/*-
 * SPDX-License-Identifier: BSD-3-Clause
 *
 * Copyright (c) 1992, 1993
 *      The Regents of the University of California.  All rights reserved.
 *
 * This code is derived from software contributed to Berkeley by
 * John Heidemann of the UCLA Ficus project.
 *
 * 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.
 *
 * Ancestors:
 *      ...and...
 */

/*
 * Null Layer
 *
 * (See mount_nullfs(8) for more information.)
 *
 * The null layer duplicates a portion of the filesystem
 * name space under a new name.  In this respect, it is
 * similar to the loopback filesystem.  It differs from
 * the loopback fs in two respects:  it is implemented using
 * a stackable layers techniques, and its "null-node"s stack above
 * all lower-layer vnodes, not just over directory vnodes.
 *
 * The null layer has two purposes.  First, it serves as a demonstration
 * of layering by proving a layer which does nothing.  (It actually
 * does everything the loopback filesystem does, which is slightly
 * more than nothing.)  Second, the null layer can serve as a prototype
 * layer.  Since it provides all necessary layer framework,
 * new filesystem layers can be created very easily be starting
 * with a null layer.
 *
 * The remainder of this man page examines the null layer as a basis
 * for constructing new layers.
 *
 *
 * INSTANTIATING NEW NULL LAYERS
 *
 * New null layers are created with mount_nullfs(8).
 * Mount_nullfs(8) takes two arguments, the pathname
 * of the lower vfs (target-pn) and the pathname where the null
 * layer will appear in the namespace (alias-pn).  After
 * the null layer is put into place, the contents
 * of target-pn subtree will be aliased under alias-pn.
 *
 *
 * OPERATION OF A NULL LAYER
 *
 * The null layer is the minimum filesystem layer,
 * simply bypassing all possible operations to the lower layer
 * for processing there.  The majority of its activity centers
 * on the bypass routine, through which nearly all vnode operations
 * pass.
 *
 * The bypass routine accepts arbitrary vnode operations for
 * handling by the lower layer.  It begins by examining vnode
 * operation arguments and replacing any null-nodes by their
 * lower-layer equivlants.  It then invokes the operation
 * on the lower layer.  Finally, it replaces the null-nodes
 * in the arguments and, if a vnode is return by the operation,
 * stacks a null-node on top of the returned vnode.
 *
 * Although bypass handles most operations, vop_getattr, vop_lock,
 * vop_unlock, vop_inactive, vop_reclaim, and vop_print are not
 * bypassed. Vop_getattr must change the fsid being returned.
 * Vop_lock and vop_unlock must handle any locking for the
 * current vnode as well as pass the lock request down.
 * Vop_inactive and vop_reclaim are not bypassed so that
 * they can handle freeing null-layer specific data. Vop_print
 * is not bypassed to avoid excessive debugging information.
 * Also, certain vnode operations change the locking state within
 * the operation (create, mknod, remove, link, rename, mkdir, rmdir,
 * and symlink). Ideally these operations should not change the
 * lock state, but should be changed to let the caller of the
 * function unlock them. Otherwise all intermediate vnode layers
 * (such as union, umapfs, etc) must catch these functions to do
 * the necessary locking at their layer.
 *
 *
 * INSTANTIATING VNODE STACKS
 *
 * Mounting associates the null layer with a lower layer,
 * effect stacking two VFSes.  Vnode stacks are instead
 * created on demand as files are accessed.
 *
 * The initial mount creates a single vnode stack for the
 * root of the new null layer.  All other vnode stacks
 * are created as a result of vnode operations on
 * this or other null vnode stacks.
 *
 * New vnode stacks come into existence as a result of
 * an operation which returns a vnode.
 * The bypass routine stacks a null-node above the new
 * vnode before returning it to the caller.
 *
 * For example, imagine mounting a null layer with
 * "mount_nullfs /usr/include /dev/layer/null".
 * Changing directory to /dev/layer/null will assign
 * the root null-node (which was created when the null layer was mounted).
 * Now consider opening "sys".  A vop_lookup would be
 * done on the root null-node.  This operation would bypass through
 * to the lower layer which would return a vnode representing
 * the UFS "sys".  Null_bypass then builds a null-node
 * aliasing the UFS "sys" and returns this to the caller.
 * Later operations on the null-node "sys" will repeat this
 * process when constructing other vnode stacks.
 *
 *
 * CREATING OTHER FILE SYSTEM LAYERS
 *
 * One of the easiest ways to construct new filesystem layers is to make
 * a copy of the null layer, rename all files and variables, and
 * then begin modifing the copy.  Sed can be used to easily rename
 * all variables.
 *
 * The umap layer is an example of a layer descended from the
 * null layer.
 *
 *
 * INVOKING OPERATIONS ON LOWER LAYERS
 *
 * There are two techniques to invoke operations on a lower layer
 * when the operation cannot be completely bypassed.  Each method
 * is appropriate in different situations.  In both cases,
 * it is the responsibility of the aliasing layer to make
 * the operation arguments "correct" for the lower layer
 * by mapping a vnode arguments to the lower layer.
 *
 * The first approach is to call the aliasing layer's bypass routine.
 * This method is most suitable when you wish to invoke the operation
 * currently being handled on the lower layer.  It has the advantage
 * that the bypass routine already must do argument mapping.
 * An example of this is null_getattrs in the null layer.
 *
 * A second approach is to directly invoke vnode operations on
 * the lower layer with the VOP_OPERATIONNAME interface.
 * The advantage of this method is that it is easy to invoke
 * arbitrary operations on the lower layer.  The disadvantage
 * is that vnode arguments must be manualy mapped.
 *
 */

