Make files opened with O_PATH to not block non-forced unmount

[FreeBSD/FreeBSD.git] / 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.
 *
 *      @(#)vfs_lookup.c        8.4 (Berkeley) 2/16/94
 */

#include <sys/cdefs.h>
__FBSDID("$FreeBSD$");

#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>

#define NAMEI_DIAGNOSTIC 1
#undef NAMEI_DIAGNOSTIC

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 __unused;
        const char *file __unused;
        int flags, line __unused;

        vp = ap->a_vp;
        lk = vp->v_vnlock;
        flags = ap->a_flags;
        file = ap->a_file;
        line = ap->a_line;

        if ((flags & LK_SHARED) == 0)
                panic("invalid lock request for crossmp");

        WITNESS_CHECKORDER(&lk->lock_object, LOP_NEWORDER, file, line,
            flags & LK_INTERLOCK ? &VI_MTX(vp)->lock_object : NULL);
        WITNESS_LOCK(&lk->lock_object, 0, file, line);
        if ((flags & LK_INTERLOCK) != 0)
                VI_UNLOCK(vp);
        LOCK_LOG_LOCK("SLOCK", &lk->lock_object, 0, 0, ap->a_file, line);
        return (0);
}

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

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

        WITNESS_UNLOCK(&lk->lock_object, 0, LOCK_FILE, LOCK_LINE);
        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 {
        struct vnode *dp;
        TAILQ_ENTRY(nameicap_tracker) nm_link;
};

/* 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);
}
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 void
nameicap_tracker_add(struct nameidata *ndp, struct vnode *dp)
{
        struct nameicap_tracker *nt;
        struct componentname *cnp;

        if ((ndp->ni_lcf & NI_LCF_CAP_DOTDOT) == 0 || dp->v_type != VDIR)
                return;
        cnp = &ndp->ni_cnd;
        nt = TAILQ_LAST(&ndp->ni_cap_tracker, nameicap_tracker_head);
        if (nt != NULL && nt->dp == dp)
                return;
        nt = malloc(sizeof(*nt), M_NAMEITRACKER, M_WAITOK);
        vhold(dp);
        nt->dp = dp;
        TAILQ_INSERT_TAIL(&ndp->ni_cap_tracker, nt, nm_link);
}

static void
nameicap_cleanup_from(struct nameidata *ndp, struct nameicap_tracker *first)
{
        struct nameicap_tracker *nt, *nt1;

        nt = first;
        TAILQ_FOREACH_FROM_SAFE(nt, &ndp->ni_cap_tracker, nm_link, nt1) {
                TAILQ_REMOVE(&ndp->ni_cap_tracker, nt, nm_link);
                vdrop(nt->dp);
                free(nt, M_NAMEITRACKER);
        }
}

static void
nameicap_cleanup(struct nameidata *ndp)
{
        KASSERT(TAILQ_EMPTY(&ndp->ni_cap_tracker) ||
            (ndp->ni_lcf & NI_LCF_CAP_DOTDOT) != 0, ("not strictrelative"));
        nameicap_cleanup_from(ndp, NULL);
}

/*
 * For dotdot lookups in capability mode, only allow the component
 * lookup to succeed if the resulting directory was already traversed
 * during the operation.  This catches situations where already
 * traversed directory is moved to different parent, and then we walk
 * over it with 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 nameicap_tracker *nt;
        struct mount *mp;

        if (dp == NULL || dp->v_type != VDIR || (ndp->ni_lcf &
            NI_LCF_STRICTRELATIVE) == 0)
                return (0);
        if ((ndp->ni_lcf & NI_LCF_CAP_DOTDOT) == 0)
                return (ENOTCAPABLE);
        mp = dp->v_mount;
        if (lookup_cap_dotdot_nonlocal == 0 && mp != NULL &&
            (mp->mnt_flag & MNT_LOCAL) == 0)
                return (ENOTCAPABLE);
        TAILQ_FOREACH_REVERSE(nt, &ndp->ni_cap_tracker, nameicap_tracker_head,
            nm_link) {
                if (dp == nt->dp) {
                        nt = TAILQ_NEXT(nt, nm_link);
                        if (nt != NULL)
                                nameicap_cleanup_from(ndp, nt);
                        return (0);
                }
        }
        return (ENOTCAPABLE);
}

static void
namei_cleanup_cnp(struct componentname *cnp)
{

        uma_zfree(namei_zone, cnp->cn_pnbuf);
#ifdef DIAGNOSTIC
        cnp->cn_pnbuf = NULL;
        cnp->cn_nameptr = NULL;
#endif
}

