & NAMEI_INTERNAL_FLAGS)));
	if (cnp->cn_flags & NOCACHE)
		KASSERT(cnp->cn_nameiop != LOOKUP,
		    ("%s: NOCACHE passed with LOOKUP", __func__));
	MPASS(ndp->ni_startdir == NULL || ndp->ni_startdir->v_type == VDIR ||
	    ndp->ni_startdir->v_type == VBAD);

restart:
	ndp->ni_lcf = 0;
	ndp->ni_loopcnt = 0;
	ndp->ni_vp = NULL;

	error = namei_getpath(ndp);
	if (__predict_false(error != 0)) {
		namei_cleanup_cnp(cnp);
		nameicap_cleanup(ndp, error);
		SDT_PROBE4(vfs, namei, lookup, return, error, NULL,
		    false, ndp);
		return (error);
	}

	cnp->cn_nameptr = cnp->cn_pnbuf;

#ifdef KTRACE
	if (KTRPOINT(td, KTR_NAMEI)) {
		ktrnamei(cnp->cn_pnbuf);
	}
#endif
	TSNAMEI(curthread->td_proc->p_pid, cnp->cn_pnbuf);

	/*
	 * First try looking up the target without locking any vnodes.
	 *
	 * We may need to start from scratch or pick up where it left off.
	 */
	error = cache_fplookup(ndp, &status, &pwd);
	switch (status) {
	case CACHE_FPL_STATUS_UNSET:
		__assert_unreachable();
		break;
	case CACHE_FPL_STATUS_HANDLED:
		if (error == 0)
			NDVALIDATE(ndp);
		else if (__predict_false(pwd->pwd_adir != pwd->pwd_rdir &&
		    (cnp->cn_flags & ISRESTARTED) == 0)) {
			namei_cleanup_cnp(cnp);
			nameicap_cleanup(ndp, ERESTART);
			NDRESTART(ndp);
			goto restart;
		}
		return (error);
	case CACHE_FPL_STATUS_PARTIAL:
		dp = ndp->ni_startdir;
		break;
	case CACHE_FPL_STATUS_DESTROYED:
		ndp->ni_loopcnt = 0;
		error = namei_getpath(ndp);
		if (__predict_false(error != 0)) {
			namei_cleanup_cnp(cnp);
			nameicap_cleanup(ndp, error);
			return (error);
		}
		cnp->cn_nameptr = cnp->cn_pnbuf;
		/* FALLTHROUGH */
	case CACHE_FPL_STATUS_ABORTED:
		MPASS(ndp->ni_lcf == 0);
		if (*cnp->cn_pnbuf == '\0') {
			if ((cnp->cn_flags & EMPTYPATH) != 0) {
				error = namei_emptypath(ndp);
				nameicap_cleanup(ndp, error);
				return (error);
			}
			namei_cleanup_cnp(cnp);
			nameicap_cleanup(ndp, ENOENT);
			SDT_PROBE4(vfs, namei, lookup, return, ENOENT, NULL,
			    false, ndp);
			return (ENOENT);
		}
		error = namei_setup(ndp, &dp, &pwd);
		if (error != 0) {
			namei_cleanup_cnp(cnp);
			nameicap_cleanup(ndp, error);
			return (error);
		}
		break;
	}

	/*
	 * Locked lookup.
	 */
	for (;;) {
		ndp->ni_startdir = dp;
		error = vfs_lookup(ndp);
		if (error != 0) {
			uint64_t was_restarted;
			bool abi_restart;

			was_restarted = ndp->ni_cnd.cn_flags &
			    ISRESTARTED;
			abi_restart = pwd->pwd_adir != pwd->pwd_rdir &&
			    error == ENOENT && was_restarted == 0;
			if (error != ERESTART && !abi_restart)
				goto out;
			nameicap_cleanup(ndp, error);
			pwd_drop(pwd);
			namei_cleanup_cnp(cnp);
			NDRESET(ndp);
			if (abi_restart)
				was_restarted = ISRESTARTED;
			ndp->ni_cnd.cn_flags |= was_restarted;
			goto restart;
		}