#include <sys/param.h>
#include <sys/systm.h>
#include <sys/conf.h>
#include <sys/kernel.h>
#include <sys/lock.h>
#include <sys/malloc.h>
#include <sys/mount.h>
#include <sys/mutex.h>
#include <sys/namei.h>
#include <sys/proc.h>
#include <sys/smr.h>
#include <sys/sysctl.h>
#include <sys/vnode.h>
#include <sys/stat.h>

#include <fs/nullfs/null.h>

#include <vm/vm.h>
#include <vm/vm_extern.h>
#include <vm/vm_object.h>
#include <vm/vnode_pager.h>

VFS_SMR_DECLARE;

static int null_bug_bypass = 0;   /* for debugging: enables bypass printf'ing */
SYSCTL_INT(_debug, OID_AUTO, nullfs_bug_bypass, CTLFLAG_RW, 
        &null_bug_bypass, 0, "");

/*
 * Synchronize inotify flags with the lower vnode:
 * - If the upper vnode has the flag set and the lower does not, then the lower
 *   vnode is unwatched and the upper vnode does not need to go through
 *   VOP_INOTIFY.
 * - If the lower vnode is watched, then the upper vnode should go through
 *   VOP_INOTIFY, so copy the flag up.
 */
static void
null_copy_inotify(struct vnode *vp, struct vnode *lvp, short flag)
{
        if ((vn_irflag_read(vp) & flag) != 0) {
                if (__predict_false((vn_irflag_read(lvp) & flag) == 0))
                        vn_irflag_unset(vp, flag);
        } else if ((vn_irflag_read(lvp) & flag) != 0) {
                if (__predict_false((vn_irflag_read(vp) & flag) == 0))
                        vn_irflag_set(vp, flag);
        }
}

/*
 * This is the 10-Apr-92 bypass routine.
 *    This version has been optimized for speed, throwing away some
 * safety checks.  It should still always work, but it's not as
 * robust to programmer errors.
 *
 * In general, we map all vnodes going down and unmap them on the way back.
 * As an exception to this, vnodes can be marked "unmapped" by setting
 * the Nth bit in operation's vdesc_flags.
 *
 * Also, some BSD vnode operations have the side effect of vrele'ing
 * their arguments.  With stacking, the reference counts are held
 * by the upper node, not the lower one, so we must handle these
 * side-effects here.  This is not of concern in Sun-derived systems
 * since there are no such side-effects.
 *
 * This makes the following assumptions:
 * - only one returned vpp
 * - no INOUT vpp's (Sun's vop_open has one of these)
 * - the vnode operation vector of the first vnode should be used
 *   to determine what implementation of the op should be invoked
 * - all mapped vnodes are of our vnode-type (NEEDSWORK:
 *   problems on rmdir'ing mount points and renaming?)
 */
int
null_bypass(struct vop_generic_args *ap)
{
        struct vnode **this_vp_p;
        struct vnode *old_vps[VDESC_MAX_VPS];
        struct vnode **vps_p[VDESC_MAX_VPS];
        struct vnode ***vppp;
        struct vnode *lvp;
        struct vnodeop_desc *descp = ap->a_desc;
        int error, i, reles;

        if (null_bug_bypass)
                printf ("null_bypass: %s\n", descp->vdesc_name);

#ifdef DIAGNOSTIC
        /*
         * We require at least one vp.
         */
        if (descp->vdesc_vp_offsets == NULL ||
            descp->vdesc_vp_offsets[0] == VDESC_NO_OFFSET)
                panic ("null_bypass: no vp's in map");
#endif

        /*
         * Map the vnodes going in.
         * Later, we'll invoke the operation based on
         * the first mapped vnode's operation vector.
         */
        reles = descp->vdesc_flags;
        for (i = 0; i < VDESC_MAX_VPS; reles >>= 1, i++) {
                if (descp->vdesc_vp_offsets[i] == VDESC_NO_OFFSET)
                        break;   /* bail out at end of list */
                vps_p[i] = this_vp_p = VOPARG_OFFSETTO(struct vnode **,
                    descp->vdesc_vp_offsets[i], ap);

                /*
                 * We're not guaranteed that any but the first vnode
                 * are of our type.  Check for and don't map any
                 * that aren't.  (We must always map first vp or vclean fails.)
                 */
                if (i != 0 && (*this_vp_p == NULL ||
                    !null_is_nullfs_vnode(*this_vp_p))) {
                        old_vps[i] = NULL;
                } else {
                        old_vps[i] = *this_vp_p;
                        *(vps_p[i]) = NULLVPTOLOWERVP(*this_vp_p);