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

        cnp = &ndp->ni_cnd;
        if ((ndp->ni_lcf & NI_LCF_STRICTRELATIVE) != 0) {
#ifdef KTRACE
                if (KTRPOINT(curthread, KTR_CAPFAIL))
                        ktrcapfail(CAPFAIL_LOOKUP, NULL, NULL);
#endif
                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 file *dfp;
        struct thread *td;
        struct pwd *pwd;
        cap_rights_t rights;
        int error;
        bool startdir_used;

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

        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 (IN_CAPABILITY_MODE(td) && (cnp->cn_flags & NOCAPCHECK) == 0) {
                ndp->ni_lcf |= NI_LCF_STRICTRELATIVE;
                ndp->ni_resflags |= NIRES_STRICTREL;
                if (ndp->ni_dirfd == AT_FDCWD) {
#ifdef KTRACE
                        if (KTRPOINT(td, KTR_CAPFAIL))
                                ktrcapfail(CAPFAIL_LOOKUP, NULL, NULL);
#endif
                        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.
         */
        ndp->ni_rootdir = pwd->pwd_rdir;
        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 {
                        rights = *ndp->ni_rightsneeded;
                        cap_rights_set_one(&rights, CAP_LOOKUP);

                        if (cnp->cn_flags & AUDITVNODE1)
                                AUDIT_ARG_ATFD1(ndp->ni_dirfd);
                        if (cnp->cn_flags & AUDITVNODE2)
                                AUDIT_ARG_ATFD2(ndp->ni_dirfd);
                        /*
                         * Effectively inlined fgetvp_rights, because
                         * we need to inspect the file as well as
                         * grabbing the vnode.  No check for O_PATH,
                         * files to implement its semantic.
                         */
                        error = fget_cap(td, ndp->ni_dirfd, &rights,
                            &dfp, &ndp->ni_filecaps);
                        if (error != 0) {
                                /*
                                 * Preserve the error; it should either be EBADF
                                 * or capability-related, both of which can be
                                 * safely returned to the caller.
                                 */
                        } else {
                                if (dfp->f_ops == &badfileops) {
                                        error = EBADF;
                                } else if (dfp->f_vnode == NULL) {
                                        error = ENOTDIR;
                                } else {
                                        *dpp = dfp->f_vnode;
                                        vref(*dpp);

                                        if ((dfp->f_flag & FSEARCH) != 0)
                                                cnp->cn_flags |= NOEXECCHECK;
                                }
                                fdrop(dfp, td);
                        }
#ifdef CAPABILITIES
                        /*
                         * If file descriptor doesn't have all rights,
                         * all lookups relative to it must also be
                         * strictly relative.
                         */
                        CAP_ALL(&rights);
                        if (!cap_rights_contains(&ndp->ni_filecaps.fc_rights,
                            &rights) ||
                            ndp->ni_filecaps.fc_fcntls != CAP_FCNTL_ALL ||
                            ndp->ni_filecaps.fc_nioctls != -1) {
                                ndp->ni_lcf |= NI_LCF_STRICTRELATIVE;
                                ndp->ni_resflags |= NIRES_STRICTREL;
                        }
#endif
                }
                if (error == 0 && (*dpp)->v_type != VDIR &&
                    (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_STRICTRELATIVE) == 0) {
                        ndp->ni_lcf |= NI_LCF_STRICTRELATIVE |
                            NI_LCF_CAP_DOTDOT;
                }
        }