		/*
		 * If not a symbolic link, we're done.
		 */
		if ((cnp->cn_flags & ISSYMLINK) == 0) {
			SDT_PROBE4(vfs, namei, lookup, return, error,
			    ndp->ni_vp, false, ndp);
			nameicap_cleanup(ndp, 0);
			pwd_drop(pwd);
			NDVALIDATE(ndp);
			return (0);
		}
		error = namei_follow_link(ndp);
		if (error != 0)
			break;
		vput(ndp->ni_vp);
		dp = ndp->ni_dvp;
		/*
		 * Check if root directory should replace current directory.
		 */
		cnp->cn_nameptr = cnp->cn_pnbuf;
		if (*(cnp->cn_nameptr) == '/') {
			/*
			 * Reset the lookup to start from the real root without
			 * origin path name reloading.
			 */
			if (__predict_false(ndp->ni_rootdir != pwd->pwd_rdir)) {
				cnp->cn_flags |= ISRESTARTED;
				ndp->ni_rootdir = pwd->pwd_rdir;
			}
			vrele(dp);
			error = namei_handle_root(ndp, &dp);
			if (error != 0)
				goto out;
		}
	}
	vput(ndp->ni_vp);
	ndp->ni_vp = NULL;
	vrele(ndp->ni_dvp);
out:
	MPASS(error != 0 && error != ERESTART);
	SDT_PROBE4(vfs, namei, lookup, return, error, NULL, false, ndp);
	namei_cleanup_cnp(cnp);
	nameicap_cleanup(ndp, error);
	pwd_drop(pwd);
	return (error);
}

static int
enforce_lkflags(struct mount *mp, int lkflags)
{

	if (mp == NULL || ((lkflags & LK_SHARED) &&
	    !(mp->mnt_kern_flag & MNTK_LOOKUP_SHARED))) {
		lkflags &= ~LK_SHARED;
		lkflags |= LK_EXCLUSIVE;
	}
	lkflags |= LK_NODDLKTREAT;
	return (lkflags);
}

static __inline int
needs_exclusive_leaf(struct mount *mp, int flags)
{

	/*
	 * Intermediate nodes can use shared locks, we only need to
	 * force an exclusive lock for leaf nodes.
	 */
	if ((flags & (ISLASTCN | LOCKLEAF)) != (ISLASTCN | LOCKLEAF))
		return (0);

	/* Always use exclusive locks if LOCKSHARED isn't set. */
	if (!(flags & LOCKSHARED))
		return (1);

	/*
	 * For lookups during open(), if the mount point supports
	 * extended shared operations, then use a shared lock for the
	 * leaf node, otherwise use an exclusive lock.
	 */
	if ((flags & ISOPEN) != 0)
		return (!MNT_EXTENDED_SHARED(mp));

	/*
	 * Lookup requests outside of open() that specify LOCKSHARED
	 * only need a shared lock on the leaf vnode.
	 */
	return (0);
}

/*
 * Various filesystems expect to be able to copy a name component with length
 * bounded by NAME_MAX into a directory entry buffer of size MAXNAMLEN.  Make
 * sure that these are the same size.
 */
_Static_assert(MAXNAMLEN == NAME_MAX,
    "MAXNAMLEN and NAME_MAX have different values");

static int __noinline
vfs_lookup_degenerate(struct nameidata *ndp, struct vnode *dp, int wantparent)
{
	struct componentname *cnp;
	struct mount *mp;
	int error;

	cnp = &ndp->ni_cnd;

	cnp->cn_flags |= ISLASTCN;

	mp = atomic_load_ptr(&dp->v_mount);
	if (needs_exclusive_leaf(mp, cnp->cn_flags)) {
		cnp->cn_lkflags &= ~LK_SHARED;
		cnp->cn_lkflags |= LK_EXCLUSIVE;
	}

	vn_lock(dp, enforce_lkflags(mp, cnp->cn_lkflags | LK_RETRY));

	if (dp->v_type != VDIR) {
		error = ENOTDIR;
		goto bad;
	}
	if (cnp->cn_nameiop != LOOKUP) {
		error = EISDIR;
		goto bad;
	}
	if (wantparent) {
		ndp->ni_dvp = dp;
		VREF(dp);
	}
	ndp->ni_vp = dp;
	cnp->cn_namelen = 0;

	if (cnp->cn_flags & AUDITVNODE1)
		AUDIT_ARG_VNODE1(dp);
	else if (cnp->cn_flags & AUDITVNODE2)
		AUDIT_ARG_VNODE2(dp);

	if (!(cnp->cn_flags & (LOCKPARENT | LOCKLEAF)))
		VOP_UNLOCK(dp);
	return (0);
bad:
	VOP_UNLOCK(dp);
	return (error);
}

struct nameidata *
vfs_lookup_nameidata(struct componentname *cnp)
{
	if ((cnp->cn_flags & NAMEILOOKUP) == 0)
		return (NULL);
	return (__containerof(cnp, struct nameidata, ni_cnd));
}

/*
 * Would a dotdot lookup relative to dvp cause this lookup to cross a jail or
 * chroot boundary?
 */
bool
vfs_lookup_isroot(struct nameidata *ndp, struct vnode *dvp)
{
	for (struct prison *pr = ndp->ni_cnd.cn_cred->cr_prison; pr != NULL;
	    pr = pr->pr_parent) {
		if (dvp == pr->pr_root)
			return (true);
	}
	return (dvp == ndp->ni_rootdir || dvp == ndp->ni_topdir ||
	    dvp == rootvnode);
}

