nullfs: add nounixbypass mount option

The option, when set, disables bypassing the unix socket vnode down
to the lower mp, effectively preventing connection to nullfs unix
socket from being acceptable from the lower mp (and vice versa).

This is done by providing a vop vector that stops bypass for unp-related
VOPs.  I believe that VFS_VOP_VECTOR_REGISTER() does the right thing
there regardless of the order of initialization.

Reviewed by:	markj
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Differential revision:	https://reviews.freebsd.org/D52983

Author: kostikbel

sys/fs/nullfs/null.h

@@ -39,6 +39,7 @@
 #include <vm/uma.h>
 
 #define	NULLM_CACHE		0x0001
+#define	NULLM_NOUNPBYPASS	0x0002
 
 struct null_mount {
 	struct mount	*nullm_vfs;
@@ -90,7 +91,7 @@ null_is_nullfs_vnode(struct vnode *vp)
 	const struct vop_vector *op;
 
 	op = vp->v_op;
-	return (op == &null_vnodeops);
+	return (op == &null_vnodeops || op == &null_vnodeops_no_unp_bypass);
 }
 
 extern uma_zone_t null_node_zone;

sys/fs/nullfs/null_subr.c

@@ -240,7 +240,9 @@ null_nodeget(struct mount *mp, struct vnode *lowervp, struct vnode **vpp)
 	 */
 	xp = uma_zalloc_smr(null_node_zone, M_WAITOK);
 
-	error = getnewvnode("nullfs", mp, &null_vnodeops, &vp);
+	error = getnewvnode("nullfs", mp, (MOUNTTONULLMOUNT(mp)->nullm_flags &
+	    NULLM_NOUNPBYPASS) != 0 ? &null_vnodeops_no_unp_bypass :
+	    &null_vnodeops, &vp);
 	if (error) {
 		vput(lowervp);
 		uma_zfree_smr(null_node_zone, xp);

sys/fs/nullfs/null_vfsops.c

@@ -87,6 +87,8 @@ nullfs_mount(struct mount *mp)
 	bool isvnunlocked;
 	static const char cache_opt_name[] = "cache";
 	static const char nocache_opt_name[] = "nocache";
+	static const char unixbypass_opt_name[] = "unixbypass";
+	static const char nounixbypass_opt_name[] = "nounixbypass";
 
 	NULLFSDEBUG("nullfs_mount(mp = %p)\n", (void *)mp);
 
@@ -222,6 +224,13 @@ nullfs_mount(struct mount *mp)
 		    &xmp->notify_node);
 	}
 
+	if (vfs_getopt(mp->mnt_optnew, unixbypass_opt_name, NULL, NULL) == 0) {
+		;
+	} else if (vfs_getopt(mp->mnt_optnew, nounixbypass_opt_name, NULL,
+	    NULL) == 0) {
+		xmp->nullm_flags |= NULLM_NOUNPBYPASS;
+	}
+
 	if (lowerrootvp == mp->mnt_vnodecovered) {
 		vn_lock(lowerrootvp, LK_EXCLUSIVE | LK_RETRY | LK_CANRECURSE);
 		lowerrootvp->v_vflag |= VV_CROSSLOCK;

sys/fs/nullfs/null_vnops.c

@@ -1256,3 +1256,11 @@ struct vop_vector null_vnodeops = {
 	.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);