[mirror_zfs-debian.git] / zfs / lib / libzpool / zfs_ctldir.c

/*
 * CDDL HEADER START
 *
 * The contents of this file are subject to the terms of the
 * Common Development and Distribution License (the "License").
 * You may not use this file except in compliance with the License.
 *
 * You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
 * or http://www.opensolaris.org/os/licensing.
 * See the License for the specific language governing permissions
 * and limitations under the License.
 *
 * When distributing Covered Code, include this CDDL HEADER in each
 * file and include the License file at usr/src/OPENSOLARIS.LICENSE.
 * If applicable, add the following below this CDDL HEADER, with the
 * fields enclosed by brackets "[]" replaced with your own identifying
 * information: Portions Copyright [yyyy] [name of copyright owner]
 *
 * CDDL HEADER END
 */
/*
 * Copyright 2008 Sun Microsystems, Inc.  All rights reserved.
 * Use is subject to license terms.
 */

#pragma ident   "%Z%%M% %I%     %E% SMI"

/*
 * ZFS control directory (a.k.a. ".zfs")
 *
 * This directory provides a common location for all ZFS meta-objects.
 * Currently, this is only the 'snapshot' directory, but this may expand in the
 * future.  The elements are built using the GFS primitives, as the hierarchy
 * does not actually exist on disk.
 *
 * For 'snapshot', we don't want to have all snapshots always mounted, because
 * this would take up a huge amount of space in /etc/mnttab.  We have three
 * types of objects:
 *
 *      ctldir ------> snapshotdir -------> snapshot
 *                                             |
 *                                             |
 *                                             V
 *                                         mounted fs
 *
 * The 'snapshot' node contains just enough information to lookup '..' and act
 * as a mountpoint for the snapshot.  Whenever we lookup a specific snapshot, we
 * perform an automount of the underlying filesystem and return the
 * corresponding vnode.
 *
 * All mounts are handled automatically by the kernel, but unmounts are
 * (currently) handled from user land.  The main reason is that there is no
 * reliable way to auto-unmount the filesystem when it's "no longer in use".
 * When the user unmounts a filesystem, we call zfsctl_unmount(), which
 * unmounts any snapshots within the snapshot directory.
 *
 * The '.zfs', '.zfs/snapshot', and all directories created under
 * '.zfs/snapshot' (ie: '.zfs/snapshot/<snapname>') are all GFS nodes and
 * share the same vfs_t as the head filesystem (what '.zfs' lives under).
 *
 * File systems mounted ontop of the GFS nodes '.zfs/snapshot/<snapname>'
 * (ie: snapshots) are ZFS nodes and have their own unique vfs_t.
 * However, vnodes within these mounted on file systems have their v_vfsp
 * fields set to the head filesystem to make NFS happy (see
 * zfsctl_snapdir_lookup()). We VFS_HOLD the head filesystem's vfs_t
 * so that it cannot be freed until all snapshots have been unmounted.
 */

#include <fs/fs_subr.h>
#include <sys/zfs_ctldir.h>
#include <sys/zfs_ioctl.h>
#include <sys/zfs_vfsops.h>
#include <sys/vfs_opreg.h>
#include <sys/gfs.h>
#include <sys/stat.h>
#include <sys/dmu.h>
#include <sys/dsl_deleg.h>
#include <sys/mount.h>
#include <sys/sunddi.h>

#include "zfs_namecheck.h"

typedef struct zfsctl_node {
        gfs_dir_t       zc_gfs_private;
        uint64_t        zc_id;
        timestruc_t     zc_cmtime;      /* ctime and mtime, always the same */
} zfsctl_node_t;

typedef struct zfsctl_snapdir {
        zfsctl_node_t   sd_node;
        kmutex_t        sd_lock;
        avl_tree_t      sd_snaps;
} zfsctl_snapdir_t;

typedef struct {
        char            *se_name;
        vnode_t         *se_root;
        avl_node_t      se_node;
} zfs_snapentry_t;

static int
snapentry_compare(const void *a, const void *b)
{
        const zfs_snapentry_t *sa = a;
        const zfs_snapentry_t *sb = b;
        int ret = strcmp(sa->se_name, sb->se_name);

        if (ret < 0)
                return (-1);
        else if (ret > 0)
                return (1);
        else
                return (0);
}

vnodeops_t *zfsctl_ops_root;
vnodeops_t *zfsctl_ops_snapdir;
vnodeops_t *zfsctl_ops_snapshot;

static const fs_operation_def_t zfsctl_tops_root[];
static const fs_operation_def_t zfsctl_tops_snapdir[];
static const fs_operation_def_t zfsctl_tops_snapshot[];

static vnode_t *zfsctl_mknode_snapdir(vnode_t *);
static vnode_t *zfsctl_snapshot_mknode(vnode_t *, uint64_t objset);
static int zfsctl_unmount_snap(zfs_snapentry_t *, int, cred_t *);

static gfs_opsvec_t zfsctl_opsvec[] = {
        { ".zfs", zfsctl_tops_root, &zfsctl_ops_root },
        { ".zfs/snapshot", zfsctl_tops_snapdir, &zfsctl_ops_snapdir },
        { ".zfs/snapshot/vnode", zfsctl_tops_snapshot, &zfsctl_ops_snapshot },
        { NULL }
};