/*
 * FAILIFEXISTS handling.
 *
 * XXX namei called with LOCKPARENT but not LOCKLEAF has the strange
 * behaviour of leaving the vnode unlocked if the target is the same
 * vnode as the parent.
 */
static int __noinline
vfs_lookup_failifexists(struct nameidata *ndp)
{
	struct componentname *cnp __diagused;

	cnp = &ndp->ni_cnd;

	MPASS((cnp->cn_flags & ISSYMLINK) == 0);
	if (ndp->ni_vp == ndp->ni_dvp)
		vrele(ndp->ni_dvp);
	else
		vput(ndp->ni_dvp);
	vrele(ndp->ni_vp);
	ndp->ni_dvp = NULL;
	ndp->ni_vp = NULL;
	NDFREE_PNBUF(ndp);
	return (EEXIST);
}

static int __noinline
vfs_lookup_cross_mount(struct nameidata *ndp)
{
	struct componentname *cnp;
	struct mount *mp;
	struct vnode *dp, *tdp;
	int error, crosslkflags;
	bool crosslock;

	cnp = &ndp->ni_cnd;
	dp = ndp->ni_vp;

	/*
	 * The vnode has been mounted on, find the root of the mounted
	 * filesystem.
	 */
	do {
		mp = dp->v_mountedhere;
		ASSERT_VOP_LOCKED(dp, __func__);
		VNPASS((vn_irflag_read(dp) & VIRF_MOUNTPOINT) != 0 && mp != NULL, dp);

		crosslock = (dp->v_vflag & VV_CROSSLOCK) != 0;
		crosslkflags = enforce_lkflags(mp, cnp->cn_lkflags);
		if (__predict_false(crosslock)) {
			/*
			 * We are going to be holding the vnode lock, which
			 * in this case is shared by the root vnode of the
			 * filesystem mounted at mp, across the call to
			 * VFS_ROOT().  Make the situation clear to the
			 * filesystem by passing LK_CANRECURSE if the
			 * lock is held exclusive, or by clearinng
			 * LK_NODDLKTREAT to allow recursion on the shared
			 * lock in the presence of an exclusive waiter.
			 */
			if (VOP_ISLOCKED(dp) == LK_EXCLUSIVE) {
				crosslkflags &= ~LK_SHARED;
				crosslkflags |= LK_EXCLUSIVE | LK_CANRECURSE;
			} else if ((crosslkflags & LK_EXCLUSIVE) != 0) {
				error = vn_lock(dp, LK_UPGRADE);
				if (error != 0) {
					MPASS(error == ENOENT);
					vrele(dp);
					if (dp != ndp->ni_dvp)
						vput(ndp->ni_dvp);
					else
						vrele(ndp->ni_dvp);
					break;
				}
				if (dp->v_mountedhere != mp) {
					/*
					 * Note that we rely on the
					 * VIRF_MOUNTPOINT loop condition to
					 * ensure we stop iterating if dp is
					 * no longer a mountpoint at all.
					 */
					continue;
				}
			} else
				crosslkflags &= ~LK_NODDLKTREAT;
		}
		if (vfs_busy(mp, 0) != 0)
			continue;
		if (__predict_true(!crosslock))
			vput(dp);
		if (dp != ndp->ni_dvp)
			vput(ndp->ni_dvp);
		else
			vrele(ndp->ni_dvp);
		vrefact(vp_crossmp);
		ndp->ni_dvp = vp_crossmp;
		error = VFS_ROOT(mp, crosslkflags, &tdp);
		vfs_unbusy(mp);
		if (__predict_false(crosslock))
			vput(dp);
		if (vn_lock(vp_crossmp, LK_SHARED | LK_NOWAIT))
			panic("vp_crossmp exclusively locked or reclaimed");
		if (error != 0)
			break;
		ndp->ni_vp = dp = tdp;
	} while ((vn_irflag_read(dp) & VIRF_MOUNTPOINT) != 0);