        /*
         * If we are auditing the kernel pathname, save the user pathname.
         */
        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 ((ndp->ni_lcf & NI_LCF_STRICTRELATIVE) != 0 &&
            lookup_cap_dotdot != 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);
        }

        if (__predict_false(error != 0))
                return (error);

        /*
         * Don't allow empty pathnames unless EMPTYPATH is specified.
         * Caller checks for ENOENT as an indication for the empty path.
         */
        if (__predict_false(*cnp->cn_pnbuf == '\0'))
                return (ENOENT);

        cnp->cn_nameptr = cnp->cn_pnbuf;
        return (0);
}

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);

        error = namei_setup(ndp, &dp, &pwd);
        if (error != 0) {
                namei_cleanup_cnp(cnp);
                goto errout;
        }

        ndp->ni_vp = dp;
        vref(dp);
        namei_cleanup_cnp(cnp);
        pwd_drop(pwd);
        ndp->ni_resflags |= NIRES_EMPTYPATH;
        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);
        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)
{
        char *cp;               /* pointer into pathname argument */
        struct vnode *dp;       /* the directory we are searching */
        struct iovec aiov;              /* uio for reading symbolic links */
        struct componentname *cnp;
        struct thread *td;
        struct pwd *pwd;
        struct uio auio;
        int error, linklen;
        enum cache_fpl_status status;

        cnp = &ndp->ni_cnd;
        td = cnp->cn_thread;
#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__));
        }
        /*
         * For NDVALIDATE.
         *
         * While NDINIT may seem like a more natural place to do it, there are
         * callers which directly modify flags past invoking init.
         */
        cnp->cn_origflags = cnp->cn_flags;
#endif
        ndp->ni_cnd.cn_cred = ndp->ni_cnd.cn_thread->td_ucred;
        KASSERT(ndp->ni_resflags == 0, ("%s: garbage in ni_resflags: %x\n",
            __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: %" PRIx64 "\n",
            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);

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

        error = namei_getpath(ndp);
        if (__predict_false(error != 0)) {
                if (error == ENOENT && (cnp->cn_flags & EMPTYPATH) != 0) 
                        return (namei_emptypath(ndp));
                namei_cleanup_cnp(cnp);
                SDT_PROBE4(vfs, namei, lookup, return, error, NULL,
                    false, ndp);
                return (error);
        }

#ifdef KTRACE
        if (KTRPOINT(td, KTR_NAMEI)) {
                KASSERT(cnp->cn_thread == curthread,
                    ("namei not using curthread"));
                ktrnamei(cnp->cn_pnbuf);
        }
#endif

        /*
         * 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);
                return (error);
        case CACHE_FPL_STATUS_PARTIAL:
                TAILQ_INIT(&ndp->ni_cap_tracker);
                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);
                        return (error);
                }
                /* FALLTHROUGH */
        case CACHE_FPL_STATUS_ABORTED:
                TAILQ_INIT(&ndp->ni_cap_tracker);
                MPASS(ndp->ni_lcf == 0);
                error = namei_setup(ndp, &dp, &pwd);
                if (error != 0) {
                        namei_cleanup_cnp(cnp);
                        return (error);
                }
                break;
        }

        /*
         * Locked lookup.
         */
        for (;;) {
                ndp->ni_startdir = dp;
                error = lookup(ndp);
                if (error != 0)
                        goto out;