                        /*
                         * The upper vnode reference to the lower
                         * vnode is the only reference that keeps our
                         * pointer to the lower vnode alive.  If lower
                         * vnode is relocked during the VOP call,
                         * upper vnode might become unlocked and
                         * reclaimed, which invalidates our reference.
                         * Add a transient hold around VOP call.
                         */
                        vhold(*this_vp_p);

                        /*
                         * XXX - Several operations have the side effect
                         * of vrele'ing their vp's.  We must account for
                         * that.  (This should go away in the future.)
                         */
                        if (reles & VDESC_VP0_WILLRELE)
                                vref(*this_vp_p);
                }
        }

        /*
         * Call the operation on the lower layer
         * with the modified argument structure.
         */
        if (vps_p[0] != NULL && *vps_p[0] != NULL) {
                error = ap->a_desc->vdesc_call(ap);
        } else {
                printf("null_bypass: no map for %s\n", descp->vdesc_name);
                error = EINVAL;
        }

        /*
         * Maintain the illusion of call-by-value
         * by restoring vnodes in the argument structure
         * to their original value.
         */
        reles = descp->vdesc_flags;
        for (i = 0; i < VDESC_MAX_VPS; reles >>= 1, i++) {
                if (descp->vdesc_vp_offsets[i] == VDESC_NO_OFFSET)
                        break;   /* bail out at end of list */
                if (old_vps[i] != NULL) {
                        lvp = *(vps_p[i]);

                        /*
                         * Get rid of the transient hold on lvp.  Copy inotify
                         * flags up in case something is watching the lower
                         * layer.
                         *
                         * If lowervp was unlocked during VOP
                         * operation, nullfs upper vnode could have
                         * been reclaimed, which changes its v_vnlock
                         * back to private v_lock.  In this case we
                         * must move lock ownership from lower to
                         * upper (reclaimed) vnode.
                         */
                        if (lvp != NULL) {
                                null_copy_inotify(old_vps[i], lvp,
                                    VIRF_INOTIFY);
                                null_copy_inotify(old_vps[i], lvp,
                                    VIRF_INOTIFY_PARENT);
                                if (VOP_ISLOCKED(lvp) == LK_EXCLUSIVE &&
                                    old_vps[i]->v_vnlock != lvp->v_vnlock) {
                                        VOP_UNLOCK(lvp);
                                        VOP_LOCK(old_vps[i], LK_EXCLUSIVE |
                                            LK_RETRY);
                                }
                                vdrop(lvp);
                        }

                        *(vps_p[i]) = old_vps[i];
#if 0
                        if (reles & VDESC_VP0_WILLUNLOCK)
                                VOP_UNLOCK(*(vps_p[i]), 0);
#endif
                        if (reles & VDESC_VP0_WILLRELE)
                                vrele(*(vps_p[i]));
                }
        }

        /*
         * Map the possible out-going vpp
         * (Assumes that the lower layer always returns
         * a VREF'ed vpp unless it gets an error.)
         */
        if (descp->vdesc_vpp_offset != VDESC_NO_OFFSET && error == 0) {
                /*
                 * XXX - even though some ops have vpp returned vp's,
                 * several ops actually vrele this before returning.
                 * We must avoid these ops.
                 * (This should go away when these ops are regularized.)
                 */
                vppp = VOPARG_OFFSETTO(struct vnode ***,
                    descp->vdesc_vpp_offset, ap);
                if (*vppp != NULL)
                        error = null_nodeget(old_vps[0]->v_mount, **vppp,
                            *vppp);
        }

        return (error);
}

static int
null_add_writecount(struct vop_add_writecount_args *ap)
{
        struct vnode *lvp, *vp;
        int error;

        vp = ap->a_vp;
        lvp = NULLVPTOLOWERVP(vp);
        VI_LOCK(vp);
        /* text refs are bypassed to lowervp */
        VNASSERT(vp->v_writecount >= 0, vp, ("wrong null writecount"));
        VNASSERT(vp->v_writecount + ap->a_inc >= 0, vp,
            ("wrong writecount inc %d", ap->a_inc));
        error = VOP_ADD_WRITECOUNT(lvp, ap->a_inc);
        if (error == 0)
                vp->v_writecount += ap->a_inc;
        VI_UNLOCK(vp);
        return (error);
}

/*
 * We have to carry on the locking protocol on the null layer vnodes
 * as we progress through the tree. We also have to enforce read-only
 * if this layer is mounted read-only.
 */
static int
null_lookup(struct vop_lookup_args *ap)
{
        struct componentname *cnp = ap->a_cnp;
        struct vnode *dvp = ap->a_dvp;
        uint64_t flags = cnp->cn_flags;
        struct vnode *vp, *ldvp, *lvp;
        struct mount *mp;
        int error;

        mp = dvp->v_mount;
        if ((flags & ISLASTCN) != 0 && (mp->mnt_flag & MNT_RDONLY) != 0 &&
            (cnp->cn_nameiop == DELETE || cnp->cn_nameiop == RENAME))
                return (EROFS);
        /*
         * Although it is possible to call null_bypass(), we'll do
         * a direct call to reduce overhead
         */
        ldvp = NULLVPTOLOWERVP(dvp);
        vp = lvp = NULL;