	return (error);
}

/*
 * Search a pathname.
 * This is a very central and rather complicated routine.
 *
 * The pathname is pointed to by cn_nameptr and is of length ni_pathlen.
 * The starting directory is taken from ni_startdir. The pathname is
 * descended until done, or a symbolic link is encountered. The cn_flags
 * has ISLASTCN or'ed if the path is completed or ISSYMLINK or'ed if a
 * symbolic link needing interpretation is encountered.
 *
 * The cn_nameiop is LOOKUP, CREATE, RENAME, or DELETE depending on
 * whether the name is to be looked up, created, renamed, or deleted.
 * When CREATE, RENAME, or DELETE is specified, information usable in
 * creating, renaming, or deleting a directory entry may be calculated.
 * If cn_flags has LOCKPARENT or'ed into it, the parent directory is returned
 * locked. If it has WANTPARENT or'ed into it, the parent directory is
 * returned unlocked. Otherwise the parent directory is not returned. If
 * the target of the pathname exists and LOCKLEAF is or'ed into the cn_flags
 * the target is returned locked, otherwise it is returned unlocked.
 *
 * Overall outline of lookup:
 *
 *	handle degenerate case where name is null string
 *
 * dirloop:
 *	identify next component of name at ndp->ni_cnd.cn_nameptr
 *	handle .. special cases related to capabilities, chroot, jail
 *	if .. and crossing mount points and on mounted filesys, find parent
 *	call VOP_LOOKUP routine for next component name
 *	    directory vnode returned in ni_dvp, unlocked unless LOCKPARENT set
 *	    component vnode returned in ni_vp (if it exists), locked.
 *	if result vnode is mounted on and crossing mount points,
 *	    find mounted on vnode
 *	if more components of name, do next level at dirloop
 *	if VOP_LOOKUP returns ERELOOKUP, repeat the same level at dirloop
 *	return the answer in ni_vp, locked if LOCKLEAF set
 *	    if LOCKPARENT set, return locked parent in ni_dvp
 *	    if WANTPARENT set, return unlocked parent in ni_dvp
 */
int
vfs_lookup(struct nameidata *ndp)
{
	char *cp;			/* pointer into pathname argument */
	char *prev_ni_next;		/* saved ndp->ni_next */
	char *nulchar;			/* location of '\0' in cn_pnbuf */
	char *lastchar;			/* location of the last character */
	struct vnode *dp = NULL;	/* the directory we are searching */
	struct vnode *tdp;		/* saved dp */
	size_t prev_ni_pathlen;		/* saved ndp->ni_pathlen */
	int docache;			/* == 0 do not cache last component */
	int wantparent;			/* 1 => wantparent or lockparent flag */
	int rdonly;			/* lookup read-only flag bit */
	int error = 0;
	int relookup = 0;		/* do not consume the path component */
	struct componentname *cnp = &ndp->ni_cnd;
	int lkflags_save;
	int ni_dvp_unlocked;

	/*
	 * Setup: break out flag bits into variables.
	 */
	ni_dvp_unlocked = 0;
	wantparent = cnp->cn_flags & (LOCKPARENT | WANTPARENT);
	KASSERT(cnp->cn_nameiop == LOOKUP || wantparent,
	    ("CREATE, DELETE, RENAME require LOCKPARENT or WANTPARENT."));
	/*
	 * When set to zero, docache causes the last component of the
	 * pathname to be deleted from the cache and the full lookup
	 * of the name to be done (via VOP_CACHEDLOOKUP()). Often
	 * filesystems need some pre-computed values that are made
	 * during the full lookup, for instance UFS sets dp->i_offset.
	 *
	 * The docache variable is set to zero when requested by the
	 * NOCACHE flag and for all modifying operations except CREATE.
	 */
	docache = (cnp->cn_flags & NOCACHE) ^ NOCACHE;
	if (cnp->cn_nameiop == DELETE ||
	    (wantparent && cnp->cn_nameiop != CREATE &&
	     cnp->cn_nameiop != LOOKUP))
		docache = 0;
	rdonly = cnp->cn_flags & RDONLY;
	cnp->cn_flags &= ~ISSYMLINK;
	ndp->ni_dvp = NULL;

	cnp->cn_lkflags = LK_SHARED;
	dp = ndp->ni_startdir;
	ndp->ni_startdir = NULLVP;

	/*
	 * Leading slashes, if any, are supposed to be skipped by the caller.
	 */
	MPASS(cnp->cn_nameptr[0] != '/');

	/*
	 * Check for degenerate name (e.g. / or "") which is a way of talking
	 * about a directory, e.g. like "/." or ".".
	 */
	if (__predict_false(cnp->cn_nameptr[0] == '\0')) {
		error = vfs_lookup_degenerate(ndp, dp, wantparent);
		if (error == 0)
			goto success_right_lock;
		goto bad_unlocked;
	}

	/*
	 * Nul-out trailing slashes (e.g., "foo///" -> "foo").
	 *
	 * This must be done before VOP_LOOKUP() because some fs's don't know
	 * about trailing slashes.  Remember if there were trailing slashes to
	 * handle symlinks, existing non-directories and non-existing files
	 * that won't be directories specially later.
	 */
	MPASS(ndp->ni_pathlen >= 2);
	lastchar = &cnp->cn_nameptr[ndp->ni_pathlen - 2];
	if (*lastchar == '/') {
		while (lastchar >= cnp->cn_pnbuf) {
			*lastchar = '\0';
			lastchar--;
			ndp->ni_pathlen--;
			if (*lastchar != '/') {
				break;
			}
		}
		cnp->cn_flags |= TRAILINGSLASH;
	}