                /*
                 * If not a symbolic link, we're done.
                 */
                if ((cnp->cn_flags & ISSYMLINK) == 0) {
                        SDT_PROBE4(vfs, namei, lookup, return, error,
                            (error == 0 ? ndp->ni_vp : NULL), false, ndp);
                        if ((cnp->cn_flags & (SAVENAME | SAVESTART)) == 0) {
                                namei_cleanup_cnp(cnp);
                        } else
                                cnp->cn_flags |= HASBUF;
                        nameicap_cleanup(ndp);
                        pwd_drop(pwd);
                        if (error == 0)
                                NDVALIDATE(ndp);
                        return (error);
                }
                if (ndp->ni_loopcnt++ >= MAXSYMLINKS) {
                        error = ELOOP;
                        break;
                }
#ifdef MAC
                if ((cnp->cn_flags & NOMACCHECK) == 0) {
                        error = mac_vnode_check_readlink(td->td_ucred,
                            ndp->ni_vp);
                        if (error != 0)
                                break;
                }
#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);
                        break;
                }
                linklen = MAXPATHLEN - auio.uio_resid;
                if (linklen == 0) {
                        if (ndp->ni_pathlen > 1)
                                uma_zfree(namei_zone, cp);
                        error = ENOENT;
                        break;
                }
                if (linklen + ndp->ni_pathlen > MAXPATHLEN) {
                        if (ndp->ni_pathlen > 1)
                                uma_zfree(namei_zone, cp);
                        error = ENAMETOOLONG;
                        break;
                }
                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;
                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) == '/') {
                        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);
        SDT_PROBE4(vfs, namei, lookup, return, error, NULL, false, ndp);
        namei_cleanup_cnp(cnp);
        nameicap_cleanup(ndp);
        pwd_drop(pwd);
        return (error);
}

static int
compute_cn_lkflags(struct mount *mp, int lkflags, int cnflags)
{

        if (mp == NULL || ((lkflags & LK_SHARED) &&
            (!(mp->mnt_kern_flag & MNTK_LOOKUP_SHARED) ||
            ((cnflags & ISDOTDOT) &&
            (mp->mnt_kern_flag & MNTK_LOOKUP_EXCL_DOTDOT))))) {
                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");

/*
 * Search a pathname.
 * This is a very central and rather complicated routine.
 *
 * The pathname is pointed to by ni_ptr 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 variable
 * ni_more is clear if the path is completed; it is set to one if a
 * symbolic link needing interpretation is encountered.
 *
 * The flag argument 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 flag has LOCKPARENT or'ed into it, the parent directory is returned
 * locked. If flag 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 flag
 * the target is returned locked, otherwise it is returned unlocked.
 * When creating or renaming and LOCKPARENT is specified, the target may not
 * be ".".  When deleting and LOCKPARENT is specified, the target may be ".".
 *
 * Overall outline of lookup:
 *
 * dirloop:
 *      identify next component of name at ndp->ni_ptr
 *      handle degenerate case where name is null string
 *      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
 *      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
lookup(struct nameidata *ndp)
{
        char *cp;                       /* pointer into pathname argument */
        char *prev_ni_next;             /* saved ndp->ni_next */
        struct vnode *dp = NULL;        /* the directory we are searching */
        struct vnode *tdp;              /* saved dp */
        struct mount *mp;               /* mount table entry */
        struct prison *pr;
        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 dpunlocked = 0;             /* dp has already been unlocked */
        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;
        /*
         * We use shared locks until we hit the parent of the last cn then
         * we adjust based on the requesting flags.
         */
        cnp->cn_lkflags = LK_SHARED;
        dp = ndp->ni_startdir;
        ndp->ni_startdir = NULLVP;
        vn_lock(dp,
            compute_cn_lkflags(dp->v_mount, cnp->cn_lkflags | LK_RETRY,
            cnp->cn_flags));

dirloop:
        /*
         * Search a new directory.
         *
         * The last component of the filename is left accessible via
         * cnp->cn_nameptr for callers that need the name. Callers needing
         * the name set the SAVENAME flag. When done, they assume
         * responsibility for freeing the pathname buffer.
         */
        for (cp = cnp->cn_nameptr; *cp != 0 && *cp != '/'; cp++)
                continue;
        cnp->cn_namelen = cp - cnp->cn_nameptr;
        if (cnp->cn_namelen > NAME_MAX) {
                error = ENAMETOOLONG;
                goto bad;
        }
#ifdef NAMEI_DIAGNOSTIC
        { char c = *cp;
        *cp = '\0';
        printf("{%s}: ", cnp->cn_nameptr);
        *cp = c; }
#endif
        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;