        /*
         * Renames in the lower mounts might create an inconsistent
         * configuration where lower vnode is moved out of the directory tree
         * remounted by our null mount.
         *
         * Do not try to handle it fancy, just avoid VOP_LOOKUP() with DOTDOT
         * name which cannot be handled by the VOP.
         */
        if ((flags & ISDOTDOT) != 0) {
                struct nameidata *ndp;

                if ((ldvp->v_vflag & VV_ROOT) != 0) {
                        KASSERT((dvp->v_vflag & VV_ROOT) == 0,
                            ("ldvp %p fl %#x dvp %p fl %#x flags %#jx",
                            ldvp, ldvp->v_vflag, dvp, dvp->v_vflag,
                            (uintmax_t)flags));
                        return (ENOENT);
                }
                ndp = vfs_lookup_nameidata(cnp);
                if (ndp != NULL && vfs_lookup_isroot(ndp, ldvp))
                        return (ENOENT);
        }

        /*
         * Hold ldvp.  The reference on it, owned by dvp, is lost in
         * case of dvp reclamation, and we need ldvp to move our lock
         * from ldvp to dvp.
         */
        vhold(ldvp);

        error = VOP_LOOKUP(ldvp, &lvp, cnp);

        /*
         * VOP_LOOKUP() on lower vnode may unlock ldvp, which allows
         * dvp to be reclaimed due to shared v_vnlock.  Check for the
         * doomed state and return error.
         */
        if (VN_IS_DOOMED(dvp)) {
                if (error == 0 || error == EJUSTRETURN) {
                        if (lvp != NULL)
                                vput(lvp);
                        error = ENOENT;
                }

                /*
                 * If vgone() did reclaimed dvp before curthread
                 * relocked ldvp, the locks of dvp and ldpv are no
                 * longer shared.  In this case, relock of ldvp in
                 * lower fs VOP_LOOKUP() does not restore the locking
                 * state of dvp.  Compensate for this by unlocking
                 * ldvp and locking dvp, which is also correct if the
                 * locks are still shared.
                 */
                VOP_UNLOCK(ldvp);
                vn_lock(dvp, LK_EXCLUSIVE | LK_RETRY);
        }
        vdrop(ldvp);

        if (error == EJUSTRETURN && (flags & ISLASTCN) != 0 &&
            (mp->mnt_flag & MNT_RDONLY) != 0 &&
            (cnp->cn_nameiop == CREATE || cnp->cn_nameiop == RENAME))
                error = EROFS;

        if ((error == 0 || error == EJUSTRETURN) && lvp != NULL) {
                if (ldvp == lvp) {
                        *ap->a_vpp = dvp;
                        vref(dvp);
                        vrele(lvp);
                } else {
                        error = null_nodeget(mp, lvp, &vp);
                        if (error == 0)
                                *ap->a_vpp = vp;
                }
        }
        return (error);
}

static int
null_open(struct vop_open_args *ap)
{
        int retval;
        struct vnode *vp, *ldvp;

        vp = ap->a_vp;
        ldvp = NULLVPTOLOWERVP(vp);
        retval = null_bypass(&ap->a_gen);
        if (retval == 0) {
                vp->v_object = ldvp->v_object;
                if ((vn_irflag_read(ldvp) & VIRF_PGREAD) != 0) {
                        MPASS(vp->v_object != NULL);
                        if ((vn_irflag_read(vp) & VIRF_PGREAD) == 0) {
                                vn_irflag_set_cond(vp, VIRF_PGREAD);
                        }
                }
        }
        return (retval);
}

/*
 * Setattr call. Disallow write attempts if the layer is mounted read-only.
 */
static int
null_setattr(struct vop_setattr_args *ap)
{
        struct vnode *vp = ap->a_vp;
        struct vattr *vap = ap->a_vap;

        if ((vap->va_flags != VNOVAL || vap->va_uid != (uid_t)VNOVAL ||
            vap->va_gid != (gid_t)VNOVAL || vap->va_atime.tv_sec != VNOVAL ||
            vap->va_mtime.tv_sec != VNOVAL || vap->va_mode != (mode_t)VNOVAL) &&
            (vp->v_mount->mnt_flag & MNT_RDONLY))
                return (EROFS);
        if (vap->va_size != VNOVAL) {
                switch (vp->v_type) {
                case VDIR:
                        return (EISDIR);
                case VCHR:
                case VBLK:
                case VSOCK:
                case VFIFO:
                        if (vap->va_flags != VNOVAL)
                                return (EOPNOTSUPP);
                        return (0);
                case VREG:
                case VLNK:
                default:
                        /*
                         * Disallow write attempts if the filesystem is
                         * mounted read-only.
                         */
                        if (vp->v_mount->mnt_flag & MNT_RDONLY)
                                return (EROFS);
                }
        }

        return (null_bypass(&ap->a_gen));
}

/*
 *  We handle stat and getattr only to change the fsid.
 */
static int
null_stat(struct vop_stat_args *ap)
{
        int error;

        if ((error = null_bypass(&ap->a_gen)) != 0)
                return (error);

        ap->a_sb->st_dev = ap->a_vp->v_mount->mnt_stat.f_fsid.val[0];
        return (0);
}

static int
null_getattr(struct vop_getattr_args *ap)
{
        int error;

        if ((error = null_bypass(&ap->a_gen)) != 0)
                return (error);

        ap->a_vap->va_fsid = ap->a_vp->v_mount->mnt_stat.f_fsid.val[0];
        return (0);
}