	/*
	 * We use shared locks until we hit the parent of the last cn then
	 * we adjust based on the requesting flags.
	 */
	vn_lock(dp,
	    enforce_lkflags(dp->v_mount, cnp->cn_lkflags | LK_RETRY));

dirloop:
	/*
	 * Search a new directory.
	 *
	 * The last component of the filename is left accessible via
	 * cnp->cn_nameptr. It has to be freed with a call to NDFREE*.
	 *
	 * Store / as a temporary sentinel so that we only have one character
	 * to test for. Pathnames tend to be short so this should not be
	 * resulting in cache misses.
	 */
	nulchar = &cnp->cn_nameptr[ndp->ni_pathlen - 1];
	KASSERT(*nulchar == '\0',
	    ("%s: expected nul at %p; string [%s]\n", __func__, nulchar,
	    cnp->cn_pnbuf));
	*nulchar = '/';
	for (cp = cnp->cn_nameptr; *cp != '/'; cp++) {
		KASSERT(*cp != '\0',
		    ("%s: encountered unexpected nul; string [%s]\n", __func__,
		    cnp->cn_nameptr));
		continue;
	}
	*nulchar = '\0';
	cnp->cn_namelen = cp - cnp->cn_nameptr;
	if (__predict_false(cnp->cn_namelen > NAME_MAX)) {
		error = ENAMETOOLONG;
		goto bad;
	}
	prev_ni_pathlen = ndp->ni_pathlen;
	ndp->ni_pathlen -= cnp->cn_namelen;
	KASSERT(ndp->ni_pathlen <= PATH_MAX,
	    ("%s: ni_pathlen underflow to %zd\n", __func__, ndp->ni_pathlen));
	prev_ni_next = ndp->ni_next;
	ndp->ni_next = cp;

	/*
	 * Something else should be clearing this.
	 */
	cnp->cn_flags &= ~(ISDOTDOT|ISLASTCN);

	cnp->cn_flags |= MAKEENTRY;
	if (*cp == '\0' && docache == 0)
		cnp->cn_flags &= ~MAKEENTRY;
	if (cnp->cn_namelen == 2 &&
	    cnp->cn_nameptr[1] == '.' && cnp->cn_nameptr[0] == '.')
		cnp->cn_flags |= ISDOTDOT;
	if (*ndp->ni_next == 0) {
		cnp->cn_flags |= ISLASTCN;

		if (__predict_false(cnp->cn_namelen == 1 && cnp->cn_nameptr[0] == '.' &&
		    (cnp->cn_nameiop == DELETE || cnp->cn_nameiop == RENAME))) {
			error = EINVAL;
			goto bad;
		}
	}

	error = nameicap_tracker_add(ndp, dp);
	if (error != 0)
		goto bad;

	/*
	 * Make sure degenerate names don't get here, their handling was
	 * previously found in this spot.
	 */
	MPASS(cnp->cn_nameptr[0] != '\0');

	/*
	 * Handle "..": five special cases.
	 * 0. If doing a capability lookup and lookup_cap_dotdot is
	 *    disabled, return ENOTCAPABLE.
	 * 1. Return an error if this is the last component of
	 *    the name and the operation is DELETE or RENAME.
	 * 2. If at root directory (e.g. after chroot)
	 *    or at absolute root directory
	 *    then ignore it so can't get out.
	 * 3. If this vnode is the root of a mounted
	 *    filesystem, then replace it with the
	 *    vnode which was mounted on so we take the
	 *    .. in the other filesystem.
	 * 4. If the vnode is the top directory of
	 *    the jail or chroot, don't let them out.
	 * 5. If doing a capability lookup and lookup_cap_dotdot is
	 *    enabled, return ENOTCAPABLE if the lookup would escape
	 *    from the initial file descriptor directory.
	 */
	if (cnp->cn_flags & ISDOTDOT) {
		if (__predict_false((ndp->ni_lcf & (NI_LCF_STRICTREL_KTR |
		    NI_LCF_CAP_DOTDOT_KTR)) == NI_LCF_STRICTREL_KTR))
			NI_CAP_VIOLATION(ndp, cnp->cn_pnbuf);
		if (__predict_false((ndp->ni_lcf & (NI_LCF_STRICTREL |
		    NI_LCF_CAP_DOTDOT)) == NI_LCF_STRICTREL)) {
			error = ENOTCAPABLE;
			goto bad;
		}
		if ((cnp->cn_flags & ISLASTCN) != 0 &&
		    (cnp->cn_nameiop == DELETE || cnp->cn_nameiop == RENAME)) {
			error = EINVAL;
			goto bad;
		}
		for (;;) {
			bool isroot;