/*
 * Root directory elements.  We have only a single static entry, 'snapshot'.
 */
static gfs_dirent_t zfsctl_root_entries[] = {
        { "snapshot", zfsctl_mknode_snapdir, GFS_CACHE_VNODE },
        { NULL }
};

/* include . and .. in the calculation */
#define NROOT_ENTRIES   ((sizeof (zfsctl_root_entries) / \
    sizeof (gfs_dirent_t)) + 1)


/*
 * Initialize the various GFS pieces we'll need to create and manipulate .zfs
 * directories.  This is called from the ZFS init routine, and initializes the
 * vnode ops vectors that we'll be using.
 */
void
zfsctl_init(void)
{
        VERIFY(gfs_make_opsvec(zfsctl_opsvec) == 0);
}

void
zfsctl_fini(void)
{
        /*
         * Remove vfsctl vnode ops
         */
        if (zfsctl_ops_root)
                vn_freevnodeops(zfsctl_ops_root);
        if (zfsctl_ops_snapdir)
                vn_freevnodeops(zfsctl_ops_snapdir);
        if (zfsctl_ops_snapshot)
                vn_freevnodeops(zfsctl_ops_snapshot);

        zfsctl_ops_root = NULL;
        zfsctl_ops_snapdir = NULL;
        zfsctl_ops_snapshot = NULL;
}

/*
 * Return the inode number associated with the 'snapshot' directory.
 */
/* ARGSUSED */
static ino64_t
zfsctl_root_inode_cb(vnode_t *vp, int index)
{
        ASSERT(index == 0);
        return (ZFSCTL_INO_SNAPDIR);
}

/*
 * Create the '.zfs' directory.  This directory is cached as part of the VFS
 * structure.  This results in a hold on the vfs_t.  The code in zfs_umount()
 * therefore checks against a vfs_count of 2 instead of 1.  This reference
 * is removed when the ctldir is destroyed in the unmount.
 */
void
zfsctl_create(zfsvfs_t *zfsvfs)
{
        vnode_t *vp, *rvp;
        zfsctl_node_t *zcp;

        ASSERT(zfsvfs->z_ctldir == NULL);

        vp = gfs_root_create(sizeof (zfsctl_node_t), zfsvfs->z_vfs,
            zfsctl_ops_root, ZFSCTL_INO_ROOT, zfsctl_root_entries,
            zfsctl_root_inode_cb, MAXNAMELEN, NULL, NULL);
        zcp = vp->v_data;
        zcp->zc_id = ZFSCTL_INO_ROOT;

        VERIFY(VFS_ROOT(zfsvfs->z_vfs, &rvp) == 0);
        ZFS_TIME_DECODE(&zcp->zc_cmtime, VTOZ(rvp)->z_phys->zp_crtime);
        VN_RELE(rvp);

        /*
         * We're only faking the fact that we have a root of a filesystem for
         * the sake of the GFS interfaces.  Undo the flag manipulation it did
         * for us.
         */
        vp->v_flag &= ~(VROOT | VNOCACHE | VNOMAP | VNOSWAP | VNOMOUNT);

        zfsvfs->z_ctldir = vp;
}

/*
 * Destroy the '.zfs' directory.  Only called when the filesystem is unmounted.
 * There might still be more references if we were force unmounted, but only
 * new zfs_inactive() calls can occur and they don't reference .zfs
 */
void
zfsctl_destroy(zfsvfs_t *zfsvfs)
{
        VN_RELE(zfsvfs->z_ctldir);
        zfsvfs->z_ctldir = NULL;
}

/*
 * Given a root znode, retrieve the associated .zfs directory.
 * Add a hold to the vnode and return it.
 */
vnode_t *
zfsctl_root(znode_t *zp)
{
        ASSERT(zfs_has_ctldir(zp));
        VN_HOLD(zp->z_zfsvfs->z_ctldir);
        return (zp->z_zfsvfs->z_ctldir);
}

/*
 * Common open routine.  Disallow any write access.
 */
/* ARGSUSED */
static int
zfsctl_common_open(vnode_t **vpp, int flags, cred_t *cr, caller_context_t *ct)
{
        if (flags & FWRITE)
                return (EACCES);

        return (0);
}

/*
 * Common close routine.  Nothing to do here.
 */
/* ARGSUSED */
static int
zfsctl_common_close(vnode_t *vpp, int flags, int count, offset_t off,
    cred_t *cr, caller_context_t *ct)
{
        return (0);
}

/*
 * Common access routine.  Disallow writes.
 */
/* ARGSUSED */
static int
zfsctl_common_access(vnode_t *vp, int mode, int flags, cred_t *cr,
    caller_context_t *ct)
{
        if (mode & VWRITE)
                return (EACCES);

        return (0);
}

/*
 * Common getattr function.  Fill in basic information.
 */
static void
zfsctl_common_getattr(vnode_t *vp, vattr_t *vap)
{
        zfsctl_node_t   *zcp = vp->v_data;
        timestruc_t     now;

        vap->va_uid = 0;
        vap->va_gid = 0;
        vap->va_rdev = 0;
        /*
         * We are a purly virtual object, so we have no
         * blocksize or allocated blocks.
         */
        vap->va_blksize = 0;
        vap->va_nblocks = 0;
        vap->va_seq = 0;
        vap->va_fsid = vp->v_vfsp->vfs_dev;
        vap->va_mode = S_IRUSR | S_IXUSR | S_IRGRP | S_IXGRP |
            S_IROTH | S_IXOTH;
        vap->va_type = VDIR;
        /*
         * We live in the now (for atime).
         */
        gethrestime(&now);
        vap->va_atime = now;
        vap->va_mtime = vap->va_ctime = zcp->zc_cmtime;
}