/*
 * Handle to disallow write access if mounted read-only.
 */
static int
null_access(struct vop_access_args *ap)
{
        struct vnode *vp = ap->a_vp;
        accmode_t accmode = ap->a_accmode;

        /*
         * Disallow write attempts on read-only layers;
         * unless the file is a socket, fifo, or a block or
         * character device resident on the filesystem.
         */
        if (accmode & VWRITE) {
                switch (vp->v_type) {
                case VDIR:
                case VLNK:
                case VREG:
                        if (vp->v_mount->mnt_flag & MNT_RDONLY)
                                return (EROFS);
                        break;
                default:
                        break;
                }
        }
        return (null_bypass(&ap->a_gen));
}

static int
null_accessx(struct vop_accessx_args *ap)
{
        struct vnode *vp = ap->a_vp;
        accmode_t accmode = ap->a_accmode;

        /*
         * Disallow write attempts on read-only layers;
         * unless the file is a socket, fifo, or a block or
         * character device resident on the filesystem.
         */
        if (accmode & VWRITE) {
                switch (vp->v_type) {
                case VDIR:
                case VLNK:
                case VREG:
                        if (vp->v_mount->mnt_flag & MNT_RDONLY)
                                return (EROFS);
                        break;
                default:
                        break;
                }
        }
        return (null_bypass(&ap->a_gen));
}

/*
 * Increasing refcount of lower vnode is needed at least for the case
 * when lower FS is NFS to do sillyrename if the file is in use.
 * Unfortunately v_usecount is incremented in many places in
 * the kernel and, as such, there may be races that result in
 * the NFS client doing an extraneous silly rename, but that seems
 * preferable to not doing a silly rename when it is needed.
 */
static int
null_remove(struct vop_remove_args *ap)
{
        int retval, vreleit;
        struct vnode *lvp, *vp;

        vp = ap->a_vp;
        if (vrefcnt(vp) > 1) {
                lvp = NULLVPTOLOWERVP(vp);
                vref(lvp);
                vreleit = 1;
        } else
                vreleit = 0;
        VTONULL(vp)->null_flags |= NULLV_DROP;
        retval = null_bypass(&ap->a_gen);
        if (vreleit != 0)
                vrele(lvp);
        return (retval);
}

/*
 * We handle this to eliminate null FS to lower FS
 * file moving. Don't know why we don't allow this,
 * possibly we should.
 */
static int
null_rename(struct vop_rename_args *ap)
{
        struct vnode *fdvp, *fvp, *tdvp, *tvp;
        struct vnode *lfdvp, *lfvp, *ltdvp, *ltvp;
        struct null_node *fdnn, *fnn, *tdnn, *tnn;
        int error;

        tdvp = ap->a_tdvp;
        fvp = ap->a_fvp;
        fdvp = ap->a_fdvp;
        tvp = ap->a_tvp;
        lfdvp = NULL;

        /* Check for cross-device rename. */
        if ((fvp->v_mount != tdvp->v_mount) ||
            (tvp != NULL && fvp->v_mount != tvp->v_mount)) {
                error = EXDEV;
                goto upper_err;
        }

        VI_LOCK(fdvp);
        fdnn = VTONULL(fdvp);
        if (fdnn == NULL) {     /* fdvp is not locked, can be doomed */
                VI_UNLOCK(fdvp);
                error = ENOENT;
                goto upper_err;
        }
        lfdvp = fdnn->null_lowervp;
        vref(lfdvp);
        VI_UNLOCK(fdvp);

        VI_LOCK(fvp);
        fnn = VTONULL(fvp);
        if (fnn == NULL) {
                VI_UNLOCK(fvp);
                error = ENOENT;
                goto upper_err;
        }
        lfvp = fnn->null_lowervp;
        vref(lfvp);
        VI_UNLOCK(fvp);

        tdnn = VTONULL(tdvp);
        ltdvp = tdnn->null_lowervp;
        vref(ltdvp);

        if (tvp != NULL) {
                tnn = VTONULL(tvp);
                ltvp = tnn->null_lowervp;
                vref(ltvp);
                tnn->null_flags |= NULLV_DROP;
        } else {
                ltvp = NULL;
        }

        error = VOP_RENAME(lfdvp, lfvp, ap->a_fcnp, ltdvp, ltvp, ap->a_tcnp,
            ap->a_flags);
        vrele(fdvp);
        vrele(fvp);
        vrele(tdvp);
        if (tvp != NULL)
                vrele(tvp);
        return (error);

upper_err:
        if (tdvp == tvp)
                vrele(tdvp);
        else
                vput(tdvp);
        if (tvp)
                vput(tvp);
        if (lfdvp != NULL)
                vrele(lfdvp);
        vrele(fdvp);
        vrele(fvp);
        return (error);
}

static int
null_rmdir(struct vop_rmdir_args *ap)
{

        VTONULL(ap->a_vp)->null_flags |= NULLV_DROP;
        return (null_bypass(&ap->a_gen));
}