			isroot = vfs_lookup_isroot(ndp, dp);
			if (__predict_false(isroot && (ndp->ni_lcf &
			    (NI_LCF_STRICTREL | NI_LCF_STRICTREL_KTR)) != 0)) {
				if ((ndp->ni_lcf & NI_LCF_STRICTREL_KTR) != 0)
					NI_CAP_VIOLATION(ndp, cnp->cn_pnbuf);
				if ((ndp->ni_lcf & NI_LCF_STRICTREL) != 0) {
					error = ENOTCAPABLE;
					goto bad;
				}
			}
			if (isroot || ((dp->v_vflag & VV_ROOT) != 0 &&
			    (cnp->cn_flags & NOCROSSMOUNT) != 0)) {
				ndp->ni_dvp = dp;
				ndp->ni_vp = dp;
				VREF(dp);
				goto nextname;
			}
			if ((dp->v_vflag & VV_ROOT) == 0)
				break;
			if (VN_IS_DOOMED(dp)) {	/* forced unmount */
				error = ENOENT;
				goto bad;
			}
			tdp = dp;
			dp = dp->v_mount->mnt_vnodecovered;
			VREF(dp);
			vput(tdp);
			vn_lock(dp,
			    enforce_lkflags(dp->v_mount, cnp->cn_lkflags |
			    LK_RETRY));
		}
	}

	/*
	 * We now have a segment name to search for, and a directory to search.
	 */
unionlookup:
#ifdef MAC
	error = mac_vnode_check_lookup(cnp->cn_cred, dp, cnp);
	if (__predict_false(error))
		goto bad;
#endif
	ndp->ni_dvp = dp;
	ndp->ni_vp = NULL;
	ASSERT_VOP_LOCKED(dp, "lookup");
	/*
	 * If we have a shared lock we may need to upgrade the lock for the
	 * last operation.
	 */
	if ((cnp->cn_flags & LOCKPARENT) && (cnp->cn_flags & ISLASTCN) &&
	    dp != vp_crossmp && VOP_ISLOCKED(dp) == LK_SHARED)
		vn_lock(dp, LK_UPGRADE|LK_RETRY);
	if (VN_IS_DOOMED(dp)) {
		error = ENOENT;
		goto bad;
	}
	/*
	 * If we're looking up the last component and we need an exclusive
	 * lock, adjust our lkflags.
	 */
	if (needs_exclusive_leaf(dp->v_mount, cnp->cn_flags))
		cnp->cn_lkflags = LK_EXCLUSIVE;
	lkflags_save = cnp->cn_lkflags;
	cnp->cn_lkflags = enforce_lkflags(dp->v_mount, cnp->cn_lkflags);
	error = VOP_LOOKUP(dp, &ndp->ni_vp, cnp);
	cnp->cn_lkflags = lkflags_save;
	if (error != 0) {
		KASSERT(ndp->ni_vp == NULL, ("leaf should be empty"));
		if ((error == ENOENT) &&
		    (dp->v_vflag & VV_ROOT) && (dp->v_mount != NULL) &&
		    (dp->v_mount->mnt_flag & MNT_UNION)) {
			tdp = dp;
			dp = dp->v_mount->mnt_vnodecovered;
			VREF(dp);
			vput(tdp);
			vn_lock(dp,
			    enforce_lkflags(dp->v_mount, cnp->cn_lkflags |
			    LK_RETRY));
			error = nameicap_tracker_add(ndp, dp);
			if (error != 0)
				goto bad;
			goto unionlookup;
		}

		if (error == ERELOOKUP) {
			vref(dp);
			ndp->ni_vp = dp;
			error = 0;
			relookup = 1;
			goto good;
		}

		if (error != EJUSTRETURN)
			goto bad;
		/*
		 * At this point, we know we're at the end of the
		 * pathname.  If creating / renaming, we can consider
		 * allowing the file or directory to be created / renamed,
		 * provided we're not on a read-only filesystem.
		 */
		if (rdonly) {
			error = EROFS;
			goto bad;
		}
		/* trailing slash only allowed for directories */
		if ((cnp->cn_flags & TRAILINGSLASH) &&
		    !(cnp->cn_flags & WILLBEDIR)) {
			error = ENOENT;
			goto bad;
		}
		if ((cnp->cn_flags & LOCKPARENT) == 0)
			VOP_UNLOCK(dp);
		/*
		 * We return with ni_vp NULL to indicate that the entry
		 * doesn't currently exist, leaving a pointer to the
		 * (possibly locked) directory vnode in ndp->ni_dvp.
		 */
		goto success;
	}

good:
	dp = ndp->ni_vp;