/*ARGSUSED*/
static int
zfsctl_common_fid(vnode_t *vp, fid_t *fidp, caller_context_t *ct)
{
        zfsvfs_t        *zfsvfs = vp->v_vfsp->vfs_data;
        zfsctl_node_t   *zcp = vp->v_data;
        uint64_t        object = zcp->zc_id;
        zfid_short_t    *zfid;
        int             i;

        ZFS_ENTER(zfsvfs);

        if (fidp->fid_len < SHORT_FID_LEN) {
                fidp->fid_len = SHORT_FID_LEN;
                ZFS_EXIT(zfsvfs);
                return (ENOSPC);
        }

        zfid = (zfid_short_t *)fidp;

        zfid->zf_len = SHORT_FID_LEN;

        for (i = 0; i < sizeof (zfid->zf_object); i++)
                zfid->zf_object[i] = (uint8_t)(object >> (8 * i));

        /* .zfs znodes always have a generation number of 0 */
        for (i = 0; i < sizeof (zfid->zf_gen); i++)
                zfid->zf_gen[i] = 0;

        ZFS_EXIT(zfsvfs);
        return (0);
}

/*
 * .zfs inode namespace
 *
 * We need to generate unique inode numbers for all files and directories
 * within the .zfs pseudo-filesystem.  We use the following scheme:
 *
 *      ENTRY                   ZFSCTL_INODE
 *      .zfs                    1
 *      .zfs/snapshot           2
 *      .zfs/snapshot/<snap>    objectid(snap)
 */

#define ZFSCTL_INO_SNAP(id)     (id)

/*
 * Get root directory attributes.
 */
/* ARGSUSED */
static int
zfsctl_root_getattr(vnode_t *vp, vattr_t *vap, int flags, cred_t *cr,
    caller_context_t *ct)
{
        zfsvfs_t *zfsvfs = vp->v_vfsp->vfs_data;

        ZFS_ENTER(zfsvfs);
        vap->va_nodeid = ZFSCTL_INO_ROOT;
        vap->va_nlink = vap->va_size = NROOT_ENTRIES;

        zfsctl_common_getattr(vp, vap);
        ZFS_EXIT(zfsvfs);

        return (0);
}

/*
 * Special case the handling of "..".
 */
/* ARGSUSED */
int
zfsctl_root_lookup(vnode_t *dvp, char *nm, vnode_t **vpp, pathname_t *pnp,
    int flags, vnode_t *rdir, cred_t *cr, caller_context_t *ct,
    int *direntflags, pathname_t *realpnp)
{
        zfsvfs_t *zfsvfs = dvp->v_vfsp->vfs_data;
        int err;

        /*
         * No extended attributes allowed under .zfs
         */
        if (flags & LOOKUP_XATTR)
                return (EINVAL);

        ZFS_ENTER(zfsvfs);

        if (strcmp(nm, "..") == 0) {
                err = VFS_ROOT(dvp->v_vfsp, vpp);
        } else {
                err = gfs_vop_lookup(dvp, nm, vpp, pnp, flags, rdir,
                    cr, ct, direntflags, realpnp);
        }

        ZFS_EXIT(zfsvfs);

        return (err);
}

static const fs_operation_def_t zfsctl_tops_root[] = {
        { VOPNAME_OPEN,         { .vop_open = zfsctl_common_open }      },
        { VOPNAME_CLOSE,        { .vop_close = zfsctl_common_close }    },
        { VOPNAME_IOCTL,        { .error = fs_inval }                   },
        { VOPNAME_GETATTR,      { .vop_getattr = zfsctl_root_getattr }  },
        { VOPNAME_ACCESS,       { .vop_access = zfsctl_common_access }  },
        { VOPNAME_READDIR,      { .vop_readdir = gfs_vop_readdir }      },
        { VOPNAME_LOOKUP,       { .vop_lookup = zfsctl_root_lookup }    },
        { VOPNAME_SEEK,         { .vop_seek = fs_seek }                 },
        { VOPNAME_INACTIVE,     { .vop_inactive = gfs_vop_inactive }    },
        { VOPNAME_FID,          { .vop_fid = zfsctl_common_fid  }       },
        { NULL }
};

static int
zfsctl_snapshot_zname(vnode_t *vp, const char *name, int len, char *zname)
{
        objset_t *os = ((zfsvfs_t *)((vp)->v_vfsp->vfs_data))->z_os;

        if (snapshot_namecheck(name, NULL, NULL) != 0)
                return (EILSEQ);
        dmu_objset_name(os, zname);
        if (strlen(zname) + 1 + strlen(name) >= len)
                return (ENAMETOOLONG);
        (void) strcat(zname, "@");
        (void) strcat(zname, name);
        return (0);
}

static int
zfsctl_unmount_snap(zfs_snapentry_t *sep, int fflags, cred_t *cr)
{
        vnode_t *svp = sep->se_root;
        int error;