/*
 * We need to process our own vnode lock and then clear the interlock flag as
 * it applies only to our vnode, not the vnodes below us on the stack.
 *
 * We have to hold the vnode here to solve a potential reclaim race.  If we're
 * forcibly vgone'd while we still have refs, a thread could be sleeping inside
 * the lowervp's vop_lock routine.  When we vgone we will drop our last ref to
 * the lowervp, which would allow it to be reclaimed.  The lowervp could then
 * be recycled, in which case it is not legal to be sleeping in its VOP.  We
 * prevent it from being recycled by holding the vnode here.
 */
static struct vnode *
null_lock_prep_with_smr(struct vop_lock1_args *ap)
{
        struct null_node *nn;
        struct vnode *lvp;

        lvp = NULL;

        vfs_smr_enter();

        nn = VTONULL_SMR(ap->a_vp);
        if (__predict_true(nn != NULL)) {
                lvp = nn->null_lowervp;
                if (lvp != NULL && !vhold_smr(lvp))
                        lvp = NULL;
        }

        vfs_smr_exit();
        return (lvp);
}

static struct vnode *
null_lock_prep_with_interlock(struct vop_lock1_args *ap)
{
        struct null_node *nn;
        struct vnode *lvp;

        ASSERT_VI_LOCKED(ap->a_vp, __func__);

        ap->a_flags &= ~LK_INTERLOCK;

        lvp = NULL;

        nn = VTONULL(ap->a_vp);
        if (__predict_true(nn != NULL)) {
                lvp = nn->null_lowervp;
                if (lvp != NULL)
                        vholdnz(lvp);
        }
        VI_UNLOCK(ap->a_vp);
        return (lvp);
}

static int
null_lock(struct vop_lock1_args *ap)
{
        struct vnode *lvp;
        int error, flags;

        if (__predict_true((ap->a_flags & LK_INTERLOCK) == 0)) {
                lvp = null_lock_prep_with_smr(ap);
                if (__predict_false(lvp == NULL)) {
                        VI_LOCK(ap->a_vp);
                        lvp = null_lock_prep_with_interlock(ap);
                }
        } else {
                lvp = null_lock_prep_with_interlock(ap);
        }

        ASSERT_VI_UNLOCKED(ap->a_vp, __func__);

        if (__predict_false(lvp == NULL))
                return (vop_stdlock(ap));

        VNPASS(lvp->v_holdcnt > 0, lvp);
        error = VOP_LOCK(lvp, ap->a_flags);
        /*
         * We might have slept to get the lock and someone might have
         * clean our vnode already, switching vnode lock from one in
         * lowervp to v_lock in our own vnode structure.  Handle this
         * case by reacquiring correct lock in requested mode.
         */
        if (VTONULL(ap->a_vp) == NULL && error == 0) {
                VOP_UNLOCK(lvp);

                flags = ap->a_flags;
                ap->a_flags &= ~LK_TYPE_MASK;
                switch (flags & LK_TYPE_MASK) {
                case LK_SHARED:
                        ap->a_flags |= LK_SHARED;
                        break;
                case LK_UPGRADE:
                case LK_EXCLUSIVE:
                        ap->a_flags |= LK_EXCLUSIVE;
                        break;
                default:
                        panic("Unsupported lock request %d\n",
                            flags);
                }
                error = vop_stdlock(ap);
        }
        vdrop(lvp);
        return (error);
}

static int
null_unlock(struct vop_unlock_args *ap)
{
        struct vnode *vp = ap->a_vp;
        struct null_node *nn;
        struct vnode *lvp;
        int error;

        /*
         * Contrary to null_lock, we don't need to hold the vnode around
         * unlock.
         *
         * We hold the lock, which means we can't be racing against vgone.
         *
         * At the same time VOP_UNLOCK promises to not touch anything after
         * it finishes unlock, just like we don't.
         *
         * vop_stdunlock for a doomed vnode matches doomed locking in null_lock.
         */
        nn = VTONULL(vp);
        if (nn != NULL && (lvp = NULLVPTOLOWERVP(vp)) != NULL) {
                error = VOP_UNLOCK(lvp);
        } else {
                error = vop_stdunlock(ap);
        }

        return (error);
}

/*
 * Do not allow the VOP_INACTIVE to be passed to the lower layer,
 * since the reference count on the lower vnode is not related to
 * ours.
 */
static int
null_want_recycle(struct vnode *vp)
{
        struct vnode *lvp;
        struct null_node *xp;
        struct mount *mp;
        struct null_mount *xmp;

        xp = VTONULL(vp);
        lvp = NULLVPTOLOWERVP(vp);
        mp = vp->v_mount;
        xmp = MOUNTTONULLMOUNT(mp);
        if ((xmp->nullm_flags & NULLM_CACHE) == 0 ||
            (xp->null_flags & NULLV_DROP) != 0 ||
            (lvp->v_vflag & VV_NOSYNC) != 0) {
                /*
                 * If this is the last reference and caching of the
                 * nullfs vnodes is not enabled, or the lower vnode is
                 * deleted, then free up the vnode so as not to tie up
                 * the lower vnodes.
                 */
                return (1);
        }
        return (0);
}

static int
null_inactive(struct vop_inactive_args *ap)
{
        struct vnode *vp;