	/*
	 * Check for symbolic link
	 */
	if ((dp->v_type == VLNK) &&
	    ((cnp->cn_flags & FOLLOW) || (cnp->cn_flags & TRAILINGSLASH) ||
	     *ndp->ni_next == '/')) {
		cnp->cn_flags |= ISSYMLINK;
		if (VN_IS_DOOMED(dp)) {
			/*
			 * We can't know whether the directory was mounted with
			 * NOSYMFOLLOW, so we can't follow safely.
			 */
			error = ENOENT;
			goto bad2;
		}
		if (dp->v_mount->mnt_flag & MNT_NOSYMFOLLOW) {
			error = EACCES;
			goto bad2;
		}
		/*
		 * Symlink code always expects an unlocked dvp.
		 */
		if (ndp->ni_dvp != ndp->ni_vp) {
			VOP_UNLOCK(ndp->ni_dvp);
			ni_dvp_unlocked = 1;
		}
		goto success;
	}

	if ((vn_irflag_read(dp) & VIRF_MOUNTPOINT) != 0 &&
	    (cnp->cn_flags & NOCROSSMOUNT) == 0) {
		error = vfs_lookup_cross_mount(ndp);
		if (error != 0)
			goto bad_unlocked;
		/*
		 * FALLTHROUGH to nextname
		 */
		dp = ndp->ni_vp;
	}

nextname:
	/*
	 * Not a symbolic link that we will follow.  Continue with the
	 * next component if there is any; otherwise, we're done.
	 */
	KASSERT((cnp->cn_flags & ISLASTCN) || *ndp->ni_next == '/',
	    ("lookup: invalid path state."));
	if (relookup) {
		relookup = 0;
		ndp->ni_pathlen = prev_ni_pathlen;
		ndp->ni_next = prev_ni_next;
		if (ndp->ni_dvp != dp)
			vput(ndp->ni_dvp);
		else
			vrele(ndp->ni_dvp);
		goto dirloop;
	}
	if (cnp->cn_flags & ISDOTDOT) {
		error = nameicap_check_dotdot(ndp, ndp->ni_dvp);
		if (error != 0)
			goto bad2;
	}
	if (*ndp->ni_next == '/') {
		cnp->cn_nameptr = ndp->ni_next;
		while (*cnp->cn_nameptr == '/') {
			cnp->cn_nameptr++;
			ndp->ni_pathlen--;
		}
		if (ndp->ni_dvp != dp)
			vput(ndp->ni_dvp);
		else
			vrele(ndp->ni_dvp);
		goto dirloop;
	}
	/*
	 * If we're processing a path with a trailing slash,
	 * check that the end result is a directory.
	 */
	if ((cnp->cn_flags & TRAILINGSLASH) && dp->v_type != VDIR) {
		error = ENOTDIR;
		goto bad2;
	}
	/*
	 * Disallow directory write attempts on read-only filesystems.
	 */
	if (rdonly &&
	    (cnp->cn_nameiop == DELETE || cnp->cn_nameiop == RENAME)) {
		error = EROFS;
		goto bad2;
	}
	if (!wantparent) {
		ni_dvp_unlocked = 2;
		if (ndp->ni_dvp != dp)
			vput(ndp->ni_dvp);
		else
			vrele(ndp->ni_dvp);
	} else if ((cnp->cn_flags & LOCKPARENT) == 0 && ndp->ni_dvp != dp) {
		VOP_UNLOCK(ndp->ni_dvp);
		ni_dvp_unlocked = 1;
	}

	if (cnp->cn_flags & AUDITVNODE1)
		AUDIT_ARG_VNODE1(dp);
	else if (cnp->cn_flags & AUDITVNODE2)
		AUDIT_ARG_VNODE2(dp);

	if ((cnp->cn_flags & LOCKLEAF) == 0)
		VOP_UNLOCK(dp);
success:
	/*
	 * FIXME: for lookups which only cross a mount point to fetch the
	 * root vnode, ni_dvp will be set to vp_crossmp. This can be a problem
	 * if either WANTPARENT or LOCKPARENT is set.
	 */
	/*
	 * Because of shared lookup we may have the vnode shared locked, but
	 * the caller may want it to be exclusively locked.
	 */
	if (needs_exclusive_leaf(dp->v_mount, cnp->cn_flags) &&
	    VOP_ISLOCKED(dp) != LK_EXCLUSIVE) {
		vn_lock(dp, LK_UPGRADE | LK_RETRY);
		if (VN_IS_DOOMED(dp)) {
			error = ENOENT;
			goto bad2;
		}
	}
success_right_lock:
	if (ndp->ni_vp != NULL) {
		if ((cnp->cn_flags & ISDOTDOT) == 0) {
			error = nameicap_tracker_add(ndp, ndp->ni_vp);
			if (error != 0)
				goto bad2;
		}
		if ((cnp->cn_flags & (FAILIFEXISTS | ISSYMLINK)) == FAILIFEXISTS)
			return (vfs_lookup_failifexists(ndp));
	}
	return (0);

bad2:
	if (ni_dvp_unlocked != 2) {
		if (dp != ndp->ni_dvp && !ni_dvp_unlocked)
			vput(ndp->ni_dvp);
		else
			vrele(ndp->ni_dvp);
	}
bad:
	vput(dp);
bad_unlocked:
	ndp->ni_vp = NULL;
	return (error);
}