        /*
         * Replace multiple slashes by a single slash and trailing slashes
         * by a null.  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.
         */
        while (*cp == '/' && (cp[1] == '/' || cp[1] == '\0')) {
                cp++;
                ndp->ni_pathlen--;
                if (*cp == '\0') {
                        *ndp->ni_next = '\0';
                        cnp->cn_flags |= TRAILINGSLASH;
                }
        }
        ndp->ni_next = cp;

        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;
        else
                cnp->cn_flags &= ~ISDOTDOT;
        if (*ndp->ni_next == 0)
                cnp->cn_flags |= ISLASTCN;
        else
                cnp->cn_flags &= ~ISLASTCN;

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

        nameicap_tracker_add(ndp, dp);

        /*
         * Check for degenerate name (e.g. / or "")
         * which is a way of talking about a directory,
         * e.g. like "/." or ".".
         */
        if (cnp->cn_nameptr[0] == '\0') {
                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;

                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);
                /* XXX This should probably move to the top of function. */
                if (cnp->cn_flags & SAVESTART)
                        panic("lookup: SAVESTART");
                goto success;
        }

        /*
         * 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.  Checks are
         *    done by ensuring that namei() already traversed the
         *    result of dotdot lookup.
         */
        if (cnp->cn_flags & ISDOTDOT) {
                if ((ndp->ni_lcf & (NI_LCF_STRICTRELATIVE | NI_LCF_CAP_DOTDOT))
                    == NI_LCF_STRICTRELATIVE) {
#ifdef KTRACE
                        if (KTRPOINT(curthread, KTR_CAPFAIL))
                                ktrcapfail(CAPFAIL_LOOKUP, NULL, NULL);
#endif
                        error = ENOTCAPABLE;
                        goto bad;
                }
                if ((cnp->cn_flags & ISLASTCN) != 0 &&
                    (cnp->cn_nameiop == DELETE || cnp->cn_nameiop == RENAME)) {
                        error = EINVAL;
                        goto bad;
                }
                for (;;) {
                        for (pr = cnp->cn_cred->cr_prison; pr != NULL;
                             pr = pr->pr_parent)
                                if (dp == pr->pr_root)
                                        break;
                        if (dp == ndp->ni_rootdir || 
                            dp == ndp->ni_topdir || 
                            dp == rootvnode ||
                            pr != NULL ||
                            ((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,
                            compute_cn_lkflags(dp->v_mount, cnp->cn_lkflags |
                            LK_RETRY, ISDOTDOT));
                        error = nameicap_check_dotdot(ndp, dp);
                        if (error != 0) {
#ifdef KTRACE
                                if (KTRPOINT(curthread, KTR_CAPFAIL))
                                        ktrcapfail(CAPFAIL_LOOKUP, NULL, NULL);
#endif
                                goto bad;
                        }
                }
        }

        /*
         * We now have a segment name to search for, and a directory to search.
         */
unionlookup:
#ifdef MAC
        error = mac_vnode_check_lookup(cnp->cn_thread->td_ucred, dp, cnp);
        if (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;
#ifdef NAMEI_DIAGNOSTIC
        vn_printf(dp, "lookup in ");
#endif
        lkflags_save = cnp->cn_lkflags;
        cnp->cn_lkflags = compute_cn_lkflags(dp->v_mount, cnp->cn_lkflags,
            cnp->cn_flags);
        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"));
#ifdef NAMEI_DIAGNOSTIC
                printf("not found\n");
#endif
                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,
                            compute_cn_lkflags(dp->v_mount, cnp->cn_lkflags |
                            LK_RETRY, cnp->cn_flags));
                        nameicap_tracker_add(ndp, dp);
                        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.
                 */
                if (cnp->cn_flags & SAVESTART) {
                        ndp->ni_startdir = ndp->ni_dvp;
                        VREF(ndp->ni_startdir);
                }
                goto success;
        }

good:
#ifdef NAMEI_DIAGNOSTIC
        printf("found\n");
#endif
        dp = ndp->ni_vp;