        vp = ap->a_vp;
        if (null_want_recycle(vp)) {
                vp->v_object = NULL;
                vrecycle(vp);
        }
        return (0);
}

static int
null_need_inactive(struct vop_need_inactive_args *ap)
{

        return (null_want_recycle(ap->a_vp) || vn_need_pageq_flush(ap->a_vp));
}

/*
 * Now, the nullfs vnode and, due to the sharing lock, the lower
 * vnode, are exclusively locked, and we shall destroy the null vnode.
 */
static int
null_reclaim(struct vop_reclaim_args *ap)
{
        struct vnode *vp;
        struct null_node *xp;
        struct vnode *lowervp;

        vp = ap->a_vp;
        xp = VTONULL(vp);
        lowervp = xp->null_lowervp;

        KASSERT(lowervp != NULL && vp->v_vnlock != &vp->v_lock,
            ("Reclaiming incomplete null vnode %p", vp));

        null_hashrem(xp);
        /*
         * Use the interlock to protect the clearing of v_data to
         * prevent faults in null_lock().
         */
        lockmgr(&vp->v_lock, LK_EXCLUSIVE, NULL);
        VI_LOCK(vp);
        vp->v_data = NULL;
        vp->v_object = NULL;
        vp->v_vnlock = &vp->v_lock;

        /*
         * If we were opened for write, we leased the write reference
         * to the lower vnode.  If this is a reclamation due to the
         * forced unmount, undo the reference now.
         */
        if (vp->v_writecount > 0)
                VOP_ADD_WRITECOUNT(lowervp, -vp->v_writecount);
        else if (vp->v_writecount < 0)
                vp->v_writecount = 0;

        VI_UNLOCK(vp);

        if ((xp->null_flags & NULLV_NOUNLOCK) != 0)
                vunref(lowervp);
        else
                vput(lowervp);
        uma_zfree_smr(null_node_zone, xp);

        return (0);
}

static int
null_print(struct vop_print_args *ap)
{
        struct vnode *vp = ap->a_vp;

        printf("\tvp=%p, lowervp=%p\n", vp, VTONULL(vp)->null_lowervp);
        return (0);
}

/* ARGSUSED */
static int
null_getwritemount(struct vop_getwritemount_args *ap)
{
        struct null_node *xp;
        struct vnode *lowervp;
        struct vnode *vp;

        vp = ap->a_vp;
        VI_LOCK(vp);
        xp = VTONULL(vp);
        if (xp && (lowervp = xp->null_lowervp)) {
                vholdnz(lowervp);
                VI_UNLOCK(vp);
                VOP_GETWRITEMOUNT(lowervp, ap->a_mpp);
                vdrop(lowervp);
        } else {
                VI_UNLOCK(vp);
                *(ap->a_mpp) = NULL;
        }
        return (0);
}

static int
null_vptofh(struct vop_vptofh_args *ap)
{
        struct vnode *lvp;

        lvp = NULLVPTOLOWERVP(ap->a_vp);
        return VOP_VPTOFH(lvp, ap->a_fhp);
}

static int
null_vptocnp(struct vop_vptocnp_args *ap)
{
        struct vnode *vp = ap->a_vp;
        struct vnode **dvp = ap->a_vpp;
        struct vnode *lvp, *ldvp;
        struct mount *mp;
        int error, locked;

        locked = VOP_ISLOCKED(vp);
        lvp = NULLVPTOLOWERVP(vp);
        mp = vp->v_mount;
        error = vfs_busy(mp, MBF_NOWAIT);
        if (error != 0)
                return (error);
        vhold(lvp);
        VOP_UNLOCK(vp); /* vp is held by vn_vptocnp_locked that called us */
        ldvp = lvp;
        vref(lvp);
        error = vn_vptocnp(&ldvp, ap->a_buf, ap->a_buflen);
        vdrop(lvp);
        if (error != 0) {
                vn_lock(vp, locked | LK_RETRY);
                vfs_unbusy(mp);
                return (ENOENT);
        }

        error = vn_lock(ldvp, LK_SHARED);
        if (error != 0) {
                vrele(ldvp);
                vn_lock(vp, locked | LK_RETRY);
                vfs_unbusy(mp);
                return (ENOENT);
        }
        error = null_nodeget(mp, ldvp, dvp);
        if (error == 0) {
#ifdef DIAGNOSTIC
                NULLVPTOLOWERVP(*dvp);
#endif
                VOP_UNLOCK(*dvp); /* keep reference on *dvp */
        }
        vn_lock(vp, locked | LK_RETRY);
        vfs_unbusy(mp);
        return (error);
}

static int
null_read_pgcache(struct vop_read_pgcache_args *ap)
{
        struct vnode *lvp, *vp;
        struct null_node *xp;
        int error;

        vp = ap->a_vp;
        VI_LOCK(vp);
        xp = VTONULL(vp);
        if (xp == NULL) {
                VI_UNLOCK(vp);
                return (EJUSTRETURN);
        }
        lvp = xp->null_lowervp;
        vref(lvp);
        VI_UNLOCK(vp);
        error = VOP_READ_PGCACHE(lvp, ap->a_uio, ap->a_ioflag, ap->a_cred);
        vrele(lvp);
        return (error);
}

static int
null_advlock(struct vop_advlock_args *ap)
{
        struct vnode *lvp, *vp;
        struct null_node *xp;
        int error;

        vp = ap->a_vp;
        VI_LOCK(vp);
        xp = VTONULL(vp);
        if (xp == NULL) {
                VI_UNLOCK(vp);
                return (EBADF);
        }
        lvp = xp->null_lowervp;
        vref(lvp);
        VI_UNLOCK(vp);
        error = VOP_ADVLOCK(lvp, ap->a_id, ap->a_op, ap->a_fl, ap->a_flags);
        vrele(lvp);
        return (error);
}

