FreeBSD/Linux Kernel Cross Reference
sys/fs/nullfs/null_vnops.c

     /*-
      * 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.
     *
     *      @(#)null_vnops.c        8.6 (Berkeley) 5/27/95
     *
     * Ancestors:
     *      @(#)lofs_vnops.c        1.2 (Berkeley) 6/18/92
     *      ...and...
     *      @(#)null_vnodeops.c 1.20 92/07/07 UCLA Ficus project
     *
     * $FreeBSD$
     */
    
    /*
     * 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 examing 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/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>
   
   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, "");
   
   /*
    * 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;
           int error;
           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 reles, i;
   
           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 && (*this_vp_p == NULLVP ||
                       (*this_vp_p)->v_op != &null_vnodeops)) {
                           old_vps[i] = NULLVP;
                   } else {
                           old_vps[i] = *this_vp_p;
                           *(vps_p[i]) = NULLVPTOLOWERVP(*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] && *vps_p[0])
                   error = VCALL(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]) {
                           lvp = *(vps_p[i]);
   
                           /*
                            * 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 != NULLVP &&
                               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);
                           }
   
                           *(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) {
                   /*
                    * 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)
                           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;
           int 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;
           KASSERT((ldvp->v_vflag & VV_ROOT) == 0 ||
               ((dvp->v_vflag & VV_ROOT) != 0 && (flags & ISDOTDOT) == 0),
               ("ldvp %p fl %#x dvp %p fl %#x flags %#x", ldvp, ldvp->v_vflag,
                dvp, dvp->v_vflag, flags));
   
           /*
            * 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 ((error == 0 || error == EJUSTRETURN) &&
               VN_IS_DOOMED(dvp)) {
                   error = ENOENT;
                   if (lvp != NULL)
                           vput(lvp);
   
                   /*
                    * 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((struct vop_generic_args *)ap));
   }
   
   /*
    *  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((struct vop_generic_args *)ap)) != 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((struct vop_generic_args *)ap)) != 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((struct vop_generic_args *)ap));
   }
   
   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((struct vop_generic_args *)ap));
   }
   
   /*
    * 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 *tdvp = ap->a_tdvp;
           struct vnode *fvp = ap->a_fvp;
           struct vnode *fdvp = ap->a_fdvp;
           struct vnode *tvp = ap->a_tvp;
           struct null_node *tnn;
   
           /* Check for cross-device rename. */
           if ((fvp->v_mount != tdvp->v_mount) ||
               (tvp && (fvp->v_mount != tvp->v_mount))) {
                   if (tdvp == tvp)
                           vrele(tdvp);
                   else
                           vput(tdvp);
                   if (tvp)
                           vput(tvp);
                   vrele(fdvp);
                   vrele(fvp);
                   return (EXDEV);
           }
   
           if (tvp != NULL) {
                   tnn = VTONULL(tvp);
                   tnn->null_flags |= NULLV_DROP;
           }
           return (null_bypass((struct vop_generic_args *)ap));
   }
   
   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.
    */
   static int
   null_lock(struct vop_lock1_args *ap)
   {
           struct vnode *vp = ap->a_vp;
           int flags;
           struct null_node *nn;
           struct vnode *lvp;
           int error;
   
           if ((ap->a_flags & LK_INTERLOCK) == 0)
                   VI_LOCK(vp);
           else
                   ap->a_flags &= ~LK_INTERLOCK;
           flags = ap->a_flags;
           nn = VTONULL(vp);
           /*
            * If we're still active we must ask the lower layer to
            * lock as ffs has special lock considerations in its
            * vop lock.
            */
           if (nn != NULL && (lvp = NULLVPTOLOWERVP(vp)) != NULL) {
                   /*
                    * 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.
                    */
                   vholdnz(lvp);
                   VI_UNLOCK(vp);
                   error = VOP_LOCK(lvp, 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(vp) == NULL && error == 0) {
                           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",
                                       ap->a_flags);
                           }
                           VOP_UNLOCK(lvp);
                           error = vop_stdlock(ap);
                   }
                   vdrop(lvp);
           } else {
                   VI_UNLOCK(vp);
                   error = vop_stdlock(ap);
           }
   
           return (error);
   }
   
   /*
    * We need to process our own vnode unlock and then clear the
    * interlock flag as it applies only to our vnode, not the
    * vnodes below us on the stack.
    */
   static int
   null_unlock(struct vop_unlock_args *ap)
   {
           struct vnode *vp = ap->a_vp;
           struct null_node *nn;
           struct vnode *lvp;
           int error;
   
           nn = VTONULL(vp);
           if (nn != NULL && (lvp = NULLVPTOLOWERVP(vp)) != NULL) {
                   vholdnz(lvp);
                   error = VOP_UNLOCK(lvp);
                   vdrop(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));
   }
   
   /*
    * 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);
           free(xp, M_NULLFSNODE);
   
           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);
           vhold(lvp);
           mp = vp->v_mount;
           vfs_ref(mp);
           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_rel(mp);
                   return (ENOENT);
           }
   
           error = vn_lock(ldvp, LK_SHARED);
           if (error != 0) {
                   vrele(ldvp);
                   vn_lock(vp, locked | LK_RETRY);
                   vfs_rel(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_rel(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);
   }
   
   /*
    * 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;
           if (vpp != NULL) {
                   vp = *vpp;
                   if (vp != NULL) {
                           vhold(vp);
                           mp = vp->v_mount;
                           lvp = NULLVPTOLOWERVP(vp);
                           if (ap->a_unlock_vp)
                                   vref(lvp);
                   }
           }
   
           res = VOP_VPUT_PAIR(ldvp, &lvp, ap->a_unlock_vp);
  
          /* lvp might have been unlocked and vp reclaimed */
          if (vp != NULL) {
                  if (!ap->a_unlock_vp && vp->v_vnlock != lvp->v_vnlock) {
                          error = null_nodeget(mp, lvp, &vp1);
                          if (error == 0) {
                                  vput(vp);
                                  *vpp = vp1;
                          }
                  }
                  if (ap->a_unlock_vp)
                          vrele(vp);
                  vdrop(vp);
          }
          vrele(dvp);
          return (res);
  }
  
  /*
   * Global vfs data structures
   */
  struct vop_vector null_vnodeops = {
          .vop_bypass =           null_bypass,
          .vop_access =           null_access,
          .vop_accessx =          null_accessx,
          .vop_advlockpurge =     vop_stdadvlockpurge,
          .vop_bmap =             VOP_EOPNOTSUPP,
          .vop_stat =             null_stat,
          .vop_getattr =          null_getattr,
          .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,
  };
  VFS_VOP_VECTOR_REGISTER(null_vnodeops);

Cache object: eaa954cb8f51d0c0cb3ca1650b0ec60f