        /*
         * Check to see if the vnode has been mounted on;
         * if so find the root of the mounted filesystem.
         */
        while (dp->v_type == VDIR && (mp = dp->v_mountedhere) &&
               (cnp->cn_flags & NOCROSSMOUNT) == 0) {
                if (vfs_busy(mp, 0))
                        continue;
                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, compute_cn_lkflags(mp, cnp->cn_lkflags,
                    cnp->cn_flags), &tdp);
                vfs_unbusy(mp);
                if (vn_lock(vp_crossmp, LK_SHARED | LK_NOWAIT))
                        panic("vp_crossmp exclusively locked or reclaimed");
                if (error) {
                        dpunlocked = 1;
                        goto bad2;
                }
                ndp->ni_vp = dp = tdp;
        }

        /*
         * 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;
        }

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_vp);
                if (error != 0) {
#ifdef KTRACE
                        if (KTRPOINT(curthread, KTR_CAPFAIL))
                                ktrcapfail(CAPFAIL_LOOKUP, NULL, NULL);
#endif
                        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 (cnp->cn_flags & SAVESTART) {
                ndp->ni_startdir = ndp->ni_dvp;
                VREF(ndp->ni_startdir);
        }
        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;
                }
        }
        if (ndp->ni_vp != NULL) {
                if ((cnp->cn_flags & ISDOTDOT) == 0)
                        nameicap_tracker_add(ndp, ndp->ni_vp);
                if ((cnp->cn_flags & (FAILIFEXISTS | ISSYMLINK)) == FAILIFEXISTS)
                        goto bad_eexist;
        }
        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:
        if (!dpunlocked)
                vput(dp);
        ndp->ni_vp = NULL;
        return (error);
bad_eexist:
        /*
         * 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.
         */
        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(ndp, NDF_ONLY_PNBUF);
        return (EEXIST);
}

/*
 * relookup - lookup a path name component
 *    Used by lookup to re-acquire things.
 */
int
relookup(struct vnode *dvp, struct vnode **vpp, struct componentname *cnp)
{
        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.
         *
         * The last component of the filename is left accessible via
         * cnp->cn_nameptr for callers that need the name. Callers needing
         * the name set the SAVENAME flag. When done, they assume
         * responsibility for freeing the pathname buffer.
         */
#ifdef NAMEI_DIAGNOSTIC
        printf("{%s}: ", cnp->cn_nameptr);
#endif

        /*
         * 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 (cnp->cn_flags & SAVESTART)
                        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.
         */
#ifdef NAMEI_DIAGNOSTIC
        vn_printf(dp, "search in ");
#endif
        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 (cnp->cn_flags & SAVESTART)
                        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 (cnp->cn_flags & SAVESTART)
                VREF(dvp);

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

/*
 * Free data allocated by namei(); see namei(9) for details.
 */
void
NDFREE_PNBUF(struct nameidata *ndp)
{

        if ((ndp->ni_cnd.cn_flags & HASBUF) != 0) {
                MPASS((ndp->ni_cnd.cn_flags & (SAVENAME | SAVESTART)) != 0);
                uma_zfree(namei_zone, ndp->ni_cnd.cn_pnbuf);
                ndp->ni_cnd.cn_flags &= ~HASBUF;
        }
}

/*
 * NDFREE_PNBUF replacement for callers that know there is no buffer.
 *
 * This is a hack. Preferably the VFS layer would not produce anything more
 * than it was asked to do. Unfortunately several non-LOOKUP cases can add the
 * HASBUF flag to the result. Even then an interface could be implemented where
 * the caller specifies what they expected to see in the result and what they
 * are going to take care of.
 *
 * In the meantime provide this kludge as a trivial replacement for NDFREE_PNBUF
 * calls scattered throughout the kernel where we know for a fact the flag must not
 * be seen.
 */
#ifdef INVARIANTS
void
NDFREE_NOTHING(struct nameidata *ndp)
{
        struct componentname *cnp;

        cnp = &ndp->ni_cnd;
        KASSERT(cnp->cn_nameiop == LOOKUP, ("%s: got non-LOOKUP op %d\n",
            __func__, cnp->cn_nameiop));
        KASSERT((cnp->cn_flags & (SAVENAME | HASBUF)) == 0,
            ("%s: bad flags \%" PRIx64 "\n", __func__, cnp->cn_flags));
}
#endif