/*
 * relookup - lookup a path name component
 *    Used by lookup to re-acquire things.
 */
int
vfs_relookup(struct vnode *dvp, struct vnode **vpp, struct componentname *cnp,
    bool refstart)
{
	struct vnode *dp = NULL;		/* the directory we are searching */
	int rdonly;			/* lookup read-only flag bit */
	int error = 0;

	KASSERT(cnp->cn_flags & ISLASTCN,
	    ("relookup: Not given last component."));
	/*
	 * Setup: break out flag bits into variables.
	 */
	KASSERT((cnp->cn_flags & (LOCKPARENT | WANTPARENT)) != 0,
	    ("relookup: parent not wanted"));
	rdonly = cnp->cn_flags & RDONLY;
	cnp->cn_flags &= ~ISSYMLINK;
	dp = dvp;
	cnp->cn_lkflags = LK_EXCLUSIVE;
	vn_lock(dp, LK_EXCLUSIVE | LK_RETRY);

	/*
	 * Search a new directory.
	 *
	 * See a comment in vfs_lookup for cnp->cn_nameptr.
	 *
	 * Check for "" which represents the root directory after slash
	 * removal.
	 */
	if (cnp->cn_nameptr[0] == '\0') {
		/*
		 * Support only LOOKUP for "/" because lookup()
		 * can't succeed for CREATE, DELETE and RENAME.
		 */
		KASSERT(cnp->cn_nameiop == LOOKUP, ("nameiop must be LOOKUP"));
		KASSERT(dp->v_type == VDIR, ("dp is not a directory"));

		if (!(cnp->cn_flags & LOCKLEAF))
			VOP_UNLOCK(dp);
		*vpp = dp;
		/* XXX This should probably move to the top of function. */
		if (refstart)
			panic("lookup: SAVESTART");
		return (0);
	}

	if (cnp->cn_flags & ISDOTDOT)
		panic ("relookup: lookup on dot-dot");

	/*
	 * We now have a segment name to search for, and a directory to search.
	 */
	if ((error = VOP_LOOKUP(dp, vpp, cnp)) != 0) {
		KASSERT(*vpp == NULL, ("leaf should be empty"));
		if (error != EJUSTRETURN)
			goto bad;
		/*
		 * If creating and at end of pathname, then can consider
		 * allowing file to be created.
		 */
		if (rdonly) {
			error = EROFS;
			goto bad;
		}
		/* ASSERT(dvp == ndp->ni_startdir) */
		if (refstart)
			VREF(dvp);
		if ((cnp->cn_flags & LOCKPARENT) == 0)
			VOP_UNLOCK(dp);
		/*
		 * We return with ni_vp NULL to indicate that the entry
		 * doesn't currently exist, leaving a pointer to the
		 * (possibly locked) directory vnode in ndp->ni_dvp.
		 */
		return (0);
	}

	dp = *vpp;

	/*
	 * Disallow directory write attempts on read-only filesystems.
	 */
	if (rdonly &&
	    (cnp->cn_nameiop == DELETE || cnp->cn_nameiop == RENAME)) {
		if (dvp == dp)
			vrele(dvp);
		else
			vput(dvp);
		error = EROFS;
		goto bad;
	}
	/*
	 * Set the parent lock/ref state to the requested state.
	 */
	if ((cnp->cn_flags & LOCKPARENT) == 0 && dvp != dp)
		VOP_UNLOCK(dvp);
	/*
	 * Check for symbolic link
	 */
	KASSERT(dp->v_type != VLNK || !(cnp->cn_flags & FOLLOW),
	    ("relookup: symlink found.\n"));

	/* ASSERT(dvp == ndp->ni_startdir) */
	if (refstart)
		VREF(dvp);

	if ((cnp->cn_flags & LOCKLEAF) == 0)
		VOP_UNLOCK(dp);
	return (0);
bad:
	vput(dp);
	*vpp = NULL;
	return (error);
}

#ifdef INVARIANTS
/*
 * Validate the final state of ndp after the lookup.
 */
static void
NDVALIDATE_impl(struct nameidata *ndp, int line)
{
	struct componentname *cnp;

	cnp = &ndp->ni_cnd;
	if (cnp->cn_pnbuf == NULL)
		panic("%s: got no buf! called from %d", __func__, line);
}

#endif