Index: sys/kern/vfs_lookup.c
===================================================================
--- sys/kern/vfs_lookup.c
+++ sys/kern/vfs_lookup.c
@@ -182,7 +182,7 @@
 	 */
 	if (error == 0 && IN_CAPABILITY_MODE(td) &&
 	    (cnp->cn_flags & NOCAPCHECK) == 0) {
-		ndp->ni_strictrelative = 1;
+		ndrequire_strict_relative_lookups(ndp, ENOTCAPABLE);
 		if (ndp->ni_dirfd == AT_FDCWD) {
 #ifdef KTRACE
 			if (KTRPOINT(td, KTR_CAPFAIL))
@@ -248,7 +248,7 @@
 			    &rights) ||
 			    ndp->ni_filecaps.fc_fcntls != CAP_FCNTL_ALL ||
 			    ndp->ni_filecaps.fc_nioctls != -1) {
-				ndp->ni_strictrelative = 1;
+				ndrequire_strict_relative_lookups(ndp, ENOTCAPABLE);
 			}
 #endif
 		}
@@ -287,7 +287,7 @@
 					ktrcapfail(CAPFAIL_LOOKUP, NULL, NULL);
 #endif
 				namei_cleanup_cnp(cnp);
-				return (ENOTCAPABLE);
+				return (ndp->ni_strictrelative);
 			}
 			while (*(cnp->cn_nameptr) == '/') {
 				cnp->cn_nameptr++;
@@ -627,7 +627,7 @@
 			if (KTRPOINT(curthread, KTR_CAPFAIL))
 				ktrcapfail(CAPFAIL_LOOKUP, NULL, NULL);
 #endif
-			error = ENOTCAPABLE;
+			error = ndp->ni_strictrelative;
 			goto bad;
 		}
 		if ((cnp->cn_flags & ISLASTCN) != 0 &&
@@ -1123,6 +1123,38 @@
 	}
 }
 
+void
+ndrequire_strict_relative_lookups(struct nameidata *ndp, int errnum)
+{
+	/*
+	 * Monotonic ordering of possible field values:
+	 *
+	 * EPERM - user-level code has requested an O_BENEATH lookup
+	 * ENOTCAPABLE - Capsicum demands strict relative lookups
+	 * 0 - allow relative lookups
+	 *
+	 * We can always move up this list but never down.
+	 */
+
+	KASSERT((errnum == EPERM) || (errnum == ENOTCAPABLE),
+	        ("invalid errno (not EPERM or ENOTCAPABLE)"));
+
+	switch (errnum) {
+	case EPERM:
+		ndp->ni_strictrelative = errnum;
+		break;
+
+	case ENOTCAPABLE:
+		if (ndp->ni_strictrelative != EPERM) {
+			ndp->ni_strictrelative = errnum;
+		}
+		break;
+
+	default:
+		KASSERT(false, ("invalid errno for strict relative lookups"));
+	}
+}
+
 /*
  * Determine if there is a suitable alternate filename under the specified
  * prefix for the specified path.  If the create flag is set, then the
Index: sys/kern/vfs_syscalls.c
===================================================================
--- sys/kern/vfs_syscalls.c
+++ sys/kern/vfs_syscalls.c
@@ -1049,7 +1049,7 @@
 	struct vnode *vp;
 	struct nameidata nd;
 	cap_rights_t rights;
-	int cmode, error, indx;
+	int beneath, cmode, error, indx;
 
 	indx = -1;
 
@@ -1058,6 +1058,7 @@
 	/* XXX: audit dirfd */
 	cap_rights_init(&rights, CAP_LOOKUP);
 	flags_to_rights(flags, &rights);
+	beneath = ((flags & O_BENEATH) == O_BENEATH);
 	/*
 	 * Only one of the O_EXEC, O_RDONLY, O_WRONLY and O_RDWR flags
 	 * may be specified.
@@ -1086,6 +1087,9 @@
 	cmode = ((mode & ~fdp->fd_cmask) & ALLPERMS) & ~S_ISTXT;
 	NDINIT_ATRIGHTS(&nd, LOOKUP, FOLLOW | AUDITVNODE1, pathseg, path, fd,
 	    &rights, td);
+	if (beneath) {
+		ndrequire_strict_relative_lookups(&nd, EPERM);
+	}
 	td->td_dupfd = -1;		/* XXX check for fdopen */
 	error = vn_open(&nd, &flags, cmode, fp);
 	if (error != 0) {
@@ -1100,7 +1104,7 @@
 		/*
 		 * Handle special fdopen() case. bleh.
 		 *
-		 * Don't do this for relative (capability) lookups; we don't
+		 * Don't do this for capability or O_BENEATH lookups: we don't
 		 * understand exactly what would happen, and we don't think
 		 * that it ever should.
 		 */
@@ -1150,8 +1154,11 @@
 		struct filecaps *fcaps;
 
 #ifdef CAPABILITIES
-		if (nd.ni_strictrelative == 1)
+		if (nd.ni_strictrelative != 0 &&
+			cap_rights_is_valid(&nd.ni_filecaps.fc_rights))
+		{
 			fcaps = &nd.ni_filecaps;
+		}
 		else
 #endif
 			fcaps = NULL;
Index: sys/sys/fcntl.h
===================================================================
--- sys/sys/fcntl.h
+++ sys/sys/fcntl.h
@@ -131,6 +131,7 @@
 
 #if __BSD_VISIBLE
 #define	O_VERIFY	0x00200000	/* open only after verification */
+#define	O_BENEATH	0x00400000	/* stay beneath a specified directory */
 #endif
 
 /*
Index: sys/sys/namei.h
===================================================================
--- sys/sys/namei.h
+++ sys/sys/namei.h
@@ -150,7 +150,8 @@
 #define	AUDITVNODE2	0x08000000 /* audit the looked up vnode information */
 #define	TRAILINGSLASH	0x10000000 /* path ended in a slash */
 #define	NOCAPCHECK	0x20000000 /* do not perform capability checks */
-#define	PARAMASK	0x3ffffe00 /* mask of parameter descriptors */
+#define	BENEATH		0x40000000 /* strict downwards-only lookup */
+#define	PARAMASK	0x7ffffe00 /* mask of parameter descriptors */
 
 /*
  * Initialization of a nameidata structure.
@@ -168,6 +169,12 @@
     enum uio_seg segflg, const char *namep, int dirfd, struct vnode *startdir,
     cap_rights_t *rightsp, struct thread *td);
 
+/*
+ * Set the ni_strictrelative field of a struct nameidata according to a
+ * monotonically-increasing policy.
+ */
+void ndrequire_strict_relative_lookups(struct nameidata*, int errnum);
+
 #define NDF_NO_DVP_RELE		0x00000001
 #define NDF_NO_DVP_UNLOCK	0x00000002
 #define NDF_NO_DVP_PUT		0x00000003