void
(NDFREE)(struct nameidata *ndp, const u_int flags)
{
        int unlock_dvp;
        int unlock_vp;

        unlock_dvp = 0;
        unlock_vp = 0;

        if (!(flags & NDF_NO_FREE_PNBUF)) {
                NDFREE_PNBUF(ndp);
        }
        if (!(flags & NDF_NO_VP_UNLOCK) &&
            (ndp->ni_cnd.cn_flags & LOCKLEAF) && ndp->ni_vp)
                unlock_vp = 1;
        if (!(flags & NDF_NO_DVP_UNLOCK) &&
            (ndp->ni_cnd.cn_flags & LOCKPARENT) &&
            ndp->ni_dvp != ndp->ni_vp)
                unlock_dvp = 1;
        if (!(flags & NDF_NO_VP_RELE) && ndp->ni_vp) {
                if (unlock_vp) {
                        vput(ndp->ni_vp);
                        unlock_vp = 0;
                } else
                        vrele(ndp->ni_vp);
                ndp->ni_vp = NULL;
        }
        if (unlock_vp)
                VOP_UNLOCK(ndp->ni_vp);
        if (!(flags & NDF_NO_DVP_RELE) &&
            (ndp->ni_cnd.cn_flags & (LOCKPARENT|WANTPARENT))) {
                if (unlock_dvp) {
                        vput(ndp->ni_dvp);
                        unlock_dvp = 0;
                } else
                        vrele(ndp->ni_dvp);
                ndp->ni_dvp = NULL;
        }
        if (unlock_dvp)
                VOP_UNLOCK(ndp->ni_dvp);
        if (!(flags & NDF_NO_STARTDIR_RELE) &&
            (ndp->ni_cnd.cn_flags & SAVESTART)) {
                vrele(ndp->ni_startdir);
                ndp->ni_startdir = NULL;
        }
}

#ifdef INVARIANTS
/*
 * Validate the final state of ndp after the lookup.
 *
 * Historically filesystems were allowed to modify cn_flags. Most notably they
 * can add SAVENAME to the request, resulting in HASBUF and pushing subsequent
 * clean up to the consumer. In practice this seems to only concern != LOOKUP
 * operations.
 *
 * As a step towards stricter API contract this routine validates the state to
 * clean up. Note validation is a work in progress with the intent of becoming
 * stricter over time.
 */
#define NDMODIFYINGFLAGS (LOCKLEAF | LOCKPARENT | WANTPARENT | SAVENAME | SAVESTART | HASBUF)
void
NDVALIDATE(struct nameidata *ndp)
{
        struct componentname *cnp;
        u_int64_t used, orig;

        cnp = &ndp->ni_cnd;
        orig = cnp->cn_origflags;
        used = cnp->cn_flags;
        switch (cnp->cn_nameiop) {
        case LOOKUP:
                /*
                 * For plain lookup we require strict conformance -- nothing
                 * to clean up if it was not requested by the caller.
                 */
                orig &= NDMODIFYINGFLAGS;
                used &= NDMODIFYINGFLAGS;
                if ((orig & (SAVENAME | SAVESTART)) != 0)
                        orig |= HASBUF;
                if (orig != used) {
                        goto out_mismatch;
                }
                break;
        case CREATE:
        case DELETE:
        case RENAME:
                /*
                 * Some filesystems set SAVENAME to provoke HASBUF, accomodate
                 * for it until it gets fixed.
                 */
                orig &= NDMODIFYINGFLAGS;
                orig |= (SAVENAME | HASBUF);
                used &= NDMODIFYINGFLAGS;
                used |= (SAVENAME | HASBUF);
                if (orig != used) {
                        goto out_mismatch;
                }
                break;
        }
        return;
out_mismatch:
        panic("%s: mismatched flags for op %d: added %" PRIx64 ", "
            "removed %" PRIx64" (%" PRIx64" != %" PRIx64"; stored %" PRIx64" != %" PRIx64")",
            __func__, cnp->cn_nameiop, used & ~orig, orig &~ used,
            orig, used, cnp->cn_origflags, cnp->cn_flags);
}
#endif

