diff --git a/share/man/man9/namei.9 b/share/man/man9/namei.9
--- a/share/man/man9/namei.9
+++ b/share/man/man9/namei.9
@@ -281,6 +281,15 @@
 .Dv O_RESOLVE_BENEATH
 flag for
 .Xr openat 2 .
+.It Dv NAMEILOOKUP
+The component is embedded in a
+.Nm
+lookup structure, and the
+.Fn vfs_lookup_nameidata
+function can be used to obtain that structure.
+This can be useful in
+.Xr VOP_LOOKUP 9
+implementations which need to obtain extra lookup metadata.
 .El
 .Sh PARAMETERS DESCRIPTORS FLAGS
 These flags are used for several purposes.
diff --git a/sys/fs/nullfs/null_vnops.c b/sys/fs/nullfs/null_vnops.c
--- a/sys/fs/nullfs/null_vnops.c
+++ b/sys/fs/nullfs/null_vnops.c
@@ -403,17 +403,25 @@
 
 	/*
 	 * 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 VOP, at least passing over lower
-	 * root.
+	 * 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 ((ldvp->v_vflag & VV_ROOT) != 0 && (flags & ISDOTDOT) != 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);
+	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);
 	}
 
 	/*
diff --git a/sys/fs/unionfs/union_vnops.c b/sys/fs/unionfs/union_vnops.c
--- a/sys/fs/unionfs/union_vnops.c
+++ b/sys/fs/unionfs/union_vnops.c
@@ -76,6 +76,21 @@
 	VNASSERT(((vp)->v_op == &unionfs_vnodeops), vp, \
 	    ("%s: non-unionfs vnode", __func__))
 
+static bool
+unionfs_lookup_isroot(struct componentname *cnp, struct vnode *dvp)
+{
+	struct nameidata *ndp;
+
+	if (dvp == NULL)
+		return (false);
+	if ((dvp->v_vflag & VV_ROOT) != 0)
+		return (true);
+	ndp = vfs_lookup_nameidata(cnp);
+	if (ndp == NULL)
+		return (false);
+	return (vfs_lookup_isroot(ndp, dvp));
+}
+
 static int
 unionfs_lookup(struct vop_cachedlookup_args *ap)
 {
@@ -149,6 +164,12 @@
 			goto unionfs_lookup_return;
 		}
 
+		if (unionfs_lookup_isroot(cnp, udvp) ||
+		    unionfs_lookup_isroot(cnp, ldvp)) {
+			error = ENOENT;
+			goto unionfs_lookup_return;
+		}
+
 		if (udvp != NULLVP)
 			dtmpvp = udvp;
 		else
diff --git a/sys/kern/vfs_cache.c b/sys/kern/vfs_cache.c
--- a/sys/kern/vfs_cache.c
+++ b/sys/kern/vfs_cache.c
@@ -5276,7 +5276,6 @@
 	struct componentname *cnp;
 	struct namecache *ncp;
 	struct vnode *dvp;
-	struct prison *pr;
 	u_char nc_flag;
 
 	ndp = fpl->ndp;
@@ -5288,15 +5287,7 @@
 	/*
 	 * XXX this is racy the same way regular lookup is
 	 */
-	for (pr = cnp->cn_cred->cr_prison; pr != NULL;
-	    pr = pr->pr_parent)
-		if (dvp == pr->pr_root)
-			break;
-
-	if (dvp == ndp->ni_rootdir ||
-	    dvp == ndp->ni_topdir ||
-	    dvp == rootvnode ||
-	    pr != NULL) {
+	if (vfs_lookup_isroot(ndp, dvp)) {
 		fpl->tvp = dvp;
 		fpl->tvp_seqc = vn_seqc_read_any(dvp);
 		if (seqc_in_modify(fpl->tvp_seqc)) {
diff --git a/sys/kern/vfs_lookup.c b/sys/kern/vfs_lookup.c
--- a/sys/kern/vfs_lookup.c
+++ b/sys/kern/vfs_lookup.c
@@ -611,12 +611,12 @@
 	}
 #endif
 	ndp->ni_cnd.cn_cred = td->td_ucred;
-	KASSERT(ndp->ni_resflags == 0, ("%s: garbage in ni_resflags: %x\n",
+	KASSERT(ndp->ni_resflags == 0, ("%s: garbage in ni_resflags: %x",
 	    __func__, ndp->ni_resflags));
 	KASSERT(cnp->cn_cred && td->td_proc, ("namei: bad cred/proc"));
 	KASSERT((cnp->cn_flags & NAMEI_INTERNAL_FLAGS) == 0,
-	    ("namei: unexpected flags: %" PRIx64 "\n",
-	    cnp->cn_flags & NAMEI_INTERNAL_FLAGS));
+	    ("namei: unexpected flags: %#jx",
+	    (uintmax_t)(cnp->cn_flags & NAMEI_INTERNAL_FLAGS)));
 	if (cnp->cn_flags & NOCACHE)
 		KASSERT(cnp->cn_nameiop != LOOKUP,
 		    ("%s: NOCACHE passed with LOOKUP", __func__));
@@ -862,6 +862,30 @@
 	return (error);
 }
 