        ASSERT(vn_ismntpt(svp));

        /* this will be dropped by dounmount() */
        if ((error = vn_vfswlock(svp)) != 0)
                return (error);

        VN_HOLD(svp);
        error = dounmount(vn_mountedvfs(svp), fflags, cr);
        if (error) {
                VN_RELE(svp);
                return (error);
        }
        VFS_RELE(svp->v_vfsp);
        /*
         * We can't use VN_RELE(), as that will try to invoke
         * zfsctl_snapdir_inactive(), which would cause us to destroy
         * the sd_lock mutex held by our caller.
         */
        ASSERT(svp->v_count == 1);
        gfs_vop_inactive(svp, cr, NULL);

        kmem_free(sep->se_name, strlen(sep->se_name) + 1);
        kmem_free(sep, sizeof (zfs_snapentry_t));

        return (0);
}

static void
zfsctl_rename_snap(zfsctl_snapdir_t *sdp, zfs_snapentry_t *sep, const char *nm)
{
        avl_index_t where;
        vfs_t *vfsp;
        refstr_t *pathref;
        char newpath[MAXNAMELEN];
        char *tail;

        ASSERT(MUTEX_HELD(&sdp->sd_lock));
        ASSERT(sep != NULL);

        vfsp = vn_mountedvfs(sep->se_root);
        ASSERT(vfsp != NULL);

        vfs_lock_wait(vfsp);

        /*
         * Change the name in the AVL tree.
         */
        avl_remove(&sdp->sd_snaps, sep);
        kmem_free(sep->se_name, strlen(sep->se_name) + 1);
        sep->se_name = kmem_alloc(strlen(nm) + 1, KM_SLEEP);
        (void) strcpy(sep->se_name, nm);
        VERIFY(avl_find(&sdp->sd_snaps, sep, &where) == NULL);
        avl_insert(&sdp->sd_snaps, sep, where);

        /*
         * Change the current mountpoint info:
         *      - update the tail of the mntpoint path
         *      - update the tail of the resource path
         */
        pathref = vfs_getmntpoint(vfsp);
        (void) strncpy(newpath, refstr_value(pathref), sizeof (newpath));
        VERIFY((tail = strrchr(newpath, '/')) != NULL);
        *(tail+1) = '\0';
        ASSERT3U(strlen(newpath) + strlen(nm), <, sizeof (newpath));
        (void) strcat(newpath, nm);
        refstr_rele(pathref);
        vfs_setmntpoint(vfsp, newpath);

        pathref = vfs_getresource(vfsp);
        (void) strncpy(newpath, refstr_value(pathref), sizeof (newpath));
        VERIFY((tail = strrchr(newpath, '@')) != NULL);
        *(tail+1) = '\0';
        ASSERT3U(strlen(newpath) + strlen(nm), <, sizeof (newpath));
        (void) strcat(newpath, nm);
        refstr_rele(pathref);
        vfs_setresource(vfsp, newpath);

        vfs_unlock(vfsp);
}

/*ARGSUSED*/
static int
zfsctl_snapdir_rename(vnode_t *sdvp, char *snm, vnode_t *tdvp, char *tnm,
    cred_t *cr, caller_context_t *ct, int flags)
{
        zfsctl_snapdir_t *sdp = sdvp->v_data;
        zfs_snapentry_t search, *sep;
        zfsvfs_t *zfsvfs;
        avl_index_t where;
        char from[MAXNAMELEN], to[MAXNAMELEN];
        char real[MAXNAMELEN];
        int err;

        zfsvfs = sdvp->v_vfsp->vfs_data;
        ZFS_ENTER(zfsvfs);

        if ((flags & FIGNORECASE) || zfsvfs->z_case == ZFS_CASE_INSENSITIVE) {
                err = dmu_snapshot_realname(zfsvfs->z_os, snm, real,
                    MAXNAMELEN, NULL);
                if (err == 0) {
                        snm = real;
                } else if (err != ENOTSUP) {
                        ZFS_EXIT(zfsvfs);
                        return (err);
                }
        }

        ZFS_EXIT(zfsvfs);

        err = zfsctl_snapshot_zname(sdvp, snm, MAXNAMELEN, from);
        if (!err)
                err = zfsctl_snapshot_zname(tdvp, tnm, MAXNAMELEN, to);
        if (!err)
                err = zfs_secpolicy_rename_perms(from, to, cr);
        if (err)
                return (err);