/*
 * Determine if there is a suitable alternate filename under the specified
 * prefix for the specified path.  If the create flag is set, then the
 * alternate prefix will be used so long as the parent directory exists.
 * This is used by the various compatibility ABIs so that Linux binaries prefer
 * files under /compat/linux for example.  The chosen path (whether under
 * the prefix or under /) is returned in a kernel malloc'd buffer pointed
 * to by pathbuf.  The caller is responsible for free'ing the buffer from
 * the M_TEMP bucket if one is returned.
 */
int
kern_alternate_path(struct thread *td, const char *prefix, const char *path,
    enum uio_seg pathseg, char **pathbuf, int create, int dirfd)
{
        struct nameidata nd, ndroot;
        char *ptr, *buf, *cp;
        size_t len, sz;
        int error;

        buf = (char *) malloc(MAXPATHLEN, M_TEMP, M_WAITOK);
        *pathbuf = buf;

        /* Copy the prefix into the new pathname as a starting point. */
        len = strlcpy(buf, prefix, MAXPATHLEN);
        if (len >= MAXPATHLEN) {
                *pathbuf = NULL;
                free(buf, M_TEMP);
                return (EINVAL);
        }
        sz = MAXPATHLEN - len;
        ptr = buf + len;

        /* Append the filename to the prefix. */
        if (pathseg == UIO_SYSSPACE)
                error = copystr(path, ptr, sz, &len);
        else
                error = copyinstr(path, ptr, sz, &len);

        if (error) {
                *pathbuf = NULL;
                free(buf, M_TEMP);
                return (error);
        }

        /* Only use a prefix with absolute pathnames. */
        if (*ptr != '/') {
                error = EINVAL;
                goto keeporig;
        }

        if (dirfd != AT_FDCWD) {
                /*
                 * We want the original because the "prefix" is
                 * included in the already opened dirfd.
                 */
                bcopy(ptr, buf, len);
                return (0);
        }

        /*
         * We know that there is a / somewhere in this pathname.
         * Search backwards for it, to find the file's parent dir
         * to see if it exists in the alternate tree. If it does,
         * and we want to create a file (cflag is set). We don't
         * need to worry about the root comparison in this case.
         */

        if (create) {
                for (cp = &ptr[len] - 1; *cp != '/'; cp--);
                *cp = '\0';

                NDINIT(&nd, LOOKUP, NOFOLLOW, UIO_SYSSPACE, buf, td);
                error = namei(&nd);
                *cp = '/';
                if (error != 0)
                        goto keeporig;
        } else {
                NDINIT(&nd, LOOKUP, NOFOLLOW, UIO_SYSSPACE, buf, td);

                error = namei(&nd);
                if (error != 0)
                        goto keeporig;

                /*
                 * We now compare the vnode of the prefix to the one
                 * vnode asked. If they resolve to be the same, then we
                 * ignore the match so that the real root gets used.
                 * This avoids the problem of traversing "../.." to find the
                 * root directory and never finding it, because "/" resolves
                 * to the emulation root directory. This is expensive :-(
                 */
                NDINIT(&ndroot, LOOKUP, FOLLOW, UIO_SYSSPACE, prefix,
                    td);

                /* We shouldn't ever get an error from this namei(). */
                error = namei(&ndroot);
                if (error == 0) {
                        if (nd.ni_vp == ndroot.ni_vp)
                                error = ENOENT;

                        NDFREE(&ndroot, NDF_ONLY_PNBUF);
                        vrele(ndroot.ni_vp);
                }
        }

        NDFREE(&nd, NDF_ONLY_PNBUF);
        vrele(nd.ni_vp);

keeporig:
        /* If there was an error, use the original path name. */
        if (error)
                bcopy(ptr, buf, len);
        return (error);
}