+struct nameidata *
+vfs_lookup_nameidata(struct componentname *cnp)
+{
+	if ((cnp->cn_flags & NAMEILOOKUP) == 0)
+		return (NULL);
+	return (__containerof(cnp, struct nameidata, ni_cnd));
+}
+
+/*
+ * Would a dotdot lookup relative to dvp cause this lookup to cross a jail or
+ * chroot boundary?
+ */
+bool
+vfs_lookup_isroot(struct nameidata *ndp, struct vnode *dvp)
+{
+	for (struct prison *pr = ndp->ni_cnd.cn_cred->cr_prison; pr != NULL;
+	    pr = pr->pr_parent) {
+		if (dvp == pr->pr_root)
+			return (true);
+	}
+	return (dvp == ndp->ni_rootdir || dvp == ndp->ni_topdir ||
+	    dvp == rootvnode);
+}
+
 /*
  * FAILIFEXISTS handling.
  *
@@ -1020,7 +1044,6 @@
 	char *lastchar;			/* location of the last character */
 	struct vnode *dp = NULL;	/* the directory we are searching */
 	struct vnode *tdp;		/* saved dp */
-	struct prison *pr;
 	size_t prev_ni_pathlen;		/* saved ndp->ni_pathlen */
 	int docache;			/* == 0 do not cache last component */
 	int wantparent;			/* 1 => wantparent or lockparent flag */
@@ -1206,13 +1229,9 @@
 			goto bad;
 		}
 		for (;;) {
-			for (pr = cnp->cn_cred->cr_prison; pr != NULL;
-			     pr = pr->pr_parent)
-				if (dp == pr->pr_root)
-					break;
-			bool isroot = dp == ndp->ni_rootdir ||
-			    dp == ndp->ni_topdir || dp == rootvnode ||
-			    pr != NULL;
+			bool isroot;
+
+			isroot = vfs_lookup_isroot(ndp, dp);
 			if (__predict_false(isroot && (ndp->ni_lcf &
 			    (NI_LCF_STRICTREL | NI_LCF_STRICTREL_KTR)) != 0)) {
 				if ((ndp->ni_lcf & NI_LCF_STRICTREL_KTR) != 0)
diff --git a/sys/sys/namei.h b/sys/sys/namei.h
--- a/sys/sys/namei.h
+++ b/sys/sys/namei.h
@@ -152,6 +152,7 @@
 #define	LOCKSHARED	0x0100	/* Shared lock leaf */
 #define	NOFOLLOW	0x0000	/* do not follow symbolic links (pseudo) */
 #define	RBENEATH	0x100000000ULL /* No escape, even tmp, from start dir */
+#define	NAMEILOOKUP	0x200000000ULL /* cnp is embedded in nameidata */
 #define	MODMASK		0xf000001ffULL	/* mask of operational modifiers */
 
 /*
@@ -249,7 +250,7 @@
 	NDINIT_PREFILL(_ndp);							\
 	NDINIT_DBG(_ndp);							\
 	_ndp->ni_cnd.cn_nameiop = op;						\
-	_ndp->ni_cnd.cn_flags = flags;						\
+	_ndp->ni_cnd.cn_flags = (flags) | NAMEILOOKUP;				\
 	_ndp->ni_segflg = segflg;						\
 	_ndp->ni_dirp = namep;							\
 	_ndp->ni_dirfd = dirfd;							\
@@ -286,6 +287,8 @@
 
 int	namei(struct nameidata *ndp);
 int	vfs_lookup(struct nameidata *ndp);
+bool	vfs_lookup_isroot(struct nameidata *ndp, struct vnode *dvp);
+struct nameidata *vfs_lookup_nameidata(struct componentname *cnp);
 int	vfs_relookup(struct vnode *dvp, struct vnode **vpp,
 	    struct componentname *cnp, bool refstart);