        /*
         * Cannot move snapshots out of the snapdir.
         */
        if (sdvp != tdvp)
                return (EINVAL);

        if (strcmp(snm, tnm) == 0)
                return (0);

        mutex_enter(&sdp->sd_lock);

        search.se_name = (char *)snm;
        if ((sep = avl_find(&sdp->sd_snaps, &search, &where)) == NULL) {
                mutex_exit(&sdp->sd_lock);
                return (ENOENT);
        }

        err = dmu_objset_rename(from, to, B_FALSE);
        if (err == 0)
                zfsctl_rename_snap(sdp, sep, tnm);

        mutex_exit(&sdp->sd_lock);

        return (err);
}

/* ARGSUSED */
static int
zfsctl_snapdir_remove(vnode_t *dvp, char *name, vnode_t *cwd, cred_t *cr,
    caller_context_t *ct, int flags)
{
        zfsctl_snapdir_t *sdp = dvp->v_data;
        zfs_snapentry_t *sep;
        zfs_snapentry_t search;
        zfsvfs_t *zfsvfs;
        char snapname[MAXNAMELEN];
        char real[MAXNAMELEN];
        int err;

        zfsvfs = dvp->v_vfsp->vfs_data;
        ZFS_ENTER(zfsvfs);

        if ((flags & FIGNORECASE) || zfsvfs->z_case == ZFS_CASE_INSENSITIVE) {

                err = dmu_snapshot_realname(zfsvfs->z_os, name, real,
                    MAXNAMELEN, NULL);
                if (err == 0) {
                        name = real;
                } else if (err != ENOTSUP) {
                        ZFS_EXIT(zfsvfs);
                        return (err);
                }
        }

        ZFS_EXIT(zfsvfs);

        err = zfsctl_snapshot_zname(dvp, name, MAXNAMELEN, snapname);
        if (!err)
                err = zfs_secpolicy_destroy_perms(snapname, cr);
        if (err)
                return (err);

        mutex_enter(&sdp->sd_lock);

        search.se_name = name;
        sep = avl_find(&sdp->sd_snaps, &search, NULL);
        if (sep) {
                avl_remove(&sdp->sd_snaps, sep);
                err = zfsctl_unmount_snap(sep, MS_FORCE, cr);
                if (err)
                        avl_add(&sdp->sd_snaps, sep);
                else
                        err = dmu_objset_destroy(snapname);
        } else {
                err = ENOENT;
        }

        mutex_exit(&sdp->sd_lock);

        return (err);
}

/*
 * This creates a snapshot under '.zfs/snapshot'.
 */
/* ARGSUSED */
static int
zfsctl_snapdir_mkdir(vnode_t *dvp, char *dirname, vattr_t *vap, vnode_t  **vpp,
    cred_t *cr, caller_context_t *cc, int flags, vsecattr_t *vsecp)
{
        zfsvfs_t *zfsvfs = dvp->v_vfsp->vfs_data;
        char name[MAXNAMELEN];
        int err;
        static enum symfollow follow = NO_FOLLOW;
        static enum uio_seg seg = UIO_SYSSPACE;

        if (snapshot_namecheck(dirname, NULL, NULL) != 0)
                return (EILSEQ);

        dmu_objset_name(zfsvfs->z_os, name);

        *vpp = NULL;

        err = zfs_secpolicy_snapshot_perms(name, cr);
        if (err)
                return (err);

        if (err == 0) {
                err = dmu_objset_snapshot(name, dirname, B_FALSE);
                if (err)
                        return (err);
                err = lookupnameat(dirname, seg, follow, NULL, vpp, dvp);
        }

        return (err);
}

/*
 * Lookup entry point for the 'snapshot' directory.  Try to open the
 * snapshot if it exist, creating the pseudo filesystem vnode as necessary.
 * Perform a mount of the associated dataset on top of the vnode.
 */
/* ARGSUSED */
static int
zfsctl_snapdir_lookup(vnode_t *dvp, char *nm, vnode_t **vpp, pathname_t *pnp,
    int flags, vnode_t *rdir, cred_t *cr, caller_context_t *ct,
    int *direntflags, pathname_t *realpnp)
{
        zfsctl_snapdir_t *sdp = dvp->v_data;
        objset_t *snap;
        char snapname[MAXNAMELEN];
        char real[MAXNAMELEN];
        char *mountpoint;
        zfs_snapentry_t *sep, search;
        struct mounta margs;
        vfs_t *vfsp;
        size_t mountpoint_len;
        avl_index_t where;
        zfsvfs_t *zfsvfs = dvp->v_vfsp->vfs_data;
        int err;

        /*
         * No extended attributes allowed under .zfs
         */
        if (flags & LOOKUP_XATTR)
                return (EINVAL);

        ASSERT(dvp->v_type == VDIR);

        if (gfs_lookup_dot(vpp, dvp, zfsvfs->z_ctldir, nm) == 0)
                return (0);

        /*
         * If we get a recursive call, that means we got called
         * from the domount() code while it was trying to look up the
         * spec (which looks like a local path for zfs).  We need to
         * add some flag to domount() to tell it not to do this lookup.
         */
        if (MUTEX_HELD(&sdp->sd_lock))
                return (ENOENT);