/*
 * Avoid standard bypass, since lower dvp and vp could be no longer
 * valid after vput().
 */
static int
null_vput_pair(struct vop_vput_pair_args *ap)
{
        struct mount *mp;
        struct vnode *dvp, *ldvp, *lvp, *vp, *vp1, **vpp;
        int error, res;

        dvp = ap->a_dvp;
        ldvp = NULLVPTOLOWERVP(dvp);
        vref(ldvp);

        vpp = ap->a_vpp;
        vp = NULL;
        lvp = NULL;
        mp = NULL;
        if (vpp != NULL)
                vp = *vpp;
        if (vp != NULL) {
                lvp = NULLVPTOLOWERVP(vp);
                vref(lvp);
                if (!ap->a_unlock_vp) {
                        vhold(vp);
                        vhold(lvp);
                        mp = vp->v_mount;
                        vfs_ref(mp);
                }
        }

        res = VOP_VPUT_PAIR(ldvp, lvp != NULL ? &lvp : NULL, true);
        if (vp != NULL && ap->a_unlock_vp)
                vrele(vp);
        vrele(dvp);

        if (vp == NULL || ap->a_unlock_vp)
                return (res);

        /* lvp has been unlocked and vp might be reclaimed */
        VOP_LOCK(vp, LK_EXCLUSIVE | LK_RETRY);
        if (vp->v_data == NULL && vfs_busy(mp, MBF_NOWAIT) == 0) {
                vput(vp);
                vget(lvp, LK_EXCLUSIVE | LK_RETRY);
                if (VN_IS_DOOMED(lvp)) {
                        vput(lvp);
                        vget(vp, LK_EXCLUSIVE | LK_RETRY);
                } else {
                        error = null_nodeget(mp, lvp, &vp1);
                        if (error == 0) {
                                *vpp = vp1;
                        } else {
                                vget(vp, LK_EXCLUSIVE | LK_RETRY);
                        }
                }
                vfs_unbusy(mp);
        }
        vdrop(lvp);
        vdrop(vp);
        vfs_rel(mp);

        return (res);
}

static int
null_getlowvnode(struct vop_getlowvnode_args *ap)
{
        struct vnode *vp, *vpl;

        vp = ap->a_vp;
        if (vn_lock(vp, LK_SHARED) != 0)
                return (EBADF);

        vpl = NULLVPTOLOWERVP(vp);
        vhold(vpl);
        VOP_UNLOCK(vp);
        VOP_GETLOWVNODE(vpl, ap->a_vplp, ap->a_flags);
        vdrop(vpl);
        return (0);
}

/*
 * Global vfs data structures
 */
struct vop_vector null_vnodeops = {
        .vop_bypass =           null_bypass,
        .vop_access =           null_access,
        .vop_accessx =          null_accessx,
        .vop_advlock =          null_advlock,
        .vop_advlockpurge =     vop_stdadvlockpurge,
        .vop_bmap =             VOP_EOPNOTSUPP,
        .vop_stat =             null_stat,
        .vop_getattr =          null_getattr,
        .vop_getlowvnode =      null_getlowvnode,
        .vop_getwritemount =    null_getwritemount,
        .vop_inactive =         null_inactive,
        .vop_need_inactive =    null_need_inactive,
        .vop_islocked =         vop_stdislocked,
        .vop_lock1 =            null_lock,
        .vop_lookup =           null_lookup,
        .vop_open =             null_open,
        .vop_print =            null_print,
        .vop_read_pgcache =     null_read_pgcache,
        .vop_reclaim =          null_reclaim,
        .vop_remove =           null_remove,
        .vop_rename =           null_rename,
        .vop_rmdir =            null_rmdir,
        .vop_setattr =          null_setattr,
        .vop_strategy =         VOP_EOPNOTSUPP,
        .vop_unlock =           null_unlock,
        .vop_vptocnp =          null_vptocnp,
        .vop_vptofh =           null_vptofh,
        .vop_add_writecount =   null_add_writecount,
        .vop_vput_pair =        null_vput_pair,
        .vop_copy_file_range =  VOP_PANIC,
};
VFS_VOP_VECTOR_REGISTER(null_vnodeops);

struct vop_vector null_vnodeops_no_unp_bypass = {
        .vop_default =          &null_vnodeops,
        .vop_unp_bind =         vop_stdunp_bind,
        .vop_unp_connect =      vop_stdunp_connect,
        .vop_unp_detach =       vop_stdunp_detach,
};
VFS_VOP_VECTOR_REGISTER(null_vnodeops_no_unp_bypass);