        ZFS_ENTER(zfsvfs);

        if (flags & FIGNORECASE) {
                boolean_t conflict = B_FALSE;

                err = dmu_snapshot_realname(zfsvfs->z_os, nm, real,
                    MAXNAMELEN, &conflict);
                if (err == 0) {
                        nm = real;
                } else if (err != ENOTSUP) {
                        ZFS_EXIT(zfsvfs);
                        return (err);
                }
                if (realpnp)
                        (void) strlcpy(realpnp->pn_buf, nm,
                            realpnp->pn_bufsize);
                if (conflict && direntflags)
                        *direntflags = ED_CASE_CONFLICT;
        }

        mutex_enter(&sdp->sd_lock);
        search.se_name = (char *)nm;
        if ((sep = avl_find(&sdp->sd_snaps, &search, &where)) != NULL) {
                *vpp = sep->se_root;
                VN_HOLD(*vpp);
                err = traverse(vpp);
                if (err) {
                        VN_RELE(*vpp);
                        *vpp = NULL;
                } else if (*vpp == sep->se_root) {
                        /*
                         * The snapshot was unmounted behind our backs,
                         * try to remount it.
                         */
                        goto domount;
                } else {
                        /*
                         * VROOT was set during the traverse call.  We need
                         * to clear it since we're pretending to be part
                         * of our parent's vfs.
                         */
                        (*vpp)->v_flag &= ~VROOT;
                }
                mutex_exit(&sdp->sd_lock);
                ZFS_EXIT(zfsvfs);
                return (err);
        }

        /*
         * The requested snapshot is not currently mounted, look it up.
         */
        err = zfsctl_snapshot_zname(dvp, nm, MAXNAMELEN, snapname);
        if (err) {
                mutex_exit(&sdp->sd_lock);
                ZFS_EXIT(zfsvfs);
                /*
                 * handle "ls *" or "?" in a graceful manner,
                 * forcing EILSEQ to ENOENT.
                 * Since shell ultimately passes "*" or "?" as name to lookup
                 */
                return (err == EILSEQ ? ENOENT : err);
        }
        if (dmu_objset_open(snapname, DMU_OST_ZFS,
            DS_MODE_USER | DS_MODE_READONLY, &snap) != 0) {
                mutex_exit(&sdp->sd_lock);
                ZFS_EXIT(zfsvfs);
                return (ENOENT);
        }

        sep = kmem_alloc(sizeof (zfs_snapentry_t), KM_SLEEP);
        sep->se_name = kmem_alloc(strlen(nm) + 1, KM_SLEEP);
        (void) strcpy(sep->se_name, nm);
        *vpp = sep->se_root = zfsctl_snapshot_mknode(dvp, dmu_objset_id(snap));
        avl_insert(&sdp->sd_snaps, sep, where);

        dmu_objset_close(snap);
domount:
        mountpoint_len = strlen(refstr_value(dvp->v_vfsp->vfs_mntpt)) +
            strlen("/.zfs/snapshot/") + strlen(nm) + 1;
        mountpoint = kmem_alloc(mountpoint_len, KM_SLEEP);
        (void) snprintf(mountpoint, mountpoint_len, "%s/.zfs/snapshot/%s",
            refstr_value(dvp->v_vfsp->vfs_mntpt), nm);

        margs.spec = snapname;
        margs.dir = mountpoint;
        margs.flags = MS_SYSSPACE | MS_NOMNTTAB;
        margs.fstype = "zfs";
        margs.dataptr = NULL;
        margs.datalen = 0;
        margs.optptr = NULL;
        margs.optlen = 0;

        err = domount("zfs", &margs, *vpp, kcred, &vfsp);
        kmem_free(mountpoint, mountpoint_len);

        if (err == 0) {
                /*
                 * Return the mounted root rather than the covered mount point.
                 * Takes the GFS vnode at .zfs/snapshot/<snapname> and returns
                 * the ZFS vnode mounted on top of the GFS node.  This ZFS
                 * vnode is the root the newly created vfsp.
                 */
                VFS_RELE(vfsp);
                err = traverse(vpp);
        }

        if (err == 0) {
                /*
                 * Fix up the root vnode mounted on .zfs/snapshot/<snapname>.
                 *
                 * This is where we lie about our v_vfsp in order to
                 * make .zfs/snapshot/<snapname> accessible over NFS
                 * without requiring manual mounts of <snapname>.
                 */
                ASSERT(VTOZ(*vpp)->z_zfsvfs != zfsvfs);
                VTOZ(*vpp)->z_zfsvfs->z_parent = zfsvfs;
                (*vpp)->v_vfsp = zfsvfs->z_vfs;
                (*vpp)->v_flag &= ~VROOT;
        }
        mutex_exit(&sdp->sd_lock);
        ZFS_EXIT(zfsvfs);

        /*
         * If we had an error, drop our hold on the vnode and
         * zfsctl_snapshot_inactive() will clean up.
         */
        if (err) {
                VN_RELE(*vpp);
                *vpp = NULL;
        }
        return (err);
}

/* ARGSUSED */
static int
zfsctl_snapdir_readdir_cb(vnode_t *vp, void *dp, int *eofp,
    offset_t *offp, offset_t *nextp, void *data, int flags)
{
        zfsvfs_t *zfsvfs = vp->v_vfsp->vfs_data;
        char snapname[MAXNAMELEN];
        uint64_t id, cookie;
        boolean_t case_conflict;
        int error;

        ZFS_ENTER(zfsvfs);

        cookie = *offp;
        error = dmu_snapshot_list_next(zfsvfs->z_os, MAXNAMELEN, snapname, &id,
            &cookie, &case_conflict);
        if (error) {
                ZFS_EXIT(zfsvfs);
                if (error == ENOENT) {
                        *eofp = 1;
                        return (0);
                }
                return (error);
        }

        if (flags & V_RDDIR_ENTFLAGS) {
                edirent_t *eodp = dp;

                (void) strcpy(eodp->ed_name, snapname);
                eodp->ed_ino = ZFSCTL_INO_SNAP(id);
                eodp->ed_eflags = case_conflict ? ED_CASE_CONFLICT : 0;
        } else {
                struct dirent64 *odp = dp;

                (void) strcpy(odp->d_name, snapname);
                odp->d_ino = ZFSCTL_INO_SNAP(id);
        }
        *nextp = cookie;

        ZFS_EXIT(zfsvfs);

        return (0);
}

/*
 * pvp is the '.zfs' directory (zfsctl_node_t).
 * Creates vp, which is '.zfs/snapshot' (zfsctl_snapdir_t).
 *
 * This function is the callback to create a GFS vnode for '.zfs/snapshot'
 * when a lookup is performed on .zfs for "snapshot".
 */
vnode_t *
zfsctl_mknode_snapdir(vnode_t *pvp)
{
        vnode_t *vp;
        zfsctl_snapdir_t *sdp;

        vp = gfs_dir_create(sizeof (zfsctl_snapdir_t), pvp,
            zfsctl_ops_snapdir, NULL, NULL, MAXNAMELEN,
            zfsctl_snapdir_readdir_cb, NULL);
        sdp = vp->v_data;
        sdp->sd_node.zc_id = ZFSCTL_INO_SNAPDIR;
        sdp->sd_node.zc_cmtime = ((zfsctl_node_t *)pvp->v_data)->zc_cmtime;
        mutex_init(&sdp->sd_lock, NULL, MUTEX_DEFAULT, NULL);
        avl_create(&sdp->sd_snaps, snapentry_compare,
            sizeof (zfs_snapentry_t), offsetof(zfs_snapentry_t, se_node));
        return (vp);
}

/* ARGSUSED */
static int
zfsctl_snapdir_getattr(vnode_t *vp, vattr_t *vap, int flags, cred_t *cr,
    caller_context_t *ct)
{
        zfsvfs_t *zfsvfs = vp->v_vfsp->vfs_data;
        zfsctl_snapdir_t *sdp = vp->v_data;

        ZFS_ENTER(zfsvfs);
        zfsctl_common_getattr(vp, vap);
        vap->va_nodeid = gfs_file_inode(vp);
        vap->va_nlink = vap->va_size = avl_numnodes(&sdp->sd_snaps) + 2;
        ZFS_EXIT(zfsvfs);

        return (0);
}

/* ARGSUSED */
static void
zfsctl_snapdir_inactive(vnode_t *vp, cred_t *cr, caller_context_t *ct)
{
        zfsctl_snapdir_t *sdp = vp->v_data;
        void *private;

        private = gfs_dir_inactive(vp);
        if (private != NULL) {
                ASSERT(avl_numnodes(&sdp->sd_snaps) == 0);
                mutex_destroy(&sdp->sd_lock);
                avl_destroy(&sdp->sd_snaps);
                kmem_free(private, sizeof (zfsctl_snapdir_t));
        }
}

static const fs_operation_def_t zfsctl_tops_snapdir[] = {
        { VOPNAME_OPEN,         { .vop_open = zfsctl_common_open }      },
        { VOPNAME_CLOSE,        { .vop_close = zfsctl_common_close }    },
        { VOPNAME_IOCTL,        { .error = fs_inval }                   },
        { VOPNAME_GETATTR,      { .vop_getattr = zfsctl_snapdir_getattr } },
        { VOPNAME_ACCESS,       { .vop_access = zfsctl_common_access }  },
        { VOPNAME_RENAME,       { .vop_rename = zfsctl_snapdir_rename } },
        { VOPNAME_RMDIR,        { .vop_rmdir = zfsctl_snapdir_remove }  },
        { VOPNAME_MKDIR,        { .vop_mkdir = zfsctl_snapdir_mkdir }   },
        { VOPNAME_READDIR,      { .vop_readdir = gfs_vop_readdir }      },
        { VOPNAME_LOOKUP,       { .vop_lookup = zfsctl_snapdir_lookup } },
        { VOPNAME_SEEK,         { .vop_seek = fs_seek }                 },
        { VOPNAME_INACTIVE,     { .vop_inactive = zfsctl_snapdir_inactive } },
        { VOPNAME_FID,          { .vop_fid = zfsctl_common_fid }        },
        { NULL }
};

/*
 * pvp is the GFS vnode '.zfs/snapshot'.
 *
 * This creates a GFS node under '.zfs/snapshot' representing each
 * snapshot.  This newly created GFS node is what we mount snapshot
 * vfs_t's ontop of.
 */
static vnode_t *
zfsctl_snapshot_mknode(vnode_t *pvp, uint64_t objset)
{
        vnode_t *vp;
        zfsctl_node_t *zcp;

        vp = gfs_dir_create(sizeof (zfsctl_node_t), pvp,
            zfsctl_ops_snapshot, NULL, NULL, MAXNAMELEN, NULL, NULL);
        zcp = vp->v_data;
        zcp->zc_id = objset;
        VFS_HOLD(vp->v_vfsp);

        return (vp);
}

static void
zfsctl_snapshot_inactive(vnode_t *vp, cred_t *cr, caller_context_t *ct)
{
        zfsctl_snapdir_t *sdp;
        zfs_snapentry_t *sep, *next;
        vnode_t *dvp;

        VERIFY(gfs_dir_lookup(vp, "..", &dvp, cr, 0, NULL, NULL) == 0);
        sdp = dvp->v_data;

        mutex_enter(&sdp->sd_lock);

        if (vp->v_count > 1) {
                mutex_exit(&sdp->sd_lock);
                return;
        }
        ASSERT(!vn_ismntpt(vp));

        sep = avl_first(&sdp->sd_snaps);
        while (sep != NULL) {
                next = AVL_NEXT(&sdp->sd_snaps, sep);

                if (sep->se_root == vp) {
                        avl_remove(&sdp->sd_snaps, sep);
                        kmem_free(sep->se_name, strlen(sep->se_name) + 1);
                        kmem_free(sep, sizeof (zfs_snapentry_t));
                        break;
                }
                sep = next;
        }
        ASSERT(sep != NULL);

        mutex_exit(&sdp->sd_lock);
        VN_RELE(dvp);
        VFS_RELE(vp->v_vfsp);

        /*
         * Dispose of the vnode for the snapshot mount point.
         * This is safe to do because once this entry has been removed
         * from the AVL tree, it can't be found again, so cannot become
         * "active".  If we lookup the same name again we will end up
         * creating a new vnode.
         */
        gfs_vop_inactive(vp, cr, ct);
}


/*
 * These VP's should never see the light of day.  They should always
 * be covered.
 */
static const fs_operation_def_t zfsctl_tops_snapshot[] = {
        VOPNAME_INACTIVE, { .vop_inactive =  zfsctl_snapshot_inactive },
        NULL, NULL
};

int
zfsctl_lookup_objset(vfs_t *vfsp, uint64_t objsetid, zfsvfs_t **zfsvfsp)
{
        zfsvfs_t *zfsvfs = vfsp->vfs_data;
        vnode_t *dvp, *vp;
        zfsctl_snapdir_t *sdp;
        zfsctl_node_t *zcp;
        zfs_snapentry_t *sep;
        int error;

        ASSERT(zfsvfs->z_ctldir != NULL);
        error = zfsctl_root_lookup(zfsvfs->z_ctldir, "snapshot", &dvp,
            NULL, 0, NULL, kcred, NULL, NULL, NULL);
        if (error != 0)
                return (error);
        sdp = dvp->v_data;

        mutex_enter(&sdp->sd_lock);
        sep = avl_first(&sdp->sd_snaps);
        while (sep != NULL) {
                vp = sep->se_root;
                zcp = vp->v_data;
                if (zcp->zc_id == objsetid)
                        break;

                sep = AVL_NEXT(&sdp->sd_snaps, sep);
        }

        if (sep != NULL) {
                VN_HOLD(vp);
                /*
                 * Return the mounted root rather than the covered mount point.
                 * Takes the GFS vnode at .zfs/snapshot/<snapshot objsetid>
                 * and returns the ZFS vnode mounted on top of the GFS node.
                 * This ZFS vnode is the root of the vfs for objset 'objsetid'.
                 */
                error = traverse(&vp);
                if (error == 0) {
                        if (vp == sep->se_root)
                                error = EINVAL;
                        else
                                *zfsvfsp = VTOZ(vp)->z_zfsvfs;
                }
                mutex_exit(&sdp->sd_lock);
                VN_RELE(vp);
        } else {
                error = EINVAL;
                mutex_exit(&sdp->sd_lock);
        }

        VN_RELE(dvp);

        return (error);
}

/*
 * Unmount any snapshots for the given filesystem.  This is called from
 * zfs_umount() - if we have a ctldir, then go through and unmount all the
 * snapshots.
 */
int
zfsctl_umount_snapshots(vfs_t *vfsp, int fflags, cred_t *cr)
{
        zfsvfs_t *zfsvfs = vfsp->vfs_data;
        vnode_t *dvp;
        zfsctl_snapdir_t *sdp;
        zfs_snapentry_t *sep, *next;
        int error;

        ASSERT(zfsvfs->z_ctldir != NULL);
        error = zfsctl_root_lookup(zfsvfs->z_ctldir, "snapshot", &dvp,
            NULL, 0, NULL, cr, NULL, NULL, NULL);
        if (error != 0)
                return (error);
        sdp = dvp->v_data;

        mutex_enter(&sdp->sd_lock);

        sep = avl_first(&sdp->sd_snaps);
        while (sep != NULL) {
                next = AVL_NEXT(&sdp->sd_snaps, sep);

                /*
                 * If this snapshot is not mounted, then it must
                 * have just been unmounted by somebody else, and
                 * will be cleaned up by zfsctl_snapdir_inactive().
                 */
                if (vn_ismntpt(sep->se_root)) {
                        avl_remove(&sdp->sd_snaps, sep);
                        error = zfsctl_unmount_snap(sep, fflags, cr);
                        if (error) {
                                avl_add(&sdp->sd_snaps, sep);
                                break;
                        }
                }
                sep = next;
        }

        mutex_exit(&sdp->sd_lock);
        VN_RELE(dvp);

        return (error);
}